EAB CONTROL COST MANUAL
(Third Edition)
EPA 450/5-87-001A
U.S. Environmental Protection Agency
Office of Air and Radiation
Office of Air Quality Planning and Standards
Economic Analysis Branch
Research Triangle Park, North Carolina 27711
February 1987
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NOTE:
A preliminary draft of this Manual was distributed in November 1986.
Since then, extensive revisions have been made to Section 5 ("Fabric
Filters") and minor changes to Sections 3 ("Thermal and Catalytic
Incinerators") and 4 ("Carbon Adsorbers"). No revisions were made to
Sections 1 and 2, however.
The pages in this Manual have been fastened together. If desired,
they may be unfastened, hole-punched, and reassembled in a three-ring
binder. This will facilitate the addition of updates when they become
available.
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OVERVIEW OF THE MANUAL*
Completion
Section Date
1 INTRODUCTION 1/86
2 MANUAL ESTIMATING METHODOLOGY 1/86
3 THERMAL AND CATALYTIC INCINERATORS 7/86
4 CARBON ADSORBERS 7/86
5 FABRIC FILTERS 9/86
6 ELECTROSTATIC PRECIPITATORS
7 DUCTWORK
8 FANS
9 FLARES
10 REFRIGERATION UNITS
11 STACKS
12 PUMPS
13 GAS ABSORBERS
14 VENTURI SCRUBBERS
15 WASTEWATER TREATMENT FACILITIES**
16 CAPTURE HOODS
17 MECHANICAL COLLECTORS
18 PRECOOLERS (SPRAY CHAMBERS AND QUENCHERS)
19 SCREW CONVEYORS
20 COMPREHENSIVE SAMPLE COST-ESTIMATING PROBLEMS
21 ESCALATING COSTS
APPENDIX A COMPOUND INTEREST FACTORS
APPENDIX B EQUIPMENT COST INDICIES
APPENDIX C CONTROL EQUIPMENT VENDORS
Date To Be
Completed
FY 87
FY 87
FY 87
FY 87
FY 87
FY 88
FY 88
FY 88
FY 88
FY 88
Not Established
Not Established
Not Established
Not Established
Not Established
Not Established
Not Established
Not Established
Not Established
*For more information about the Manual, contact Albert Wehe, Robert Pahel-
Short, or William Vatavuk (at FTS:629-5610).
**These are facilities that would be required to treat only the wastewater
generated by venturi scrubbers, quenchers, spray chambers, and gas absorbers.
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TABLE OF CONTENTS
Section
1 INTRODUCTION
1.1 Role of Cost in the Setting of Regulations 1-1
1.2 Purpose of Manual 1-2
1.3 Organization of the Manual 1-2
1.4 "Uniqueness" of the Manual 1-5
References for Section 1 1-7
2 MANUAL ESTIMATING METHODOLOGY
2.1 Types of Cost Estimates 2-1
2.2 Cost Categories Defined 2-3
2.2.1 Elements of Total Capital Investment 2-3
2.2.2 Elements of Total Annual Cost 2-7
2.3 Engineering Economy Concepts 2-9
2.3.1 Time Value of Money 2-9
2.3.2 Cash Flow 2-9
2.3.3 Annualization Methods 2-14
2.4 Estimating Procedure 2-15
2.4.1 Facility Parameters and Regulatory Options 2-15
2.4.2 Control System Design 2-17
2.4.3 Sizing the Control System 2-18
2.4.4 Estimating Total Capital Investment 2-20
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TABLE OF CONTENTS
Section Page
2 (Cont'd)
2.4.5 Retrofit Cost Considerations 2-23
2.4.6 Estimating Annual Costs 2-25
References for Section 2 . 2-33
3 THERMAL AND CATALYIC INCINERATORS
3.1 Process Description 3-1
3.1.1 Elements of Combustion 3-1
3.1.2 Types of Incinerators 3-6
3.2 Design Procedure 3-8
3.2.1 Design and Operating Features 3-8
3.2.2 Design Calculations ' 3-10
3.3 Estimating Total Capital Investment 3-23
3.3.1 Thermal Incinerator Equipment Costs 3-24
3.3.2 Catalytic Incinerator Equipment Costs 3-27
3.3.3 Total Capital Investment for Incinerators 3-29
3.4 Estimating Total Annual Cost 3-30
3.4.1 Direct Annual Costs 3-30
3.4.2 Indirect Annual Costs 3-34
References for Section 3 3-35
Appendix 3A
Appendix 3B
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TABLE OF CONTENTS
Section
4 CARBON ADSORBERS
4.1 Process Description 4-1
4.1.1 Introduction 4-1
4.1.2 Types of Adsorbers 4-2
4.1.3 Adsorption Theory 4-6
4.2 Design Procedure 4-11
4.2.1 Sizing Parameters 4-11
4.2.2 Determining Adsorption and Desorption Times 4-13
4.2.3 Estimating Carbon Requirement 4-15
4.3 Estimating Total Capital Investment 4-20
4.3.1 Fixed-Bed Systems 4-21
4.3.2 Cannister Systems 4-26
4.4 Estimating Total Annual Cost 4-28
4.4.1 Direct Annual Costs 4-28
4.4.2 Indirect Annual Costs 4-34
4.4.3 Recovery Credits 4-35
4.4.4 Total Annual Cost 4-36
References for Section 4 4-37
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TABLE OF CONTENTS
Section Page
5 FABRIC FILTERS
5.1 Process Description 5-1
5.1.1 Introduction 5-1
5.1.2 Types of Fabric Filters 5-2
5.1.3 Auxiliary Equipment 5-5
5.1.4 Fabric Filtration Theory 5-7
5.2 Design Procedures 5-15
5.2.1 Gas-to-Cloth Ratio 5-15
5.2.2 Pressure Drop 5-23
5.2.3 Particle Characteristics 5-24
5.2.4 Gas Stream Characteristics 5-25
5.2.5 Pressure or Suction Housings 5-27
5.2.6 Standard or Custom Construction 5-28
5.2.7 Filter Media 5-29
5.3 Estimating Total Capital Investment 5-30
5.3.1 Equipment Cost 5-30
5.3.2 Total Purchased Cost 5-40
5.3.3 Total Capital Investment 5-41
5.4 Estimating Total Annual Costs ' 5-41
5.4.1 Direct Annual Cost 5-41
5.4.2 Indirect Annual Cost 5-46
5.4.3 Recovery Credits 5-47
5.4.4 Total Annual Cost 5-47
5.4.5 Example Problem 5-48
References for Section 5 5-54
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Section 1
INTRODUCTION
William M. Vatavuk
Economic Analysis Branch, OAQPS
1.1 Role of Cost in the Setting of Regulations
Cost has an important role in setting many state and federal air
pollution control regulations. The extent of this role varies with the
type of regulation. For some types of regulations, cost is expl icitly used
in determining their stringency. This use may involve a balancing of costs
and environmental impacts, costs and dollar valuation of benefits, or
environmental impacts and economic consequences of control costs.
For other types of regulations cost analysis is used to choose among
alternative regulations with the same level of stringency. For these
regulations, the environmental goal is determined by some set of criteria
which do not include costs. However, cost-effectiveness analysis is employed
to determine the minimum cost way of achieving the goal.
For some regulations, cost influences enforcement procedures or
requirements for demonstration of progress towards compliance with an air
quality standard. For example, the size of any monetary penalty assessed
for noncompliance as part of an enforcement action needs to be set with
awareness of the magnitude of the control costs being postponed by the non-
complying facility. For regulations without a fixed compliance schedule,
demonstration of reasonable progress towards the goal is sometimes tied to
the cost of attaining the goal on different schedules.
Cost is a vital input into two other types of analyses that also
sometimes have a role in standard setting. Cost is needed for a benefit-cost
analysis that addresses the economic efficiency of alternative regulations.
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Cost is also an input into any analysis of the economic impact of each
regulatory alternative. An economic impact analysis deals with the consequences
of the regulation for small businesses, employment, prices, and industry
structure.
1.2 Purpose of Manual
The purpose of this Manual is two-fold: (1) to compile up-to-date
capital costs, operating and maintenance expenses, and other costs for
"add-on" air pollution control systems and (2) to provide a comprehensive,
concise, consistent, and easy-to-use procedure for estimating and (where
appropriate) escalating these costs. ("Add-on" systems are those installed
downstream of an air pollution source to control its emissions.)
The Manual estimating procedure rests on the notion of the "factored" or
"study" estimate, nominally accurate to within +_ 30%. This type of estimate
is well suited to estimating control system costs intended for use in
regulatory development. Study estimates are sufficiently accurate, yet do
not require the detailed, site-specific data inputs needed to make "definitive"
or other more accurate types of estimates.
1.3 Organization of the Manual
This Manual is a major revision of the 1978 edition of the EAB Control
Cost Manual,U) which, in turn, was a revision of the original edition,
completed in 1976. This third edition of the Manual includes a more thorough
discussion of estimating methodology and more detailed design procedures
for an enlarged set of equipment types. The appendices have been revised
to delete some infrequently used material, and to include certain new and
more useful material.
The format of the Manual has been changed to one which will permit more
flexibility in its updating and expansion. To achieve this flexibility
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this edition will be issued in self-contained sections. Each section will
address a logically separate topic, which can be either of a general nature
(e.g., this introduction) or of a more specific, equipment-oriented nature
(e.g., fabric filters). The sections which comprise this portion of the
Manual are listed in Table 1-1, alongside the sections in the 1978 Manual
they will replace.
Two changes in the 1978 Manual are indicated by Table 1-1. First,
the numbering scheme is different. In the third edition, each type of
equipment, background topic, etc., is given its own number, for ease of
identification and to reinforce the intent that each section should "stand
alone". Second, the auxiliary equipment items (e.g., ductwork), which were
collected into a single chapter in the previous Manuals, are now also stand-
alone sections. This was mainly done to eliminate the confusion that arises
when classifying auxiliaries like mechanical collectors which can either
support a primary control device or be control devices in their own right.
The notion of a stand alone section is also new. Where in the 1976
and 1978 Manuals, the various capital and annual cost factors were collected
in an introductory section (old Section 3), now they are dispersed among the
sections covering the equipment types. Each of these sections contains a:
o Process description, where the types, uses, and operating modes of
the equipment item and (if applicable) its auxiliaries are discussed;
o Design procedure, which enables one to use the parameters of the
pollution source (e.g., gas volumetric flowrate) to size the equipment
item(s) in question;
o Capital and annual costs for the equipment and suggested factors to
use in estimating these costs from equipment design and operational
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Table 1-1 Format of the EAB Control Cost Manual (Third Edition)
Number
1
2
3
4
5
New Section
Title
"Introduction"
"Manual Estimating Methodology"
"Thermal and Catalytic Incinerators"
"Carbon Adsorbers"
"Fabric Filters"
(Other sections to be developed)
Number(s)
1
2,3
5.4
5.5
5.3
Old Section(s) Replaced
Title
"Introduction"
"Application to Industry"; "Cost Estimating
Procedures"
"Thermal and Catalytic Incinerator Systems"
"Adsorbers"
"Fabric Filters"
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1-5
(e.g., operating hours) parameters. These costs are presented in
both graphical and equational forms wherever possible.
1.4 "Uniqueness" of the Manual
The Manual presents a different perspective on estimating air pollution
control system costs than other cost-oriented reports, such as:
o The Cost Digest: Cost Summaries of Selected Envi ronmental Control
Technologies^)
o A Standard Procedure for Cost Analysis of Pollution Control Operations^)
o Evaluation of Control Technologies for Hazardous Air Pollutants^4)
Although these reports (as well as many of the NSPS Background
Information Documents) contain costs for add-on control systems, they do
not duplicate the Manual for one or more of the following reasons: (1)
their costs have been based either wholly or partly on data in the previous
Manuals; (2) they apply to specific source categories only, whereas the
Manual data may be applied generally; (3) their estimating procedures and
costs are of less than study estimate quality; or (4) they are not intended for
estimating costs used in regulatory development.
Reason (3) applies to the Cost Digest, for example, as this report,
designed for use by non-technical personnel, contains procedures for making
"order-of-magnitude" estimates (+_ 30% accuracy or worse). A Standard Procedure,
conversely, was primarily intended for estimating costs for R&D cases (e.g.,
demonstration projects), where some site-specific data are available. Further,
although the latter report contains a thorough list of equipment installation
factors, it contains few equipment costs. The report, Evaluation of Control
Technologies, used data and estimating procedures from the 1978 Manual to
provide sound generalized procedures for estimating thermal and catalytic
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1-6
incinerator costs. The third edition of the Manual updates and expands
this information.
Finally, the second edition of the Manual (published December 1978),
was one of the earliest of its kind. It has been extensively used in
Agency regulatory development efforts. Accordingly, the Manual 's role in
the speciality of air pollution control system cost estimating is both
unique and secure.
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1-7
References for Section 1:
1. Neveril, R. B. (GARD, Inc.) Capital and Operating Costs for Selected
Air Pollution Control Systems, EPA, Office of Air Quality Planning
and Standards, Economic Analysis Branch, December 1978 (EPA 450/5-
80-002).
2. DeWolf, Glenn, et al. (Radian, Inc.) The Cost Digest: Cost Summaries
of Selected Environmental Control Technologies. EPA, ORD, Office of
Environmental Engineering and Technology, October 1984 (EPA-600/884-010).
3. Vhl, Vincent W. ^ Standard Procedure for Cost Analysis ^f Poll ution
Control Operations, Volumes I and II. EPA, ORD, Industrial Environmental
Research Laboratory, June 1979 (EPA-600/8-79-018a).
4. Katari, Vishnu (Pacific Environmental Services) Evaluation of Control
Technologies for Hazardous Air Pollutants. EPA, Air and Energy Engineering
Research Laboratory, ORD, October 1985.
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Section 2
MANUAL ESTIMATING METHODOLOGY
William M. Vatavuk
Economic Analysis Branch, OAQPS
This section presents a methodology that will enable the user, having
knowledge of the source being controlled, to produce study-level cost
estimates for a control system to control that source. The methodology,
which applies to each of the control systems included in this Manual, is
general enough to be used with other "add-on" systems as well. Further,
the methodology may also be applicable to estimating costs of fugitive
emission controls and of other nonstack abatement methods.
Before presenting this methodology in detail, we should first discuss
the various kinds of cost estimates and then define the cost categories and
engineering economy concepts employed in making the estimates.
2.1 Types of Cost Estimates
As noted above, the costs and estimating methodology in this Manual
are directed toward the "study11 estimate, of +_ 30% accuracy. According to
Perry's Chemical Engineer's Handbook, a study estimate is ".... used to
estimate the economic feasibility of a project before expending significant
funds for piloting, marketing, land surveys, and acquisition ... [However]
it can be prepared at relatively low cost with minimum data."(l) Specifically,
to make a study estimate, the following must be known:
o Location of the source within the plant;
o Rough sketch of the process flow sheet (i.e., the relative locations
of the equipment in the system);
o Preliminary sizes of, and material specifications for, the system
equipment items;
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2-2
o Approximate sizes and types of construction of any buildings required
to house the control system;
o Rough estimates of utility requirements (e.g., electricity);
o Preliminary flow sheet and specifications for ducting and piping;
o Approximate sizes of motors required.(l)
In addition, an estimate of the labor hours required for engineering
and drafting is needed, as the accuracy of an estimate (study or otherwise)
is highly dependent on the amount of engineering work expended on the
project.
There are, however, four other types of estimates, three of which
are more accurate than the study estimate. These are:(l)
o "Order-of-magnitude"--"a rule of-thumb procedure applied only to
repetitive types of plant installations for which there exists good
cost history". Its accuracy is >+30%. (However, according to
Perry's, "...no limits of accuracy can safely be applied to it.")
The sole input required for making this level of estimate is the
control system's capacity (often measured by the maximum volumetric
fl owrate of the gas passing through the system). So-called "six-
tenths factor" estimates (not to be confused with factored
estimates) are examples of this type.
o "Scope'V'Budget authorization"/"Preliminary". This estimate,
nominally of _+ 20% accuracy, requires more detailed knowledge than
the study estimate regarding the site, flow sheet, equipment,
buildings, etc. In addition, rough specifications for the insulation
. and instrumentation are also needed.
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2-3
o "Project control "/"Definitive" estimates, accurate to within + 10%,
require yet more information than the scope estimates, especially
concerning the site, equipment, and electrical requirements.
o "Firm"/"Contractor's"/"Detai1ed". This is the most accurate (+_ 5%)
of the estimate types, requiring complete drawings, specifications,
and site surveys. Further, "[t]ime seldom permits the preparation
of such estimates prior to an approval to proceed with the project."^)
For the purposes of regulatory development, study estimates have been
found to be acceptable, as they represent a compromise between the less
accurate "order-of-magnitude" and the more accurate estimate types. The
former are too imprecise to be of much value, while the latter are not only
very expensive to make, but require detailed site and process-specific
knowledge that most Manual users will not have available to them.
2.2 Cost Categories Defined
The names given certain "categories" of costs and what they contain
vary considerably throughout the literature. Certain words like "capital
cost" can have vastly different meanings, which can often lead to confusion,
even among cost estimators. To avoid this confusion and, at the same time,
provide uniformity in the Manual , basic terms are defined in this Section
and will be used throughout. The terminology used is adapted from that of
the American Association of Cost Engineers^). Although it has been
developed for general use, it is readily adaptable to air pollution control
system costing.
2.2.1 Elements of Total Capital Investment
First, two general kinds of costs are estimated, total capital investment
(TCI) and total annual cost (TAC). The total capital investment includes
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2-4
all costs required to purchase equipment needed for the control system
(termed "purchased equipment" costs), the costs of labor and materials for
installing that equipment (termed "direct installation" costs), site
preparation, building costs, and certain other costs which are termed
"indirect installation" costs. Direct installation costs include costs for
foundations and supports, erecting and handling the equipment, electrical
work, piping, insulation, and painting. Indirect installation costs include
such costs as engineering costs; construction and field expenses (i.e.,
costs for construction supervisory personnel, office personnel, rental of
temporary offices, etc.); contractor fees (for construction and engineering
firms involved in the project); start-up and performance test costs (to get
the control system running and to verify that it meets performance guarantees);
and contingencies. Contingencies is a catch-all category that covers unfore-
seen costs that may arise, including (but certainly not limited to) "...
possible redesign and modification of equipment, escalation increases in
cost of equipment, increases in field labor costs, and delays encountered
in start-up."(2)
These elements of total capital investment are displayed in Figure
2-1. Note that the sum of the purchased equipment cost, direct and
indirect installation costs, site preparation, and buildings costs
comprise the battery limits estimate. By definition, this is the total
estimate "... for a specific job without regard to required supporting
facilities which are assumed to already exist..."(2) at the plant. This
would mainly apply to control systems installed in existing plants, though
it could also apply to those systems installed in new plants when no special
facilities for supporting the control system would be required.
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Figure 2-1
Elements of Total Capital Investment
o Primary Control Device
o Auxiliary Equipment
(including ductwork)
o Instrumentation3
o Sales Taxes3
o Freight3
o Foundations and Supports
o Handling and Erection
o Electrical
o Piping
o Insulation
o Painting
Purchased
•Equipment
Cost
Land6
Working Capital6
Total
Nondepreciable
Investment
o Engineering
o Construction and Field
Expenses
o Contractor Fees
o Start-up
o Performance Test
o Contingencies
Direct
-Installation
Cost0
Site Preparation0
Buildingsd
Indirect
Installation
Costb
Total
Direct
Cost
Total
Indirect
Cost
"Battery
Limits"
Cost
Off-Site
fac1Htes6
Total
Depreciable
Investment
Total
Capital
Investment
ro
en
a
b
c
d
These costs are factored from the sum of the control device and auxiliary equipment costs.
These costs are factored from the purchased equipment cost.
Usually required only at "grass roots" installations.
Unlike the other direct and indirect costs, costs for these items are not factored from the purchased equipment
cost. Rather, they are sized and costed separately.
Normally not required with add-on control systems.
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2-6
Where required, these supporting facilities would encompass units to
produce steam, electricity, and treated water; laboratory buildings, railroad
spurs, roads, and the like. It is unusual, however, for a control system
to have one of these units (e.g., a power plant) dedicated to it. The
system needs are rarely that great. However, it may be necessary—especially
in the case of control systems installed in new or "grass roots" plants--
for extra capacity to be built into the site generating plant to service
the system. (A venturi scrubber, which often requires large amounts of
electricity, is a good example of this.) It is customary for the utility costs
to be charged to the project as operating costs at a rate which covers both
the investment and operating costs for the utility.
As Figure 2-1 shows, there are two other costs which may be included
in the total capital investment for a control system. These are "working
capital", and "land". The first of these, working capital, is a fund set
aside to cover the initial costs of fuel, chemicals, and other materials,
as well as labor and maintenance. It usually does not apply to control
systems, for the quantities of utilities, materials, labor, etc., they
require are usually small. (An exception might be an oil-fired thermal
incinerator, where a small supply (e.g., 30-day) of distillate fuel would
have to be available during its initial period of operation.)
Land may also be required. But, since most add-on control systems
take up very little space (a quarter-acre or less) this cost would be
relatively small. (Certain control systems, such as those used for flue
gas desulfurization, require larger quantities of land for the process
equipment, chemicals storage, and waste disposal.)
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2-7
Note also in Figure 2-1 that the working capital and land are
"nondepreciable" expenses. In other words, these costs are "recovered"
when the control system reaches the end of its useful life (generally in 10
to 20 years). Conversely, the other capital costs are "depreciable", in
that they cannot be recovered and are included in the calculation of the
income tax credit and depreciation allowance, whenever such taxes are
considered in a cost analysis. (In the Manual methodology, however,
income taxes are not considered. See Section 2.3.)
Notice that when 100% of the system costs are depreciated, m> salvage
value is taken for the system equipment at the conclusion of its useful life.
This is a reasonable assumption for add-on control systems, as most of the
equipment, which is designed for a specific source, cannot be used elsewhere
without modifications. Even if it were reusable, the cost of disassembling
the system into its components could be as high (or higher) than the salvage
value.
2.2.2 Elements of Total Annual Cost
The Total Annual Cost (TAG) for control systems is comprised of three
elements: "direct" costs, (DC) "indirect" costs, (1C) and "recovery credits"
(RC), which are related by the following equation:
TAC = DC H- 1C - RC (2-1)
Clearly, the basis of these costs is one year, as this period allows for
seasonal variations in production (and emissions generation) and is directly
usable in profitability analyses. (See Section 2.3.)
Pi rect costs are those which tend to be proportional or partially
proportional to the quantity of exhaust gas processed by the control system
per unit time. These include costs for raw materials, utilities (steam,
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2-8
electricity, process and cooling water, etc.). waste treatment and disposal,
maintenance materials, replacement parts, and operating, supervisory, and
maintenance labor. Of these direct costs, costs for raw materials, utilities,
and waste treatment and disposal are variable, in that they tend to be a
direct function of the exhaust flowrate. That is, when the flowrate is at
its maximum rate, these costs are highest. Conversely, when the flowrate
is zero, so are the costs.
Semi variable direct costs are only partly dependent upon the exhaust
flowrate. These include all kinds of labor, maintenance materials, and
replacement parts. Although these costs are a function of the gas flowrate,
they are not 1 inear functions. Even while the control system is not
operating, some of the semi variable costs continue to be incurred.
Indirect, or "fixed", annual costs are those whose values are totally
independent of the exhaust flowrate and, in fact, would be incurred even if
the control system were shut down. They include such categories as overhead,
property taxes, insurance, and capital recovery.
Finally, the direct and indirect annual costs are offset by recovery
credits, taken for materials or energy recovered by the control system,
which may be sold, recycled to the process, or reused elsewhere at the
site. These credits, in turn, must be offset by the costs necessary for
their purification, storage, transportation, and any other costs required
to make them reusable or resalable. Great care and judgement must be
exercised in assigning values to recovery credits since materials recovered
may be of small quantity or of doubtful purity, resulting in their having less
value than virgin material.
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2-9
The various annual costs and their interrelationships are displayed in
Figure 2-2. A more thorough description of these costs and how they may
be estimated is given in Section 2.4.
2.3 Engineering Economy Concepts
As mentioned previously, the estimating methodology presented in
Section 2.4 rests upon the notion of the "factored" or study estimate.
However, there are other concepts central to the cost analyses which must
be understood. These are (1) the time value of money, (2) cash flow, and
(3) annualization.
2.3.1 Time Value of Money
The "time value of money" is based on the truism that "...a dollar now
is worth more than the prospect of a dollar... at some later date."(3)
A measure of this value is the interest rate which "...may be thought of as
the return obtainable by the productive investment of capital."(3)
2.3.2 Cash Flow
During the lifetime of a project, various kinds of cash expenditures
are made and various incomes are received. The amounts and timing of these
expenditures and incomes constitute the cash flows for the project. In
control system costing it is normal to consider expenditures (negative cash
flows) and unusual to consider income (positive cash flows), except for
product or energy recovery income. By the simplifying convention recommended
by Grant, Ireson, and Leavenworth(4), each annual expenditure (or payment)
is considered to be incurred at the end of the year, even though the payment
will probably be made sometime during the year in question. (The error
introduced by this assumption is minimal, however.) Figure 2-3, which
sh6ws three hypothetical cash flow "diagrams", illustrates these end-of-
year payments. In these diagrams, "P" represents the capital investment,
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2-10
Figure 2-2 Elements of Total Annual Cost
o Raw materials
o Utilities
- Electricity
- Steam
- Water
- Others
Variable
o Labor
- Operating
- Supervisory
- Maintenance
o Maintenance materials
o Replacement parts
•Semi variable
Di rect
Annual
Costs
o Overhead
o Property Taxes
o Insurance
o Capital Recovery
o
o
Materials
Energy
Indirect
Annual
Costs
Recovery
Credits
Total
Annual
Cost
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Figure 2-3 Hypothetical Cash Flow Diagrams
I. P
II. P
All Values Are Constant Year (Real) Dollars
34567
8
10 Year
1 J I
A!I A2r A3j. A4
1 Y \
?
A5
?
\
f
A7
f
\
AS Ag
{
A10
P
1
A
i
A
, i
A,
A
\
A,
A
i
A
*• <
f
A
f \
*
A
10 Year
III.
.7
10 Year
i
i
.1
i
. A11
r >
, Al,
. Al,
, 1
' 1
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2-12
while the A's denote the end-of-year annual payments. Note that in all
diagrams, the cash flows are in "constant" ("real") dollars, meaning that
they do not reflect the effects of inflation. Also note that in the top
diagram (I), the annual payments are different for each year. (These
represent the control system annual costs (exclusive of capital recovery)
described in Section 2.2.) In reality, these payments would be different,
as labor and maintenance requirements, labor and utility costs, etc., would
vary from year to year. A generally upward trend in annual costs would be
seen, however.
In diagram II, these fluctuating annual payments have been converted
to equal payments. This can be done by calculating the sum of the present
values of each of the annual payments shown in diagram I and annualizing
the total net present value to equivalent equal annual payments via a capital
recovery factor. (See discussion in the following paragraphs and in Section
2.3.3.) Alternatively, it is adequate to choose a value of A equal to the
sum of the direct and indirect annual costs estimated for the first year.
This assumption is in keeping with the overall accuracy of study estimates
and allows for easier calculations.
Finally, notice diagram III. Here, the annual costs (A^) are again
equal, while the capital investment (P) is missing. Put simply, P has been
incorporated into A1 so that A1 reflects not only the various annual costs
but the investment as well. This was done by introducing another term, the
capital recovery factor (CRF), defined as follows: "when multiplied by a
present debt or investment , [the CRF] gives the uniform end-of-year payment
necessary to repay the debt or investment in n years with interest rate i."(5)
The product of the CRF and the investment (P) is the capital recovery cost (CRC):
CRC = CRF x P (2-2)
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2-13
where:
CRF = 1(1 + i)n (2-3)
Therefore, A* is the sum of A and the CRC, or:
. A1 = A + CRF x P (2-4)
In this context, "n" is the control system economic life, which,
as stated above, typically varies from 10 to 20 years. The interest rate
("i") used in this Manual is a real rate of 10% (annual). This value is
used in most of the OAQPS cost analyses and is in keeping with current EAB
guidelines^) and the Office of Management and Budget recommendation for use
in regulatory analyses.'^'
It may be helpful to illustrate the difference between "real" and
"nominal" interest rates. The mathematical relationship between them is
straightforward: (6)
(1 + 1n) = (1 + 1)(1 + r) (2-5)
where:
inj = the annual nominal and real interest rates, respectively
r = the annual inflation rate
Clearly, the "real" rate does not consider inflation and is in keeping
with the expression of annual costs in constant (i.e., real) dollars.
EAB guidelines also recommend the exclusion of income tax considerations
from cost analyses.(6) Not only does this simplify the analysis, but it
also allows for the calculation of the "economic" costs of air pollution
control--i.e., the true cost to society. Income taxes generally represent
transfer payments from one segment of society to another and as such are not
properly part of the economic costs.
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2.3.3 Annual ization Methods
The above method of "smoothing out" the investment into equal end-of-
year payments, is termed the "equivalent uniform annual cash flow" (EUAC)
method.(8) in addition to its inherent simplicity, this method is very
useful when comparing the costs of two or more alternative control systems
(i.e., those which are designed to control the same source to an equivalent
degree). In fact, the EUAC's--or simply the total annual costs—of two
competing systems may be compared even if both the systems have different
economic lives, say 10 and 20 years. We recommend that the EUAC method be
used for control cost work unless particular circumstances preclude its
use.
