EPA-660/2 73-026
NOVEMBER 1973
Environmental Protection Technology Series
Technical and Economic
Evaluations of Cooling Systems
Slowdown Control Techniques
Office of Research and Development
U.S. Environmental Protection Agency
Washington, D.C. 20460
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RESEARCH REPORTING SERIES
Research reports of the Office of Research and Development,
U.S. Environmental Protection Agency, have been grouped into
five series. These five broad categories were established to
facilitate further development and application of environmental
technology. Elimination of traditional grouping was consciously
planned to foster technology transfer and a maximum interface
in related fields. The five series are:
1. Environmental Health Effects Research
2. Environmental Protection Technology
3. Ecological Research
4. Environmental Monitoring
5. Socioeconomic Environmental Studies
This report has been assigned to the ENVIRONMENTAL PROTECTION
TECHNOLOGY STUDIES series. This series describes research performed
to develop and demonstrate instrumentation, equipment and methodology
to repair or prevent environmental degradation from point and
non-point sources of pollution. This work provides the new or
improved technology required for the control and treatment of
pollution sources to meet environmental quality standards.
EPA REVIEW NOTICE
This report has been reviewed by the Office of Research and
Development, U.S. Environmental Protection Agency, and approved
for publication. Approval does not signify that the contents
necessarily reflect the views and policies of the U.S. Environmental
Protection Agency, nor does mention of trade names or comnerical
products constitute endorsement or recommendation for use.
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EPA-660/2-73-026
November 1973
TECHNICAL AND ECONOMIC
EVALUATIONS OF COOLING SYSTEMS
SLOWDOWN CONTROL TECHNIQUES
By
David B. Boies
James E. Levin
Bernard Baratz
Contract No. 68-03-0233
Program Element 1BB392
Project Officer
Guy R. Nelson
Pacific Northwest Environmental Research Laboratory
National Environmental Research Center
200 S.W. 35th Street
Corvallis, Oregon 97330
Prepared For
Office of Research and Development
U.S. Environmental Protection Agency
Washington, D.C. 20460
For sale by the Superintendent of Documents, U.S. Government Printing Office, Washington, D.C. 20402 - Price $1.20
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ABSTRACT
This report presents descriptions of methods which are either currently
applied or commercially available to reduce the pollution impact of blow-
down streams from large cooling systems such as those found in the new
1000 mw scale steam-electric power plants.
Treatment equipment descriptions, capabilities and compatibilities are dis-
cussed. Where appropriate, broad ranges for both capital costs and oper-
ating expenses for the various treatment methods are provided, usually as
a function of power plant output capacity.
Methods described include the application and design of cooling systems
(such as evaporative cooling towers, dry towers, cooling lakes, spray pcnds),
makeup water treatment, recirculating water chemical treatment, on-line con-
denser tube cleaning, and blowdown treatment and/or disposal. Automatic
control of overall cooling system operation is also covered.
The most common practices found for reducing the pollution impact of blow-
down streams involve using corrosion resistant materials in the condensers
and piping, minimizing the chemicals added to the recirculating water, and
controlling the cycles of concentration in the recirculating water to
minimize scaling. This approach has minimal environmental impact. Few
large systems use zinc or chromate corrosion inhibitors to treat recircu-
lating water, and the use of low toxicity scale preventive chemical treat-
ment programs appears to be growing, particularly where water supply avail-
ability is a problem or where zero discharge of blowdown must be practiced.
This report was submitted in fulfillment of Contract No. 68-03-0233 by
WAPORA, Inc. under the sponsorship of the Environmental Protection Agency.
Work was completed as of July 1973.
ii
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TABLE OF CONTENTS
ABSTRACT
LIST OF TABLES
LIST OF FIGURES
ACKNOWLEDGMENTS vii
SECTIONS
I. CONCLUSIONS AND RECOMMENDATIONS 1
II. INTRODUCTION 4
III. IMPACT OF THE CHOICE OF COOLING SYSTEM ON BLOWDOWN
CONTROL 9
- Introduction 9
- Mechanical Draft Evaporative Cooling Towers 9
- Natural Draft Evaporative Cooling Towers 14
- Natural Draft or Mechanical Draft Dry Cooling
Towers 17
- Hybrid Cooling Towers 20
- Cooling Lakes 22
- Spray Canals and Ponds 24
- New Cooling System Developments 25
IV. THE USE OF CORROSION RESISTANT CONSTRUCTION
MATERIALS 30
- Introduction 30
- Corrosion Resistant Materials in Cooling Towers 30
- Corrosion Resistant Materials in Recirculating
Systems 31
- Corrosion Resistant Materials in Condensers 31
- Corrosion Resistant Materials in Spray Coolers 31
- Resistant Pretreatments and Coatings 31
V. MAKEUP AND RECIRCULATING WATER TREATMENT 33
- Introduction 33
- Influent Treatment Methods 37
- Physical Treatment of Condenser Tubes 45
VI. PROCESS CONTROL 53
- Introduction 53
- Makeup Control 53
- pH Adjustment 54
- Residual Chlorine 54
- Temperature Control 55
- Corrosion Detection 55
- Chemical Additions 55
iii
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TABLE OF CONTENTS
(Continued)
VII. SLOWDOWN TREATMENT AND REUSE 57
- Introduction 57
- Discharge Methods 57
- Treatment Methods 60
- Zero Slowdown Treatment and Reuse 61
- Developments in Slowdown Handling Technology 63
VIII. EUROPEAN PRACTICES 68
- Cooling Tower Types 68
- Feedwater Pretreatment 68
- In-Towers Treatment 68
- Slowdown Treatment 69
- Other Problems 70
- Unique Facilities 71
IV
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No.
1. Projection of Power Generating Capacity 5
2. Projection of Large New Power Generating Sites 5
3. Capital and Operating Costs for Alternative Cooling
Systems 15
4. Capital Costs of Selected Cooling Ponds 23
5. Effect of Cycles of Concentration on Cooling System
Makeup Rate for a 1,000 mw Power Station 34
6. Effect of Cycles of Concentration on Heat Discharge
Rate from a 1,000 raw Power Station 35
7. Annual Chemical Use of Selected Steam Electric Power
Stations, Operating Cooling Ponds and Lakes 40
8. Typical Sponge Rubber Ball Tube Cleaning System Costs 49
9. Capital and Operating Costs for On-Line Tube Cleaning
Equipment 50
10. Effect of Cycles of Concentration on Cooling Tower
Effluents - Diffusion System Costs 59
11. Slowdown Treatment System Costs 62
v
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LIST OF FIGURES
No. Page
1. Mechanical Draft Evaporative Cooling Tower 10
2. Cooling Tower Use in Once Through Cooling Systems 12
3. Cooling Tower Use in Recirculating Cooling Systems 13
4. Natural Draft Evaporative Cooling Tower 16
5. Dry Cooling Tower Design for Indirect Cooling 18
6. Dry Cooling Tower Design for Direct Cooling 19
7. Wet/Dry Cooling Tower 21
8. Disinfecting Efficiency of Chlorine 44
9. Sponge Rubber Ball Tube Cleaning Process 47
10. On-Line Condenser Tube Cleaning With Plastic Brushes 48
11. Multiport Diffusion System for Large Lakes 58
12. Slowdown Evaporation System Flow Diagram 65
13. Cable Tower 72
vi
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ACKNOWLEDGMENTS
The authors appreciate the assistance of the following WAPORA, Inc. staff
members on this project:
J. I. Bregman
Henry Handler
Stephen Megregian
Patricia Thompson
In addition, the authors extend their thanks to the following people who
have made significant contributions to the content of this report:
T. Ashton
L. Baldwin
J. Bechthold
G. Bingaman
R. Britt
R. Brough
R. Grouse
H. Crutchfield
R. Cunningham
J. Delgado
J. Donohue
R. Drake
G. Eicher
K. Fredericks
A. Freedman
E. Galloway
R. Garner
E. Goldman
R. Goss
R. Hicks
R. Higdon
M. Hiscox
P. Hoffmeir
R. Hopkins
P. Kelleher
W. Kern
G. Kimmons
0. Kirchner
K. Ladd
R. Landon
F. McGilbra
J. Meakim
R. McGinnis
J. McNey
R. Menke
Pacific Power & Light Company
Pennsylvania Electric Company
Northern States Power Company
Wallace & Tiernan Division-Pennwalt
Corporation
Southern California Edison Company
Hercules Inc.
Toledo Edison Company
Utah Power and Light Company
Zimmite Corporation
City Public Service Board
San Antonio, Texas
Betz Laboratories, Inc.
Amertap Corporation
Portland General Electric Company
Metropolitan Edison Company
Nalco Chemical Company
Cincinnati Gas & Electric Company
Arizona Public Service Company
Bechtel Corporation
Ecodyne Corporation
Consumers Power Company
Permutit Company
Utah Power and Light Company
Cincinnati Gas & Electric Company
GPU Electric Power Companies
Bechtel Corporation
Amertap Corporation
Tennessee Valley Authority
Resources Conservation Company
Southwestern Public Service Company
Marley Company
Public Service Company of Oklahoma
Betz Laboratories, Inc.