Comparisons of systems with differing economic lives cannot be made,
however, using the other two annualization (i.e., profitability analysis)
methods--"present worth" and "internal rate of return". The "present
worth" (or "discounted cash flow") method involves the "discounting" of all
cash flows occuring after year "0" (i.e., the system startup date) back to
year "0". These cash flows are discounted by multiplying each by a "discount
factor", 1 , where "m" is the number of years from year 0 to the year
(1 + i)m
in which the cash flow is incurred. The sum of these discounted cash flows
is then added to the capital investment to yield the "present worth" of the
project. The alternative having the highest present worth would be selected
(in control system costing this is usually a negative number). But when
comparing the present worths of alternative systems, the system lifetimes
must be equal for the comparison to be valid.(9)
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2-15
The third annualization method, "internal rate of return" (IRR), is
similar to the present worth method, in that it involves the discounting of
a series of unequal cash flows. However, where with the PW method the
interest rate, "i", is set beforehand, in the IRR method the interest rate
is solved for (usually via trial-and-error) after arbitrarily setting the
PW to zero. When comparing alternative systems, the one with the highest
IRR is selected.(10) But again, the alternative systems compared must have
equal economic lives.
2.4 Estimating Procedure
The estimating procedure used in the Manual consists of five steps:
(1) obtaining the "facility parameters" and "regulatory options" for a
given facility; (2) "roughing out" the control system design; (3) sizing
the control system components; (4) estimating the costs of these individual
components; and (5) estimating the costs (capital and annual ized) of the
enti re system.
2.4.1 Facility Parameters and Regulatory Options
Obtaining the facility parameters and regulatory options involves
not only assembling the parameters of the air pollution source (i.e., the
quantity, temperature, and composition of the emission stream(s)),
but also compiling data for the facility's operation. (Table 2-1 lists
examples of these.) Note that two kinds of facility parameters are identi-
fied -- "intensive" and "extensive". The former are simply those variables
whose values are independent of quantity or dimensions — i.e., the "extent"
of the system. Conversely, "extensive" parameters encompass all size-dependent
variables, such as the gas volumetric flowrate.
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2-16
Table 2-1 Facility Parameters and Regulatory Options
FACILITY PARAMETERS
Intensive
- Facility status (new or existing, location)
- Gas characteristics (temperature, pressure, moisture content)
- Pollutant concentration(s) and/or particle size distribution
Extensive
- Facility capacity
- Facility life
- Gas flow rate
- Pollutant emission rate(s)
REGULATORY OPTIONS
o No control
o "Add-on" devices
- Emission 1imits
- Opacity "
o Process modifications
- Raw material changes
- Fuel substitution
o Others
- Coal desulfurization
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2-17
Like the facility parameters, the "regulatory options" are usually
specified by others. These options are ways to achieve a predetermined
emission limit. They range from "no control" to maximum control technically
achievable. The option provided will depend, firstly, on whether the
emission source is a stack (point source), a process leak ("process fugitives"
source) or an unenclosed (or partly closed) area, such as a storage pile
("area fugitives" source). Stacks are normally controlled by "add-on"
devices. As discussed above, this Manual will deal primarily with these
add-on devices. (However, some of these devices can be used to control
process fugitives in certain cases, such as a fabric filter used in conjunction
with a building evacuation system.) Add-ons are normally used to meet a
specified emission level, although in the case of particulate emissions,
they may also be required to meet an opacity level.
2.4.2 Control System Design
Step 2 — roughing out the control system design -- first involves
deciding what kinds of systems will be priced (a decision that will depend
on the pollutants to be controlled, gas stream conditions, and other factors),
and what auxiliary equipment will be needed. When specifying the auxiliary
equipment, several questions need to be answered:
o What type of hood (if any) will be needed to capture the emissions
at the source?
o Will a fan be needed to convey the exhaust through the system?
o Is a cyclone or another pre-cleaner needed to condition the exhaust
before it enters the control device?
o Will the captured pollutants be disposed of or recycled? How will
this be done?
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2-18
o Can the on-site utility capacity (e.g., electricity) accomodate the
added requirements of the control system?
The kinds of auxiliary equipment selected will depend on the answers
to these and other site-specific questions. However, regardless of the
source being controlled, each system will likely contain, along with the
control device itself, the following auxiliaries:
o Hood, or other means for capturing the exhaust;
o Ductwork, to convey the exhaust from the source, to, through, and
from the control system;
o Fan system (fan, motor, starter, inlet/outlet dampers, etc.), to
move the exhaust through the system;
o Stack, for dispersing the cleaned gas into the atmosphere.
2.4.3 Sizing the Control System
Once the system components have been selected, they must be sized.
Sizing is probably the most critical step, because the assumptions made in
this step will more heavily influence the capital investment than any other.
Before discussing how to size equipment, we need to define the term. For
the purposes of this Manual, "sizing" is the calculation (or estimation) of
certain "critical" design parameters for a control device against which
the purchased cost of that device is most accurately correlated. For
instance, the purchased cost of an electrostatic precipitator (ESP) is most
often correlated with its collecting area. This, in turn, is a function
of the exhaust volumetric flowrate, the overall collection efficiency and
the empirically-determined drift velocity, the ESP "critical" parameter.
(Table 2-2 lists examples of these parameters.)
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2-19
Table 2-2 Examples of Typical Control Device Parameters'H'
GENERAL
Material of construction: e.g., carbon steel
Insulated? Yes
Economic life: 20 yr
Redundancy3: none
DEVICE-SPECIFIC
Air-to-cloth ratio ("critical parameter"): 7.5 to 1
Pressure drop: 6.0 in w.g. (inches water gauge)
Construction: suction (vs. pressurized)
Duty: continuous (vs. intermittent)
Other features: dilution air port (for exhaust temperature regulation)
a Refers to whether there are any extra equipment items installed (e.g.,
fans) to function in case the basic item becomes inoperative, so as to avoid
shutting down the entire system.
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2-20
Also listed in Table 2-2 are "general" parameters which must also be
specified before the purchased cost of the system equipment can be estimated.
Note that, unlike the control device parameters, these may apply to any
kind of control system. These include materials of construction (which may
range from carbon steel to various stainless steels to fiberglass-reinforced
polyester), presence or absence of insulation, and the economic or "useful"
life of the system. As indicated in Section 2.3.2, this last parameter is
required for estimating the annual capital recovery costs. The lifetime
not only varies according to the type of the control system, but with the
severity of the environment in which it is installed. (Representative
values for this and the other control device parameters will be presented
in those Sections of the Manual covering them.)
2.4.4 Estimating Total Capital Investment
The fourth step is estimating the total purchased cost of the control
system equipment. These costs are available from this Manual for the most
commonly used add-on control devices and auxiliary equipment. Each type of
equipment is covered in a separate section. (See Table of Contents.)
Most of these costs, in turn, have been based on data obtained from
control equipment vendors. There are over one hundred of these firms, many
of whom fabricate and erect a variety of control systems.-^2) They have
readily available, current price lists of their equipment, usually indexed
by model designation. If the items for which costs are requested are
fabricated, "off-the-shelf" equipment, then the vendor can provide a
written quotation listing their costs, model designations, date of quotation,
estimated shipment date, and other information. (See Figure 2-4 for a
sample quotation.) Moreover, the quote is usually "F.O.B." ("free-on-board")
-------
Figure 2-4 Typical Vendor Quotation
QUOTATION
ocess Equipment Plant (NOTE: ComPa"y name and address hW been deleted.)
r
MAIL DROP # 12
U.S. EPA
RESEARCH TRIANGLE PARK
DURHAM. NC 27711
ATTN: MR. BILL VATAVUK
QUOTATION NO.
DATE
REFERENCE
85S23382
9-23-85
VERBAL - BUDGET
J
Thank you for your inquiry. We are pleased to submit our quotation as follows:
QUANTITY
DESCRIPTION
PRICE
ITEM #1 PREHEATER
! $ 7,147.00 EA.
MODEL 191-19 SIZE #9 IMPERVITE SHELL & TUBE HEAT EXCHANGER WITH
55.8 SQ. FT. OF HEAT TRANSFER AREA AND CODE STAMPED
ITEM #2 CONDENSER
7,430.00 EAc
MODEL 191-19 SIZE #12 IMPERVITE SHELL & TUBE HEAT EXCHANGER WITH
74.5 SQ. FT. OF HEAT TRANSFER AREA AND CODE STAMPED
APPROVAL DWG'S 2-3 WEEKS AFTER RECEIPT OF ORDER.
THIS QUOTATION IS IN CONFIRMATION OF OUR PHONE CONVERSATION OF
9/18/85.
ESTIMATED SHIPMENT _6_tO_8__ WEEKS AFTER
RECEIPT OF ORDER
RECEIPT OF DRAWING APPROVAL
Prices are F.O.B. >. Net 30 Days. - •
Unless otherwise stated these prices are subject to acceptance within 30 days from date.
By.
ANY PURCHASE ORftER RESULTIMfi FROM THIS QUOTATION WILL BE SUBJECT TO THE CONTRACT TERMS AND CONDITIONS PRINTED ON TNE REVERSE SIDE OF THIS PAGE.
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2-22
the vendor, meaning that no taxes, freight, or other charges.are included.
However, if the items are not off-the-shelf, they must be custom fabricated
or, in the case of very large systems, constructed on-site. In such cases,
the vendor can still give quotations—but will likely take much longer to
do so and may even charge for this service, to recoup the labor and overhead
expenses of his estimating department.
As discussed in Section 2.2 in this Manual , the total capital investment
is "factored" from the "purchased equipment cost", which in turn, is the
sum of the base equipment cost (control device plus auxiliaries), freight,
instrumentation, and sales tax. The values of these installation factors
depend on the type of the control system installed and are, therefore,
listed in the individual Manual sections dedicated to them.
The costs of freight, instrumentation, arid sales tax are calculated
differently from the direct and indirect installation costs. These items
are "factored" also, but from the base equipment cost (F.O.B. the vendor(s)).
But unlike the installation factors, these factors are essentially equal
for all control systems. Values for these are as follows:
Cost Range Typical
Freight
Sales tax
Instrumentation
0.01
0
0.05
- 0.10
- 0.08
- 0.30
0.05
0.03
0.10
The range in freight costs reflects the distance between the vendor and the
site. The lower end is typical of major U.S. metropolitan areas, while the
latter would reflect freight charges to remote locations such as Alaska and
Hawaii.U1) The sales tax factors simply reflect the range of local and
state tax rates currently in effect in the U.S.(13)
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2-23
The range of instrumentation factors is also quite large. For systems
requiring only simple continuous or manual control, the lower factor would
apply. However, if the control is intermittent and/or requires safety backup
instrumentation, the higher end of the range would be appl icabl e.(^)
Finally, some "package" control systems (e.g., incinerators covered in
Section 3) have built-in controls, whose cost is included in the base
equipment cost. In those cases, the factor to use would, of course, be zero.
2.4.5 Retrofit Cost Considerations
The installation factors listed elsewhere in the Manual apply primarily
to systems installed in new facilities. These factors must be adjusted
whenever a control system is sized for, and installed in (i.e, "retrofitted")
an existing facility. However, because the size and number of auxiliaries
are usually the same in a retrofit situation, the total purchased cost of
the control system would probably not be different from the new plant
purchased cost. An exception is the ductwork cost, for in many retrofit
situations exceptionally long duct runs are required to tie the control
system into the existing process.
Each retrofit installation is unique; therefore, no general factors
can be developed. Nonetheless, some general information can be given
concerning the kinds of system modifications one might expect in a retrofit:
1. Auxiliaries. Again, the most important component to consider is
the ductwork cost. In addition, to requiring very long duct runs,
some retrofits require extra tees, elbows, dampers, and other
fittings.
2. Handling and Erection. Because of a "tight fit", special care may
need to be taken when unloading, transporting, and placing the
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2-24
equipment. This cost could increase significantly if special
means (i.e., helicopters) are needed to get the equipment on roofs
or to other inaccessible places.
3. Piping, Insulation, and Painting. Like ductwork, large amounts
of piping may be needed to tie in the control device to sources of
process and cooling water, steam, etc. Of course, the more piping
and ductwork required, the more insulation and painting will be
needed.
4. Site Preparation. Unlike the other categories, this cost may
actually decrease, for most of this work would have been done when
the original facility was built.
5. Facilities. Conceivably, retrofit costs for this category could
be the largest. For example, if the control system requires large
amounts of electricity (e.g., a venturi scrubber), the facility's
power plant may not be able to service it. In such cases, the
facility would have to purchase the additional power from a public
utility, expand its power plant, or build another one. In any
case, the cost of electricity supplied to that control system
would likely be higher than if the system were installed in
a new facility where adequate provision for its electrical needs
would have been made.
6. Engineering. Designing a control system to fit into an existing
plant normally requires extra engineering, especially when the
system is exceptionally large, heavy, or utility-consumptive. For
the same reasons, extra supervision may be needed when the
installation work is being done.
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2-25
7. Lost Production. This cost is incurred whenever a retrofit control
system cannot be tied into the process during normally scheduled
maintenance periods. Then, part or all of the process may have to
be temporarily shut down. The revenue lost during this shutdown
period is a bonafide retrofit expense.
8. Contingency. Due to the uncertain nature of retrofit estimates,
the contingency (i.e., uncertainty) factor in the estimate should
be increased.
From the above points, it is apparent that some or most of these instal-
lation costs would increase in a retrofit situation. However, there may be
other cases where the retrofitted installation cost would be less than the
cost of installing the system in a new plant. This could occur when one
control device, say an ESP, is being replaced by a more efficient unit—a
baghouse, for example. The ductwork, stack, and other auxiliaries for the
ESP may be adequate for the new system, as perhaps would the support
facilities (power plant, etc.).
2.4.6 Estimating Annual Costs
Determining the total annual cost is the last step in the estimating
procedure. As mentioned in Section 2.2 the TAC is comprised of three
components -- direct and indirect annual costs and recovery credits. Unlike
the installation costs, which are "factored" from the purchased equipment
cost, annual cost items are usually computed from known data on the
system size and operating mode, as well as from the facility and control
device parameters.
Following is a more detailed discussion of the items comprising the
total annual cost. (Values/factors for these costs are also given in the
sections for the individual devices.)
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2-26
2.4.6.1 Raw Materials
Raw materials are generally not required with control systems. Exceptions
would be chemicals used in absorbers or venturi scrubbers as absorbents or
to neutralize acidic exhaust gases (e.g., hydrochloric acid). Chemicals
may also be required to treat wastewater discharged by scrubbers or absorbers
before releasing it to surface waters. But, these costs are only considered
when a wastewater treatment system is exclusively dedicated to the control
system. In most cases, a pro-rata waste treatment charge is applied. (See
section 2.4.6.5.)
Quantities of chemicals required are calculated via material balancies,
with an extra 10 to 20% added for miscellaneous losses. Costs for chemicals
are available from the Chemical Marketing Reporter and similar publications.
2.4.6.2 Operating Labor
The amount of labor required for a system depends on its size, complexity,
level of automation, and operating mode (i.e., batch or continuous). The
labor is usually figured on an hours-per-shift basis. As a rule, though,
data showing explicit correlations between the labor requirement and capacity
are hard to obtain. A typical correlation is logarithmic:
Ll
where: Lj , Lg = labor requirements for systems 1 and 2
vl» V2 = capacities of systems 1 and 2 (as measured by the
gas flow rate, for instance)
y = 0.2 to 0.25 (typical ly) (14)
The exponent in equation (2-6) can vary considerably, however. Conversely,
in many cases, the amount of operator labor required for a system will be
approximately the same regardless of its size.
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2-27
A certain amount must be added to operating labor to cover supervisory
requirements. Fifteen per cent of the operating labor requirement is
representative. (15)
To obtain the annual labor cost, multiply the operating and supervisory
labor requirements by the respective wage rates (in $/hr) and the system
operating factor (number of hours per year the system is in operation).
The wage rates also vary widely, depending upon the source category,
geographical location, etc. These data are tabulated and periodically
updated by the U.S. Department of Labor, Bureau of Labor Statistics, in its
Monthly Labor Review and in other publications. Finally, note that these
are base labor rates, which do not include payroll and plant overhead.
(See overhead discussion below.)
2.4.6.3 Maintenance
Maintenance labor is calculated in the same way as operating labor
and is influenced by the same variables. The maintenance labor rate,
however, is normally higher than the operating labor rate, mainly because
more skilled personnel are required. A 10% wage rate premium is typical.U5)
Further, there are expenses for maintenance materials -- oil, other
lubricants, duct tape, etc., and a host of small tools. Costs for these
items can be figured individually, but since they are normally so small,
they are typically factored from the maintenance labor. Reference 15
suggests a factor of 100% of the maintenance labor.
2.4.6.4 Utilities
This cost category covers many different items, ranging from electricity
to compressed air. Of these, only electricity is common to all control
devices, where fuel oil and natural gas are generally used only by
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2-28
incinerators; water and water treatment, by venturi scrubbers, quenchers,
and spray chambers; steam, by carbon adsorbers; and compressed air, by
pulse-jet fabric filters.
Techniques and factors for estimating utility costs for specific devices
are presented in their respective sections. However, because nearly every
system requires a fan to convey the exhaust gases to and through it, a
general expression for computing the fan electricity cost (Ce) is given
Ce = 0.746 Q AP s 9 Pp
6356 n (2-7)
where:
Q = gas flowrate (actual ft^/min)
AP = pressure drop through system (inches of water, gauge)
(Values for AP are given in the sections covering the equipment
items.)
s = specific gravity of gas relative to air (1.000, for all practical
purposes)
9 = operating factor (hr/yr)
ri = combined fan and motor efficiency (usually 0.60 to 0.70)
Pe = electricity cost ($/kwhr).
A similar expression can be developed for calculating pump motor electricity
requirements.
2.4.6.5 Water Treatment and Disposal
Though often overlooked, there can be a significant cost associated
with treating and/or disposing of waste material captured by a control system
that neither can be sold nor recycled to the process.
Liquid waste streams, such as the effluent from a venturi scrubber,
are usually processed before being released to surface waters. The type
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2-29
and extent of this processing will, of course, depend on the characteristics
of the effluent. For example, the waste can first be sent to one (or more)
clarifiers, for coagulation and removal of suspended solids. The precipitate
from the clarifier is then conveyed to a rotary filter, where most of the
liquid is removed. The resulting filter cake is then disposed of, via land
filling, for example.
The annual cost of this treatment is relatively high--$1.00 to
$2.00/thousand gallons treated or more.(16) Tne solid waste disposal costs
(via land filling, for example) typically would add another $20 to $30/ton
disposed of.(^) This, however, would not include transportation to the
disposal site. More information on these technologies and their costs is
found in References (16) and (17).
2.4.6.6 Replacement Parts
This cost is computed separately from maintenance, because it is a
large expenditure, incurred one or more times during the useful life of a
control system. This category includes such items as carbon (for carbon
adsorbers), bags (for fabric filters) and catalyst (for catalytic
incinerators), along with the labor for their installation.
The annual cost of the replacement materials is a function of the
initial parts cost, the parts replacement labor cost, the life of the
parts, and the interest rate, as follows:
CRCp = (Cp + Cpl)CRFp (2-8)
where: CRCp = capital recovery cost of replacement parts ($/yr)
Cp = initial cost of replacement parts, including taxes
and freight ($)
Cpi = cost of parts replacement labor ($)
CRFp = capital recovery factor (defined in Section 2.3).
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2-30
In the Manual methodology, replacement parts are treated the same as
any other investment, in that they are also considered an expenditure that
must be amortized over a certain period. Also, the useful life of the parts
(typically 2 to 5 years) is generally less than the useful life of the rest
of the control system.
Replacement part labor will vary, depending upon the amount of the
material, its workability, accessibility of the control device, and other
factors.
2.4.6.7 Overhead
This cost is easy to calculate, but often difficult to comprehend.
Much of the confusion surrounding overhead is due to the many different
ways it is computed and to the several costs it includes, some of which
may appear to be duplicative.
There are, generally, two categories of overhead, payroll and pi ant.
Payroll overhead includes expenses directly associated with operating,
supervisory, and maintenance labor, such as: workmen's compensation, Social
Security and pension fund contributions, vacations, group insurance, and
other fringe benefits. Some of these are fixed costs (i.e., they must be
paid regardless of how many hours per year an employee works). Payroll
overhead is traditionally computed as a percentage of the total annual
labor cost (operating, supervisory, and maintenance).
Conversely, pi ant (or "factory") overhead account for expenses not
necessarily tied to the operation and maintenance of the control system,
including: plant protection, control laboratories, employee amenities,
plant lighting, parking areas, and landscaping. Some estimators compute
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2-31
plant overhead by taking a percentage of all labor plus maintenance material s(18)
while others factor it from the total labor costs alone.(19)
For "study" estimates, it is sufficiently accurate to combine payroll
and plant overhead into a single indirect cost. This is done in this Manual .
Also, overhead will be factored from the sum of all labor (operating,
supervisory, and maintenance) plus maintenance materials, the approach
recommended in reference 18. The factors recommended therein range from 50
to 70% (18) /\n average value of 60% is used in this Manual .
2.4.6.8 Property Taxes, Insurance, and Administrative Charges
These three indirect operating costs are factored from the system total
capital investment, and typically comprise 1,1, and 2% of it, respectively.
Taxes and insurance are self-explanatory. "Administrative charges" covers
sales, research and development, accounting, and other home office expenses.
(It should not be confused with plant overhead, however.) For simplicity,
the three items are usually combined into a single, 4% factor* This value,
incidentally, is standard in all OAQPS cost analyses.
2.4.6.9 Capital Recovery
As discussed in Section 2.3, the annualization method used in the Manual
\
is the equivalent uniform annual ized cost method. Recall that the cornerstone
of this method is the capital recovery factor which, when multiplied by
the total capital investment, yields the capital recovery cost. (See
equation 2-2.)
However, whenever there are parts in the control system that must
be replaced before the end of its useful life, equation 2-2 must be adjusted,
to avoid double-counting.
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That i s:
CRCS = CRFS [TCI - (Cp + Cpl)] (2-9)
where:
CRCS = capital recovery cost for control system (3/yr)
TCI = total capital investment for entire system ($).
CRFS = capital recovery factor for control system
The term (Cp+Cp-|) accounts for the cost of those parts that would be
replaced during the useful life of the control system and the labor for
replacing them. Clearly, CRFS and CRFp will not be equal unless the control
system and replacement part lives are equal.
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References for Section 2:
1. Perry, Robert H., and Chi 1 ton, Cecil H., Perry's Chemical Engineer's
Handbook (Fifth Edition). New York: McGraw-Hill, 1973, pp. 25-12 to
25-16.
2. Humphries, K. K. and Katell, S., Basic Cost Engineering. New York:
Marcel Dekker, 1981, pp. 17-33.
3. Grant, E.L., I reson, W.G., Leavenworth, R.S., Principles ^f Engineering
Economy. Sixth Edition. New York: John Wiley & Sons, 1976, p.25.
4. Ibid., p. 35.
5. Ibid., p. 36.
6. EAB Guideline Memo: "Interest Rates for Regulatory Impact Analyses
(RIA)", May 27, 1982.
7. "Interim Regulatory Impact Analysis Guidance," Office of Management
and Budget, June 5, 1981.
8. Grant, et al., Op. Cit., p. 67.
9. Ibid., p. 91.
10. Ibid., pp. 107-131.
11. Vatavuk, W. M. and Neveril, R. B., "Estimating Costs of Air-Pollution
Control Systems—Part I: Parameters for Sizing Systems," Chemical
Engineering. October 6, 1980, pp. 165-168.
12. Pollution Equipment News, Pittsburgh: Rimbach Publishing, 1982, pp.
71-72.
13. Internal Revenue Service, Form 1040, 1985.
14. Peters, M. S. and Timmerhaus, K. D., Plant Design and Economics
for Chemical Engineers (Third EditionJTNew York: McGraw-Hill, 1980,
p. 195.
15. Vatavuk, W. M. and Neveril, R. B., "Estimating Costs of Air-Pollution
Control Systems—Part II: Factors for Estimating Capital and Operating
Costs," Chemical Engineering, November 3, 1980, pp. 157-162.
16. Ibid., "Part XVII: Particle Emissions Control," Chemical Engineering,
April 2, 1984, pp. 97-99.
17. The RCRA Risk-Cost Analysis Model: U.S. Environmental Protection Agency,
Office of Solid Waste, January 13, 1984.
18. Peters and Timmerhaus, Op. Cit., p. 203.
19. Humphries and Katell, Op. Cit.. pp. 64-68.
-------
Section 3
THERMAL AND CATALYTIC INCINERATORS
i Vishnu S. Katari
Pacific Environmental Services (Durham, NC 27707)
William M. Vatavuk
Economic Analysis Branch, OAQPS
3.1 Process Description
3.1.1 Elements of Combustion
Fume incineration, a controlled oxidation process, is a technique
used for destruction of vaporous volatile organic compound (VOC) emis-
sions from industrial waste gases. In the process, the VOC content of
waste gases reacts at high temperatures with oxygen to form carbon
dioxide and water, while liberating heat. Three parameters: temperature,
residence time (also referred to as "retention time" or "dwell time")
and turbulence (the "three Ts") have an interrelated effect upon the
final combustion performance. To achieve good oxidation rates and
obtain release of the full heat content of the combustion products,
emission effluents must be held for sufficient residence times at
combustion temperatures 100°F or more above reported ignition
temperatures. Further, turbulent flow conditions must be maintained in
the incinerator for good mixing of the fuel combustion products with
incoming effluents, so that a thorough homogeneity of combustion elements
(VOC and oxygen) may be achieved. The ignition temperature of a given
VOC is the minimum temperature at which the VOC reacts instantaneously
with oxygen. Below this temperature (if the supply of heat were inter-
rupted) combustion would slow down and gradually stop.
Typically, the rudimentary combustion parameters for an incinerator
design are established empirically and no calculations are involved in
their estimation. Of the 3 Ts, time and turbulence are fixed by
-------
•:' 3-2
incinerator design and air flow rate, and only temperature can be signi-
ficantly controlled. The type of VOCs present in the waste gas usually
dictates the combustion temperature for a required VOC destruction
rate. Lower or higher combustion temperatures can be used to a limited
extent, with corresponding variations in the residence time (e.g., an
incinerator can be designed for a lower residence time but must operate
at a correspondingly higher combustion temperature). The possibilities
of varying these parameters are limited, because the combustion
temperature must always be above the ignition temperature. Moreover,
the cost savings realized from reducing the residence time (and, in
turn, the incinerator size) generally are not large enough to offset the
additional expenses (e.g., fuel) resulting from compensating increases
in the combustion temperature.
In general, the design of an incinerator system is based on
providing the required amounts of: (1) heat to bring the waste gas to
the oxidation temperature and (2) oxygen for complete combustion. The
waste gas heat and oxygen contents determine, respectively, the auxiliary
heat and oxygen requirements (i.e., the lower the waste gas heat and
oxygen contents, the higher are the auxiliary requirements). Either
natural gas or fuel oil can be used as an auxiliary heat source while
ambient air is the usual source of oxygen. Natural gas is preferred
and used in most incinerators. Oil-fired burners have limited turn-
down capabilities and require high maintenance. Depending upon the
fuel availability and geographic location, some incinerators are designed
for a dual fuel-burning capability (natural gas with standby oil). In
practice, a flue gas oxygen content of at least 3 percent (by volume)
is used empirically to insure that a sufficient amount of oxygen is
available for combustion.
-------
3-3
The waste gas heat content measures the potential heat of VOC that
would be released as heat of combustion upon the oxidation of the VOC. The
heat and VOC contents of a waste gas are proportional to, and can provide
a measure of, each other. They are also inherently related to, and can
be expressed empirically, in terms of the VOC property "flammabil ity."
Flammability is characterized by two limits: the lower explosive limit
(LEL) and the upper explosive limit (UEL). These limits represent,
respectively, the smallest and largest amounts of VOCs which, when
mixed with air, will burn without a continuous application of external
heat. Major reference books present VOC flammabil ity (LEL and UEL)
data. Table 3A-1 in Appendix 3A presents the flammability data for
several VOCs commonly encountered in industrial waste gases. To avoid
potential explosions, the VOC content of industrial waste gases released
to the atmosphere is normally outside the flammability limits. The
majority of waste gases contain low concentrations of VOCs. For safety
reasons, their composition is typically limited to below 25 percent and
will seldom exceed 50 percent of the LEL level. It has been empirically
determined that, upon combustion, most VOCs release approximately 50
Btu/scf of waste gas, if their concentration is at 100 percent of the
LEL. Therefore, the VOC content of a waste gas at less than 25 percent
LEL, the safe upper limit for incineration, will have a potential heat
content of less than 13 BTU/scf.
The LEL and heat content values of a given waste gas can also be
related empirically to the temperature rise of the combustion flue
gases. Upon combustion, each one percent of LEL of the VOC (i.e., a
potential heat content of 0.50 BTU/scf of waste gas) can raise the
temperature of one ft^ of the waste gas by 27°F based on a heat capacity
-------
3-4
of 0.018 BTU/ft3°F for dry air. These empirical relations can sometimes
be used in making qualitative observations to determine if a given waste
gas can be incinerated. They can also be used to make quick and approxi-
mate estimates of the auxiliary fuel requirement.
Most industrial waste gases incinerated are dilute mixtures of
VOCs, air, and inert gas. Thei r VOC content is very low, and their
oxygen content exceeds that required for combustion of both the VOC and
the auxiliary fuel. If a waste gas with a VOC content over 25 percent
LEL is encountered, it is diluted to bel ow 25 percent LEL prior to
incineration by adding outside air (thus increasing the waste gas
volume flow rate to be treated). However, a waste gas with a VOC content
from 25 to 50 percent LEL can be incinerated without adding dilution air,
provided the waste gas VOC levels in the system are continuously monitored
via LEL monitors, to satisfy fire protection regulations.
A low-oxygen content waste gas mixture of VOC, air, and inert gases
could disrupt the burner flame stability. Therefore, when a waste yas
with less than 13 to 16 percent oxygen content is incinerated, the portion
of such waste gas used for fuel combustion is augmented with ambient air.
In a few applications, the waste gas is an inert gas with a low VOC content
and negligible or zero oxygen content. In such cases ambient air is
provided for burning of both the waste gas VOC and auxiliary fuel. In
rare cases, the waste gas is a rich VOC stream that can support combustion
without auxiliary fuel. Such a rich VOC waste gas is treated as a fuel
and burned, either premixed or oxygen free. This process is sometimes
referred to as "direct flame incineration." Figure 3-1 provides a f 1 ow
chart for categorizing a waste gas to determine its suitability for
incineration. (For more information on waste gas characterization,
see Appendix 3B.)