Northern States Power Company
Southern California Edison Company
Dearborn Chemical Division-CHEMED
Corporation
vii
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NAME AFFILIATION
E. Myers Pacific Gas & Electric Company
J. Nelson Marley Company
M. Pappenfus Northern States Power Company
J. Parmley Public Service Company of Oklahoma
M. Pitts Taulman Company
R. Poppe Northern States Power Company
P. Puckorius Zimmite Corporation
J. Reynolds Consumers Power Company
D. Schnitz City Public Service Board-
San Antonio, Texas
J. Schaub Consumers Power Company
G. Schweitzer Calgon Corporation
A. Sierra Cities Service Oil Company
J. Walko Betz Laboratories, Inc.
J. Wegscheider American M.A.N. Corporation
A. Williford Texas Electric Service Corporation
viii
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SECTION I
CONCLUSIONS AND RECOMMENDATIONS
The results of this study have indicated that in practice or theory, there
are techniques available to reduce the environmental impact of practically
any cooling system problem which may be encountered. As the complexity of
the techniques increases, and as zero blowdown is approached, costs tend to
increase. Large cooling system designs should be based on minimum environ-
mental impact and economic optimization of cost factors such as water supply
availability, fuel, transportation, power transmission and alternative plant
locations in more environmentally favorable areas.
The general investigation of current practice and available technology led to
the following conclusions:
1. The method of operation of cooling tower systems which probably
provides the least environmental impact consists of the following:
a. Use corrosion-resistant materials of construction so
that the need for corrosion inhibitors in the recir-
culating water is either eliminated or minimized.
b. Reduce the amount of chemical biocide required to
control microbiological growth by the use of mecha-
nical devices to keep condenser tubes clean and by
other practices outlined in the report.
c. Provide a settling basin for blowdown before discharge
to reduce the suspended solids content, residual
chlorine, and heat. As an alternative, where conser-
vation of land is required, a clarifier and/or small
mechanical-draft cooling tower can be used to reduce
suspended solids and heat as required.
,'
d. Where corrosion inhibitors are necessary because of
high dissolved solids content in the recirculating
water, high temperatures, non-resistant materials of
concentration, or other factors, the corrosion inhibi-
tion system must be chosen with a regard for environ-
mental factors. Because of its toxicity and the
promulgation of effluent standards, chromate must
be removed from the blowdown before discharge;
methods for its removal and/or recovery are rela-
tively expensive, so chromate is not being normally
used.
Alternative methods for corrosion protection usually
involve operation at a higher pH to reduce corrosivity
and the use of inhibitors based on combinations of
phosphorus compounds, zinc, and organic materials.
The higher pH operation requires the use of an organic
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dispersant and/or mechanical tube cleaning devices to
control, and are 2-3 times more expensive than opera-
tion at a lower pH with chromate, considering chemical
costs alone. However, when the costs of chromate re-
moval from the blowdown must be considered, the higher
pH operation usually becomes more economically attractive.
e. Operation at higher recirculating water concentrations
may make treatment of makeup necessary in order to con-
trol suspended solids and scaling in the tower. Settling
or lime-softening of makeup water is being practiced in
certain areas.
2. Chlorination is the primary microbiological control measure;
chlorine is normally fed on a slug basis for several hours at
a time to produce a given level of free chlorine. Invervals
between chlorine feedings range from 8 hours to two weeks.
Non-oxidizing biocides are seldom used, and are applied
primarily as a supplement to chlorination, usually slug-fed
on a monthly basis during warm weather.
The presence of residual chlorine in the blowdown is of
ecological concern. The methods being used to reduce this
at the present time include controls to reduce excess chlo-
rination and settling ponds to allow dissipation before dis-
charge. In addition, chemical reduction of free chlorine with
sulfur dioxide or sulfite compounds has been proposed for use
in at least one new power station.
3. Various water reuse schemes are being practiced which result
in reduced environmental impact by lessening the amount of
water which must be withdrawn from the supply.
a. In coal-fired plants, the use of the blowdown
for ash-transport water is frequently practiced.
This is useful not only to reduce total water
demand, but also retention in the ash pond re-
duces the suspended solids and chlorine residual.
b. Proposed schemes to use blowdown for stack gas
scrubbing provide a reuse function.
c. The use of sewage treatment plant effluent for
cooling tower makeup provides for water reuse.
This practice also has been found to act as a
tertiary treatment for the sewage.
4. The announced environmental goal of zero discharge is being
met by certain cooling systems in the following way:
a. Dry cooling towers are in operation here, in
Germany and other foreign countries, but none
serve power plants sized for over 350mw. Dry
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cooling appears to be practical only in areas of
extreme water shortage.
Ion exchange demineralization of feed has been
practiced in units up to 150mw in size in Germany;
again this practice is only practical where plant
location factors outweigh the added water treat-
ment costs.
Lime-soda softening of the blowdown, where the
makeup water contains primarily calcium bicar-
bonate, with subsequent return to the tower has
allowed one system to operate with zero blowdown.
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SECTION II
INTRODUCTION
A great concern in the United States has developed over the past several
years about the impact of waste heat on the aquatic environment. Discharges
of heated water can contribute to physical and biological changes in a re-
ceiving stream which can be beneficial, detrimental or insignificant depend-
ing on the ecology of the particular water body and its desired uses.l
Guided by the 1972 Amendments to the Federal Water Pollution Control Act,
Federal, regional, and state pollution control agencies have imposed thermal
standards on discharge streams and receiving water bodies. There are four
types of standards which have been applied. The first is simply the imposi-
tion of absolute maximum temperature levels. The second involves an allow-
able temperature increase of the receiving body above the temperature which
it would have without the addition of heat from a discharge stream. A less
frequently used standard relates temperature rise to time and allows gradual
increases in temperature up to a point. For example, a standard may allow
an increase of 0.6°C (1.0°F) per hour over a twelve hour period each day,
but with a total temperature increase not to exceed 2.8" - 3.9°C (5.0° -
7.0°F). The final standard implements the concept of a mixing zone, a
relatively small area where heated water is allowed to discharge directly
into the receiving water body. Thermal standards are then imposed to the
receiving water outside the mixing zone.
Other discharge and receiving water quality standards have been imposed in
addition to temperature. Suspended solids, dissolved solids, metal ions
and organic contaminants (usually as measured by the five day biochemical
oxygen demand test, 6005) are some of the more common parameters regulated
by pollution control agencies. In addition, there are currently a few
streams like the Colorado River, into which it is unlawful to discharge
any water, regardless of quality or temperature.
Cooling systems are being built by industry in an effort to meet these
standards. This is particularly true in the power industry where roughly
two-thirds of the energy supplied by fuel systems must be dissipated as
waste heat to the environment. The growth of the power industry as pro-
jected in Table 1, and the installation of new power stations, as shown
in Table 2 (many of which have capacities of 1000 megawatts [mw] or more),
will require the use of relatively large cooling systems. For example,
there are over eighty cooling towers and more than forty cooling lakes in
use or under construction which have recirculating capacities of more than
51,000 cubic meters (m3) per hour (500 ft3/sec or 224,000 gpm).
There are several types of cooling systems that can be applied to large
power plants. The best system for a given plant will depend upon the
following factors:
1. Water supply availability
2. Effluent limitations or receiving water quality standards
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Table 1. PROJECTION OF POWER GENERATING CAPACITY3
1970 1980 1990
Gigawatts3 Percent Gigawatts Percent Gigawatts Percent
Fossil Steam 261 76 396 59 559 44
Nuclear 12 3 147 22 500 40
Conventional Hydro 51 15 68 10 82 6
Internal Combustion
& Gas Turbine 16 5 30 5 50 4
Pumped Storage Hydro 4 __1 27 4 70 6
TOTAL 344 100 668 100 1261 100
a. One gigawatt is equivalent to 1,000 megawatts or 1,000,000 kilowatts.
Table 2. PROJECTION OF LARGE NEW POWER GENERATING SITES4
1971 - 1980 1981 - 1990 Total (1971 - 1990)
Fossil Fueled Steam3 95 40 135
Nuclear Fueled Steam3 75 90 165
TOTAL 170 130 300
a. Each plant has a minimum generation capacity of 500 raw.
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3. Climate conditions
4. Land availability and cost.
The types of systems commercially available are as follows:
1. Evaporating cooling towers
2. Dry cooling towers (closed systems)
3. Hybrid cooling towers (a combination of wet and dry)
4. Cooling lakes and ponds
5. Spray ponds and canals.
However, the use of these cooling devices does not offer a panacea to pollu-
tion problems. Chemicals added to the cooling systems for corrosion, scal-
ing or slime control may be present in the blowdown streams and have harmful
effects on aquatic life. Fogging caused by some cooling devices, cooling
tower plumes, and the poor aesthetic aspects of having large cooling towers
marring the landscape are some other problems that can be encountered.
A study has been made to determine the new methods which are in use or are
being designed to reduce the pollution characteristics of blowdown streams
from cooling systems having recirculation rates of 51,000 m^/hr (224,000 gpm)
or more. The physical characteristics, capabilities, compatabilities and
costs associated with each pollution abatement measure were assessed. The
project goal was to prepare an up-to-date report that could be used by rep-
resentatives of industry and regulatory agencies to readily compare the
general technical aspects, economic considerations and environmental impli-
cations of the various methods of blowdown stream pollution abatement.
However, to provide a more meaningful document, it has been necessary to
also consider blowdown control measures which appear to be commercially
available for use but which have not as yet been implemented.