-------
3-5
No
Auxiliary air may
be required tor
Incineration
The majority of mdmtrtd MM gnu tor Inelntmaon tal Mo omoory 1.
CticuWim tor cngory 1 ira iddntMd m tr» turt tnd cticultliora tor
•I cragorio (i.t. c«l»goo«i 1 through 6) in •ddmcKl m Apptndta 3B
Figure 3-1. Flow chart for categorization of a waste
gas to determine its suitability for incineration
and need for auxiliaries.
-------
3-6
3.1.2 Types of Incinerators
Two types of incinerators are in use: thermal and catalytic. The
combustion process, as well as equipment, design concept, and calculations,
are essentially the same with both incinerator types. The only exception
is that catalytic incineration is essentially a flame!ess combustion
process, wherein a catalyst bed is used to initiate the combustion reaction
at much lower temperatures.
Historically, the greatest reason for using a catalytic incinerator
has been to reduce fuel consumption. However, because the application of
either a thermal or a catalytic incinerator system without a heat exchanger
(for recovery of flue gas heat) is now rare, the savings associated with
catalytic incinerators are less significant. Also, this fuel economy for
catalytic incinerators is partially offset by increased operating costs for
maintenance (i.e., periodic cleaning and replacement of catalyst). Catalysts
undergo a gradual loss of activity through thermal aging, fouling, and
erosion of their surfaces. Certain poisonous contaminants, such as phospho-
rus, arsenic, antimony, lead, and zinc, also cause catalyst deactivation.
Catalytic incineration is not recommended for waste gases containing sig-
nificant concentrations of particulate matter (either organic or inorganic)
that cannot be vaporized.
The catalysts used in catalytic incinerator systems for gaseous
VOC control are usually precious or base metals or their salts, either
supported on inert carriers, such as alumina or porcelain, or unsupported.
Precious metal oxide catalysts are less brittle and more expensive than
base metal types, and are used in lesser amounts per unit of waste gas
volume. Of the precious metal oxide catalysts, platinum/palladium
oxides are preferred. Others include rhodium, nickel, and gold.
-------
3-7
Manganese dioxide is the most commonly used base metal oxide catalyst.
The types of VOCs present in the waste gas determine the operating
temperature required in a catalytic unit. The more stable VOCs are
generally the least reactive and require higher inlet catalytic operating
temperatures. Methane is an example of a stable, low molecular weight
compound that requires a relatively high catalytic conversion temperature,
about 1,000°F. Hydrogen, on the other hand, is extremely reactive,
having a conversion temperature of about 200°F. To achieve a VOC
destruction efficiency of at least 90 percent, the catalytic ignition
temperature for most hydrocarbons must be between 400 and 500°F.
Higher temperatures are required to obtain higher VOC destruction
efficiencies. When methane is present along with other less stable
VOCs in a waste gas, preheat temperatures lower than 1,000°F can be
used, because the less stable VOCs burn first and generate heat.
The auxiliary fuel requirement is the most significant operating
expense of an incinerator. Further, the temperature rise of the
combustion products due to the VOC heat release can be substantial.
Therefore, to minimize auxiliary fuel expenses, a part of the heat from
the incinerator flue gases is recovered. This heat is usually recovered
in recuperative heat exchangers, in which the heat from the flue gases
is exchanged with the waste gas. Both countercurrent and cross-flow
types of heat exchangers are used for this purpose. When feasible,
heat may be recovered indirectly, by producing low-pressure steam in a
waste heat boiler. Further, heat recovery from the flue gases after
primary heat recovery (PHR) can be achieved by employing secondary heat
recovery (SHR) units. Of course, SHR can be economical, only if the
secondary heat can be consumed on-site and cheaper heat sources
-------
3-8
are unavailable. The PHR and SHR combinations are most suitable for
large installations where recovered secondary heat may be used on-site
for water heating, air heating, and other purposes.
In general, a typical incinerator system may include the following
components: (1) a fan to move the waste gas; (2) a fan to supply
ambient air, if required; (3) a combustion unit (i.e., a refractory chamber
with burner for thermal units, and a preheat chamber with burner and
catalyst bed for catalytic units); (4) heat recovery equipment
(optional, but almost always used); (5) controls, instrumentation, and
control panel; (6) a stack; and (7) in the case of catalytic units, a
filter/mixer to assure flow distribution, protect the catalyst bed from
flame impingement, and remove noncombustible particulate matter. In
addition, auxiliary equipment, such as ductwork, may be required in the
system.
3.2 Design Procedure
3.2.1 Design and Operating Features
The minimum waste gas characteristics data needed to perform
incinerator design calculations are the waste gas volume f 1 ow rate
and temperature, and the VOC composition. The waste gas flow rate
primarily determines the quantity of combustion flue gases generated
which, in turn, dictates the size of an incinerator system. As discussed
in Section 3.1, the waste gas composition determines the combustion air
requirements. The waste gas and combustion temperatures, along with
the waste gas volume flow rate and VOC content, determine the auxiliary
heat requirement and the heat exchanger size. Finally, the types of
VOC present in the waste gas determine the combustion temperature
required in the incinerator for optimum oxidation.
-------
3-9
In the incineration process, a waste gas is introduced to the
combustion chamber, (or "preheat chamber," in the case of a catalytic
incinerator), where the waste gas temperature is raised to the
appropriate combustion temperature by burning auxiliary fuel. Because
of the high combustion temperatures maintained, refractory chambers are
used in thermal incinerators, while stainless steel or carbon steel
chambers are used in catalytic incinerators. In thermal incinerators,
the waste gas is heated and retained for 0.3 to 1.0 seconds in the
combustion chamber at 100°F or more above the ignition temperature,
which ranges from 1,000 to 1,400°F for most VOCs. At these temperatures,
95 to 99 percent of the VOCs in the waste gas are combusted. As discussed
in Section 3.1, the resulting flue gases are exhausted via a stack to
the atmosphere, after a part of their sensible heat is recovered via
direct exchange with the incoming waste gas.
In catalytic incinerators, the waste gas temperature is typically
raised in the preheat chamber to 500 to 600°F. This is above the
catalytic ignition temperature of 400 to 550°F theoretically required
for 90 percent destruction of most VOCs. (Thermal and catalytic ignition
temperature data for VOCs are well documented in the literature. Table
3A-2 in Appendix A presents catalytic ignition temperatures for several
common VOCs.) The thoroughly mixed gaseous effluents from the preheat
chamber (where partial oxidation may occur) are subsequently passed
through specially designed units containing catalyst elements, on the
surface of which oxidation occurs at an accelerated rate at temperatures
of 700-900°F--much lower than typical thermal incineration combustion
chamber temperatures. As the linear velocity through the catalyst bed
is high (600 to 1,200 ft/min.), the residence time is negligible.
-------
3-10
For this reason residence time is rarely an important factor in the
design of catalytic incinerators. The heat of reaction from the oxidation
of the VOCs in the catalyst bed causes the gas temperature to increase as it
passes across the catalyst bed. The amount of VOCs present in the
waste gas determines the temperature increase in the catalyst bed.
The desired catalyst bed outlet temperature is typically 700 to
900°F. The maximum temperature to which the catalyst bed can be exposed
continuously is limited to about 1,200°F. Therefore, the heat released
from the combustion reaction and, accordingly, the VOC content of the
waste gas are limited to about 20 percent LEL. (See Section 3.2.2. for
more details.)
Depending upon the catalyst bed temperature swings (i.e., the
frequency at which it is subjected to extreme temperature excursions),
the operating and maintenance practices, and the particulate matter and
specific catalyst poisons encountered, the catalyst would have an
effective life of 2 to 10 years. The amount of catalyst required
(measured by the standard hourly flue gas volume flow rate per unit
volume of catalyst or the "space velocity," hr'1), depends on the type of
catalyst used, but increases with the required VOC destruction efficiency.
The space velocity, incinerator gas velocity, and pressure drop used
in the system design are determined experimentally.
Finally, as in thermal incinerators, the flue gases exiting the
catalyst bed are exhausted to a stack, usually after heat exchange with
the incoming waste gas.
3.2.2 Design Calculations
This section presents calculations for designing an incineration
system to the level of detail required by a study cost estimate. Further,
these calculations only apply to dilute mixtures of VOCs, air, and inert gas.
-------
3-11
These mixtures are typical of most industrial waste gases (i.e., those
cases which require no outside air for dilution or for combustion
of fuel and waste gas VOC--Category 1 in Figure 3-1 and Table 3B-1,
Appendix 3B). Appendix 3B presents a general procedure applicable
to all types of waste gas compositions, including those requiring the
use of outside air for combustion or dilution purposes (i.e., Categories
1 through 6 in Figure 3-1 and Table 3B-1, Appendix 3B).
Figure 3-2 is a simplified schematic of an incinerator system. A
thermocouple in a thermal incinerator combustion chamber measures
temperature, and appropriate control circuitry alters the rate of
auxiliary fuel entering the incinerator to maintain the desired combustion
temperature. In catalytic incinerators, thermocouples installed in the
preheat chamber and catalytic bed perform the same function.
The incinerator design calculations for the waste gas cases where
no outside air is added for either combustion or dilution and where the
VOC content is low (i.e., Category 1) can be summarized by the following
step-wise approach. The items that must be calculated or estimated
in the design of any emission control system are those which determine
the system size, performance, and capital and operating costs. In the
case of incinerators, these items are: (1) the auxiliary fuel requirement
and flue gas flow rate; (2) for catalytic incinerators, the amount of
catalyst required; and (3) the pressure drop across the system.
Item 1. Calculate Auxiliary Fuel Requirement and Flue Gas Flow Rate.
The calculations of the auxiliary fuel requirement and the flue gas flow
rate constitute an important incinerator design item. The flue gas flow rate
determines the incinerator system size and, consequently, its capital cost.
The auxiliary fuel can be a major operating cost for an incinerator system,,
-------
Uaste gas from the process
(0)
Combustion
A1r« (4)
Auxiliary
Fuel (3)
Combustion
Chamberb
i
Heat Loss
L
-------
3-13
Calculations of the auxiliary fuel requirement and flue gas
flow rate are considered as one item, because they are interrelated.
The specified flue gas conditions determine the amount of auxiliary
fuel consumed which, in turn, becomes a part of the flue gas. The
necessary waste gas and combustion information to be compiled and
the calculations to be performed are presented in the following
steps:
Step 1. Identify the waste gas composition data. These
include: the volume flow rate (Qj), scfm; temperature
(Tl), °F; VOC content, % LEL; and heat content (hi),
BTU/scf of waste gas. The heat content of the waste
gas is a function of the VOC content. (The waste gas
pressure is not a design consideration, because the
majority of waste gases enter the incinerator at
atmospheric pressure.)
Step 2. For thermal incinerators, determine the combustion
temperature (T^) based on the desired VOC destruction
efficiency. Suggested combustion temperature (T$)
values for waste gases containing nonhalogenated VOCs
are 1,600°F and 1,800°F, respectively, for 98 and 99
percent VOC destruction efficiencies. These temperatures
correspond to a 0.75-second residence time in the
incinerator. Higher temperatures of about 2,000°F
(and 1-second residence time) are required for the
destruction of halogenated VOCs by 98 percent or
more.
-------
3-14
For catalytic incinerators, select the preheat
temperature (Tg) considering the amount and type of VOCs
present in the waste gas. Upon oxidation in the catalyst
bed, the heat content of the waste gas VOCs is released
in the bed. Consequently, the temperature of the bed and
waste gas increase, as explained in Section 3.1.1, by
about 27°F per each one percent of VOC LEL. For most
VOCs oxidized on precious metal catalysts, the suggested
preheat temperature (15) is 600°F. For a waste gas content
of 4 to 10 percent LEL, this results in an average catalyst
operating temperature of 700 to 900°F (i.e., 600°F + [4
to 10]% LEL x 27°F/%LEL = 700 to 900°F).
Less stable VOCs, such as monohydric alcohols,
aromatic hydrocarbons or propylene, can use lower catalyst
operating temperatures, on the order of 500°F.
In rare cases, where the VOC content in the waste
gases is less than 2 percent LEL, higher preheat temperatures
(700-750°F), or more catalyst than typically required, may
have to be used to obtain equivalent 700° to 900°F catalyst
operating temperature and VOC destruction. However, to
avoid this increase in fuel/catalyst costs, the VOC content
of the waste gas is increased when possible by repeated
recycling of waste gases within the process being controlled,
before introducing them to the incinerator.
Waste gases with VOC contents higher than 20 percent
LEL are not suitable for catalytic incineration, because
-------
3-15
the VOC heat content released increases the catalyst bed
temperature to beyond 1,200°F, the maximum permissible
temperature to which the catalyst bed can be exposed
continuously.
Conversely, for the destruction of more stable VOCs,
such as methane, higher preheat temperatures (900-1,000°F)
are used. This, however, requires a lower maximum allowable
VOC content--i.e., 7 to 11%.
Step 3. (Applies to only catalytic incinerators.) Calculate the
catalyst bed outlet temperature (Tfi). As indicated in
Step 2, Tfi should not exceed 1,200°F. Further, as explained
in Section 3.1.1, the heat of combustion released by the
VOC in the waste gas increases the waste gas temperature
by 27°F for each one percent of LEL. Because the average
heat capacity of air is approximately 0.018 BTU/scf-°F,
the waste gas heat content required to increase the waste
gas temperature by 27°F would be equivalent to 0.5 BTU/scf.
(0.018 x 27). It follows that a waste gas VOC heat content
of 1 BTU/scf would, when released through combustion,
increase the waste gas temperature by about 55°F upon
release (2 x 27°F). Therefore, we can write:
T6 = T5 + (Qi/Q5)(55)(h1) (3-1)
where 1§ = Preheat temperature, °F, determined from
Step 2
hi = Waste gas heat content, (BTU/scf of waste
gas)
= Ratio of the waste gas flow rate at the
incinerator inlet to the flue gas flow rate
at the preheat chamber exit. For dilute
waste gases consider the ratio to be equal
to 1, by neglecting the increase in flue
gas volume due to the fuel addition.
1200°F
-------
3-16
Step 4. Determine the waste gas temperature at the incinerator inlet
(T?) = the temperature at the heat exchanger outlet.
With thermal incineration, Tg must be no higher than
1,000 to 1,1000F, to avoid preignition of the waste gas
before it reaches the combustion chamber. T£ is also
limited by the heat VOC content of the inlet waste gas. As
eq. 3-1 indicates, the temperature rise realized in the
combustion chamber is proportional to this heat content.
For example, if a 20 percent LEL stream were incinerated,
the temperature rise would be about 540°F (20 x 27°).
Subtracting this from a combustion temperature of 1500°F
yields a maximum T£ of about 950°F. For catalytic
incinerators, the waste gas temperature (12) entering the
preheat chamber can be as high as the preferred preheat
temperature (15), 600°F.
Broadly speaking, as ~\2 increases, the auxiliary fuel
requirement (and cost) decreases. But at the same time,
the size and cost of the recuperative heat exchanger
increases, driving up the total capital investment of the
system. Thus, there is a trade-off between capital and
operating costs, the extent of which depends upon the value
of 1"2 selected. However, as this selection depends
on the results of a process optimization analysis, no
firm guidance can be given for selecting T"2—except that
it should not exceed the design limits noted above. The
discussion that follows may guide one in performing such an
optimization.
-------
3-17
First of all, the waste gas temperature entering the
combustion chamber/preheat chamber is determined by the
heat exchanger heat transfer performance (denoted by (j)
or HE) and the waste gas inlet temperature (TI).
For incinerator systems with jru> heat exchangers
T£ = TI (the temperature of the entering waste gas).
For incinerator systems with primary (recuperative)
heat exchangers, T£ is calculated from the known value of
the system's heat exchanger heat transfer performance
($) and the following expressions:
T"2 = TI + (p (15 - TI) for thermal incinerators
1"2 = TI + (p (Ts - TI) for catalytic incinerators
(Note: Use of a secondary heat exchanger has no affect
on the T"2 value.)
Theoretically, a heat exchanger heat transfer performance
(0) approaching 100 percent is possible. However, as the heat
exchanger performance increases the required heat transfer
area, the heat exchanger cost increases enormously,
approaching infinity as (p approaches 100 percent.
Therefore, heat exchanger (p is selected based on economic
considerations, as well as the waste gas and flue gas data.
Included among the waste gas and flue gas data which impose
limitations on (j) are: (.1) the flue gas temperature at
-------
3-18
the heat exchanger outlet (Ty) in the case of either type
of incinerator and (2) the waste gas VOC content in the
case of thermal incinerators.
Depending upon the flue gas constituents, mainly
moisture and corrosive elements, Ty should be about SOOT,
due to possible equipment corrosion and condensation
that may occur if it falls below this temperature.
The value of Ty is related to (|) by the following
equation:
() = Q" [1 " Tg - ij ] for thermal incinerators (3_4)
Cl - TQ - TJ3 for catalytic incinerator (3_5)
where Qi, 0.5, and 0.5 = waste gas flow rate and flue gas flow rates
at the exit of combustion chamber and
catalyst bed respectively (scfm)
CP2» CP5» and CP6 = Mean neat capacities of waste gas and
of flue gas at the exit of the combustion
chamber and catalyst bed, respectively
(Btu/ft3°F)
It is evident from the above equation that maximum
possible $ increases with the value of T]_. However, T^ can
never equal or exceed Ty.
The following heat transfer performance ((])) capabili-
ties are commonly reported for typical modular heat exchangers:
35 to 40 percent for 1-pass, 45 to 50 percent for 2-pass, and
65 to 70 percent for 3-pass units.
Step 5. Calculate the amount of auxiliary fuel required (0.3) based
on the following energy balance around the combustion chamber/
preheat chamber (see Figure 3-2):
-------
3-19
Amount of
sensible heat
leaving
Amount of
sensible heat
entering
Amount of
heat of VOCs
released
from
combustion
Amount of
heat of fuel
released from
combustion
(H5
(H5
HL) -
HL) -
where
(H2 + H3)
(H2 + H3)
HL =
\\2 =
H3 =
0.3 =
h]_ =
h3 =
QI hi + Q3 (13 (for thermal incinerators) (3-6)
03 h3 (for catalytic incine- (3-7)
rators, because VOC heat
content is not released
in the preheat chamber).
Sensible heat of flue gas at the combustion
chamber/preheat chamber exit (BTU/min)
Heat loss = 10% HS (assumed)
Sensible heat of waste gas at the incinerator/
preheat chamber inlet (BTU/min)
Sensible heat of fuel used (BTU/min)
0 (for fuel entering at ambient con-
ditions)
Waste gas flow rate (scfm)
Auxiliary fuel flow rate (scfm)
Waste gas heat content (BTU/scf of waste gas)
Lower heating value (LHV) of fuel (BTU/scf).
By substituting Hn- = Q-j Cpj ATj , where Cp^ represents the
mean heat capacity for the temperature difference of AT-,-
(Tj-Tr, the reference temperature) the sensible heat
values of the gas streams leaving and entering the
chamber can be expressed by the following equations:
H5 = Q5 CP5 AT5
(3-8)
where
= Mean heat capacity of the flue gas for the
temperature interval of AJ5) from the
reference temperature (70°F) to the combus-
tion temperature (in the case of thermal
incinerators) or to the preheat temperature
(in the case of catalytic incinerators)^
Cp5 = 0.0194, 0.0196, and 0.0198 BTU/ft3°F
for thermal incinerator combustion
temperatures of 1,600, 1.800, 2,000°F, respect-
ively, and 0.0183 BTU/ft3°F for a catalytic
incinerator preheat temperature of 600°F
H2 = 0,2
(3-9)
-------
3-20
where Cp2 = Mean heat capacity of the waste gas for
the temperature interval AT2 from the
reference temperature (70°F) to the
temperature at the inlet of the incinerator/
preheat chamber (T2). Cp2 = 0.0181 BTU/ftJ°F
for waste gas temperatures (Tj) up to 300°F
Q2 = Ql-
Substituting the above enthalpy values for H2 and HS
and QI + 0,3 = Q2 + Qa for 0.5 in eq. (3-6) or (3-7),
rearranging the resulting equation, and accounting for
the heat loss yields the following (see Appendix 3B for
details):
Ch 33
Q = Fuel used, ft /std. ft of waste gas
1.1 CPR AT 5 - Cp? AT? - hi
h3 - 1.1 C5 AT5 (3-10)
Step 6. Calculate the gas flow rate leaving the combustion
chamber/preheat chamber (Qs). In the case of a thermal
incinerator, because the combustion takes place pri-
marily in the combustion chamber, the resulting flue
gas consists of the products of combustion of the waste
gas VOCs and the auxiliary fuel. In the case of catalytic
incinerators, because fuel is added in the preheating
chamber to raise the waste gas temperature for subsequent
combustion in the catalyst bed, the gases exiting the
preheat chamber consist of the waste gas and the products
of fuel combustion.
In the incineration of dilute VOC waste gases the
flue gas flow rate is approximately equal to the total
of the waste gas flow rate and fuel flow rate used for
-------
3-21
combustion, because no outside combustion air is used
(i.e., Q4 =0). (This is because, in the case of thermal
incinerators, the increase in flow rate due to combustion
of the waste gas VOCs and fuel is negligible.) Therefore,
as indicated above:
Q5 = Q2 + Q3 (3-11)
where Q5 = Flue gas flow rate (scfm)
Q2 = Waste gas flow rate (scfm)
0,3 = Auxiliary fuel burned (scfm)
Item 2. Establish the Amount of Catalyst Required.
The cost of catalyst consumed or replaced represents a significant
catalytic incinerator operating cost item. Therefore, the amount of
catalyst used and its expected life must be accurately determined in
the initial design of a catalytic incinerator system.
The amount of catalyst required usually depends upon the type and
age of catalyst used, types and amounts of VOCs encountered, the
destruction efficiency required, and the amounts of potential reactants
present that impair the catalyst activity, which is usually highest when
the catalyst is fresh. Because in most cases only limited information
is available about the potential reactants and noncombustible particul ate
matter that may be present in the waste gas, most catalytic incinerator
systems are overdesigned to compensate for the unexpected. The amounts
of catalyst required for the destruction of various types of VOCs are
usually established by laboratory tests by catalyst manufacturers.
The amounts of precious metal catalysts commonly used are 1.5 and
2 ft3 per 1,000 scfm waste gas (equivalent to space velocities of
40,000 and 30,000 hr1) for 90 and 95 percent VOC destruction
efficiencies, respectively.(*) The corresponding amounts of base metal
-------
3-22
catalysts used are 4 to 6 ft^/1,000 scfm (i.e., space velocities of
15,000 to 10,000 hr'1).^)
Item 3. Determine the Pressure Drop Required Across the System.
The total pressure drop required across an incinerator system
determines the waste gas fan size and horsepower requirements, which,
in turn, determine the fan capital cost and electricity consumption.
The total pressure drop across an incinerator system depends on
the number and types of equipment included in the system and on
design considerations. The estimation of actual pressure drop require-
ments involves complex calculations based on the specific system's
waste gas and flue gas conditions and equipment used. For the purposes
of this section, however, the following approximate values can be used:
Pressure drop (AP, in. t^O) across
Thermal incinerators = 4
Catalytic incinerators = 6
Heat exchangers: 35% = 4
50% = 8
70% = 15
Once the total pressure drop required is estimated (as a summation
of the pressure drops across all pieces of equipment in the incinerator
system), the blower electricity requirements can be estimated from the
basic fan horsepower requirement equation. For example, at a combined
fan-motor efficiency of 62 percent, the fan horsepower equation dictates
that 0.19 kWh of electricity per hour is required for moving 1,000 acfm
of flow rate at a 1-in. water pressure drop. Therefore, the fan
power requirement is estimated by multiplying the total pressure drop
(inches of water) by the total flue gas flow rate (thousand acfm) and
the 0.19 kWh/hr factor. For the cases where an additional fan is
-------
3-23
used to add outside air to the system, its power requirement must be
calculated separately. (See Section 2 of this Manual for more on how
to estimate electricity requirements.)
3.3 Estimating Total Capital Investment
This section provides a general methodology for developing "study"
estimates of capital costs (April 1986 dollars) for thermal and
catalytic incinerator systems. The precision of the "study" cost
estimate (i.e., +30 percent) applies to the estimates presented herein.
In this method, the total purchased cost of the system equipment is
the basis to which predetermined factors are applied, to estimate the
system direct and indirect installation costs. (See Section 2 for a
more thorough discussion of this estimating method.)
The purchased cost of an incinerator varies widely depending upon
several design factors. Therefore, discretion is needed when using
generic incinerator cost information. Among the factors that influence
the incinerator purchased cost are supplier's design experience, materials
of construction, instrumentation, the type of heat exchanger used and the
nature of the installation, i.e., whether indoor, outdoor, ground level,
or roof top.
The nature of the installation has a particular effect on the system
design and, consequently, the cost. Specifically, incinerator systems
for roof top installations are made of light weight material, while
equipment for outdoor locations can be preassembled in larger modules
than for indoor locations. Traditionally, for thermal incinerators
stainless steel combustion chambers are used to achieve a maximum
equipment life (typically 15 years). Several manufacturers offer
-------
3-24
carbon steel units at costs of 60 to 70 percent of that for stainless
steel units. Of course, the life of such equipment will be lower—
approximately 10 years. Catalytic incinerators will tolerate lower grade
material (e.g., carbon steel) better than thermal units because of their
lower operating temperatures. However, catalyst bed enclosures are mostly
fabricated of stainless steel.
3.3.1 Thermal Incinerator Equipment Costs
Figure 3-3 presents thermal incinerator equipment costs (April 1986
dollars) as a function of the waste gas volume flow rate at standard
conditions of 70°F and 0 psig. This figure was developed from cost
information received from three incinerator manufacturers for three
volume flow rates each.(3»4»5) Analytical equations for these equipment
cost curves are presented in Table 3-1. (The table also presents
analytical equations for catalytic incinerators, the cost curves for
which are discussed in Section 3.3.2.) The equipment costs listed
represent all the equipment in an incinerator system including a com-
bustion chamber with burner, waste gas fan, inlet and outlet plenums,
prepiping and prewiring, instrumentation and controls, a 10-ft stack,
and in the case of heat recovery, a primary heat exchanger. The cost
data apply to the cases of dilute VOC content waste gases (to which no
outside air is added) containing up to 25 percent LEL VOC and incinerated
at a 1500°F combustion temperature.
The Figure 3-3 (or Table 3-1) cost data can be applied to other cases
of waste gas compositions including those requiring slightly different
combustion temperatures without introducing significant errors to the costs.
When these data are applied to different combustion temperatures,
-------
3-25
1,000
vo
oo
Ol
(A
o
Q.
•r-
cr
O No HE A 50% HE
D 35% HE O 70% HE
HE » Heat Exchange j=g
Volume Flow Rate, 1,000 scfm
Figure 3-3. Thennal incinerator equipment cost estimates
-------
TABLE 3-1. EQUATIONS FOR INCINERATOR EQUIPMENT COSTS3
(APRIL 1986 DOLLARS)
Extent of
heat
exchange
None
35%
50%
70%
1 n Eq =
In Eq =
In Eq =
In Eq =
Equipment cost equation for
thermal incinerators"
[14,402 -
[16,175 -
[15,784 -
[20,608 -
992(1 nQ)+
l,262(lnQ)+
l,165(lnQ)+
2,119(lnQ)+
70 (In
85 (In
81 (In
131 (1
Q)2] (IO-3)
Q)2] (IO-3)
Q)2] (IO-3)
n Q)2](10-3)
In Eq
In Eq
In Eq
In Eq
Equipment cost equation for
catalytic incinerators
= [24,086 -
= [27,170 -
= [26,497 -
= [21,685 -
3,252(1 n
3,789(1 n
3,650(ln
2,643(1 n
Q)+
Q)+
Q)+
Q)+
•».-»-.•«=»•
205
231
225
174
=»^a— e--a
(lnQ)2](10-3
(lnQ)2](lO-3
(lnQ)2](10-3
(lnQ)2](lO-3.
aThese equations should not be extrapolated outside the flowrate range of 5,000 to 50,000 scfm.
co
i
ro
15 Eq - Equipment cost in April 1986 dollars, Q - Volume flow rate in scfm.
-------
3-27
it is assumed that the system will tolerate minor adjustments
to the incinerator face velocity and residence time values resulting
from these temperatures. Even if the system size were adjusted to
compensate for these different temperatures,the resulting changes in
the equipment cost would be minor. For example, if a combustion chamber
were sized for a combustion temperature of 1,700°F instead of 1,500°F
the cost increase due to the increase in combustion chamber size would
be less than 5 percent.
The Figure 3-3 and Table 3-1 cost data can also be applied to
cases of waste gas compositions to which outside ambient air is added
for combustion. In these cases, the flue gai flow rate at standard
conditions must be substituted for the waste gas flow rate in the
figure.
3.3.2 Catalytic Incinerator Equipment Costs
Figure 3-4 presents catalytic incinerator equipment costs (April
1986 dollars) as a function of the waste gas volume flow rate at standard
conditions (70°F and 0 psig), developed from cost information received
from five equipment manufacturers.(6-10) Analytical equations repre-
senting these cost data are presented in Table 3-1. The equipment
cost data represent all the equipment in an incinerator system, including
the burner, fan, housing, skid mounting, instrumentation and controls,
a 10-ft stack, catalyst, and where heat recovery is used, a primary
heat exchanger. The cost data apply to dilute VOC waste gases requiring
a temperature of 600°F at the preheat chamber exit/catalytic bed inlet.
The cost data can be applied to waste gases to which outside air is
added by substituting the flue gas flow rate at standard conditions for
the waste gas flow rate in Figure 3-4.
-------
3-28
1,000
O No HE
D 35Z HE
A 50* HE y
O 702 HE !