The program involved studies in the following areas:
1. Types of cooling systems available and their blowdown
characteristics
2. Use of corrosion-resistant cooling system construction
materials to minimize or eliminate need for commercial
treatment
3. Ways to increase cycles of concentration to reduce blowdown
rate through makeup water treatment, sidestream treatment,
and recirculating water chemical and physical treatment.
4. Automatic control of cooling systems, blowdown, and related
operations
5. Methods of blowdown water treatment and processes for
recycle.
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Information was collected from three basic sources:
1. A survey was made of applicable journal articles, reports,
books and conference proceedings.
2. Contacts and visits were made with users of cooling systems
in America and Europe to determine actual practices.
3. Contacts and visits were made with cooling tower manufacturers,
water treatment chemical suppliers, water treatment equipment
suppliers and instrumentation and controls vendors.
The economics data accumulated were separated into capital cost and operating
cost catagories. To allow crude comparisons of processes, capital costs
were annualized at a rate of 15 percent per year and then added to operating
costs to obtain a total annualifced operating cost.
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REFERENCES
1. Problems in Disposal of Waste Heat from Steam Electric Plants.
Federal Power Commission. Bureau of Power. 1969. 53 p.
2. Bregman, J. I. Trends in Thermal Pollution Control Requirements.
WAPORA, Inc. (Presented at Cooling Tower Institute Semi-Annual
Meeting. Aspen. June 22-24, 1970.) 8 p.
3. Warren, F. H. Electric Power and Thermal Output. In: Electric
Power and Thermal Discharges, Eisenbud, M., and G. Gleason, (eds.).
New York, Gordon and Breach, 1969, p. 21-46.
4. Jimeson, R. M., and G. G. Adkins. Waste Heat Disposal in Power
Plants. Chemical Engineering Progress. 67: 64-69, July 1971.
8
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SECTION III
IMPACT OF THE CHOICE OF
COOLING SYSTEM ON BLOWDOWN CONTROL
INTRODUCTION
The choice of the cooling system to be used in a power plant will have signi-
ficant effect on the rate and composition of the blowdown stream. At the
present time, mechanical draft and natural draft evaporative cooling towers,
cooling lakes (or ponds - the two being used interchangably), and spray ponds
and canals are being used to cool large power plants. But in addition,
the technology apparently exists to also build large scale dry cooling towers
and hybrid (wet/dry) cooling towers.
The basic characteristics of each of these cooling methods will be briefly
discussed below. More detailed technical discussions and comparisons of
cooling systems, particularly cooling towers, are available in the
literature.l"13
MECHANICAL DRAFT EVAPORATIVE
COOLING TOWERS
This type of cooling system is by far the most popular method used for cool-
ing large steam-electric power plants. Several wet mechanical draft towers
with recirculation rates exceeding 51,000 m^/hr (224,000 gpm) have also been
constructed for the petroleum refining and gaseous diffusion industries. A
sketch of a mechanical (induced) draft cooling tower is shown in Figure 1.
In this type of a tower, heat transfer is accomplished mainly by the evapo-
ration of 1-3 percent of the recirculating water. This heat loss effectively
cools the water by 5° - 15°C (10 - 30°F). A mechanical draft tower usually
consists of a redwood, concrete or fir structure packed with wood or plastic
fill over which warm water from the condensers falls forming small droplets
which enhances heat transfer efficiency. Of the two major types of pack-
aging available, splash packing is the more prevalent. Film packing, which
causes the falling water to form thin closely spaced sheets, is usually
found in smaller towers. Air guiding louvers are generally made from plas-
tic or asbestos concrete board (ACB). Air is induced through the tower and
past the falling water by large fans mounted on top of the structure. There
are two basic methods for air and water contact: crossflow and counterflow.
Most large mechanical draft towers have a crossflow arrangement where the
flow of air is perpendicular to the flow of the water. Lower air pressure
drops are obtained when the crossflow arrangement is used, thus minimizing
the fan horsepower requirements. In the counterflow arrangement, the air
flows directly past the falling water, providing the most efficient arrange-
ment in terms of heat transfer since the coolest water contacts the coolest
air.
Drift eliminators are installed beyond the tower packing to prevent excessive
loss of water droplets through the fans and out the tower with the air and
water vapor in the plume. Drift losses in towers can normally be maintained
below 0.008 percent of the recirculating water flow and recently towers have
been built having guaranteed drift losses less than 0.002 percent. Despite
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Figure 1. Mechanical (Induced) Draft Crossflow Cooling Tower. (Sketch
provided by Northern States Power Company; Minneapolis,
Minnesota)
10
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these improvements to drift losses, special care must be taken to locate
towers where their plumes will not cause ground fogging or icing conditions.
Cooling towers are normally used in one of two ways to dissipate heat from
a power plant. The first way, shown in Figure 2, is simply to cool hot
water from condensers before it is discharged to a river or lake. Several
large power plants use cooling towers in this manner. The second method
involves cooling hot water in the tower, then recirculating cool water from
the tower basin back to the condensers. This approach, which is the most
popular, is shown in Figure 3. The six major advantages of mechanical
draft evaporative towers, as:cited by Oleson and Boyle,13 are:
1. Positive control over air movement
2. Close control over cold water temperature
3. Relatively low pumping head
4. Ambient air humidity has little effect on tower performance
5. More available fill per unit volume of tower
6. Lower capital costs than a natural draft tower.
Inherent disadvantages of this type of tower include:
1. May have a blowdown disposal problem
2. Subject to mechanical failure of the fans
3. Possible recirculation of humid exhaust air back through
the tower
4. Higher maintenance and operating costs than with natural
draft towers. Tower fans consume up to 0.8 percent of
power generator capacity
5. Exhaust air can cause icing and fogging problems
6. Possible mixing of vapor and stack gases outside the
tower can be troublesome
7. Dissolved solids in the drift losses may violate air
pollution codes.
Several sources have estimated the overall costs of mechanical draft evapo-
rative cooling tower systems.3,5,6,8,11,12,13-16
Capital costs will vary depending on many design criteria, including design
wet bulb temperature, temperature range, temperature approach, relative
humidity, land and construction costs. However, most of the estimates
show that the cost of a cooling tower, basin, pumps, fans, recirculating
water conduit and makeup system will be $6.5 - $8.6/kw or $1,600 - $2,200
11
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Figure 2. Cooling Tower Use in Once Through Cooling System. (Sketch
provided by Northern States Power Company; Minneapolis,
Minnesota)
PUN7
V V
TOWER Loss
RIVER
FLOW
7
12
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Figure 3. Cooling Tower Use in Recirculating Cooling Systems, (Sketch
provided by Northern States Power Company; Minneapolis,
Minnesota)
MAKE-PI
PLANT
COOLIKJG
v v v
TOWER LOSS
UT E%% OF FLOW;
&LOW-
DOV^M
IL'-'itfj S W JlffKiTj*C3ftl
PLOW
RIVER
FLOW
13
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per million BTU/hr of heat rejected through the tower. Operating costs for
power usage, maintenance, labor and capital charges at 15 percent/yr will be
.3 - .4 mills/kwh. These costs are compared with those of other cooling
methods in Table 3.
NATURAL DRAFT EVAPORATIVE
COOLING TOWERS
Although natural draft towers have been used for years in Europe, they were
not applied in this country until 1962. Like the mechanical draft towers
discussed above, the wet natural draft towers rely on water evaporation for
cooling effect. However, fans are not used to induce air through the tower.
Instead, the tower is designed so that air will naturally flow through the
tower fill as a result of: (1) density differences between ambient air and
warm moist air inside the tower, and (2) the chimney effect of the tower's
tall structure. Natural draft towers are often selected over mechanical
draft towers in areas where low wet bulb temperatures and high relative
humidities prevail. These conditions are characteristic of the northeastern
United States. Power1 notes that natural draft towers can also be attractive;
when: (1) a combination of broad range and long cooling approach exists,
(2) heavy winter heat load is possible, and (3) a long amortization period
can be arranged. Natural draft (or hyperbolic) towers can have heights and
diameters as large as 500 ft. A sketch of this type of tower is shown in
Figure 4.
Hyperbolic towers are generally constructed of concrete to avoid fire hazards.
Either crossflow or counterflow air-water contact arrangements are available,
but crossflow is probably the most prevalent. Plastic (usually polyvinyl
chloride), ACB or treated wood splash-type fill can be supplied along with
the ACB louvers (for crossflow towers) .
Major advantages of hyperbolic towers are:
1. No mechanical or electrical components - thus lower
operating and maintenance costs than a mechanical
draft tower
2. Smaller land areas generally required than for mechanical
draft towers
3. Relatively large water loading capacities.
The main disadvantages are:
1. Possible blowdown disposal problems
2. Higher capital cost than mechanical draft tower
3. Internal resistance to air flow must be minimal
4. Great tower height required to produce draft
14
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Table 3 . CAPITAL AND OPERATING COSTS FOR ALTERNATIVE
COOLING SYSTEMS0» d
COOLING SYSTEM
Mechanical draft
evaporative towers
Natural draft
evaporative towers
Mechanical draft
dry towers"
Wet/dry evaporative
towers
Cooling lakes
Sptay ponds or canals
CAPITAL COSTS3
$/106BTU/hr.