HE = Heat Exchange
50
Volume Flow Rate, 1,000 scfm
Figure 3-4. Catalytic incinerator equipment cost estimates
-------
3-29
3.3.3 Total Capital Investment for Incinerators
The total incinerator capital investment as a percentage of equip-
ment cost varies with the type of installation, i.e., whether custom or
package. The type of incinerator installation is an important consider-
ation. For most nonchemical industry operations, such as drying ovens,
painting facilities, printing and coating operations and other processes
where incinerators are commonly used for VOC control, waste gas flow rates
generated are small. Modular incinerator units, prefabricated and
preassembled, are most suitable for such flow rates. The size of
skid-mounted modular units is limited to handling flows up to about
20,000 cfm, mainly because of limitations to the sizes of trucks carrying
them and access doors at the plant sites. Of course, larger units,
prefabricated and partially assembled, are possible for outdoor install-
ations. Some incinerators handling up to 100,000 cfm, which are partly
field-fabricated and field-assembled, can be found in chemical plant
applications. Typically, heat exchangers can accommodate gas flows from
3,000 to 20,000 cfm. For larger flowrates, multiple heat exchangers
units are usually installed in parallel in a single incinerator system.
For incinerators, the total capital investment varies from a small
percentage (about 110 percent) to more than 200 percent of the total
equipment purchased cost. The custom installation charges of an incinerator
system, including incinerator and heat exchanger units, ductwork, fans,
and a tall stack if required, may amount to 300 to 400 percent of the
total equipment purchased cost, depending upon the installation.
The skid-mounted units ready to be placed on a concrete pad at a
prepared site incur the lowest installation expenses. Roof top instal-
lations will require higher costs, field-assembled units even higher,
-------
3-30
and field-fabricated and assembled units the highest costs.
For estimating purposes, the total capital investment (TCI) of a
skid-mounted modular unit can be calculated at 125 percent of the total
purchased equipment cost. Conversely, Table 3-2 presents typical
estimated capital cost factors applicable to custom installations of
both thermal and catalytic incinerator systems. No cost factor is
included for estimating ductwork cost in the table because the ductwork
size and length and, consequently, its cost will vary with the instal-
lation and the waste gas flowrate. For both packaged and custom incine-
rator units, the ductwork capital cost must be estimated separately.
Alternatively, in the case of custom units, the purchased cost of the
ductwork and other auxiliary equipment can be added to the total incine-
rator equipment cost prior to applying the Table 3-2 factors for estimating
the total capital investment. (Note: As Section 2 of the Manual
indicates, the TCI also includes costs for land, working capital, and
off-site facilities, which are not included in the direct/indirect
installation factor. However, as these items are rarely required with
incinerator systems, they will not be considered here. Further, no
factor has been provided for site preparation (S.P.), or buildings (Bldg.)
since these site-specific costs depend very little on the purchased equipment
cost. Lastly, for the incinerator cases in which outside air is added
for combustion or dilution, the cost of the fan used for this purpose
also must be estimated separately.)
3.4 Estimating Total Annual Cost
3.4.1 Direct Annual Costs
For incinerator systems with no heat exchanger, the cost o'f auxiliary
fuel is the major direct annual cost. Fuel costs are considerably
-------
3-31
TABLE 3-2. CAPITAL COST FACTORS FOR CUSTOM THERMAL AND CATALYTIC INCINERATORS3
Cost Item
Cost Factor (Fraction of Indicated Cost)
DIRECT COSTS
1) Purchased equipment cost
Incinerator
Auxiliary equipment^
Instruments and controls0
Taxes
Freight
Total Purchased Equipment cost
2) Direct installation costs
Foundations and supports
Erection and handling
Electrical
Piping
Insulation
Painting
Site preparation (S.P) Buildings
(Bldg.)
Total Installation Direct Costs
Total Direct Cost
INDIRECT COSTS
As
As
required K
required )"
0.1 A
0.03 A
0.05 A
B(=1.18A)
0.08 B
0.14 B
0.04 B
0.02 B
0.01 B
0.01 B
As required
0.30 B + S.P. + Bldg.
1.30 B + S.P. + Bldg.
Engineering and supervision 0.10 B
Construction and field expenses 0.05 B
Construction fee 0.10 B
Start-up 0.02 B
Performance test 0.01 B
Contingency 0.03 B
Total Indirect Costs 0.31 B
Total Direct and Indirect costs = Total Capital Investment |1.61 B + S.P. + Bld"g<
Reference 11.
^Includes ductwork and any other equipment normally not included with unit furnished
by incinerator vendor.
cCost of instrumentation and controls often furnished by vendor.
-------
3-32
lower for catalytic incinerators than for thermal units because of the
lower operating temperature. Besides the fuel, the utility requirements
for an incinerator include electricity for the waste gas fan and, if
used, the ambient air fan. Operating labor is usually small for both
types of incinerators--0.5 hr/shift.(H)
Maintenance costs of well-designed and maintained thermal or catalytic
incinerators of either type are low, with a substantial portion required
for the heat exchanger maintenance. (Well-designed and fabricated heat
exchangers, i.e., stainless heat exchangers, usually have lower maintenance
expenses.) The incinerator unit maintenance requirements may include
repairs to the refractory lining and blowers, maintenance of control
instruments, and cleaning of flame rods, if used. Maintenance labor
may be estimated at 0.5 hr/shift. The maintenance materials may be assumed
to equal the maintenance labor cost.(H)
Depending upon the incinerator and how it is operated, the life
of a given load of catalyst may be 2 to 10 years. A conservative
estimate of catalyst replacement cost can be based on the lower life
time--2 years. The initial costs of precious metal and base metal
(manganese dioxide) catalysts are $3,000/ft3(1) and $600/ft3 (2),
respectively. Item 2 of Section 3.2.2 showed how to estimate the
catalyst requirement (ft3). Finally, the catalyst replacement labor is
minimal compared to the catalyst cost.
Based on these values and the estimating procedure shown in Section 2,
the catalyst replacement cost (CRCcat) would be:
= Ccat x 1.08 x CRFcat (3-12)
-------
3-33 ,
TABLE 3-3. SUGGESTED FACTORS FOR ESTIMATING INCINERATOR ANNUAL COSTS
Item
Suggested Factor
Direct Operating Costs
Operating labor9
Supervisory labor3
Maintenance labor
Maintenance materials9
Replacanent parts
Utilities:
Fuel
Electricity15'0
Indirect Operating Costs
Overhead
Administrative charges
Property tax
Insurance
Capital recovery cost6
0.5 hr/shift
15% of operating labor
0.5 hr/shift
100% of maintenance labor
Thermal incinerators: None
Catalytic incinerators: (See eq. 3-12)
The amount of fuel required is calculated from
Step 5 (Qs) of Section 3.2.
Use the following A P values in estimating elec-
tricity requirements:
Thermal incinerators = 4 in. water
Catalytic incinerators = Sin. water
Heat exchange of
35%
50%
70%
Ductwork and stack
= 4 in. water
= Sin. water
=15 in. water
= As required
60% of sum of operating, supervisory, and maintenance
labor and maintenance materials
2% X TCI?
1% X TCI°
1% X TCI
CRFs X [TCI - 1.08 x Crat]
Reference 11.
total A P of an incinerator system is the sum of base (i.e., incinerator)
A P + heat exchanger A P.
cAn equation to calculate electricity requirements (kWh per hour) is given
in Item 3 of Section 3.2.2.
dTCI = Total capital investment.
CRFs (system capital recovery factor) is a function of the equipment life
(10 years, typically) and the opportunity cost of the capital (i.e., interest
rate). For instance, for a 10-year life and a 10% interest rate, CRFS = 0.1628.
-------
3-34
where: Ccat = initial catalyst cost ($)
1.08 = factor accounting for taxes and freight
CRFcat = capital recovery factor for catalyst (e.g., for a
2-year life and a 10% interest rate, the factor would
be 0.5762).
3.4.2 Indirect Annual Costs
As Table 3-3 shows, the indirect (fixed) annual operating costs
include capital recovery; overhead; and property taxes, insurance, and
administrative charges. The last three costs can be estimated at 1
percent, 1 percent, and 2 percent of the total capital investment,
respectively. The system capital recovery cost is based on an estimated
10-year equipment life. (The incinerator system life varies from 5 to
15 years depending on the material of construction used and the durability
of the design.) However, as Section 2 indicates, the system capital
recovery cost is the product of the system capital recovery factor
(CRFs) and the total capital investment (TCI) less the purchased cost
of the catalyst ("Crat x 1.08"), This offset is necessary to avoid
double-counting.
Finally, overhead may be estimated at 60% of the sum of operating,
supervisory, and maintenance labor and maintenance materials. (See
Section 2 of the Manual for a detailed discussion of the items comprising
the total annual cost.)
-------
3-35
References for Section 3
1. Telephone conversation: Robert M. Yarrington (Engelhard Corpora-
tion, Union, NJ) with Vishnu S. Katari (Pacific Environmental Services,
Inc., Durham, NC), June 2, 1986.
2. Telephone conversation: Bob Campbell (Pillar Technologies, Inc.,
Hartland, WI) with Tom Rhoads (Pacific Environmental Services, Inc.,
Durham, NC), February 28, 1986.
3. Correspondence: Ralph N. Stettenbenz (C-E Air Preheater, Combustion
Engineering, Inc., Wellsville, NY), to Vishnu S. Katari, April 3, 1986.
4. Correspondence: Ronald J. Baschiere (Hi rt Combustion Engineers,
Montebello, CA) to Vishnu S. Katari, April 7, 1986.
5. Correspondence: C.L. Bumford (Peabody Engineering, Stamford, CT)
to Michael K. Sink (Pacific Environmental Services, Inc., Durham, NC),
March 24, 1986.
6. Correspondence: Robert M. Yarrington (Engelhard Corporation, Union,
NJ) to Vishnu S. Katari, April 30, 1986.
7. Correspondence: Kent Lewis (C-E Ai r Preheater, Combustion Engineering,
Inc., Wellsville, NY) and Vishnu S. Katari, June 16, 1986.
8. Correspondence: Robert Hablewitze (Pillar Technologies, Inc., Hartland,
WI) to Vishnu S. Katari, March 3, 1986.
9. Correspondence: Thomas Schmidt (Energy Development Associates, Itascon,
IL) to Michael K. Sink, March 7, 1986.
10. Correspondence: Richard J. Ihde (TEC Systems, DePere, WI) to Michael K.
Sink, March 11, 1986.
11. Vatavuk, William M. and Neveril, Robert. "Estimating Costs of Air-
Pollution Control Systems, Part II: Factors for Estimating Capital and
Operating Costs." Chemical Engineering, November 3, 1980, pp. 157-162.
-------
APPENDIX 3A. LOWER AND UPPER EXPLOSIVE LIMITS FOR VOCs
-------
3A-2
TABLE 3A-1. FLAMMABILITY CHARACTERISTICS OF COMBUSTIBLE
ORGANIC COMPOUNDS IN AIRa»b
Methane
Ethane
Propane
n-Butane
n-Pentane
n-Hexane
n-Heptane
n-Octane
n-Nonane
n-Decane
n-Undecane
n-Dodecane
n-Tridecane
n-Tetradecane
n-Pentadecane
n-Hexadecane
Ethyl ene
P ropyl ene
Butene-1
cis-Butene-2
Isobutylene
3-Methyl -Butene-1
Propadiene
1,3-Butadiene
Acetyl ene
Methyl acetyl ene
Benzene
Toluene
Ethyl benzene
o-Xyl ene
m-Xylene
p-Xyl ene
Cumene
p-Cymene
Cyclopropane
Cyclobutane
Cyclopentane
Cycl ohexane
Ethyl cyclobutane
Mol.
wt.
16.04
30.07
44.09
58.12
72.15
86.17
100.20
114.23
128.25
142.28
156.30
170.33
184.36
108.38
212.41
226.44
28.05
42.08
56.10
56.10
56.10
70.13
40.06
54.09
78.11
92.13
106.16
106.16
106.16
106.16
120.19
134.21
42.08
56.10
70.13
84.16
84.16
LEL,C
% vol .
5.0
3.0
2.1
1.8
1.4
1.2
1.05
0.95
0.85
0.75
0.68
0.60
0.55
0.50
0.46
0.43
2.7
2.4
1.7
1.8
1.8
1.5
2.6
2.0
2.5
1.7
1.3
1.2
1.0
1.1
1.1
1.1
0.88
0.85
2.4
1.8
1.5
1.3
1.2
UEL,d
% vol .
15.0
12.4
9.5
8.4
7.8
7.4
6.7
3.2
2.9
5.6
36
11
9.7
9.7
9.6
9.1
12
100
7.0
7.1
6.7
6.4
6.4
6.6
6.5
6.5
10.4
7.8
7.7
-------
3A-3
TABLE 3A-1.
FLAMMABILITY CHARACTERISTICS OF COMBUSTIBLE
ORGANIC COMPOUNDS IN
(concluded)
Cycloheptane
Methyl cycl ohexane
Ethyl cycl opentane
Ethyl cycl ohexane
Methyl al cohol
Ethyl alcohol
n-Propyl alcohol
n-Butyl al cohol
n-Amyl al cohol
n-Hexyl alcohol
Dimethyl ether
Di ethyl ether
Ethyl propyl ether
Diisopropyl ether
Acetaldehyde
Propionaldehyde
Acetone
Methyl ethyl ketone
Methyl propyl ketone
Di ethyl ketone
Methyl butyl ketone
Mol.
wt.
98.18
98.18
98.18
112.21
32.04
46.07
60.09
74.12
88.15
102.17
46.07
74.12
88.15
102.17
44.05
58.08
58.08
72.10
86.13
86.13
100.16
LEL,
% vol .
1.1
1.1
1.1
0.95
6.7
3.3
2.2
1.7
1.4
1.2
3.4
1.9
1.7
1.4
4.0
2.9
2.6
1.9
1.6
1.6
1.4
UEL,
% vol .
6.7
6.7
6.7
6.6
36
19
14
12
10
7.9
27
36
9
7.9
36
14
13
10
8.2
8.0
Reference 1.
^Most common handbooks (e.g., Reference 2) provide
flammability information for VOCs.
CLEL - lower explosive limit
- upper explosive limit
-------
3A-4
TABLE 3A-2. CATALYTIC IGNITION TEMPERATURE
FOR 90% CONVERSION3
Component Temperature, °F
Hydrogen 220
Acetylene 395
Carbon monoxide 425
Propyne 460
Propadiene 480
Propylene 500
Ethyl ene 550
jv-Heptane 575
Benzene 575
Toluene 575
Xylene 575
Ethanol 600
Methyl ethyl ketone 700
Methyl isobutyl ketone 700
Propane 770
Ethyl acetate 775
Dimethyl formamide 800
Ethane 810
Cyclopropane 850
Methane 920
Reference 3.
-------
3A-5
References for Appendix 3A
1. Hilado, Carlos J., "How to Predict if Materials Burn," Chemical
Engineering, December 14, 1970, pp. 174-178.
2. Handbook of Chemistry and Physics, 54th Edition. Cleveland: The
Chemical Rubber Co., 1973-1974, pp. D85-D92.
3. Organic Chemical Manufacturing Volume 4: Combustion Control Devices,
(EPA-450/3-80-026T;Research Triangle Park: U.S. Environmental
Protection Agency, Emissions Standards and Engineering Division.
December 1980. Report 3, p. 1-2.
-------
APPENDIX 3B
This appendix presents equations to estimate flue gas and fuel
requirements of incineration systems combusting all types of waste
gas compositions. These equations have been derived from energy and
material balances around the incinerator system. Figure 3B-1, a
reproduction of Figure 3-2, Section 3.2, is a schematic of an incinerator
system. (All streams shown in Figure 3B-1 are also identified by the
same subscripts in the energy and material balances)
Haste gas fro* the process
(0)
CoBfaustton
Air* (4)
Auxiliary
Fuel (3)
Conbustlon
Chaad>ero
(2)
Heat Loss U
(HI) p
Haste Gas (1)
I
. Catalyst . Heat
F1
Gi
"«,., "*»' <•> (Optional)
I
• Dilution Air* (0)
Flue Gas to
•* Vent/Stack/
ABblent (7)
(a) Hhen required.
(b) Referred to as Preheat Chamber In
the case of catalytic Incinerators.
(c) Included only In catalytic Incinerators.
Figure 38-1. Schematic diagram of an incinerator system.
Energy Balance
The basic energy balance equation around the combustion chamber is:
1 Sensible heat
I leaving
Sensible heat
entering J
Heat released
from the
combustion of
VOCs
From Figure 3B-1, (H5 + HL) - (H2 + H3 + H4) =
where:
tHeat released
from the combustion
of fuel
+ Q3h3
•J
H = Sensible heat of the stream identified by the
subscript, Btu/min
Q = Flow rate of the stream identified by the sub-
script, scfm
-------
3B-2
hi = Waste gas heat content released upon combustion,
BTU/scf of waste gas
h3 = Lower heating value (LHV) of fuel (BTU/scf)
Assume H3 = 0 for fuel at ambient temperature
H4 = 0 for ambient air
HL = 10% of H5
Therefore, 1.1 HS - H2 = QI hi + 0.3 113
Substituting the sensible heat expression H = Q Cp AT in the above equation
yields:
1.1 Q5 CP5 AJ5 - Q2 CP2 A T2 = QI h: + Q3 h3 (3B-1)
where Q = Flow rate (scfm)
CD = Mean heat capacity for the temperature interval
v of AT, BTU/ft3 °F
AT = Temperature interval from TR (70°F) to T identi-
fied by the subscript
hi = Waste gas heat content (BTU/scf of waste gas)
h3 = Lower heating value (LHV) of natural gas
BTU/ft3
Material Balance (see Figure 3B-1)
An (approximate) material balance around the combustion chamber yields:*
Q5 = Q2 + Q3 + Q4 (3B-2)
where QS = Flue gas flow rate (scfm)
Qg = Waste gas flow rate entering the system (scfm)
Q3 = Auxiliary fuel burned (scfm)
Q4 = Ambient air used for combustion (scfm)
The quantities of Q£, 0.3, and 0,4 are estimated as follows:
1) Waste gas entering the combustion chamber (Q?)
= QO For cases of waste gases to which no dilution air
is added (i.e., Cases 1,2, and 4 through 6 in
Table 3B-1)
= QO (he/hd) For cases of waste gases to which dilution
air is added (i.e., Case 3 in Table 3B-1) (3B-3)
where he = Waste gas heat content before dilution, BTU/scf
hd = Waste gas heat content after dilution.
*Strictly speaking, equation 3B-2 is a mole balance, not a material balance.
However, because the moles (or volumes) of the reactants (VOC, fuel, air,
etc.) entering the combustion chamber/catalyst bed approximately equal the
moles of products leaving (H20, C02, N2, etc.), equation 3B-2 is accurate—
at least for purposes of this Manual.
-------
3B-3
2) Ambient air used (Q&)
Q4 = 0 For dilute VOC waste gases (category 1 in
Table 3B-1)
Q4 = Q4,VOC + Q4,Fuel (3B-4)
where Q4 \jnr = Ambient air used for the combustion of the
VOC
= Q2 [ani (X + Y/4 - Z/2)i - HIQ ] 4.79
2
where m-j = Volume content of ith VOC component in the waste
gas
X, Y, Z = Atoms of C, Hg, and 02 in the VOC
mo = Volume content of oxygen in the waste gas
2
0.4,Fuel = Ambient air used for the combustion of the fuel
= 9.58 (base amount of fuel burned)(1 + E)
where E = Excess air used, % of total stoichiometric air
requi rement for fuel only
3) Auxiliary fuel burned (Q-Q
0,3 = Base amount of fuel burned (calculated at zero
excess air) + additional amount of fuel burned to
ensure raising the excess air to the combustion
temperature, (i.e., 9.58E [base amount of fuel])
= Base amount (1 + K)
where K = Excess amount used, % of base amount
The value of K can be determined as follows:
(Additional fuel used)
= [9.58 (base amount of fuel) E + Additional fuel] Cp5 ATs * (13*
Therefore,
Additional fuel used (3B-5)
K = Base amount of fuel
= 9.58E/[(h3/CP5 AT5) - 1]
= Constant for a given fuel and known values of E and "t§
= 0 for dilute VOC waste gases.
*In other words, the "additional fuel" must heat both the excess air used
in combustion of the base fuel and itself, from the ambient (reference)
temperature to the combustion temperature
-------
38-4
Estimation of Combustion Flue Gases Generated
Substituting the values of Q2, QS, and (fy into the basic material
balance equation (Equation 3B-2) yields the following expression:
Q5 = Q2 + Q3 + Q2 l>i (* + Y/4 - Z/2), - mg ] 4.79 (3B-6)
+ 9.58 (base amount of fuel)(l + E)
However, the base amount of fuel = Q3/(l + K), so that:
Q5 = Q2 + Q3 + Q2 l>1 (X + Y/4 - 1/2) \ - m0 ] 4.79
2
+ 9.58 Q3 (1 + E)/(l + K) (3B-7)
= Q2 + Q3 (f°r dilute VOC waste gases)
The above equation applies to the waste gases to which ambient air
is added for combustion purposes. However, in many cases, the waste gas
contains air sufficient to provide partial or complete combustion air
requirements. Table 3B-1 identifies various waste gas types and their
ambient air and auxiliary fuel requirements. Table 3B-2 presents the
equations for estimating the flue gas (Qs) flow rate from the combustion
of each type of waste gas.
Estimation of Actual Amount of Auxiliary Fuel Burned (($3)
Substituting the material balance equation (Equation 3B-2) into the
energy balance equation (Equation 3B-1) and solving for Q3/Q2 (i.e., the
amount of fuel required per unit of waste gas flow rate) yields the
following expression:
?L_ LI [1 + Umi (X + Y/4 - 112)t- mQ2)4.793 CpsATs-CCpoATo+hi]
Q2 " (h3 - 1.1 Cp5 AT5) - iO.54 (1 + E) Cp5 AT5/(I + K) (3B-8)
1.1 Cps ATt; - Cp? AT? - hT , ,., fc
• h!| - 1.1 Cps AT5 (for dTlute waste gases)
The values of all items in the above equation are specific to the
waste gas composition and fuel type. Therefore, the equation can be further
-------
3B-5
TABLE 3B-1. CATEGORIZATION OF WASTE GAS STREAMS
Category
Waste gas
Composition
Auxiliaries and other requirements
Mixture of VOC, air, and inert gas with >16% 02
and a VOC content <25% LEL (i.e., heat content
<13 Btu/ft3)
Mixture of VOC, air, and inert gas with >16% 02
and a VOC content between 25 and 50% LEL (i.e.,
heat content between 13 to 26 Btu/ft3)
Mixture of VOC, air, and inert gas with <16% 02
Mixture of VOC and inert gas with zero to
negligible amount of 02 (air) and <100 Btu/scf
heat content
Mixture of VOC and inert gas with zero to
negligible amount of 02 (air) and >100 Btu/scf
heat
Mixture of VOC and inert gas with zero to negligi-
ble amount of 02 and heat content insufficient to
raise the waste gas to the combustion temperature
Auxiliary fuel is required. No auxiliary
air is requi red.
Dilution air is required to lower the heat
content to <13 Btu/ft3. (Alternative to
dilution air is installation of LEL monitors.)
Treat this waste stream the same as cate-
gories 1 and 2, except augment the portions
of the waste gas used for fuel burning with
outside air to bring its 02 content to above
16 percent.
Oxidize it directly with a sufficient amount
of ai r.
Premix and use it as a fuel.
Auxiliary fuel and combustion air for both
the waste gas VOC and fuel are required.
-------
3B-6
Waste gas
category
TABLE 3B-2. EQUATIONS FOR ESTIMATING FLUE GAS QUANTITIES
Equations for flue gas flow rate (Qs), scfm
1
2
3
5
6
Q5 = Q2 + Q3» where Q2 * Qo
Q5 = Q2 + Q3. where Q2 = QO (he/hd)
Q5 = Q2 + Q3 + Q4,Fuel > where
°.4,Fuel = 9>58 (1+E) (Base amount of fuel used)
= 9.58 Q3 (1+E)/(1+K)
Q5 = Q2 + Q2 [aii (X + Y/4 - Z/2)i - mo ] 4.79 (1+E)
2
Assumes no auxiliary fuel except for pilot is needed
P remix and use it as a fuel.
Q5 = Q2 + Q3 + 9.58 Q3 (1
Q2 \^\ (X + Y/4 - Z/2)i -
m0 ] 4.79
2
-------
3B-7
simplified by substituting known values of these items.
The values of Cp and K are established based on the waste gas
temperature, flue gas combustion temperature, and the type of fuel used
The overall Cp value of the flue gas is calculated more accurately from
the Cp values of the flue gas components, which include C02, ^0, 02,
and N2. Alternatively, as an approximation, the Cp value of air can be
substituted for Cp values of the flue gas and the waste gas. The mean
heat capacity values for air are 0.0194, 0.0196, and 0.0198 BTU/ft3°F
for combustion temperatures of 1,600, 1,800, and 2,000°F, respectively.
The mean heat capacity value of air, to use with a waste gas at 100°F,
is 0.018 BTU/ft3°F.
At a combustion temperature of 1,600°F, K = 0.033 for a LHV of 900
BTU/ft3 for natural gas, an excess air of 10 percent, and a reference
temperature of 70°F. When this value of K and the values of Cps' and
ATs1 are substituted into equation 3B-8, assuming the waste gas
temperature to be 100°F, we obtain:
- U8 hi + 293 < E mi
-------
3B-8
TABLE 3B-3. EQUATIONS FOR ESTIMATING FUEL REQUIREMENTS
Waste gas
category
T^
2a
3b,c
4
5
6
1.1 CpsAT^ - Cjp^AT? - hi
h3 - 1.1 Cp5AT5
1.1 Cp5At5 - Cp2ATo - h}
h3 - 1.1 Cp5AT^
1.1 CpciATt; - Cp2A
(h3 - 1.1 CP5AT5) - 10.
1.1 CpRATR + 1.1 [Smi (X
Waste gas is premixed an
Q3 1.1 [1 + (Emi (X +
Q2 " (h3 - LI
General eguation for fuel,
ft3/ft3 of waste gas
T, - h,
54 Cps AT5 (1+E)_/(1+K)
+ Y/4 - Z/2)i] 4.79 (1+E) Cp^ATR - Cp?AT? - hi
d used as fuel .
Y/4 - Z/2)i - m02) 4.79] Cp5AT5 - [Cp9AT9 + h, ]
Cp5AT5) - 10.54 (f+E) Cp5AT5/(l+K)
aThe waste gas for this category must be adjusted for dilution (See Table 3B-2).
bE - Excess ai r used
CK - Constant = 9.58E/[(h3/Cp5AT5) - 1].
-------
Section 4
CARBON ADSORBERS
William M. Vatavuk
Economic Analysis Branch, OAQPS
William L. Klotz and Robert L. Stallings
Research Triangle Institute
(Research Triangle Park, NC 27709)
4.1 Process Description
4.1.1 Introduction
In air pollution control, adsorption is employed to remove volatile
organic compounds (VOC's) from low to medium concentration gas streams,
when a stringent outlet concentration must be met and/or recovery of the
VOC is desired. Adsorption itself is a phenomenon where gas molecules
passing through a bed of solid particles are selectively held there by
attractive forces which are weaker and less specific than those of
chemical bonds. During adsorption, a gas molecule migrates to the surface
of the solid where it is held by physical atttraction releasing energy—
the "heat of adsorption", which approximately equals the heat of condensation.
Adsorptive capacity of the solid for the gas tends to increase with the
gas phase concentration.
Some gases form actual chemical bonds with the adsorbent surface
groups. This phenomenon is termed "chemisorption".
Most gases ("adsorbates") can be removed ("desorbed") from the
adsorbent by heating to a sufficiently high temperature, usually via
steam, or by reducing the pressure to a sufficiently low value (vacuum
desorption). The physically adsorbed species in the smallest pores of
the solid and the chemisorbed species may require rather high temperatures
to be removed, and for all practical purposes cannot be desorbed during
'regeneration. For example, approximately 3 to 5 percent of organics
adsorbed on virgin activated carbon is either chemisorbed or very
-------
4-2
strongly physically adsorbed and, for all intents, cannot be desorbed
during regeneration.(1)
Adsorbents in large scale use include activated carbon, silica gel,
activated alumina, synthetic zeolites, fuller's earth, and other clays.
This Section is oriented toward the use of activated carbon, a commonly
used adsorbent for VOCs.
4.1.2 Types of Adsorbers
Five types of adsorption equipment are used in collecting gases: (1)
fixed regenerable beds; (2) disposable/rechargable cannisters; (3)
traveling bed adsorbers; (4) fluid bed adsorbers; and (5) chromatographic
baghouses.C2) Of these, the most commonly used in air pollution control are
the fixed bed and cannister types.
4.1.2.1 Fixed-bed Units
Fixed-bed units are normally used for controlling continuous, VOC-
containing streams over a wide range of flowrates, ranging from several
thousand to several hundred thousand cubic feet/minute (cfm). The VOC
concentration of streams treated by fixed-bed adsorbers can be as low as
several parts per billion (ppbv) by volume in the case of some toxic
chemicals or as high as 25% of the VOCs' lower explosive limit (LEL).
For most VOCs, this ranges from 2500 to 10,000 ppmv.(^) Fixed-bed adsorbers
may be operated in either intermittent or continuous modes. In intermittent
operation, the adsorber removes VOC for a specified time (the "adsorption
time"), which corresponds to the time during which the controlled source
is emitting VOC. After the adsorber and the source are shut down (e.g.,
overnight), the unit begins the desorption cycle during which the captured
VOC is removed from the carbon. This cycle, in turn, consists of three
-------
:. 4-3
steps: (1) regeneration of the carbon by blowing steam through the bed
in the direction opposite to the gas flow; (2) drying of the bed, with
compressed air or a fan; and (3) cooling the bed to its operating tempera-
ture via a fan. (In some designs, the same fan can be used both for bed
drying and cooling.) At the end of the desorption cycle (which usually
lasts 1 to 1 1/2 hours), the unit sits idle until the source starts up
again.