Rejected
1,600 - 2,200
2,600 - 3,400
5,000 - 7,500
3,000 - 4,500
2,000 - 2,700
2,500 - 4,200
$/kw
6.5 - 8.6
10.5 - 13.5
20 - 30
12 - 18
ANNUALIZED CAPITAL
Mills/kwh
.11 - .15
.18 - .23
.34 - .51
.14 - .26
8-11 ! .14 - .19
10-17 .17 - -29
OPERATING AND MAINTENANCE COSTS
$/Yr-10'3 BTU/hr
Rejected
440 - 550
285 - 350
440 - 880
307 - 350
20 - 200
220 - 260
Mills/kwh
.20 - .25
.13 - .16
.20 - .40
.14 - .16
.01 - .09
.10 - .12
TOTAL ANNUAL COSTS
Mills/kwh
.31 - .40
.31 - .39
.54 - .91
.28 - .42
.15 - .28
.27 - .41
a. Capital costs include make-up water system, towers, pumps, piping, controls, dams, dikes, ponds, and
condensers. Plant capacity losses and interest during construction are excluded. Operating and maintenance
costs include allowances for labor, power and water costs, but not chemicals.
b. Estimated costs. No large towers yet in operation.
c. Costs suited to fossil -fueled steam electric power plant having capacities in the range of 1,000 mw. To
roughly estimate costs for nuclear-fueled units, multiply $/kw and mills/kwh by 1.5.
d. Power plant operation assumed at 8,760hrs/yr.
-------
Figure 4. Natural Draft Evaporative Counterflow Cooling Tower. (Sketch
from Industrial Waste Guide on Thermal Pollution by FWPCA -
Northwest Region, Pacific Northwest Water Laboratory)
DRIFT
HOT WATER /ELIMINATOR
DISTRIBUTION' *
N
FILL
-^s^:v^I^^
COLD WATER
BASIN
16
-------
6. Exact control of cold water temperature difficult
7. Possible mixing of vapor and stack gases outside the
tower can be troublesome.
Cost estimates for natural draft towers are also plentiful.3,5,6,8,11,12,13-16
Investment costs for hyperbolic systems will vary depending on climate con-
ditions, range, approach, land and construction costs, but should be in a
range of $2,600 - $3,400 per million BTU/hr of heat rejected, or $10.5 -
$13.5/kw. These costs include a tower, basin, pumps, recirculating water
system conduit, and makeup system. Operating and maintenance costs are
.13 - .16 mills/kwh. These costs are summarized in Table 3.
NATURAL DRAFT OR MECHANICAL
DRAFT DRY COOLING TOWERS
Despite all the research being done on dry cooling towers, no large units
have as yet been built for power plants in this country. A few dry towers
have been built in Europe, but none are in existence in the 51,000 m^/hr
recirculation rate range being considered in this report. A dry tower is
now beginning construction for a 340 mw power plant in the Black Hills.
In a dry tower, air is drawn over banks of tubes fitted with fins to increase
the effective heat transfer area. Since only sensible heat transfer is in-
volved in this type of operation, it is not nearly as efficient as the eva-
porative cooling methods. Thus, costs for dry towers are substantially
higher than for evaporative or wet systems.
Two processes can be used with a dry tower. The Heller, or indirect type,
is shown in Figure 5. Exhaust steam from the turbine is condensed in a
barometric or direct-contact condenser. Part of the condensate is returned
to the feedwater circuit while the remainder is sent to cooling tubes in
the cooling tower.
Figure 6 illustrates the direct type of dry cooling. Turbine exhaust steam
is piped directly to the cooling tower where it is condensed in the tubes.
Condensate is then sent back to the boiler or nuclear reactor.
Dry towers have several environmental advantages:13
1. No blowdown problem
2. No icing or fogging
3. Water availability near the proposed plant site not a
problem.
However, the reasons why dry towers have not been built to date for large
power plants are evident from the following disadvantages:
1. Capital costs significantly higher than for evaporative
towers
2. High maintenance costs
17
-------
Figure 5. Dry Cooling System Design For Indirect Cooling. (From Rossie
& Cecil. Research on Dry-Type Cooling Towers for Thermal
Electric Generation: Part I19)
STEAM
TURBINE
}:::
NATURAL-
DRAFT TOWER
... COOLING COILS
:;;i
EXHAUST
STEAM
DIRECT-CONTACT
.CONDENSER
CIRCULATING PUMP
MOTOR
WATER RECOVERY
TURBINE
•«3-
o
CIRCULATING
WATER PUMP
£».
TO BOILER
FEEDWATER
CIRCUIT
18
-------
Tl
STEAM
HEADER
COOLING
COILS
CONDENSATE
HEADER
TO BOILER
FEEDWATER
CIRCUIT
EXHAUST STEAM
CONDENSATE
HEADER
EXHAUST-
STEAM
TRUNK
STEAM .