In continuous operation a regenerated carbon bed is always available
for adsorption, so that the controlled source can operate continuously
without shut down. For example, two carbon beds can be provided: while
one is adsorbing, the second is desorbing/idled. As each bed must be
large enough to handle the entire gas flowrate while adsorbing, twice as
much carbon- must be provided than an intermittent system handling the
,j. ___________
same flowrate. If the desorption cycle is significantly shorter than the
adsorption cycle, it may be more economical to have three, four, or even
more beds operating in the system. This can reduce the amount of extra
carbon capacity needed or provide some additional benefits, relative to
maintaining a low VOC content in the effluent. (See Section 4.2 for a more
thorough discussion of this.)
A typical two-bed, continuously operated adsorber system is shown in
Figure 4-1. One bed is adsorbing at all times, while the second is
desorbing/idled. As shown here, the VOC-laden gas enters vessel #1
through valve A, passes through the carbon bed (shown by the shading) and
exits through valve B, from whence it passes to the stack. Meanwhile,
vessel #2 is in the desorption cycle. Steam enters through valve C,
passes up through the bed and exits through D. The steam-VOC vapor
-------
(Drying/Cooling
Air)
Waste Gas
(From
Source)
System Fan
(Drying/ M
Cooling A1r) ^^
Steam
Steam-
VOC Vapor^
Out
In
Cooling
Hater
1
Total
Condenser
VOC
Condensate ^
(To Storage,
Processing)
Decanter
r Mater
(To Treatment/
Sewer)
(To Stack)
Figure 4-1. Typical Two-Bed, Continuously Operated Fixed-Bed Carbon Adsorber System
-------
4-5
mixture passes to a condenser, where cooling water condenses the entire
mixture. The condensate next passes to a decanter, where the VOC
and water layers are separated. The VOC layer is conveyed to storage.
If impure, it may receive additional processing, such as distillation.
Depending on its quality (i.e., quantity of dissolved organics), the
water layer is discharged either to a wastewater treatment facility or to
the sewer.
Once steaming is completed, valves C and D are closed and valve E is
opened, to allow, air to enter to dry and cool the bed. After this is
done, the bed is placed on standby until vessel #1 reaches the end of its
adsorption cycle. At this time, the VOC-laden gas is valved to vessel #2,
while vessel #1 begins its desorption cycle, and the above process is
repeated.
In Figure 4-1, the system fan is shown installed ahead of the vessels,
though it could also be placed after them. Further, this figure does not
show the pumps needed to bring cooling water to the condenser. Nor does
it depict the solvent pump which conveys the VOC condensate to storage.
Also missing are preconditioning equipment used to cool, dehumidify, or
remove particulate from the inlet gases. Such equipment may or may not
be needed, depending on the condition of the inlet gas. In any case,
preconditioning equipment will not be covered in this Section.
4.1.2.2 Cannister Units
Canm'ster-type adsorbers differ from fixed-bed units, in that they are
normally limited to controlling low-volume, (typically 100 ft3/min, maximum)
intermittent gas streams, such as those emitted by storage tank vents, where
process economics dictate that toll regeneration or throw-away cannisters
-------
4-6
are appropriate. The carbon cannisters may not be desorbed. Alternatively,
the carbon may be regenerated at a central facility. Once the carbon
reaches a certain VOC content, the unit is shut down, replaced with
another, and disposed of or regenerated by the central facility. Each
cannister unit consists of a vessel, activated carbon, inlet connection
and distributer leading to the carbon bed, and an outlet connection for
the purified gas stream.(4) in one design (Calgon's Ventsorb®), 150 Ibs
of carbon are installed on an 8-inch gravel bed, in a 55-gallon drum.
The type of carbon used depends on the nature of the VOC to be treated.
In theory, a cannister unit would remain in service no longer than a
regenerable unit would stay in its adsorption cycle. Doing so would
prevent the allowable outlet concentration from being exceeded. In
reality, however, poor operating practice may result in the cannister
remaining connected until the carbon is near saturation. This is because:
(1) the carbon (and often the vessel) will probably be disposed of, so
there is the temptation to operate it until the carbon is saturated; and
(2) unlike fixed-bed units, whose outlet VOC concentrations are monitored
continuously (via flame ionization detectors, typically), cannisters are
usually not monitored. Thus, the user can only guess at the outlet
loading, and could tend to leave a unit in place longer.
4.1.3 Adsorption Theory
At equilibrium, the quantity of gas that is adsorbed on activated
carbon is a function of the adsorption temperature and pressure, the
chemical species being adsorbed, and the carbon characteristics, such as
carbon particle size and pore structure. For a given adsorbent-VOC
combination at a given temperature, an adsorption isotherm can be constructed
-------
4-7
which relates the mass of adsorbate per unit weight of adsorbent
("equilibrium adsorptivity") to the partial pressure of the VOC in the
gas stream. The adsorptivity increases with increasing VOC partial
pressure and decreases with increasing temperature.
A family of adsorption isotherms having the shape typical of adsorption
on activated carbon is plotted in Figure 4-2. This and other isotherms whose
shapes are convex upward throughout, are designated "Type I" isotherms. The
Freundlich isotherm can be fit to a Type I curve; and it is commonly used
in industrial design.(2)
we = kPm (4-1)
where: we = equilibrium adsorptivity
(Ib adsorbate/lb adsorbent)
P = partial pressure of VOC in gas
stream (psia)
k,m = empirical parameters
The treatment of adsorption from gas mixtures is complex and beyond
the scope of this Section. Except where the VOC in these mixtures have
nearly identical adsorption isotherms, one VOC in a mixture will tend
displace another on the carbon surface. Generally, VOCs with lower vapor
pressures will displace those with higher vapor pressures, resulting in
the former displacing the latter previously adsorbed. Thus, during the
course of the adsorption cycle the carbon's capacity for a higher vapor
pressure constituent decreases. This phenomonen should be considered
when sizing the adsorber. To be conservative, one would normally base
the adsorption cycle requirements on the least adsorbable component in a
mixture and the desorption cycle on the most adsorbable component.^)
The equilibrium adsorptivity is the maximum amount of adsorbate the
carbon can hold at a given temperature and VOC partial pressure. In
-------
I
o
W)
41
.a
o
o
CF)
(Note:
Adsorbate Partial Pressure (psia)
Figure 4-2. FreundHch (Type I) Adsorption Isotherms
For Hypothetical Adsorbate
-------
4-9
actual control systems, however, the carbon bed is never allowed to reach
equilibrium. Instead, once the outlet concentration reaches a preset
limit (the "breakthrough concentration"), the adsorber is shut down for
desorption or (in the case of cannister units) replacement and disposal.
At the point where the vessel is shut down, the bed VOC concentration may
only be 50% or less of the equilibrium concentration.
As equation 4-1 indicates, the Freundlich isotherm is a power function
that plots as a straight line on log-log paper. Conveniently, for the
concentrations/partial pressures normally encountered in carbon adsorber
operation, most VOC - activated carbon adsorption conforms to equation
4-1. At very low concentrations, typical of breakthrough concentrations,
a linear approximation to the Freundlich isotherm is adequate. However,
the Freundlich isotherm does not accurately represent the isotherm at
high gas concentrations and thus should be used with care as such concen-
trations are approached.
Adsorptivity data for several VOCs were obtained from an activated
carbon vendor and fitted to the Freundlich equation.(5) These VOCs are
listed in Table 4-1. The adsorbates listed include aromatics (e.g.,
benzene, toluene), chlorinated aliphatics (dichloroethane), and one
ketone (acetone). However, the list is far from all-inclusive. Additional
isotherm data are available from the activated carbon vendors, handbooks
(such as Perry's Chemical Engineer's Handbook), and the literature.
Notice that a range of. partial pressures is listed with each set of
parameters, "k" and "m". (Note: In one case (m-xylene) the isotherm was
so curvilinear that it had to be split into two parts, each with a different
set of parameters.) This is the range to which the parameters apply.
-------
4-10
Table 4-1 Parameters for Selected Adsorption Isotherms3'15
Adsorbate
(1) Benzene
(2) Chlorobenzene
(3) Cyclohexane
(4) Dichloroethane
(5) Phenol
(6) TMchloroethane
(7) Vinyl Chloride
(8) m-Xylene
(9) Acrylonitrile
(10) Acetone
(11) Toluene
Adsorption
Temp. (°F)
77
77
100
77
104
77
100
77
77
100
100
77
Isotherm
Parameters
k m
0.597
1.05
0.508
0.976
0.855
1.06
2.00
0.708
0.527
0.935
0.412
0.551
0.176
0.188
0.210
0.281
0.153
0.161
0.477
0.113
0.0703
0.424
0.389
0.110
Range of
isotherm0
(psia)
0.0001-0.05
0.0001-0.01
0.0001-0.05
0.0001-0.04
0.0001-0.03
0.0001-0.04
0.0001-0.05
0.0001-0.001
0.001 -0.05
0.0001-0.015
0.0001-0.05
0.0001-0.05
a Reference 5.
b Each isotherm is of the form: w = kPm. (Set text for definition of terms)
Data are for adsorption on Calgon type "BPL" carbon (4 x 10 mesh).
c Equations should not be extrapolated outside these ranges.
-------
4-11
Extrapolation beyond this range—especially at the high end—can introduce
inaccuracy to the calculated adsorptivity.
But high-end extrapolation may not be necessary, as the following
will show. In most air pollution control applications, the system pressure
is approximately one atmosphere (14.696 psia). The upper end of the
partial pressure ranges in Table 4-1 goes from 0.04 to 0.05 psia.
According to Dal ton's Law, at a total system pressure of one atmosphere
this corresponds to an adsorbate concentration in the waste gas of 2,720
to 3,400 ppmv. However, as discussed in Section 4.1.2, the adsorbate
concentration is usually kept at 25% of the lower explosive limit (LEL).
For many VOCs, the LEL ranges from 1 to 1.5 volume %, so that 25% of the
LEL would be 0.25 to 0.375% or 2.500 to 3.750 ppmv, which approximates
the high end of the partial pressure ranges in Table 4-1.
Finally, each set of parameters applies to a fixed adsorption
temperature, ranging from 77° to 104° F. These temperatures reflect
typical operating conditions, although adsorption can take place as low
as 32°F and even higher than 104°F. As the adsorption temperature increases
to much higher levels, however, the equilibrium adsorptivity decreases to
such an extent that VOC recovery by carbon adsorption may become economically
impractical.
4.2 Design Procedure
4.2.1 Si zing Parameters
Data received from adsorber vendors indicate that the size and purchase
cost of a fixed-bed or cannister carbon adsorber system primarily depend on
four parameters:
-------
4-12
(1) The volumetric flowrate of the VOC laden gas passing through
the carbon bed(s);
(2) The inlet and outlet VOC mass loadings of the gas stream;
(3) The adsorption time (i.e., the time a carbon bed remains on-line
to adsorb VOC before being taken off-line for desorption of
the bed);
(4) The working capacity of the activated carbon.
In addition, the cost could also be affected by other stream conditions,
such as the presence/absence of excessive amounts of particulate, moisture,
or other substances which would require the use of extensive pretreatment
and/or corrosive-resistant construction materials.
The purchased cost depends to a large extent on the volumetric
flowrate (usually measured in actual ft^/min). The flowrate, in turn,
determines the size of the vessels housing the carbon, the capacities of the
fan and motor needed to convey the waste gas through the system, and the
diameter of the internal ducting.
Also important are the VOC inlet and outlet gas stream loadings,
the adsorption time, and the working capacity of the carbon. These
variables determine the amount and cost of carbon charged to the system
initially and, in turn, the cost of replacing that carbon after it is
exhausted (typically, five years after startup). Moreover, the amount of
the carbon charge affects the size and cost of the auxiliary equipment
(condenser, decanter, bed drying/cooling fan), because the sizes
of these items are tied to the amount of VOC removed by the bed. The
amount of carbon also has a bearing on the size and cost of the vessels.
To illustrate this effect, for each of a range of flowrates, the VOC
inlet concentration was increased ten-fold from 500 to 5000 ppm, while
-------
4-13
the respective flowrates and adsorption times were held constant. The
resulting purchased costs obtained from a vendor increased by an averaye
of 27%.(6) Part of these increases was needed to pay for the additional
carbon required. However, some was also needed for enlarging the adsorber
vessels to accomodate the added carbon and for the additional structural
steel needed to support the larger vessels. Also, larger condensers,
decanters, cooling water pumps, etc., were necessary to treat the more
concentrated VOC streams. (See Section 4.3.)
The VOC inlet loading is set by the source parameters, while the
outlet loading is set by the VOC emission limit. (For example, in many
states, the average VOC outlet concentration from adsorbers may not
exceed 25 ppm.)
4.2.2 Determining Adsorption and Desorption Times
The relative times for adsorption and desorption and the adsorber
bed configuration (i.e., whether single or multiple and series or parallel
adsorption beds are used) establish the adsorption/desorption cycle
profile. The cycle profile is important in determining carbon and vessel
requirements and in establishing desorption auxiliary equipment and
utility requirements. An example will illustrate. In the simplest case,
an adsorber would be controlling a process which emits a relatively small
amount of VOC intermittently—say, during one 8-hour shift per day.
During the remaining 16 hours the system would either be desorbing or on
stand-by. Such a system would only require a single bed, which would
contain enough carbon to treat eight hours worth of gas flow at the
specified inlet concentration, temperature, and pressure. Multiple beds,
operating in parallel, would be needed to treat large gas flows (>100,OOU
std. ft^/min, generally)^), as there are practical limits to the sizes
-------
4-14
to which adsorber vessels can be built. But, regardless of whether a
single bed or multiple beds were used, the system would only be on-line
for part of the day.
However, if the process were operating continuously, extra carbon
capacity would have to be built into the system. The amount of this
extra capacity would depend on the number of carbon beds that would be
adsorbing at any one time, the length of the adsorption period relative
to the desorption period, and whether the beds were operating in parallel
or in series. If one bed were adsorbing, a second would be needed to
come on-line when the first was shut down for desorption. In this case,
100% extra capacity would be needed. Similarly, if five beds in parallel
were adsorbing at any given time, again only one extra bed would be
needed and the extra capacity would be 20% (i.e., l/S>)--provided, of
course, that the adsorption time were at least five times as long as the
desorption time. The relationship between adsorption time, regeneration
time, and the required extra capacity can be generalized.
Mc = Mcl x.f (4-2)
where: Mc, Mcj = amounts of carbon required for continuous or intermittent
control of a given source, respectively (Ibs)
f = extra capacity factor (dimensionless)
The factor, f, is related to the number of beds adsorbing (NA) and
desorbing (NQ) in a continuous system as follows:
ND
f= i + /NA (4-3)
(Note: NA is also the number of beds in an intermittent system that would be
adsorbing at any given time. The total number of beds in the system would
be NA + N0.)
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4-15
It can be shown that the number of desorbing beds required in a continuous
system (NQ) is related to the desorption time (Op), adsorption time (0^), and
the number of adsorbing beds, as follows:
NO
Q D ie A( /NA) (4-4)
(Note: e o is the total time needed for bed regeneration, drying, and cooling.)
For instance, for an eight-hour adsorption time, in a continuously operated
system of seven beds (six adsorbing, one desorbing) 0 Q would have to be 1 1/3
hrs or less (8 hrs/6 beds). Otherwise, additional beds would have to be added to
provide sufficient extra capacity during desorption.
4.2.3 Estimating Carbon Requirement
4.2.3.1 Carbon Estimation Procedures Developed
Obtaining the carbon requirement (Mc or Mc ) is not as straightforward as
I
determining the other adsorber design parameters. When estimating the carbon
charge, the depth of the approach used depends on the data and calculational
tools available. In preparing this Section of the Manual, we have developed
two procedures for estimating the carbon requirement. The first procedure,
described in more detail below, is based to a large extent on rules-of-thumb.
This procedure, sometimes employed by adsorber vendors, is relatively
simple and easy to use, though it normally yields results incorporating a
large safety margin.
The second procedure yields a more accurate estimate of the carbon
requirement but requires additional input data. In addition, the procedure
is of such mathematical complexity that a microcomputer is needed to use
it. This procedure is centered on the "BED_SIZE" model, developed by
William L Klotz of Research Triangle Institute. The program "...uses a
detailed mathematical description of the adsorption process to predict the
bed size needed to* maintain the effluent [outlet] concentration below a
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4-16
defined allowable maximum when the inlet concentration is constant over
the time period for operation."^) Inputs needed to run the program
include the inlet and desired outlet ("breakthrough") VOC concentrations,
gas stream temperature, and the "breakthrough time," which is usually
equal to the adsorption time. Other inputs needed are the carbon particle
size and bed void fraction and the gas superficial velocity through the
bed. (Superficial velocity is discussed in Section 4.3.1.) As outputs,
the program provides the carbon equilibrium capacity, working capacity
(see below), the required bed size (as measured by the bed depth) and
pressure drop, and the bed efficiency--"the percent removal of all ad-
sorbate entering the bed up to the breakthrough time." This efficiency
is calculated by integrating the ratio of VOC removed to the VOC inlet
rate over the entire adsorption cycle. (See Section 4.4.3. for more on
this.)
These BED_SIZE outputs, along with the bed superficial velocity and
other parameters, are input to a second model ("CARADS"), developed by
the Economic Analysis Branch. This model is used to determine the carbon
requirement, size the adsorber vessels, and estimate the system capital
and annual costs in a manner similar to the rule-of-thumb procedure
detailed in the next sections. However, where the rule-of-thumb procedure
may be done by hand, a microcomputer is needed to utilize the CARADS
model.
4.2.3.2. Carbon Estimation Procedure Used in Manual
The rule-of-thumb carbon esimation procedure is based on the "working
capacity" (wc, Ib VOC/lb carbon). This is the difference per unit mass
of carbon between the amount of VOC on the carbon at the end of the
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4-17
adsorption cycle and the amount remaining on the carbon at the end of the
desorption cycle. It should not be confused with the "equilibrium capacity" (we)
defined above in Section 4.1.3. Recall that the equilibrium capacity
measures the capacity of virgin activated carbon when the VOC has been in
contact with it (at a constant temperature and partial pressure) long
enough to reach equilibrium. In adsorber design, it would not be feasible
to allow the bed to reach equilibrium. If it were, the outlet concentration
would rapidly increase beyond the allowable outlet (or "breakthrough")
concentration until the outlet concentration reached the inlet concentration.
During this period the adsorber would be violating the emission limit.
The working capacity is some fraction of the equilibrium capacity.
Table 4-2 compiles working capacities for selected VOCs. These data were
obtained from an adsorber manufacturer.^) For comparison, Table 4-2 also
shows the equilibrium capacities for some of these VOCs, which were computed
from the parameters given above in Table 4-1 in Section 4.1. Note that
the working capacities range from 24 to 100% of the respective capacities
at equilibrium, with an average value of 48%. This average approximates
the rule-of-thumb used by adsorber vendors--that is, working capacity
equals 50% of equilibrium capacity. Further, like the equilibrium adsorp-
tivity, the working capacity depends upon the temperature, the VOC partial
pressure, and the VOC composition. The working capacity also depends on
the flow rate and the carbon bed parameters.
The working capacity, along with the adsorption time and VOC inlet
loading, is used to compute the carbon requirement for a cannister adsorber
or for an intermittently operated fixed-bed adsorber as follows:
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4-1-8
Table 4-2. Equilibrium vs. Working Carbon Capacity for
Selected VOCs(8)
Adsorption
Compound Temp. (°F)a
(1) Acetone
(2) Benzene
(3) Cyclohexane
(4) Toluene
(5) m-Xylened
100
77
100
77
77
Partial Carbon Capacities0
Press. (psia)b Equilibrium(we) Working(wc)
0.0147
0.000147
0.00441
0.00294
0.00147
0.0798
0.126
0.163
0.290
0.333
0.08
0.06
0.06
0.07
0.10
wc/we Ratio
1.0
0.48
0.37
0.24
0.30
Average U.48
a Temperature at which equilibrium capacity was measured.
b Directly proportional to the VOC concentration in the influent (ppmv).
Calculated as follows:
Partial pressure = Cone, (ppmv) x 14.696 psia x 10"6
c Measured as Ib of VOC/lb. of carbon. Equilibrium capacities were calculated
using parameters in Table 4-1; working capacities were obtained from Reference
8.
d Actually, the compound listed in Reference 8 was "xylene." The precise
structure -- i.e., ortho, meta, or para -- was not given. However, as the
molecular weight of all three is the same, we have assumed that the working
capacity data would apply to m-xylene, as well as to the other two isomers.
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4-19
Mci = mvoc efl (4-5)
wc
where: "WOC = VOC inlet loading (Ib/hr)
Combining this with equations (4-2) and (4-3) yields the general equation for
estimating the system total carbon charge:
Mc = mVOC 6fl (1 + ND/NA) (4-6)
wc
Values for wc may be obtained from Table 4-2 or similar data sources. If
no value for wc is available for the VOC (or VOC mixture) in question,
the working capacity may be estimated at 50% of the equilibrium capacity, as
follows:
wc = 0.5 we(max) (4-7)
where: we(max) = the equilibrium capacity (Ib VOC/lb carbon) taken at
the adsorber inlet (i.e., the point of maximum VOC
concentration).
(Note: To be conservative, this 50% figure should be lowered if short
desorption cycles, very high vapor pressure consituents, or difficult-
to-desorb VOCs are involved.)
* * *
Example: A source emitting 100 Ib/hr toluene is to be collected in a
carbon adsorber. The system operates continuously, with two beds adsorbing
at all times. For convenience, adsorption and desorption times of 12 and
1.5 hours, respectively, have been chosen. The total gas flowrate is
35,000 ft3/min at 77°F and 1 atmosphere (or approximately 200 ppmv of VOC).
Assume negligible quantities of particulate and moisture. Calculate the
system carbon requirement. Solution: From Table 4-2, we find the working
capacity of toluene to be 0.07 Ib/lb carbon. Since the regeneration time
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4-20
(1.5 hrs) is less than 1/2 the adsorption time (see eq. 4-4), one extra bed
will be adequate. Substituting these values into equation 4-6 yields the
carbon requirement:
Mc = (100 1b VOC/hr)(12 hr) (1 + 1/2)
0.07 Ib VOC/lb carbon
= approximately 25,700 Ibs.
As equation 4-6 shows, the carbon requirement is directly proportional
to the adsorption time. This would tend to indicate that a system could
be designed with a shorter adsorption time to minimize the carbon requirement
(and purchased cost). There is a trade-off here not readily apparent from
equation 4-6, however. Certainly, a shorter adsorption time would require
less carbon. But, it would also mean that a carbon bed would have to be
desorbed more frequently. This would mean that the regeneration steam
would have to be supplied to the bed(s) more frequently to remove (in the
long run) the same amount of VOC. Further, each time the bed is regenerated
the steam supplied must heat the vessel and carbon, as well as drive off
the adsorbed VOC. And the bed must be dried and cooled after each desorption,
regardless of the amount of VOC removed. Thus, if the bed is regenerated
too frequently, the bed drying/cooling fan must operate more often, increasing
its power consumption. Also, more frequent regeneration tends to shorten
the carbon life. As a rule-of-thumb, the optimum regeneration frequency for
fixed-bed adsorbers treating streams with moderate to high VOC inlet
loadings is once every 8 to 12 hours.(1)
4.3 Estimating Total Capital Investment
An entirely different procedure should be used in estimating the
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4-21
purchased costs of fixed-bed and canm'ster-type adsorbers. Therefore,
they will be discussed separately.
4.3.1 Fixed-Bed Systems
As indicated in the previous section, the purchased cost is a function
of the volumetric flowrate, VOC inlet and outlet loadings, the adsorption
time, and the working capacity of the activated carbon. As Figure 4-1
shows, the adsorber system is made up of several different items. Of
these, the adsorber vessels and the carbon comprise nearly 3/4 of the
purchased cost. There is also auxiliary equipment, such as fans, pumps,
condensers, decanters, and internal piping. But because these usually
comprise a small part of the total purchased cost, they may be "factored"
from the costs of the two major items without introducing significant
error. The costs of these major items will be considered separately.
4.3.1.1 Carbon Cost
This cost (Cc,$) is simply the product of the initial carbon require-
ment (Mc) and the current price of carbon. As adsorber vendors buy carbon
in very large quantities (million-pound lots or larger), their cost is
somewhat lower than the list price. Current vendor costs typically
range from $1.60 to $2.00/lb (spring 1986 dollars). Taking the midpoint
of this range, we have:
Cc = 1.80 Mc (4-8)
4.3.1.2 Vessel Cost
The cost of an adsorber vessel is primarily determined by its dimen-
sions which, in turn, depend upon the amount of carbon it must hold
and the superficial gas velocity through the bed that must be maintained
for optimum adsorption. The desired superficial velocity is used to calculate
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4-22
the cross-sectional area of the bed perpendicular to the gas flow. An
acceptable superficial velocity is established empirically, considering
desired removal efficiency, the carbon particle size and bed porosity,
and other factors. For example, one adsorber vendor recommends a superficial
bed velocity of 85 ft/min(6), while an activated carbon manufacturer cautions
against exceeding 60 ft/min in systems operating at one atmosphere.(5)
Another vendor uses a 65 ft/min. superficial face velocity in sizing its
adsorber vessels.(9) Lastly, there are practical limits to vessel dimensions
which also influence their sizing. That is, due to shipping restrictions,
vessel diameters rarely exceed 12 feet, while their length is generally
limited to 50 feet.(9)
The cost of a vessel is usually correlated with its weight. However,
as the weight is often difficult to obtain or calculate, the cost may be
determined from the external surface area. This is true because the
vessel material cost — and the cost of fabricating that material —
is directly proportional to its surface area. The surface area (S, ft^)
of a vessel is a function of its length (L) and diameter (D), which in turn,
depend upon the superficial bed face velocity, the L/0 ratio, and other
factors.
Most commonly, adsorber vessels are cylindrical in shape and erected
horizontally (as in Figure 4-1). In these cases, it can be shown that:
D = 0.127M V (4-9)
Q
and: L = 7.87(q'/vb)2 (4-10)
'c
where: v& = bed superficial velocity (ft/min)
i
MC = carbon requirement per vessel (Ibs)
-------
4-23
Q' = volumetric flowrate per^ adsorbing vessel (ft^/min)
Equations (4-9) and (4-10) also assume that the carbon occupies 1/3 of
the vessel volume(6»9) and that the carbon's bulk density is 30 lb/ft3.
Finally, for a cylinder:
S = » D(L + D/2) (4-11)
Similar equations can be developed for other vessel shapes, configurations,
etc.
Based on vendor data, we developed a correlation between adsorber
vessel cost and surface area:(9)
Cv = exp [18.827 - 3.3945 InS + 0.3090 (InS)2] (4-12)
where: Cv = vessel cost (spring 1986 $), F.O.B. vendor
and: 228
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4-24
points derived from costs supplied by an equipment vendor (9), we found
that, depending on the total gas flowrate (Q), the cost of the carbon and
vessels together comprised from 50% to 85% of the total adsorber equipment cost.
These data points spanned a gas flowrate range of 4,000 to 500,000 scfm. The
average of these points was 72%, with a standard deviation of 16%. Taking the
reciprocal of this average (i.e., 1/0.72 = 1.39), we can write:
CA = 1.39[CC + CVJ (4-13)
where: Cc = cost of carbon
Cv = cost of vessel(s)
C/\ = cost of adsorber equipment
4.3.1.4. Total Capital Investment
As discussed in Section 2, in the methodology used in this Manual, the
total capital investment (TCI) is estimated from the total purchased cost via
an overall direct/indirect installation cost factor. A breakdown of that
factor for carbon adsorbers is shown in Table 4-3. As Section 2 indicates,
the TCI also includes costs for land, working capital, and off-site facilities,
which are not included in the direct/indirect installation factor. However,
as these items are rarely required with adsorber systems, they will not be
considered here. Further, no factors have been provided for site preparation
(S.P.) and buildings (lildg.), as these site-specific costs depend very
little on the purchased equipment cost.
Note that the installation factor is applied to the total purchased
equipment cost, which includes the cost of the stack and external ductwork
and such costs as freight and sales taxes (if applicable). ("External ductwork"
is that ducting needed to convey the exhaust gas from the source to the
adsorber system, and then from the adsorber to the stack. Costs for ductwork
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4-25
Table 4-3. Direct and Indirect Installation Factors for Carbon Adsorbers3
Cost Item Cost Factor (Fraction of Indicated Cost)
DIRECT COSTS
1) Purchased equipment cost
Adsorber As required)
Auxiliary equipment" As requi redj "
Instruments and controls0 0.1 A
Taxes . 0.03 A
Freight 0.05 A
Total Purchased Equipment cost B(=1.18A)
2) Installation direct costs
Foundations and supports 0.08 B
Erection and handling 0.14 B
Electrical 0.04 B
Piping 0.02 B
Insulation 0.01 B
Painting 0.01 B
Site preparation (S.P.), Buildings (Bldg.) As requi red
Total Installation Direct Costs 0.30 B + S.P. + Bldg.
Total Direct Cost 1.30B + S.P. + Bldg.
INDIRECT COSTS
Engineering and supervision 0.10 B
Construction and field expenses 0.05 B
Construction fee 0.10 B
Start-up 0.02 B
Performance test 0.01 B
Contingency 0.03 B
Total Indirect Cost 0.31B
Total Direct and Indirect Costs = Total Capital p..616 + S.P. + BIcTgTl
Investment
a Reference 11.
b Includes external ductwork, stack, and any other equipment normally
not included with unit furnished by adsorber vendor.
c Instrumentation cost usually included in cost of vendor-supplied adsorber
unit.
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4-26
and stacks are shown elsewhere in this Manual.) Normally, the adjustment
would also cover the instrumentation cost, but this cost is usually included
with the adsorber purchased cost. Finally, note that these factors reflect
"average" installation conditions and could vary considerably, depending upon
the installation circumstances.
4.3.2 Cannister Systems
Once the carbon requirement is estimated using the above procedure,
the number of cannisters is determined. This is done simply by dividing
the total carbon requirement (Mc) by the amount of carbon contained by each
cannister (typically, 150 Ibs.). This quotient, rounded to the next
highest digit, yields the required number of cannisters to control the
vent in question.
Costs for a typical cannister (Calgon's Ventsorb®) are listed in
Table 4-4. These costs include the vessel, carbon, and connections,
but do not include freight or installation charges. Note that the cost
per unit decreases as the quantity purchased increases. Each cannister
contains Calgon's "BPL" carbon (4 x 10 mesh), which is commonly used in
industrial adsorption. However, to treat certain VOCs, more expensive
speciality carbons (e.g., "FCA 4 x 10") are needed. These carbons can
increase the equipment cost by 60% or more.(4)
As fewer installation materials and labor are required to install a
cannister unit than a fixed-bed system, the composite installation factor
is consequently lower. The only costs required are those needed to place
the cannisters at, and connect them to, the source. This involves a
small amount of piping only; little or no electrical work, painting,
foundations, or the like would be needed. Twenty percent of the sum of
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4-27
Table 4-4 Equipment Costs (Spring 1986 $) for a Typical Cannister Adsorber9
Quantity Equipment Cost (each)b
1-3 $687
4-9 659
10-29 622
_>30 579
a Reference 4.
b These costs are F.O.B., Pittsburgh, PA. They do not include taxes and freight
charges.
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4-28
the cannister(s) cost, freight charges, and applicable sales taxes would
cover this installation cost.
4.4. Estimating Total Annual Cost
As Section 2 of this Manual explains, the annual operating cost is
comprised of three components: direct costs, indirect costs, and
recovery credits. These will be considered separately.
4.4.1 Direct Annual Costs
These include the following expenditures: steam, cooling water,
electricity, carbon replacement, operating and supervisory labor, and
maintenance labor and materials. Of these, only electricity and solid
waste disposal would apply to the cannister-type adsorbers.
4.4.1.1 Steam:
As explained in Section 4.1, steam is used during the desorption
cycle. The quantity of steam required will depend on the amount of
carbon in the vessel, the vessel dimensions, the type and amount of VOC
adsorbed, and other variables. Experience has shown that the steam
requirement ranges from approximately ^ tp_ 4_ Ibs of steam/lb of adsorbed
VOC.(6»9) Using the midpoint of this range, we can develop the following
expression for the annual steam cost:
Cs = 3.50X10"3 mvoc 9s ps (4-14)
where: Cs = steam cost ($/yr)
9S = system operating hours (hr/yr)
mvoc = voc inlet loading (Ibs/hr)
ps = steam price ($/thous. Ibs)
If steam price data are unavailable, one can estimate its cost at
120% of the fuel cost. For example, if the local price of natural gas
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4-29
were $5.00/million BTU, the estimated steam price would be $6.00/million
BTU » $6.00/thousand Ibs. (The 20% factor covers the capital and annual
costs of producing the steam.)
4.4.1.2 Cooling Hater;
Cooling water is consumed by the condenser in which the steam-VOC
mixture leaving the desorbed carbon bed is totally condensed. Most of
the condenser duty is comprised of the latent heat of vaporization (AHV)
of the steam and VOC. As the VOC AHV are usually small compared to the
steam AHV (about 1000 BTU/lb), the VOC AHV may be ignored. So may the sensible
heat of cooling the water-VOC condensate from the condenser inlet temperature
(about 212°F) to the outlet temperature. Therefore, the cooling water
requirement is essentially a function of the steam usage and the allowable
temperature rise in the coolant, which is typically 30° to 40°F(6). Using
the average temperature rise (35°F), we can write:
CCW = 3.43CS Pcw (4-15)
where: Ccw = cooling water cost ($/yr)
pcw = cooling water price ($/thous. gal.)
If the cooling water price is unavailable, use $0.15 to $0.30/thousand
gallons.
4,4.1.3 Electricity:
In fixed-bed adsorbers, electricity is consumed by the system fan, bed
drying/cooling fan, cooling water pump, and solvent pump(s). Both the
system and bed fans must be sized to overcome the pressure drop through
the carbon beds. But, while the system fan must continuously convey the
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4-30
total gas flow through the system, the bed cooling fan is only used during
a part of the desorption cycle (one-half hour or less).
For both fans, the horsepower needed depends both on the gas flowrate
and the pressure drop through the carbon bed. The pressure drop through
the bed (A PD) depends on several variables, such as the adsorption temperature,
bed velocity, bed characteristics (e.g., void fraction), and thickness.
But, for a given temperature and carbon, the pressure drop per unit thickness
depends solely on the gas velocity. For instance, for Calgon's "PCB"
carbon (4 x It) mesh), the following relationship holds:(5)
APb/tb =0.03679 vb + 1.107 x UT4 vb2 (4-16)
where: APD/tb = pressure drop through bed (inches of water/foot of
carbon)
Vb = superficial bed velocity (ft/min)
The bed thickness (tfc.ft) is the quotient of the bed volume (V^) and the
bed cross-sectional area (Ajj). For a 30 lb/ft3 carbon bed density, this
becomes:
tb = V^ = 0.0333Mc' (4-17)
(For instance, for horizontally-erected cylindrical vessels, Ab=LD.)
Once APb is known, the system fan horsepower requirement (hpsf) can be
calculated:
hpsf = 2.50 x 10"4 Q APS (4-18)
where: Q = gas volumetric flowrate through system (ft-^/min)
APS = total system pressure drop = APb + 1
(The extra inch accounts for miscellaneous pressure losses through
the external ductwork and other parts of the system.(6))
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4-31
This equation incorporates a fan efficiency of 70% and a motor
efficiency of 90%, or 63% overall.
The horsepower requirement for the bed drying/cooling fan (hpcf)
is computed similarly. While the bed fan pressure drop would still
be APt,, the gas flowrate and operating times would be different. (For
example, to cool a bed from the regeneration temperature to the adsorption
temperature, would require from 3 to 3.5 SCFM/lb of carbon, based on a
30-minute cooling/drying time.)
The cooling water pump horsepower requirement (hpcwp) would be computed
as follows:
hpcwp = 2.52 x IP"4 qcwHs (4-19)
n
where: qcw = cooling water flowrate (gal/min)
H = required head (normally 100 feet of water)
s = specific gravity of fluid relative to water at 60°F.
n = combined pump-motor efficiency
Equation 4-18 may also be used to compute the solvent pump horsepower
requirement. In the latter case, the flowrate (qs) would be different, of
course, although the same head—100 ft. of water--could be used. The specific
gravity would depend on the composition and temperature of the condensed
solvent. For example, the specific gravity of toluene at 100°F would be
approximately 0.86 at 70° F. (However, the solvent pump horsepower is usually
very small—usually <0.1 hp.--so its electricity consumption can usually
be neglected.)
Once the various horsepowers are calculated, the electricity usage
(in kWh) is calculated, by multiplying each horsepower value by 0.746 (the
factor for converting hp to kilowatts) and the number of hours each fan or
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4-32
pump operates annually. For the system fan and cooling water pump, the hours
would be the annual operating hours for the system (es).
But for the bed drying/cooling fan, the time (%) would be much
shorter. This is dependent on the time the bed fan is run during a
desorption cycle, the number of desorption cycles per year, the adsorption
time (OA)» and other factors. For instance, for a three-bed system, with
a 12-hour adsorption time and a 0.5-hour bed cooling/drying time per
desorption cycle, the bed fan would operate 8.33 hrs. for every 10U hrs
of on-stream (system) time.
To obtain the annual electricity cost, simply multiply kWh by the
electricity price (in $/kWh) that applies to the facility being controlled
For cannister units, use equation 4-17 to calculate the fan horsepower
requirement. However, instead of AP^, use the following to compute the
total cannister pressure drop (APc, inches of water):(4) .
APC = 0.0471QC + 9.29 x 10"4QC2 . (4-20)
•J
where: Qc = flowrate through the cannister (ft /min)
4.4.1.4. Carbon Replacement:
As discussed above, the carbon has a different economic life than
the rest of the adsorber system. Therefore, its replacement cost must be
calculated separately. Employing the procedure detailed in Section 2,
we have:
CRCC = CRFC (1.08CC + Ccl) (4-21)
where: CRFC = capital recovery factor for the carbon
1.08 = taxes and freight factor
cc» ccl = initial cost of carbon and carbon replacement
labor cost, respectively ($/yr) (F.O.B. vendor)
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4-33
The replacement labor cost covers the labor cost for removing spent
carbon from vessels and replacing it with virgin or regenerated carbon.
The cost would vary with amount of carbon being replaced, the labor rates, and
other factors. For example, to remove and replace a 50,000 - pound charge
would require about 16 person-days, which, at typical wage rates, is equivalent
to approximately $0.05/lb repl aced.(12)
A typical life for the carbon is five years. However, if the inlet
contains VOCs that are very difficult to desorb, tend to polymerize, or react
with other constituents, a shorter carbon 1 ifetime--perhaps as low as two
years—would be likely.'•*•' For a five-year life and 10% interest rate, CRFC
= 0.2638.
4.4.1.5 Solid Waste disposal:
Disposal costs are rarely incurred with fixed-bed adsorbers, because
the carbon is almost always regenerated in place, not discarded. In certain
cases, the carbon in cannister units is also regenerated, either off-site
or at a central regeneration facility on-site. However, most cannister
adsorbers are disposed of once they become saturated. The entire
cannister--carbon, drum, connections, etc.--is shipped to a secure landfill.
The cost of landfill disposal could vary considerably, depending on the
number of cannisters disposed of, the location of the landfill, etc.
Based on data obtained from two large landfills, for instance, the
disposal cost would range from approximately $35 to $65 per cannister
excluding transportation costs.(13,14)
4.4.1.6 Operating and Supervisory Labor:
The operating labor for adsorbers is relatively low, as most systems
are automated and require little attention. One-half operator hour per
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4-34
shift is typical. (11) Add to this 15% to cover supervisory labor, as
Section 2 suggests. The annual labor cost would then be the product of
these labor requirements and their respective wage rates ($/hr) which,
naturally, would vary according to the facility location, type of industry,
etc.
4.4.1.7 Maintenance Labor and Materials:
Use 0.5 hours/shift for maintenance labor and the applicable
maintenance wage rate. If the latter data are unavailable, estimate the
maintenance wage rate at 110% of the operating labor rate, as Section 2
suggests. Finally, for maintenance materials, add an amount equal to the
maintenance labor, also per Section 2.
4.4.2 Indirect Annual Costs
These include such costs as capital recovery, property taxes, insurance,
overhead, and administrative costs ("G&A"). The capital recovery cost is
based on the equipment lifetime and the annual interest rate employed. (See
Section 2 for a thorough discussion of the capital recovery cost and the
variables that determine it.) For adsorbers, the system lifetime is typically
ten years, except for the carbon, which, as stated above, typically needs to
be replaced after five years. Therefore, when figuring the system capital
recovery cost, one should base it on the installed capital cost less the cost
of replacing the carbon (i.e., the carbon cost plus the cost of labor necessary
to replact it). Substituting the initial carbon and replacement labor
costs from equation 4-21, we obtain:
CRCS = [TCI - (1.08CC + Ccl)] CRFs (4-22)
where: CRCS = capital recovery cost for adsorber system ($/yr)
TCI = total capital investment ($)
-------
4-35
1.08 = taxes and freight factor
Cci = initial carbon cost (F.O.B. vendor) and carbon replacement
cost, respectively ($)
CRFS = capital recovery factor for adsorber system (defined in
Section 2)„
For a ten-year life and a 10% annual interest rate, the CRFS would be
0.1628.
As Section 2 suggests, the suggested factor to use for property taxes,
insurance, and administrative charges is $% of the TCI. Finally, the overhead
is calculated as 60% of the sum of operating, supervisory, and maintence
labor, and maintenance materials.
The above procedure applies to cannister units as well, except that,
in most cases, the carbon is not repl aced--the entire unit is. Cannisters
are generally used in specialized applications. The piping and ducting
cost can usually be considered a capital investment with a useful life of
ten years. However, whether the cannister itself would be treated as a
capital or an operating expense would depend on the particular application
and needs to be evaluated on a case-by-case basis.
4.4.3 Recovery Credits
These could apply to the VOC which is adsorbed, then desorbed, condensed,
and separated from the steam condensate. However, if the VOC layer contained
impurities or were a mixture of compounds, it would require further treatment,
such as distillation. Purification and separation costs are beyond the scope
of this Section. Suffice it to say that the costs of these operations could
offset and, if large enough, obliterate any recovery credits. In any case,
the following equation can be used to calculate these credits:
-------
4-36
where: PVOC = resale value of the captured VOC ($/lb)
RC = recovery credit ($/yr)
E = adsorber VOC control efficiency
By definition, the efficiency (E) is the difference between the inlet
and outlet VOC mass loadings, divided by the inlet loading. However, during
an adsorption cycle the outlet VOC loading will increase from essentially
zero at the start of the cycle to the breakthrough concentration at the end
of the cycle. Because the efficiency is a function of time, it should be
calculated via integration over the length of the adsorption cycle. To do
this would require knowledge of the temporal variation of the outlet loading
during the adsorption cycle. If this knowledge is not available to the
Manual user, a conservative approximation of the efficiency may be made by
setting the outlet loading equal to the breakthrough concentration.
4.4.4 Total Annual Cost
Finally, as explained in Section 2, the total annual cost (TAC) is
the sum of the direct and indirect annual costs, less any recovery credits,
or:
TAC = DC + 1C - RC (4-24)
-------
4-37
References for Section 4
1. Correspondence: Robert L. Stallings and William Klotz (Research Triangle
Institute, Research Triangle Park, NC) to William M. Vatavuk (U.S.
Environmental Protection Agency, Office of Air Quality Planning and
Standards, Economic Analysis Branch, Research Triangle Park, NC),
June 24, 1986.
2. Calvert, Seymour and Englund, Harold M. (eds.). Handbook of Air Pollution
Control Technology. New York: John Wiley & Sons, 1984, pp. 135-192.
3. Handbook of Chemistry and Physics, 54th Edition. Cleveland: The Chemical
Rubber Company, 1973-74, pp. D85-D92.
4. "Calgon Ventsorb® for Industrial Ai r Purification" (Bui letin 23-56a).
Pittsburgh: Calgon Corporation, 1986.
5. Adsorption Handbook. Pittsburgh: Calgon Corporation, 1980.
6. Correspondence: Richard Selznick (Baron Blakeslee, Inc., Westfield, NJ)
to William M. Vatavuk, April 23, 1986.
7. Klotz, William L., "Documentation for Program 'Bed_Size'". Research
Triangle Institute (Research Triangle Park, NC), September 3, 1986.
8. Manzone, R.R. et al. "Profitability of Recycling Solvents from Process
Systems." Pollution Engineering, October 1973.
9. Correspondence: James Jessup (M&W Industries, Inc., Rural Hall, NC)
to William M. Vatavuk, May 16, 1986.
10. Matley, Jay (ed.). Modern Cost Engineering. New York: McGraw-Hill
Publications Co., 1984, p. 142.
11. Vatavuk, William M. and Neveril, Robert. "Estimating Costs of Air
Pollution Control Systems, Part II: Factors for Estimating Capital
and Operating Costs." Chemical Engineering, November 3, 1980, pp. 157-
162.
12. Telephone conversation: Robert L. Stallings (Research Triangle Institute,
Research Triangle Park, NC) with William M. Vatavuk, September 11, 1986.
13. Correspondence: William Kitto (Chemwaste, Sulphur, LA) to William M.
Vatavuk, July 25, 1986.
14. Correspondence: Jerry Lockl ear (6SX, Pinewood, SC) to William M.
Vatavuk, July 25, 1986.
-------
Section 5
FABRIC FILTERS
James H. Turner
Andrew S. Viner
Research Triangle Institute
(Research Triangle Park, N.C. 22709)
John 0. McKenna
ETS, Inc.
Roanoke, VA 24018-4394
Richard E. Jenkins
William M. Vatavuk
Economic Analysis Branch, OAQPS
5.1 Process Description
5.1.1 Introduction
A fabric filter unit consists of one or more isolated compartments
containing rows of fabric filter bags or tubes. Particle-laden gas passes up
(usually) along the surface of the bags then radially through the fabric.
Particles are retained on the upstream face of the bags while the cleaned gas
stream is vented to the atmosphere. The filter is operated cyclically
alternating between relatively long periods of filtering and short periods of
cleaning. During cleaning, dust that has accumulated on the bags is removed
from the fabric surface and deposited in a hopper for subsequent disposal.
This device will collect particle sizes ranging from submicron to several
hundred microns in diameter at efficiencies generally in excess of 99 or 99.9
percent. The dust cake collected on the fabric is primarily responsible for
such high efficiency. Gas temperatures up to 500 *F, with surges to 550 °F
can be accommodated routinely. Most of the energy used to operate the system
-------
5-2
appears as pressure drop across the bags and associated hardware and ducting.
Typical values range from 5 to 20 in. of water gauge. Fabric filters are used
where high-efficiency particle collection is required. Limitations are
imposed by gas characteristics (temperature and corrosivity) and particle
characteristics (primarily stickiness) that affect the fabric or its operation
and that cannot be accommodated economically.
Important process variables include particle characteristics, gas
characteristics, and fabric properties. The most important design parameter
is the air- or gas-to-cloth ratio (volumetric flow rate divided by fabric
3 2
area, (ft /min)/ft ), and the usual operating parameter of interest is
pressure drop across the filter system. The major distinguishing operating
feature of fabric filters is the ability to renew the filtering surface
periodically by cleaning.
5.1.2 Types of Fabric Filters
Fabric filters can be categorized by several means, including type of
cleaning (shaker, reverse-air, pulse-jet), direction of gas flow (from inside
the bag towards the outside or vice versa), location of the system fan
(suction or pressure), or size (low, medium, or high gas flow quantity).
Cleaning methods are discussed more fully in this section, and the other
categories are described in Section 5.2.
5.1.2.1 Shaker Cleaning
For any type of cleaning, enough energy must be imparted to the fabric to
overcome the adhesion forces holding dust to the bag. In shaker cleaning,
used with inside to outside gas flow, this is accomplished by suspending the
bag from a motor-driven hook or framework that oscillates. Motion may be
imparted to the bag in several ways, but the general effect is to create a
sine wave along the fabric. As the fabric moves outward, accumulated dust on
-------
5-3
the surface moves with the fabric. When the fabric reaches the limit of its
extension, the patches of dust have enough inertia to tear away from the
fabric and descend to the hopper.
For small, single-compartment baghouses, a lever attached to the shaker
mechanism may be operated manually at appropriate intervals, typically at the
end of a shift. In multicompartment baghouses, a timer or a pressure sensor
responding to system pressure drop initiates bag shaking automatically. The
compartments operate in sequence so that one compartment at a time is cleaned.
Forward gas flow to the compartment is stopped, dust is allowed to settle,
residual gas flow stops, and the shaker mechanism is switched on for several
seconds to a minute or more. The settling and shaking periods may be
repeated, then the compartment is brought back online for filtering. Many
large-scale shaker systems employ a small amount of reverse air during the
shaker cycle to assist cleaning by deflating the bags.
Parameters that affect cleaning include the amplitude and frequency of
the shaking motion and the tension of the mounted bag. The first two
parameters are part of the baghouse design and generally are not changed
easily. Typical values are about 4 Hz for frequency and 2 to 3 in. for
amplitude (half-stroke).' ' The tension is set to about 2 Ib/in. of bag
circumference when bags are installed. Some installations allow easy
adjustment of bag tension, while others require that the bag be loosened and
reclamped to its attaching thimble.
The vigorous action of shaker systems tends to stress the bags and
requires heavier and more durable fabrics. In the United States, woven
(2\
fabrics are used almost exclusivelyv ' for shaker cleaning. European practice
allows the use of felted fabrics at somewhat higher filtering velocities.
-------
5-4
5.1.2.2 Reverse-air Cleaning
When glass fiber fabrics were introduced, a gentler means of cleaning the
bags was needed to prevent premature degradation. Reverse-air cleaning was
developed as a less intensive way to impart energy to the bags. In this
method, gas flow to the bags is stopped in the compartment being cleaned, and
a reverse flow of air is directed through the bags. This reversal of gas flow
gently collapses the bags and dust is removed from the fabric surface by shear
forces developed between the dust and fabric as the latter changes its
contours. Another difference between reverse-air and shaker cleaning is the
installation of sewn-in rings to prevent complete collapse of the bag, which
may be greater than 30 ft long, during cleaning. Without these rings,
collected dust tends to choke the bag as the fabric collapses in on itself.
As with multicompartment shaker baghouses, the same cycle takes place in
reverse-air baghouses of stopping forward gas flow and allowing dust to settle
before cleaning action begins.
The source of reverse air is generally a separate fan capable of
supplying air for one or two compartments at a gas-to-cloth ratio similar to
that of the forward gas flow.
5.1.2.3 Pulse-jet Cleaning
This form of cleaning uses compressed air to force a burst of air down
through the bag and expand it violently. As with shaker baghouses, the fabric
reaches its extension limit and the dust separates from the bag. In pulse
jets, however, gas flows are opposite in direction when compared with shaker
or reverse-air baghouses. Bags are mounted on wire cages to prevent collapse
while the dusty gas flows from outside the bag to the inside. Instead of
attaching both ends of the bag to the baghouse structure, the bag and cage
-------
5-5
assembly generally is attached only at the top. The bottom end of the
assembly tends to move in the turbulent gas flow and may contact other bags,
which accelerates wear.
Although some pulse-jet baghouses are compartmented, most are not. Bags
are cleaned by rows when a timer initiates the burst of cleaning air through a
quick-opening valve. Usually 10% of the collector is pulsed at a time by
zones. A pipe across each row of bags carries the compressed air. The pipe
is pierced above each bag so that cleaning air exits directly downward into
the bag. Some systems direct the air through a short venturi that is intended
to entrain additional cleaning air. The pulse interrupts forward gas flow .
only for a few tenths of a second. However, the quick resumption of forward
flow redeposits most of the dust back on the clean bag or on adjacent bags.
An advantage of online pulse-jet cleaning is the reduction in baghouse size
allowed by not having to build an extra compartment for offline cleaning.
Pulse jets normally operate at two or more times the gas-to-cloth ratio of
reverse-air baghouses.
5.1.3 Auxiliary Equipment
The typical auxiliary equipment associated with fabric filter systems is
shown in Figure 5-1. Along with the fabric filter itself, a control system
typically includes the following auxiliary equipment: a capture device (i.e.,
hood or direct exhaust connection); ductwork; dust removal equipment (screw
conveyor, etc.); fans, motors, and starters; and a stack. In addition, spray
chambers, mechanical collectors, and dilution air ports may be needed to
precondition the gas before it reaches the fabric filter. Capture devices are
usually hoods that exhaust pollutants into the ductwork or direct exhaust
couplings attached to a process vessel. Hoods are more common, yet poorly
-------
Hood
Direct Exhaust
Dilution Air
Spray Cooler
Fabric Filter
w
Ul
I
en
Stack
Fan
Dust Removal
Mechanical Collector
Figure 5-1. Typical alternative auxiliary equipment items used with fabric filter control systems.
-------
5-7
designed hoods will allow pollutants to escape. Direct exhaust couplings are
less common, requiring sweep air to be drawn through the process vessel, and
may not be feasible in some processes. Ductwork provides a means of moving
the exhaust stream to the control device. Spray chambers and dilution air
ports are used to decrease the temperature of the pollutant stream to protect
the filter fabric from excessive temperatures. When a substantial portion of
the pollutant loading consists of relatively large particles, mechanical
collectors such as cyclones are used to reduce the load on the fabric filter
itself. The fans provide motive power for air movement and can be mounted
before (pressure baghouse) or after (suction baghouse) the filter. A stack,
when used, vents the cleaned stream to the atmosphere. Screw conveyors are
often used to remove captured dust from the bottom of the hoppers. Air con-
veying systems and direct dumping into containers are also used.
5.1.4 Fabric Filtration Theory
The key to designing a baghouse is to determine^the face velocity that
produces the optimum balance between pressure drop (operating cost) and
baghouse size (capital cost). Major factors that affect design face velocity
(or gas-to-cloth ratio), discussed in Section 5.2, include particle and fabric
characteristics and gas temperature. Although collection efficiency is
another important measure of baghouse performance, it is generally assumed
that a properly designed and well run baghouse will be highly efficient.
Therefore, the design process focuses on the pressure drop. There are several
contributions to the pressure drop across a baghouse including the pressure
drop from the flow through the inlet and outlet ducts, from flow through the
hopper regions, and from flow through the bags. The pressure drop through the
baghouse (excluding the pressure drop across the bags) depends largely on the
-------
5-8
baghouse design and ranges from 1 to 2 Inches of H2o' ' in conventional
designs and up to 3 inches of H^O in designs having complicated gas flow
paths. This loss can be kept to a minimum (i.e., 1 inch of H^O or less) by
investing in a flow modeling study of the proposed design. A study of this
sort would cost on the order of $50,000 (in 1986). The pressure drop across
the bags (also called the tubesheet pressure drop) can be as high as 10 inches
of H20 or more. The duct and hopper losses are constant and can be minimized
effectively through proper design based on a knowledge of the flow through the
baghouse. (No.te: A procedure for estimating duct pressure losses is given in
the "Ductwork" section of this Manual.) The tubesheet pressure drop is a
complex function of the physical properties of the dust and fabric and the
manner in which the baghouse is designed and operated.
Fabric filtration is inherently a batch process that has been adapted to
continuous operation through clever engineering. One requirement for a
continuously operating baghouse is that the dust collected on the bags must be
removed periodically. Shaker and reverse-air baghouses are similar in the
sense that they both normally use woven fabric bags, run at relatively low
face velocities, and the filtration mechanism is cake filtration. That is,
the fabric merely serves as a substrate for the formation of a dust cake that
is the actual filtration medium. Pulse-jet baghouses generally use felt
fabrics and run with a high face velocity (about double that of shaker or
reverse-air baghouses). Some investigators feel that the felt fabric plays a
much more active role in the filtration process. This distinction between
cake filtration and fabric filtration has important implications for calcu-
lating the rate of pressure loss across the filter bags. The theoretical
description of cake filtration is quite different from that for fabric
filtration, and the design processes are quite different.
-------
5-9
The general equations used to design a baghouse follow beginning with the
reverse air/shake deflate type of baghouse.
5.1.4.1 Reverse Air/Shake Deflate Baghouses
The construction of a baghouse begins with a set of specifications
including average pressure drop, total gas flow, and other requirements; a
maximum pressure drop is always specified. Given these specifications, the
designer must determine the maximum face velocity that can meet these
requirements. The standard way to relate baghouse pressure drop to face
velocity is given by the relation:
AP(0) = Ssys(*)Vave (5-1)
where:
AP(0) = the pressure drop across the filter, in. H20 (a
function of time, 6)
S (0) = system drag, in. H,,0/(ft/min) (a function of time)
V = average (i.e., design) face velocity (ft/min)
(essentially constant)
For a multicompartment baghouse, the system drag is determined as the sum of
several parallel resistances representative of several compartments. For the
typical case where the pressure drop through each compartment is the same, it
can be shown that:
sys
M
i=l
-1
1
E
* (5-2)
where
M = number of compartments in the baghouse
S.. (6) = drag across compartment i
-------
5-10
The compartment drag is a function of the amount of dust collected on the bags
in that compartment. In general, the dust will be distributed in a very
nonuniform manner. That is, there will be a variation of dust load from one
bag to the next and within a given bag there will also be a variation of dust
load from one area to another. For a sufficiently small area j within
compartment i, it can be assumed that the drag is a linear function of dust
load:
where:
S = drag of a dust-free (freshly cleaned) filter bag
K2 = dust cake flow resistance, [in. H20/(ft/min)]/(lb/ft2)
W.- .-(0) = dust mass per unit area of area j in compartment i
1 1 J
If there are N different areas of equal size within compartment i, each with a
different drag S. ., then the total drag for compartment i can be computed in
' i J
a manner analogous to equation (5-2):
' -i X" / • »f*-L»/*-'*.j\w/j» V ** * /
The constants Sg and K~ depend upon the fabric and the nature and size of the
dust. The relationships between these constants and the dust and fabric
properties are not understood well enough to permit accurate predictions and
so must be determined empirically, either from prior experience with the
dust/fabric combination or from laboratory measurements. The dust mass as a
function of time is defined as:
6
I (CinV.(0)d0} (5-5)
o
-------
5-11
where:
W = dust mass per unit area remaining on a "clean" bag
C^ = dust concentration in the inlet gas (gr/ft )
Vn. (0} = face velocity in compartment i
It is assumed that the inlet dust concentration and the filter area are
constant. The face velocity through each compartment changes with time,
starting at a maximum value just after cleaning and steadily decreasing as
dust builds up on the bags. The individual compartment face velocities are
related to the average face velocity by the expression:
Vave = E(V1(5)A1}/E{A1} (5-6)
= E(V.}/M (for M compartments with equal area)
Equations (5-1) through (5-6) reveal that there is no explicit relationship
between the design face velocity and the tubesheet pressure drop. On the
contrary, the pressure drop that results from a given design can only be
determined by the simultaneous solution of equations (5-1) through (5-5), with
equation (5-6) as a constraint on that solution. This conclusion has several
implications for the design process. The design requires an iterative
procedure: one must begin with a known target for the average pressure drop,
propose a baghouse design (number of compartments, length of filtration
period, etc.), assume a face velocity that will yield that pressure drop, and
solve the system of equations (5-1) through (5-6) to verify that the calcu-
lated pressure drop equals the target pressure drop. This procedure is
repeated until the specified face velocity yields an average pressure drop
(and maximum pressure drop, if applicable) that is sufficiently close to the
design specification.
-------
5-12
5.1.4.2 Pulse-Jet Baghouses
The distinction between pulse-jet baghouses and reverse-air and shake
baghouses is basically the difference between cake filtration and composite
dust/fabric filtration (noncake filtration). This distinction is more a
matter of convenience than physics. In reality, pulse-jet baghouses have been
designed to operate in a variety of modes. Some pulse jets remain online at
all times and are cleaned frequently. Others are taken offline for cleaning
at relatively long intervals. Obviously, if a compartment remains online long
enough without being cleaned, then the filtration mechanism becomes that of
cake filtration. A complete model of pulse-jet filtration therefore must
account for the depth filtration occurring on a relatively clean pulse-jet
filter, the cake filtration that inevitably results from prolonged periods
online, and the transition period between the two regimes.
Besides the question of filtration mechanism, there is also the question
of cleaning method. If a compartment is taken off-line for cleaning, then the
dust that is removed from the bags will fall into the dust hopper before
forward gas flow resumes. If a compartment is cleaned while online, then only
a small fraction of the dust removed from the bag will fall to the hopper.
The remainder of the dislodged dust will be redeposited (i.e., "recycled") on
the bag by the forward gas flow. The redeposited dust layer has different
pressure drop characteristics than the freshly deposited dust. The modeling
work that has been done to date focuses on the online cleaning method. Dennis
(4}
and Klenrnr ' proposed the following model of drag across a pulse-jet filter:
S = Se + (K2)CWC + K2W0 (5-7)
where:
S = drag across the filter
-------
5-14
The disadvantage of the model represented by equations (5-7) and (5-8) is
that the constants, S , 1C, and W , cannot be predicted at this time. Con-
sequently, correlations of laboratory data must be used to determine the value
of (PE). . For the fabric-dust combination of Dacron felt and coal fly ash,
Dennis and Klemnr ' developed an empirical relationship between (PE)*wi the
face velocity, and the cleaning pulse pressure. This relationship (converted
from metric to English units) was as follows:
-0.65
(PE)&W = 6.08 VP.. (5-10)
where:
V = face velocity (ft/min)
P.. = pressure of the cleaning pulse (usually 60 to 100 psig; see Section
J 5.4.1.8)
It is not known how well the constants in equation (5-10) would fit the data
for a different dust/fabric combination. Based on a limited amount of data,
it appears that the power law form of equation (5-10) may be a valid model for
(PE)«W. However, equation (5-10) can be used as a first approximation to
estimate (PE).W for other fabric-dust combinations.
Another model that shows promise in the prediction of noncake filtration
pressure drop is that of Leith and Ellenbecker' ' as modified by Koehler and
Leith.^ ' In this model, the tubesheet pressure drop is a function of the
clean fabric drag, the system hardware, and the cleaning energy.
Specifically:
AP = (1/2) [Ps + KjVf - ]{(PS - KjVf)2 - 4W0K2/K3}] + KyVf2 (5-11)
where:
P = maximum static pressure achieved in the bag during cleaning
-------
5-15
K, = clean fabric resistance
Vf = face velocity
K2 = dust deposit flow resistance
K3 = bag cleaning efficiency coefficient
K = loss coefficient for the venturi at the inlet to the bag
Comparisons of laboratory data with pressure drops computed from equation
(5-11)^ are in close agreement for a variety of dust/fabric combinations.
The disadvantage of equation (5-11) is that the constants K,, 1C, and K3 must
be determined from laboratory measurements. The most difficult one to
determine is the K3 value, which can only be found by making measurements in a
pilot-scale pulse-jet baghouse.
5.2 Design Procedures
5.2.1 6as-to-Cloth Ratio
The gas-to-cloth ratio is difficult to estimate from first principles.
However, shortcut methods of varying complexity allow rapid estimation.
Descriptions of three methods of increasing difficulty follow. For shaker and
reverse-air baghouses, the third method is best performed with publicly
available computer model programs.
5.2.1.1 Gas-to-Cloth Ratio From Similar Applications
Net gas-to-cloth ratio is equal to the total actual volumetric flow rate
in cubic feet per minute divided by the net cloth area in square feet. This
ratio reduces to units of feet per minute and affects pressure drop and bag
life. After a fabric has been selected, the gas-to-cloth ratio can be
determined using Table 5-1. Column 1 shows the type of dust; column 2 shows
the gas-to-cloth ratios for woven fabric; and column 3 shows gas-to-cloth
ratios for felted fabrics. The net cloth area is determined by dividing the
-------
5-13
S = drag of a just-cleaned filter
(IC)- = specific dust resistance of the recycling dust
W = areal density of the recycling dust
l<2 = specific dust resistance of the freshly deposited dust
W = areal density of the freshly deposited dust
This model has the advantage that it can easily account for all three regimes
of filtration in a pulse-jet baghouse, i.e., cake filtration, depth filtra-
tion, and filtration in the transition region. As in equations (5-1) to
(5-6), the drag and areal densities are functions of time (0). However, for a
pulse-jet baghouse with online cleaning, the filtration velocity is relatively
constant. The pressure drop can thus be expressed as the sum of a relatively
constant term and a term that increases due to dust build-up:
AP = (PE)Aw + K2WQV (5-8)
where:
AP = pressure drop (in. H20)
V = filtration velocity (ft/min)
(PE)Aw = [Se + (K2)CWC] V (5-9)
Equation (5-8) describes the pressure drop behavior of an individual bag. To
extend this single bag result to a multiple-bag compartment, equation (5-7)
would be used to determine the individual bag drag and the total baghouse drag
would then be computed as the sum of the parallel resistances as in equation
(5-2). Pressure drop would then be calculated as in equation (5-1). It seems
reasonable to extend this analysis to the case where the dust is distributed
unevenly on the bag and then apply equation (5-7) to each area on the bag,
followed by an equation analogous to (5-4) to compute the overall bag drag.
The difficulty in doing this is that one must assume values for W for each
different area to be modeled.
-------
5-16
Table £-1 Gas-tp-Cloth Ratios^7)
(ffVnnn)/(firof cloth area)
Dust
Alumina
Asbestos
Bauxite
Carbon Black
Coal
Cocoa, Chocolate
Clay
Cement
Cosmetics
Enamel Frit
Feeds, Grain
Feldspar
Fertilizer
Flour
Fly Ash
Graphite
Gypsum
Iron Ore
Iron Oxide
Iron Sulfate
Lead Oxide
Leather Dust
Lime
Limestone
Mica
Paint Pigments
Paper
Plastics
Quartz
Rock Dust
Sand
Sawdust (Wood)
Silica
Slate
Soap, Detergents
Spices
Starch
Sugar
Talc
Tobacco
Zinc Oxide
Shaker/Woven
Reverse-A1 r/Woven
2.5
3.0
2.5
1.5
2.5
2.8
2.5
2.0
1.5
2.5
3.5
2.2
3.0
3.0
2.5
2.0
2.0
3.0
2.5
2.0
2.0
3. -5
2.5
2.7
2.7
2.5
3.5
2.5
2.8
3.0
2.5
3.5
2.5
3.5
2.0
2.7
3.0
2.0
2.5
3.5
2.0
Pulse Jet/Felt
8
10
8
5
8
12
9
8
10
9
14
9
8
12
5
5
10
11
7
6
6
12
10
8
9
7
10
7
9
9
10
12
7
12
5
10
8
7
10
13
5
Generally safe design values—application requires consideration of particle
size and grain loading.
-------
5-17
gas-to-cloth ratio into the actual cubic feet per minute flow of the exhaust
gas stream. For an intermittent-type baghouse that is shut down for cleaning,
this is the total, or gross, cloth area. However, for continuously operated
filters, the area must be increased to allow the shutting down of one or more
compartments for cleaning. Table 5-2 provides a guide for adjusting the net
area to the gross area, which determines the size of a continuously cleaned
filter.
5.2.1.2 Gas-to-Cloth Ratio From Manufacturer's Methods
Manufacturers have developed nomographs and charts that allow rapid
estimation of the gas-to-cloth ratio. Two examples are given below, one for
shaker-cleaned baghouses and the other for pulse-jet cleaned baghouses.
For shaker baghouses, Table 5-3 gives a factor method for estimating the
ratio. Ratios for several materials in different operations are presented,
but are modified by factors for particle size and dust load. Directions and
an example are included. Gas-to-cloth ratios for reverse-air baghouses would
be about the same or a little more conservative compared to the Table 5-3
values.
(g\
For pulse-jet baghouses, another factor methodv ' has been modified with
equations to represent temperature, particle size, and dust load:
V = A x B x 2.647T"0'2335 x (0.7471 + 0.0853 In D) x 1.0873 |_'0'06021
(5-12)
where:
V = gas-to-cloth ratio, ft/min
A = material factor, from Table 5-4
B = application factor, from Table 5-4
T = temperature, *F (between 50 and 275)
-------
5-18
Table 5-2 Approximate Guide to Estimate Gross Cloth Area
(8)
Net Cloth Area
(ft2)
1-4,000
4,001-12,000
12,001-24,000
24,001-36,000
36,001-48,000
48,001-60,000
60,001-72,000
72,001-84,000
84,001-96,000
96,001-108,000
108,001-132,000
132,001-180,000
above 180,001
Gross Cloth Area
(ft2)
Multiply by 2
1.5
1.25
1.17
1.125
1.11
1.10
1.09
1.08
1.07
1.06
1.05
1.04
-------
Table 5-3 Manufacturer's Factor Method for Estimating Gas-to-Cloth
Ratios for Shaker Baghouses
.MATJRJAL
lardboaid
ceds
lour
irain
Leather Dust
Tobacco
Supply Air
Hood. Dust. Chips
RATIO
OPERATION
1
2.3.4.5.6.7
2.3.4.5.6.7
2,3,4.5.6,7
1.7.8
1.4.6.7
13
1.6.7
3/1 RATIO
MATERIAL
Asbestos
Aluminum Dust
Fibrous Mat'l.
Cellulose Mat'l.
Gypsum
Lime (Hydraled)
Per lite
Rubber Chem.
Salt
Sand0
Iron Scale
Soda Ash
Talc
Machining Operation
OPERATION
7.8
7.8
4.7,8
4.7.8
3,5.6.7
4,6.7
4.5.6
5.6.7.8
3.4.5,6,7
5.6.7.9.15
1.7.8
4.6.7
3.4,5.6.7
1,8
2.5/1 RATIO
MATERIAL
Alumina
Carbon Black
Cement
Coke
Ceramic Pigm.
Clay& Brick Dust
Coal
Kaolin
Limestone
Rock, Ore Dust
Silica
Sugar
OPERATION
2.3,4.5.6
4.5.6.7
3,4,5.6,7
2.3,5.6
4,5.6,7
2,4.6.12
2.3.6.7.12
4,5.7
2.3,4,5,6,7
2.3.4.5.6,7
2.3.4.5.6.7
3,4,5.6,7
MATERIAL
2/F"RATib"_'~""_"
OPERATION
Ammonium Phos-
phate Perl.
Dialomaceous
Earth
Dry Pelrochem.
Dyes
Fly Ash
Metal Powders
Plastics
Resins
Silicates
Starch
Soaps
2.3.4.5,6,7
4,5.6,7
2.3.4,5,6,7,14
2,3,4,5.6,7
10
2, 3.4.5.6.7,14
2,3,4,5.6.7.14
2,3.4.5.6,7,14
2.3.4.5,6,7.14
6.7
3,4.5.6,7
MATERIAL
Activated Charcoal
arbon Black
Detergents
Metal Fumes,
Oxides and
other Solid
Dispersed
Products
OPERATION
2,4,5.6.7
11.14
2.4,5,6.7
10,11
CUTTING
CRUSHING
PULVERIZING -
- 1
-2
3
MIXING
-4
SCREENING - 5
STORAGE - 6
CONVEYING - 7
GRINDING -8
SHAKEOUT -9
FURNACE FUME - 10
REACTION FUME- II
DUMPING - 12
INTAKE CLEANING - 13
PROCESS - 14
BLASTING - 15
B
FINENESS FACTOR
MICRON SIZE
>100
50-100
10-50'
3-10
1-3
<1
FACTOR
1.6
DUST LOAD FACTOR
loading GR. CU. FT.
1 -3
4-8
9- 17
18-40
40
Factor
1.2
1.0
.95
.90
.85
cn
This information constitutes a guide for commonly encountered situations and should not be con-
sidered a "hard-and-fast" rule. Air-lo-clolh ratios ore dependent on dust loading, size distribution,
particle shape and "cohesiveness" of the deposited dust. These conditions must be evaluated for
each application. The longer the interval between bag cleaning "Hie lower the air-to-cloth ratio
must be. Finely-divided, uniformly sized particles generally form more dense filter cakes and re-
quire lower air-lo-clolh ratios than when larger particles are interspersed with the fines. Sticky,
oily particles, regardless of shape or size, form dense filler cakes and require lower air-to-cloth
ratios.
EXAMPLE: Foundry shakeout unit handling 26000 CFM and collecting 3500 #/ hr. of sand. The
particle distribution shows 90% greater than 10 microns. The air is to exhaust to room
in winter, to atmosphere in summer.
3500 */„ , -5- 60 3$ - 26000 ^^ X 7000 ° '/« =15.7 ^
•Chart A = 3/1 ratio. Chart B — Factor 1.0, Chart C — .95; 3 ic 1 x .95 -- 2.9 air
to cloth ratio. 26000 ~ 2.9 — 9,000 sq. ft.
Reprinted with permission from Buffalo Forge Company Bulletin AHD-29.
-------
5-20
Table 5-4 Factors for Pulse-Jet Gas-to-Cloth Ratios
(9)
A. Material Factor
15a
Cake mix
Cardboard dust
Cocoa
Feeds
Flour
Grain
Leather dust
Sawdust
Tobacco
12
Asbestos
Buffing dust
Fibrous and
cellulosic
material
Foundry shakeout
Gypsum
Lime (hydrated)
Perlite
Rubber chemicals
Salt
Sand
Sandblast dust
Soda ash
Talc
10
Alumina
Aspirin
Carbon black
(finished)
Cement
Ceramic pig-
ments
Clay and
brick dusts
Coal
Fluorspar
Gum, natural
Kaolin
Limestone
Perchlorates
Rock dust,
ores and
minerals
Silica
Sorbic acid
Sugar
9.0
Ammonium
phosphate-
fertilizer
Cake
Diatomaceous
earth
Dry petro-
chemicals
Dyes
Fly ash
Metal powder
Metal oxides
Pigments,
metallic
and synthetic
Plastics
Resins
Silicates
Starch
Stearates
Tannic acid
6.0b
Activated
carbon
Carbon
black
(molec-
ular)
Deter-
gents
Fumes
and
other
dis-
persed
products
direct
from
reac-
tions
Powdered
milk
Soaps
B. Application Factor
Nuisance Venting
Relief of transfer
points, conveyors,
packing stations, etc.
Product Collection
Air conveying-venting
mills flash driers
classifiers, etc.
Process Gas Filtration
Spray driers, kilns,
reactors, etc.
1.0
0.9
0.8
In general physically and chemically stable materials.
DAlso includes those solids that are unstable in their physical or chemical
state due to hygroscopic nature, sublimation, and/or polymerization.
-------
5-21
D = mass mean diameter of particle, pm (between 3 and 100)
L = inlet dust loading, gr/ft3 (between 0.05 and 100)
For temperatures below 50 *F, use T = 50 but expect decreased accuracy;
for temperatures above 275 °F, use T = 275. For particle mass mean diameters
less than 3 /tm, the value of D is 0.8, and for. diameters greater than 100 /jm,
D is 1.2. For dust loading less than 0.05 gr/ft , use L = 0.05; for dust
loading above 100 gr/ft , use L = 100.
5.2.1.3 Gas-to-Cloth Ratio From Theoretical/Empirical Equations
5.2.1.3.1 Shaker and reverse-air baghouses—The system described by
equations (5-1) through (5-6) is complicated; however, numerical methods can
be used to obtain an accurate solution. A critical weakness in baghouse
modeling that has yet to be overcome is the lack of a fundamental description
of the bag cleaning process. That is, to solve equations (5-1) through (5-6),
the value of W (the dust load after cleaning) must be known. Clearly, there
must be a relationship between the amount and type of cleaning energy and the
degree of dust removal from a bag. Dennis et al.^ ' have developed
correlations for the removal of coal fly ash from woven fiberglass bags by
shaker cleaning and by reverse air cleaning. These correlations have been
incorporated into a computer program that generates the solution to the above
system of equations.' ''' If one were to apply the correlations
developed with coal ash and woven glass fabrics to other dust/fabric
combinations, the accuracy of the results would depend on how closely that
dust/fabric combination mimicked the coal ash/woven glass fabric system.
Physical factors that affect the correlation include the particle size
distribution, adhesion properties of the dust and fabric, and fabric weave, as
well as cleaning energy. More research is needed in this area of fabric
filtration.
-------
5-22
The rigorous design of a baghouse thus involves several steps. First,
the design goal for average pressure drop (and maximum pressure drop, if
necessary) must be specified along with total gas flow rate and other
parameters, such as $e and K2 (obtained either from field or laboratory
measurements). Second, a face velocity is assumed and the number of
compartments in the baghouse is computed based on the total gas flow, face
velocity, bag size, and number of bags per compartment (typical compartments
in the U.S. electric utility industry use bags 1 ft in diameter by 30 ft long
with 400 bags per compartment). Standard practice is to design a baghouse to
meet the specified pressure drop when one compartment is off-line for
maintenance and a second compartment off-line for cleaning. The third step is
to specify the operating characteristics of the baghouse (i.e., filtration
period, cleaning period, and cleaning mechanism). Fourth, the designer must
specify the cleaning efficiency so that the residual dust load can be
estimated. Finally, the specified baghouse design is used to establish the
details for equations (5-1) through (5-6), which are then solved numerically
to establish the pressure drop as a function of time. The average pressure
drop is then computed by integrating the instantaneous pressure drop over the
filtration cycle and dividing by the cycle time. If the computed average is
higher than the design specification, then the face velocity must be reduced
\
and the procedure repeated. If the computed average pressure drop is
significantly lower than the design specification, then the proposed baghouse
was oversized and should be made smaller by increasing the face velocity and
repeating the procedure. When the computed average pressure drop comes
sufficiently close to the assumed specified value, then the design has been
determined. A complete description of the modeling process can be found in
-------
5-23
the reports by Dennis et al.* ' ' A critique on the accuracy of the.model
is presented by Viner et al.* '
5.2.1.:3.2 Pulse-Jet baghouses—The overall process of designing a pulse
jet baghouse is actually simpler than that required for a reverse-air or
shaker baghouse if the baghouse. remains online for cleaning. The first step
is to specify what the desired average tubesheet pressure drop should be.
Second, the operating characteristics of the baghouse must be established
(e.g., online time, cleaning energy). Third, the designer must obtain values
for the coefficients in either equation (5-10) or equation (5-11) from field,
pilot plant, or laboratory measurements. Fourth, a value is estimated for the
face velocity and the appropriate equation [(8) or (11)] is solved for the
pressure drop as a function of time for the duration of the filtration cycle.
This information is used to calculate the cycle average pressure drop. If the
calculated pressure drop matches the specified pressure drop, then the
procedure is finished. If not, then the designer must adjust the face
velocity and repeat the procedure.
5.2.2 Pressure Drop
Pressure drop for the bags can be calculated rigorously from the
equations given in the preceding section if values for the various parameters
are known. Frequently they are not known. For quick estimation, a maximum
pressure drop of 5- to 10-in. H20 across the baghouse and 10- to 20-in H20
across the entire system can be assumed if it contains much ductwork.
A comparable form of equations (5-1) and (5-3) that may be used for
pressure drop across the fabric in a shaker or reverse-air baghouse is:
AP = SeV + K2C.V20 (5-13)
-------
5-24
where:
AP = pressure drop (In. H20)
Sg = effective residual drag of the fabric [in. H20/(ft/min)]
V = superficial face velocity or gas-to-cloth ratio (ft/min)
K0 = specific resistance coefficient of the dust [in.
*• H20/(ft/min)]/(lb/ftz)
C. = inlet dust concentration (lb/ft3)
9 = filtration time, min
Although there is much variability, values for Sg may range from about
0.2 to 2 in. H20/(ft/min) and for K2 from 1 or 2 to 30 or 40 [in. H20/(ft/
min)]/lb/ft. Typical values for coal fly ash are 1 to 4. Inlet concen-
trations vary from less than 0.05 gr/ft to more than 100 gr/ft , but a more
nearly typical range is from 0.5 to 10 gr/ft . Filtration times may range
from 20 minutes to 8 hours for continuous duty reverse-air and shaker bag-
houses, but 30 minutes to 4 hours is more frequently found. Filtration times
for pulse-jet baghouses range from 2 to 60 minutes, but 5 to 20 minutes is
typical. For pulse-jet baghouses, use equations (5-8) and (5-10) to estimate
AP, after substituting C^t for WQ and (PE)Aw for SQ V.
5.2.3 Particle Characteristics
Particle size distribution and adhesiveness are the most important
particle properties that affect design procedures. Smaller particle sizes can
form a denser cake, which increases pressure drop. As shown in Table 5-3 and
equation (5-12), the effect of decreasing average particle size is a lower
applicable gas-to-cloth ratio.
Sticky particles may require installing equipment that injects a
precoating material onto the bag surface, which acts as a buffer that traps
-------
5-25
the particles and prevents them from blinding or permanently plugging the
fabric pores.
5.2.4 Gas Stream Characteristics
Moisture and corrosives content are the major gas stream characteristics
requiring design consideration. The baghouse and associated ductwork should
be insulated and possibly heated to avoid condensation. Both the structural
and fabric components must be considered, as either may be damaged. Where
structural corrosion is likely, stainless steel substitution for mild steel
may be required, provided that chlorides are not present. (Most austenitic
stainless steels are susceptible to chloride corrosion.)
5.2.4.1 Temperature
The temperature of the pollutant stream to be cleaned must be above and
remain above the dew point of any condensables in the stream. If the
temperature is high and it can be lowered without approaching the dew point,
spray coolers or dilution air can be used to drop the temperature so that
temperature limits of the fabric will not be exceeded. The additional cost of
a precooler will have to be weighed against the higher cost of bags with
greater temperature resistance. The use of dilution air to cool the stream
also constitutes a tradeoff between a less expensive fabric and a larger
filter necessary to accommodate the additional volume of the dilution air.
Generally, precooling would not be necessary if fabric that will handle the
temperature and the chemical action of the pollutant stream is available.
(Costs for spray chambers, quenchers, and other precoolers are found in the
"Precoolers" section of the Manual.) Table 5-5 lists several of the fabrics
in current use and provides information on temperature limits and chemical
resistance. The column labeled "Flex Abrasion" indicates the fabric's
suitability for cleaning by mechanical shakers.
-------
5-26
Table 5-5 Properties of Leading Fabric Materials
(14)
Fabric
Cotton
Cres1anb
Dacron0
Dyne1c
Fiberglasd
Filtron6
Gore-Tex
Nomex0
Nylonc
Orlonc
Polypro-
pylene
Teflon0
Wool
Temp,
.pa
180
250
275
160
500
270
Depends
on
backing
375
200
260
200
450
200
Acid
Resistance
Poor
Good in mineral
acids
Good in most
mineral acids;
di solves par-
tially in con-
centrated H2S04
Little effect
even at high
concentration
Fair to good
Good to
excellent
Depends on
backing
Fair
Fair
Good to excel-
lent in mineral
acids
Excellent
Inert except
to fluorine
Very good
Alkali
Resistance
Very good
Good in weak
alkali-
Good in weak
alkali; fair
in strong
alkali
Little effect
even in high
concentration
Fair to good
Good
Depends on
backing
Excellent at
low temperature
Excellent
Fair to good
in weak alkali
Excellent
Inert except
to tri fluoride,
chlorine, and
molten alkaline
metals
Poor
Flex
Abrasion
Very good
Good to
very good
Very good
Fair to
good
Fair
Good to
Very good
Fair
Excellent
Excellent
Good
Excellent
Fair
Fair to good
Maximum continuous operating temperatures recommended by the Industrial Gas
Cleaning Institute.
American Cyanamid registered trademark.
GDu Pont registered trademark.
Owens-Corning Fiberglas registered trademark.
eW. W. Criswell Div. of Wheelabrator-Fry, Inc., trade name.
W. L. Gore and Co., registered trademark.
-------
5-27
5.2.4.2 Pressure
Standard fabric filters can be used in pressure or vacuum service but
only within the range of about +25 inches of water gauge. Because of the
sheet metal construction of the house, they are not generally suited for more
severe service. However, for special applications, high-pressure shells can
be built.
5.2.5 Pressure or Suction Housings
The location of the baghouse with respect to the fan in the gas stream
affects the capital cost. A suction-type baghouse, with the fan located on
the downstream side of the unit, must withstand high negative pressures and
therefore must be more heavily constructed and reinforced than a baghouse
located downstream of the fan (pressure baghouse). The negative pressure in
the suction baghouse can result in outside air infiltration, which can result
in condensation, corrosion, or even explosions if combustible gases are being
handled. In the case of toxic gases, this inward leakage can have an advan-
tage over the pressure-type baghouse, where leakage is outward. The main
advantage of the suction baghouse is that the fan handling the process stream
is located at the clean-gas side of the baghouse. This reduces the wear and
abrasion on the fan and permits the use of more efficient fans (backward-
curved blade design). However, because for some designs the exhaust gases
from each compartment are combined in the outlet manifold to the fan, locating
compartments with leaking bags may be difficult and adds to maintenance costs.
Pressure-type baghouses are generally less expensive because the housing
must only withstand the differential pressure across the fabric. In some
designs the baghouse has no external housing. Maintenance also is reduced
because the compartments can be entered and leaking bags can be observed while
-------
5-28
the compartment is in service. With a pressure baghouse, the housing acts as
the stack to contain the fumes with the subsequent discharge at the roof of
the structure, which makes it eas-ier to locate leaking bags. The main dis-
advantage of the pressure-type baghouse is that the fan is exposed to the
dirty gases where abrasion and wear on the fan blades may become a problem.
Also, some applications require a stack for dispersion of gaseous pollutants,
negating some of the construction economics.
5.2.6 Standard or Custom Construction
The design and construction of baghouses are separated into two groups,
(0\
standard and custom,v ' which are further separated into low, medium, and high
capacity. Standard baghouses are predesigned and factory built as complete
off-the-shelf units that are shop-assembled and bagged for low-capacity units
(hundreds to thousands of acfm throughput). Medium-capacity units (thousands
to less than 100,000 acfm) have standard designs, are shop-assembled, may or
may not be bagged, and have separate bag compartment and hopper sections.
High-capacity baghouses (larger than 50,000 or 100,000 acfm) can be designed
as shippable modules requiring only moderate field assembly. These modules
may have bags installed and can be shipped by truck or rail. Upon arrival,
they can be operated singly or combined to form units for larger-capacity
applications. Because they are preassembled, field labor for installation is
less costly.
The custom baghouse, also high capacity, is designed for a specific
application and is usually built to the specifications prescribed by the
customer. Generally, these units are much larger than standard baghouses.
For example, many are used on power plants. The cost of the custom baghouse
is much higher per square foot of fabric because it is not an off-the-shelf
-------
5-29
item and requires special setups for manufacture and expensive field labor for.
assembly upon arrival. The advantages of the custom baghouse are many and are
usually directed towards ease of maintenance, accessibility, and other
customer preferences. In some very small baghouses, a complete set of bags
must be replaced in a compartment at one time because of the difficulty in
locating and replacing single leaking bags, whereas in custom baghouses,
single bags are accessible and can be replaced one at a time as leaks develop.
5.2.7 Filter Media
The type of filter material used in baghouses is dependent on the
specific application in terms of chemical composition of the gas, operating
temperature, dust loading, and the physical and chemical characteristics of
the particulate. A variety of fabrics, either felted or woven, is available
and the selection of a specific material, weave, finish, or weight is based
primarily on past experience. The type of yarn (filament, spun, or staple),
the yarn diameter, and twist are also factors in the selection of suitable
fabrics for a specific application. For some difficult applications, Gore-
Tex, a polytetrafluoroethylene (PTFE) membrane laminated to a substrate fabric
(felt or woven) or altered-surface fabrics may be used. Because of the
violent agitation of mechanical shakers, spun or heavy weight staple yarn
fabrics are commonly used with this type of cleaning, while lighter weight
filament yarn fabrics are used with reverse-air cleaning.
The type of material will limit the maximum operating gas temperature for
the baghouse. Cotton fabric has the least resistance to high temperatures
(about 180 °F), while fiberglass has the most (about 500 *F). The temperature
of the exhaust-gas stream must be well above the dew point of any of its
contained condensables as liquid particles will usually plug the fabric pores
-------
5-30
quickly. However, the-temperature must be below the maximum limit of the
fabric in the bags. These maximum limits are given in Table 5-5,
5.3 Estimating Total Capital Investment
Total capital investment includes costs for the baghouse structure, the
initial complement of bags, auxiliary equipment, and the usual direct and
indirect costs associated with installing or erecting new structures. These
costs are described below.
5.3.1 Equipment Cost
5.3.1.1 Bare Baqhouse Costs
Six types of baghouses will be considered:
Preassembled units
Intermittent Shaker Figure 5-2
Continuous Shaker Figure 5-3
Continuous Pulse-jet (common housing) Figure 5-4
Continuous Pulse-jet (modular) Figure 5-5
Continuous Reverse-air Figure 5-6
Field-assembled units
Continuous Any method Figure 5-7
Each figure gives costs for the filter without bags and additional costs
for stainless steel construction and for insulation. All curves are based on
a number of actual quotes. A least squares line has been fitted to the quotes
and the line's equation is given. However, extrapolation should not be used.
The reader should not be surprised if he obtains quotes that differ from these
curves by as much as +25%. Significant savings can be obtained by soliciting
multiple quotes. All units include inlet and exhaust manifolds, supports,
-------
5-31
platforms, handrails, and hopper discharge devices. The indicated prices are
flange to flange. Note that the scales on both axes change from one figure to
another to accommodate the differing gas flow ranges over which the various
types of baghouses operate.
The 304 stainless steel add-on cost is used when such construction is
necessary to prevent the exhaust gas stream from corroding the interior of the
baghouse. Stainless steel is substituted for all metal surfaces that are in
contact with the exhaust gas stream.
Insulation costs are for 3 inches of shop-installed glass fiber encased
in a metal skin. One exception is the custom baghouse, which has field-
installed insulation. Costs for insulation include only the flange-to-flange
baghouse structure on the outside of all areas in contact with the exhaust gas
stream. Insulation for ductwork, fan casings, and stacks must be calculated
separately as discussed later.
The first baghouse type is the intermittent service baghouse cleaned by a
mechanical shaker. This baghouse is shut down and cleaned at convenient
times, such as the end of the shift or end of the day. Although few units are
sold, they are applicable for operations that require infrequent cleaning.
Figure 5-2 presents the unit cost with price in dollars plotted against the
15
gross square feet of cloth required. Because intermittent service baghouses
do not require an extra compartment for cleaning, gross and net fabric areas
are the same. The plot is linear because baghouses are made up of modular
compartments and thus have little economy of scale. Because of the modular
construction, the price line should not be extrapolated downward. Costs for
both types of shaker baghouse include the shaker mechanism.
-------
Caution: Do not extrapolate.
Cost without bags- —
Stainless steel add on
Insulation add on
$ = 1,875 + 0.450 ft 2
0
Source: ETS, Inc.
6 8 10 12 14
Gross Cloth Area (1000 ft ^
Figure 5-2. Equipment costs for intermittent shaker filters.
16
18
en
co
ro
-------
5-33
Figure 5-3 presents the same costs for a continuously operated baghouse
cleaned by mechanical shaker. ' Again, price is plotted against the gross
cloth area in square feet. As in Figure 5-2, the units are modular in
construction. Costs for these units, on a square foot basis, are higher
because of increased complexity and generally heavier construction.
The third and fourth types are common-housing pulse jets and modular
pulse jets. The latter are constructed of separate modules that may be
arranged for offline cleaning, and the former have all bags within one
housing. The costs for these units are shown in Figures 5-4 and 5-5,
respectively. The cleaning system compressor is not included. Note that in
the single-unit (common-housing) pulse jet, for the range shown, the height
and width of the unit are constant and the length increases; thus, for a
different reason than that for the modular units discussed above, the cost
increases linearly with size. Because the common housing is relatively
inexpensive, the stainless steel add-on is proportionately higher than for
modular units. Added material costs and setup and labor charges associated
with the less workable stainless steel account for most of the added expense.
15
Figure 5-6 shows the costs for the reverse-air baghouses. The construction
is modular and the reverse-air fan is included. The final type is the custom
baghouse which, because of its large size, must be field assembled. It is
often used on power plants, steel mills, or other applications too large for
the factory-assembled baghouses. Prices for these units are shown in
Figure 5-7.15
5.3.1.2 Bag Costs
Table 5-6 gives the price per square foot of bags by type of fabric and
by type of cleaning system used. The prices represent about a 10% range. In
calculating the cost, the gross area as determined from Table 5-2 should be
-------
g 600
o>
0
Caution: Do not extrapolate.
Cost without ba
Stainless steel add on--
Insulation add on
i i i i i i i i i i r
10
20 30 40 50 60 70
Gross Cloth Area (1000ft2)
Source: ETS, Inc.; Fuller Co.
80
90
en
CO
Figure 5-3. Equipment costs for continuous shaker filters.
-------
Caution: Do not extrapolate.
Insulation add on
o
Source: ETS, Inc. GrossCI* Area (10001,2)
"9- 5.4. Equipment cos(s ,„ pu(M^ niun (commm h^
en
CO
en
-------
CD
CO
CD
0)
I
O
T3
O
O
O
150
125
100
75
50
Caution: Do not extrapolate.
8
O
c
0)
Q.
"5
S o
25
i i i i i i i i i
Cost without bags
Stainless steel add on
Insulation add on
0
Source: ETS, Inc.
6 8 10 12 14
Gross Cloth Area (1000 ft ^
Figure 5-5. Equipment costs for pulse-jet filters (modular).
16
18
cn
i
GO
-------
0
Caution: Do not extrapolate.
Cost without baas
Stainless steel add on
Insulation add on
01
I
co
10
Source: ETS, Inc.
20 30 40 50 60 70
Gross Cloth Area (1000 ft ^
Figure 5-6. Equipment costs for reverse-air filters.
80
90
-------
O)
g 2500
O
-g 2000
o
o
o
o
o
O
0
Q.
"5
(7
111
1500
1000
500
0
Caution: Do not extrapolate.
Cost without bags
Stainless steel add on
Insulation add on
0
Source: ETS, Inc.
100 200 300
Gross Cloth Area (1000 ft ^
Figure 5-7 Equipment costs for custom-built filters.
400
en
i
co
oo
-------
5-39
Table 5-6 Bag Prices,
(3rd quarter 1986
Type of Cleaning
Pulse jet, TRD
Pulse jet, BBR
Shaker
Strap top
Loop top
Reverse air with
rings
Reverse air w/o
rl ngs
Bag Diameter
(Inches)
4-1/2 to 5-1/8
6 to 8
4-1/2 to 5-1/8
6 to 8
5
5
8
11-1/2
8
11-1/2
Type
0
0
0
0
0
0
0
0
0
0
PE
.59
.43
.37
.32
.45
.43
.46
.47
.32
.32
PP
0.61
0.44
0.40
0.33
0.48
0.45
NA
NA
NA
NA
1
1
1
1
1
1
1
1
1
1
NO
.88
.56
.37
.18
.28
.17
.72
.69
.20
.16
of Material3
HA
0.92
0.71
0.66
0.58
0.75
0.66
NA
NA
NA
NA
FG
1.
1.
1.
0.
NA
NA
0.
0.
0.
0.
29
08
24
95
99
76
69
53
CO
NA
NA
NA
NA
0.44
0,39
NA
NA
NA
NA
TF
9.05
6.80
8.78
6.71
NA
NA
NA
NA
NA
NA
NA = Not applicable.
Materials:
PE = 16-oz polyester FG = 16-oz fiberglass with 10% Teflon
PP = 16-oz polypropylene CO = 9-oz cotton
NO = 14-oz nomex TF = 22-oz Teflon felt
HA = 16-oz homopolymer acrylic
Bag removal methods:
TR = Top bag removal (snap in)
BBR = Bottom bag removal
cldentified as reverse-air bags, but used in low pressure pulse applications.
NOTE: For pulse-jet baghouses, all bags are felts except for the fiberglass,
which 1s woven. For bottom access pulse jets, the cage price for one
cage can be calculated from the single-bag fabric area using:
In 50 cage lots
In 100 cage lots
In 500 cage lots
Mild steel cage
$ * 4.941 + 0.163 ftf
$ = 4.441 + 0.163 ft;
$ = 3.941 + 0.163 ft*
Stainless steel cage
$ = 23.335 + 0.280 ftf
$ = 21.791 -i- 0.263 ft;
$ = 20.564 + 0.248 ft''
These costs apply to 4-1/2-1 n. or 5-5/8-1n diameter, 8-ft and 10-ft
cages made of 11 gauge mild steel and having 10 vertical wires and
"Roll Band" tops. For flanged tops, add $1 per cage. If flow control
Venturis are used (as they are in about half of the pulse-jet manufac-
turers' designs), add $5 per cage.
For shakers and reverse air baghouses, all bags are woven. All prices are for
finished bags, and prices can vary from one supplier to another. For Gore-Tex
bag prices, multiply base fabric price by factors of 3 to 4.5.
Source: ETS, Inc.
(15)
-------
5-40
used. Gore-Tex fabric costs are a combination of the base fabric cost and a
premium for the PTFE laminate and its application. As fiber market conditions
change, the costs of fabrics relative to each other also change. The bag
prices are based on typical fabric weights, in ounces/square yard, for the
fabric being priced. Sewn-in snap rings are included in the price, but other
mounting hardware, such as clamps or cages, is an added cost.
5.3.1.3 Auxi1iary Equipment
The auxiliary equipment depicted in Figure 5-1 is discussed elsewhere in
the Manual. Because hoods, precoolers, cyclones, fans, motors, and stacks are
common to many pollution control systems, they are given extended treatment in
the following tentatively numbered separate sections: capture hoods in
Section 16, ductwork in Section 7, precoolers (spray chambers and quenchers)
in Section 18, cyclones in Section 17, fans and motors in Section 8, and
stacks in Section 11. If dust-removal equipment is to be considered,
Section 19 discusses screw conveyors.
5.3.2 Total Purchased Cost
The total purchased cost of the fabric filter system is the sum of the
costs of the baghouse, bags, auxiliary equipment, instruments and controls;
and of taxes and freight. The last three items generally are taken as
percentages of the estimated total cost of the first three items. Typical
values, from Section 2 of the Manual, are 10% for instruments and controls, 3%
for taxes, and 5% for freight.
Bag costs can vary from less than 15% to more than 100% of bare baghouse
cost, depending on type of fabric required. This situation makes it inadvis-
able to estimate total purchased cost without considering both costs, and
prevents effective use of factors to estimate a single cost for the baghouse
and bags.
-------
5-41
5.3.3 Total Capital Investment
Using the Section 2 methodology, the total capital investment (TCI) is
estimated from a series of factors applied to the purchased equipment cost to
obtain direct and indirect costs for installation. The TCI is the sura of
these three costs. The required factors are given in Table 5-7. Because bag
costs can have such a large effect on total purchased equipment cost, the
factors may cause overestimation of total capital investment when expensive
bags are used. Using stainless steel components may also cause overestima-
tion. Because baghouses may vary from small units installed within existing
buildings to large, separate structures, specific factors for site preparation
or for buildings are not given. However, costs for buildings may be obtained
from such references as Means Square Foot Costs 1986.' ' Land, working
capital, and offsite facilities are excluded from the table, as they are not
normally required. For very large installations, however, they may be needed
and would be estimated on an as-needed basis.
Note that the factors given in Table 5-7 are for average installation
conditions. Considerable variation may be seen with other-than-average
installation circumstances.
5.4 Estimating Total Annual Costs
5.4.1 Direct Annual Cost
Direct annual costs include operating and supervisory labor, operating
materials, replacement bags, maintenance (labor and materials), utilities, and
dust disposal. Most of these costs are discussed individually below. They
vary considerably with location and time, and, for this reason, should be
obtained to suit the specific baghouse system being costed. For example,
current labor rates may be found in such publications as the Monthly Labor
Review, published by the U.S. Department of Labor, Bureau of Labor Statistics.
-------
5-42
Table 5-7 Capital Cost Factors for Fabric Filters
(8)
Direct Costs
Factor
Purchased equipment costs;
Fabric filter
Bags A =
Auxiliary equipment
Instruments & controls
Taxes
Freight
Total purchased equipment cost B =
Installation direct costs
Foundations & supports
Erection & handling
Electrical
Piping
Insulation for ductwork
Painting
Site preparation (S.P.)
Buildings (Bldg.)
Total installation direct costs
Total direct costs
Indirect costs
Engineering & supervision
Construction and field expense
Construction fee
Startup fee
Performance test
Contingencies
Total indirect costs
Total direct and indirect costs = Total capital investment
As
estimated
Sum of As
estimated
As
estimated
0.10 A
0.03 A
0.05 A
1.18 A
0.04 B
0.50 B
0.08 B
0.01 B
0.07 B
0.02 B
As required
As required
0.72 B +
S.P. +
Bldg.
1.72 B +
S.P. +
Bldg.
0.10 B
0.20 B
0.10 B
0.01 B
0.01 B
0.03 B
0.45 B
2.17 B +
S.P. +
Bldg.
If ductwork dimensions have been established, cost may be estimated based on
p
$10-12/ft of surface for field application. Fan housings and stacks may also
be insulated.15
-------
5-43
5.4.1.1 Operating and Supervisory Labor
Typical operating labor requirements are 2 to 4 hours per shift for a
fa\
wide range of filter sizes.v ' Small or we11-performing units may require
less time, while very large or troublesome units may require more.
Supervisory labor is taken as 15% of operating labor.
5.4.1.2 Operating Materials
Operating materials are generally not required for baghouses. An
exception is the use of precoat materials injected on the inlet side of the
baghouse to provide a protective dust layer on the bags when sticky or
corrosive particles might harm them. Adsorbents may be similarly injected
when the baghouse is used for simultaneous particle and gas removal. Costs
for these materials should be included on a dollars-per-mass basis (e.g.,
dollars per ton).
5.4.1.3 Maintenance
Maintenance labor varies from 1 to 2 hours per shift.' ' As with
operating labor, these values may be reduced or exceeded depending on the size
and operating difficulty of a particular unit. Maintenance materials costs
/0\
are assumed to be equal to maintenance labor costs.v '
5.4.1.4 Replacement Parts
The major replacement part items are filter bags, which have a normal
operating life of 1 to 5 years with about 2 years being typical. The
following formula is used for computing the bag replacement cost:
CRCg = (CB -i- CL) x CRFg (5-14)
where:
CRCg = bag capital recovery cost ($/year)
C = initial bag cost including taxes and freight ($)
-------
5-44
CL = bag replacment labor ($)
CRFB = capital recovery factor whose value is a function of the annual
interest rate and the useful life of the bags. (For instance, for
a 10% interest rate and a 2-year life, CRFg = 0.5762.)
The bag replacement labor cost (C.) will depend on such factors as the
number, size, and type of the bags; their accessibility; how they are
connected to the baghouse tubesheet; etc. For example, in a reverse-air
baghouse it would probably take from 10 to 20 man-minutes to change an 8-in.
by 24-ft bag that is clamped in place. This bag has a filtering surface area
o
of approximately 50 ft . If the replacement labor rate were $21.12/h
2
(including overhead), C. would be from $0.07 to $0.14/ft of bag area. As
Table 5-6 shows, for some bags (e.g., cotton), this range of C. would
constitute a significant fraction of the purchase cost. For pulse jets,
replacement time would be about 5 to 10 man-minutes for a 5-in. by 10-ft bag
in a top-access baghouse. These bag replacement times are based on changing a
minimum of an entire module and on having typical baghouse designs. Times
would be significantly longer if only a few bags were being replaced or if the
design for bag attachment or access were atypical.
This method treats the bags as an investment that is amortized over the
useful life of the bags, while the rest of the control system is amortized
over its useful life (typically 20 years; see Section 5.4.2). Values of CRFD
D
for bag lives different from 2 years can be calculated from equation (2-3) of
the Manual.
5.4.1.5 Electricity
Power is required to operate system fans and cleaning equipment. Fan
power for primary gas movement can be calculated from equation (2-7) of the
Manual. After substituting into this equation a combined fan-motor efficiency
-------
5-45
(IB)
of 0.65 and a specific gravity of 1.000, we obtain:v '
P.P. = 0.000181(0) (AP)(0) (5-15)
where:
P.P. = fan power requirement (kWh/yr)
Q = system flow rate (acfm)
AP = system pressure drop (in. hLO)
Q = operating time (h/yr)
Cleaning energy for reverse-air systems can be calculated from the number
of compartments to be cleaned at one time (usually one, sometimes two), and
the reverse gas-to-cloth ratio (from about one to two times the forward gas-
to-cloth ratio). Reverse-air pressure drop varies up to 6 or 7 in. H^O
depending on location of the fan pickup (before or after the main system
(IQ\
fan).v ' The reverse-air fan generally runs continuously.
Typical energy consumption in kWh/yr for a shaker cleaning system
(2\
operated 8,760 h/yr can be calculated from:v '
P = 0.053A (5-16)
where:
2
A = gross fabric area (ft )
5.4.1.6 Fuel
If the baghouse or associated ductwork is heated to prevent condensation,
fuel costs should be calculated as required. These costs can be significant,
but may be difficult to predict. For methods of calculating heat transfer
requirements, see Perry.* '
5.4.1.7 Water
Cooling process gases to acceptable temperatures for fabrics being used
can be done by dilution with air, evaporation with water, or heat exchange
-------
5-46
with normal equipment. The last two cases require consumption of plant water,
although costs are not usually significant. Section 4.4 of the Manual
provides information on estimating cooling-water costs.
5.4.1.8 Compressed Air
Pulse-jet filters use compressed air at pressures of about 60 to
(2\
100 psig. Typical consumption is about 2 scfm/1,000 cfm of gas filtered.v '
For example, a unit filtering 20,000 cfm of gas uses about 40 scf of
compressed air for each minute the filter is operated.
5.4.1.9 Dust Disposal
If collected dust cannot be recycled or sold, it must be landfilled or
disposed of in some other manner. Disposal costs are site-specific, but they
may typically run $20 or $30 per ton exclusive of transportation (see Sec-
tion 2.4) of the Manual.
5.4.2 Indirect Annual Cost
These include such costs as capital recovery, property tax, insurance,
administrative costs ("G&A"), and overhead. The capital recovery cost is
based on the equipment lifetime and the annual interest rate employed. (See
Section 2 for a thorough discussion of the capital recovery cost and the
variables that determine it.) For fabric filters, the system lifetime varies
from 5 to 40 years, with 20 years being typical. However, this does not apply
to the bags, which usually have much shorter lives. (See Section 5.4.1.4)
Therefore, as Section 2 of the Manual suggests, when figuring the system
capital recovery cost, one should base it on the installed capital cost less
the cost of replacing the bags (i.e., the purchased cost of the bags plus the
cost of labor necessary to replace them). In other words;
CRCS = [TCI-CB-CL] CRFS (5-17)
-------
5-47
where:
CRCg = capital recovery cost for fabric filter system ($/yr)
TCI = total capital investment ($)
Co = initial cost of bags including taxes and freight ($)
C. = labor cost for replacing bags ($)
CRF = capital recovery factor for fabric filter system (defined in
s Section 2).
For example, for a 20-year system life and a 10% annual interest rate, the
CRFS would be 0.1175.
The suggested factor to use for property taxes, insurance, and
administrative charges is 4% of the TCI. Finally, the overhead is calculated
as 60% of the sum of operating, supervisory, and maintenance labor, and
maintenance materials.
5.4.3 Recovery Credits
For processes that can reuse the dust collected in the baghouse or that
can sell the dust in a local market, such as fly ash sold as an extender for
paving mixes, a credit should be taken. As used below, this credit (RC)
appears as a negative cost.
5.4.4 Total Annual Cost
Total annual cost for owning and operating a fabric filter system is the
sum of the components listed in Sections 5.4.1 through 5.4.3, i.e.:
TAG = DC + 1C - RC (5-18)
where:
TAC = total annual cost ($)
DC = direct annual cost ($)
1C = indirect annual cost ($)
RC = recovery credits (annual) ($)
-------
5-48
5.4.5 Example Problem
Assume a baghouse is required for controlling fly ash emissions from a
coal -fired boiler. The flue gas stream is 50,000 acfm at 325 *F and has an
ash loading of 4 gr/ft . Analysis of the ash shows a mass median diameter of
7 ^m. Assume the baghouse operates for 8,640 h/yr (360 d).
Design Gas-to-Cloth Ratio
The gas-to-cloth ratio (G/C) can be taken from Table 5-1 as 2.5, for
woven fabrics in shaker or reverse-air baghouses, or 5, for felts used in
pulse-jet baghouses. If a factor method were used for estimating G/C,
Table 5-3 for shakers would yield the following values: A = 2, B = 0.9, and
C = 1.0. The gas-to-cloth ratio would be:
2 x 0.9 x 1.0 = 1.8.
This value could also be used for reverse-air cleaning. For a pulse-jet unit,
Table 5-4 gives a value of 9.0 for factor A and 0.8 for factor B. Equation
(5-12) becomes:
V = 9.0 x 0.8 x 2.647 (275)~°*2335 x (0.7471 + 0.0853 In 7)
x 1.0873
= 4.69
Because this value is so much greater than the shaker/reverse-air G/C, we
conclude that the pulse-jet baghouse would be the least costly design.*
*This conclusion is based on the inference that a much higher G/C would
yield lower capital and, in turn, annual costs. However, to make a more
rigorous selection, we would need to calculate and compare the total annual
costs of all three baghouse designs (assuming all three are technically
acceptable}. The reader is invited to make this comparison. Further
discussion of the effects of G/C increases, and accompanying pressure drop
increases, on overall annual costs will be found in Reference 21.
-------
5-49
Assume the use of online cleaning in a common housing structure and, due to
the high operating temperature, the use of glass filter bags (see Table 5-5).
At a gas-to-cloth ratio of 4.69, the fabric required is:
50,000 acfm/4.69 fpm = 10,661 ft2.
Baqhouse Cost
From Figure 5-4, the cost of the baghouse ("common housing" design) is:
Cost = 9,688 + 5.552 (10,661) (5-20)
= $68,878
Insulation is required. The insulation add-on cost from Figure 5-4 is:
Cost = 1,428 + 0.931 (10,661) (5-21)
= $11,353
Bag and Cage Cost
From Table 5-6, bag costs are $1.24/ft for 5-1/8-in diameter glass
fiber, bottom removal bags. Total bag cost is:
10,661 ft2 x $1.24/ft2 = $13,220.
For 10-ft long cages, fabric area per cage = 5-1/8 in./12 in./ft x x x 10 ft =
13.42 ft The number of cages = II
Table 5-6, individual cage cost is:
Total cage cost is:
13.42 ft2' The number of cages = 10,661 ft2/13.42 ft2 = 795 cages. From
3.941 + 0.163 (13.42 ft2) = $6.128.
795 cages x $6.128/cage = $4,872.
Costs of Auxiliaries
Assume the following auxiliary costs have been estimated from data in
other parts of the Manual;
-------
5-50
Ductwork $16,000
Fan 16,000
Motor 7,500
Starter 4,000
Dampers 7,200
Screw conveyor 4,000
Stack 8,000
$62,700
Total Capital Investment
Direct costs for the fabric filter system, based on the factors in Table
5-7, are given in Table 5-8. (Again, we assume site preparation and buildings
costs to be negligible.) Total capital investment is $412,315.
Annual Costs—Bags
Table 5-9 gives the direct and indirect annual costs, as calculated from
the factors given in Section 5.4. For bag replacement labor, assume 10 min
per bag for each of the 795 bags. At a maintenance labor rate of $21.12
(including overhead), the labor cost is $2,809 for 133 h. The bags are
assumed to be replaced every 2 yr. The replacement cost is calculated using
equation (5-14).
Annual Costs—Pressure Drop
Pressure drop (for energy costs) can be calculated from equations (5-8)
through (5-10), with assumed values of 15 [in. H20/(ft/min)]/(lb/ft2) for KZ,
100 psig for P., and a cleaning interval of 10 min. We further assume that
J
the G/C (4.69 ft/min) is a good estimate of the mean face velocity over the
duration of the filtering cycle.
-------
5-51
Table 5-8 Example Costs.for Fabric Filter System
Purchased Equipment Costs
Fabric filter (with insulation) $ 80,231
Bags and cages 18,092
Auxiliary equipment 62,700
$161,023 = A
Instruments and controls, 0.1A 16,102
Taxes, 0.03A 4,831
Freight, 0.05A 8,051
Total purchased equipment cost $190,007 = B
Installation Direct Costs
Foundation and supports, 0.04B 7,600
Erection and handling, 0.50B 95,004
Electrical, 0.08B 15,201
Piping, 0.01B 1,900
Insulation for ductwork, 0.07B 13,300
Painting, 0.02B 3,800
Site preparation
Facilities and buildings —
Total installation direct costs $136,805
Total direct costs $326,812
Indirect Costs
Engineering and supervision, 0.10B 19,001
Construction and field expense, 0.20B 38,001
Construction fee, 0.10B 19,001
Startup fee, 0.01B 1,900
Performance test, 0.01B 1,900
Contingencies, 0.03B 5,700
Total indirect costs $85,503
Total capital investment $412,315
-------
5-52
Table 5-9 Example Annual Costs for Fabric Filter Systems
Direct Annual Costs
Operating labor
Operator, 6 h/day x 360 d/yr x $12/h = $25,920
Supervisor, 15% of operator = 3,888
Operating materials
Maintenance
Labor, 3 h/day x 360 d/yr x $13.20/h = 14,256
Material, equal to labor costs 14,256
Replacement parts, bags, [2,809 + (13,220 x 1.08*)] x 0.5762 = 9,845
Utilities
Electricity, 0.000181 x 50,000 acfm x 10.3 in. H~0
x 8,640 h/yr x $0.06/kWh = * 48,323
Compressed air (dried and filtered), 2 scfm/1,000 acfm
x 50,000 acfm x $0.16/1,000 scfm x 60 min/h x 8,640 h/yr = 8,294
Waste disposal, at $20/ton onsite for essentially 100%
collection efficiency:
1 1b y 50.000 ft3.. 60 min 8.640 h 1 ton $20 _
7,000 gr * mm * R * yr * 2,000 Ib * Ton "
Total direct annual costs $272,896
Indirect Annual Costs
Overhead, 0.6 x (25,920 + 3,888 + 14,256 + 14,256) = 34,992
Property tax, 0.01 x 412,315 = 4,123
Insurance, 0.01 x 412,315= 4,123
Administration, 0.02 x 412,315= 8,246
Capital recovery cost, (412,315 - 2,809 - 13,220 x 1.08) 46.439
x 0.1175
Total indirect annual costs 97,923
Total annual cost $371,000
(rounded)
*The "1.08" accounts for freight and sales tax on the bags.
-------
5-53
VJ = c.V0 = 4 gr/ft3 x 7 LI^ x 4.69 ft/min x 10 min = 0.0268 lb/ft2
o i /,uuu gr (5-22)
-0.65 -
AP = 6.08 x 4.69 ft/min x [100 psig] + 15 [In. H20/ft/min)]/(lb/ft^)
' x 0.0268 lb/ft2 x 4.69 ft/min = 3.32 in. H,0 across the fabric (when
fully loaded). i
Assume the baghouse structure and the ductwork contribute an additional 3 in.
H20 and 4 in. H^O, respectively. The total pressure drop is, therefore, 10.3
in.
Total Annual Cost
The total annual cost is $371,000, nearly half of which is for ash
disposal. If a market for the flyash could be found, the total annual cost
would be greatly reduced. For example, if $2/ton were received for the ash,
the total annual cost would drop to $208,000 ($370,819 - $148,114 - $14,811),
or 56% of the cost when no market exists. Clearly, the total annual cost is
extremely sensitve to the value chosen for the dust disposal cost in this
case. In this and in similar cases, this value should be selected with care.
5.4.6 Acknowledgment
We gratefully acknowledge the following companies for contributing data to
this section:
Aget
BACT
BHA
Dustex
Fuller
Griffin
W. W. Sly
Zurn
-------
5-54
References for Section 5
1. Turner, J. H., "Bag Filtration," in Handbook of Multiphase Systems, ed.
by G. Hetsroni, Hemisphere, 1982.
2. Turner, J. H., and J. D. McKenna, "Control of Particles by Filters," in
Handbook of Air Pollution Technology, ed. by S. Calvert and H. Englund,
New York:John Wiley & Sons, 19847
3. Donovan, R. P., Fabric Filtration For Combustion Sources, New York:
Marcel Dekker, Inc., 1985, p. 201.
4. Dennis, R., and H. A. Klemm, "Modeling Concepts for Pulse Jet
Filtration." JAPCA. 30(1), January 1980.
5. Leith, D. and M. J. Ellenbecker, "Theory for Pressure Drop in a Pulse Jet
Cleaned Fabric Filter." Atm. Environment. 14, 1980, pp. 845-852.
6. Koehler, J. L. and D. Leith, "Model Calibration for Pressure Drop in a
Pulse-Jet Cleaned Fabric Filter," Atm. Environment, 17(10), 1983, pp.
1909-1913.
7. Northrop Services, Inc. Fabric Filter Workshop Reference Materials. 1977
Workshop. Air Pollution Training Institute.
8. Vatavuk, W. M., and R. B. Neveril, "Estimating Costs of Air-Pollution
Control Systems, Part II: Factors for Estimating Capital and Operating
Costs," Chemical Engineering. November 3, 1980, pp. 157-162.
9. Frey, R. F., and T. V. Reinauer, "New Filter Rate Guide," Air
Engineering. 30 April 1964.
10. Dennis, R., et al. Filtration Model for Coal Fly Ash with Glass Fabrics,
August 1977 (EPA-600/7-77-084 [NTIS PB 276489JJ.
11. Owen, M. K. and A. S. Viner, Microcomputer Programs for Particulate
Control, June 1985 (EPA-600/8-85-025a).
12. Dennis, R. and H. A. Klemm. Fabric Filter Model Change; Vol. I. Detailed
Technical Report. February 1979 (EPA-600/7-79-043a (NlIS PB 293551JJ.
13. Viner, A. S., et al. "Comparison of Baghouse Test Results with the
GCA/EPA Design Model." JAPCA. Vol. 34(8), August 1984.
14. Reigel, S. A. and R. P. Bundy. "Why the Swing to Baghouses?" Power.
121-1, pp. 68-73, January 1977 (modified).
15. ETS, Inc., Suite C-103, 3140 Chaparral Dr. SW, Roanoke, VA 24018-4394.
16. Fuller Company, P.O. Box 2040, Bethlehem, PA 18001.
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17. R. S. Means Company, Inc., Means Square Foot Costs 1986, Kingston, MA.
18. Perry, Robert H., et al., Perry's Chemical Engineers' Handbook (Fourth
Edition), New York: McGraw-Hill, 1963, p. 6-20.
19. Personal communication from Gary Greiner, ETS, Inc., to Jim Turner,
Research Triangle Institute, October 24, 1986.
20. Perry, Robert H., et al.. Perry's Chemical Engineers' Handbook (Sixth
Edition), New York: McGraw-Hill, 1984.
21. McKenna, J. D., J. H. Turner, D. Furlong, and D. S. Beachler, Fabric
Filters-Baghouses, I. Theory, Design and Selection, in preparation, ETS,
Inc., Suite C-103, 3140 Chaparral Dr. 5W, Roanoke, VA 24018-4394.
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