TURBINE
CONDENSATE
RECEIVER
CONDENSATE
PUMP
EXHAUST
STEAM
CO
05
rt O O
1 ft 4
H- O <<
n H-
M n
o • o
o> o
3 M
(0 » H-
4 (D P
r+ fl>
H- p CQ
O H "<
3 O 01
•• S1 rt-
CD
O S
4 a
a
TO
01
COV?
o
o a
o H-
0 O
*• o
m M
*•< H-
en 3
m
HI •
o
(D O
3
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en
W 01
l_i M.
0) tt)
o
I R°
-------
3. Huge volumes of air must be circulated
4. Decreased power plant output at high dry bulb temperature
5. Larger land area requirements than for evaporative towers.
Since no dry towers have been built for any large power stations, capital and
operating costs can only be roughly estimated. Previous studies^* H» 20
indicate that dry towers will cost $20 - $30/kw or $5,000 - $7,500 per million
BTU/hr heat rejected. Operating and maintenance costs are projected at
$440 - $880 per year per million BTU/hr or .20 - .40 mills/kwh. Assuming
annual capitalization of 15 percent, annual coats will total .54 - .91 mills/
kwh.
HYBRID COOLING TOWERS
Cooling tower suppliers have recently developed a hybrid between the evapora-
tive and dry mechanical draft cooling towers.1»23-25 Although the largest
hybrid, or wet/dry, cooling tower built thusfar has only a 180,000 gpm recir-
culation rate, the technology apparently exists to build these towers in
much larger sizes. Although these towers can be built for practically any
ratio of wet-to-dry cooling, towers typically have been built to-date with
the following heat transfer loads:
Percent Evaporative Cooling Percent Dry Cooling
Summer Conditions 95 5
Winter Conditions 80 20
This type of tower, as shown in Figure 7, combines many of the advantages of
wet and dry towers as indicated below:
1. Reduced blowdown rates and makeup water requirements
over ordinary evaporative towers
2. Plume abatement possible to reduce or eliminate fogging
problems
3. Lower capital costs than for dry .towers
4. Versatility to allow greater use of evaporative cooling
during warm weather and increased use of dry cooling in
cold weather.
The following disadvantages are also inherent with wet/dry towers:
1. Higher capital and maintenance costs than with ordinary
mechanical draft evaporative towers
2. Possible mixing of vapor and stack gases outside the
tower which could be troublesome
3. Higher operating costs than a natural draft towers
4. Possible mechanical failure of the overhead fans.
20
-------
Figure 7. Wet/Dry Cooling Tower. (Sketch provided by The Marley Company;
Mission, Kansas)
SUB-SATURATED AIR MIXTURE
AIR COOLED
HEAT
EXCHANGERS
HINGED SUMMER
DAMPER DOOR
SUMMER
HOT WATER FLOW ONLY X-
INLETPIPE
INTERMEDIATE
WATER
NORMAL
AMBIENT
AIR INLET
COLD WATER
BASIN
j EVAPORATIVE
FILL
SECTION
21
-------
Although no wet/dry towers have as yet been built for power plants in the
1,000 raw size range, it is estimated that capital costs for such units would
be roughly $3,000 - $4,500/miliion BTU/hr of heat rejected or $12 - $18/kw.
Operating and maintenance costs would probably run about $307 - $350 per
million BTU/hr or .14 - .16 mills/kwh. Assuming annualized capitalization
at the rate of 15 percent, total yearly costs would be .28 - .42 mills/kwh.
COOLING LAKES
Large lakes or ponds are sometimes used to dissipate power plant waste heat
through evaporation, radiation and conduction. There are three general types
of cooling lakes:
1. Completely mixed lakes which have the same temperature
throughout
2. Flow through lakes (probably the most common) where the
temperature decreases away from the warm water discharge
point
3. Internally recirculating lakes where warm water flows on
the surface while cool water flows to the intake underneath.
This cooling method is generally most appropriate in relatively dry climates.
Thusfar, half of the nation's cooling lakes are located in the Southwest, Texas
and Oklahoma, a quarter in the Southeast and the remainder are found mainly in
the Midwest. The overall attractiveness of cooling lakes depends on climate
conditions and land topography, availability and cost. Although mathematical
models have been developed to size these lakes, a widely used rule-of-thumb
calls for 1 acre per megawatt (where 1 acre = 4,047 m2) for a fossil fueled
plant and 1 1/2 - 2 acres/mw for a nuclear plant. This generally provides
for a 1° - 2°C (2° - 3°F) temperature approach. It should be noted that
while mechanical and natural draft cooling towers have recirculating water
retention times of roughly 15-20 minutes, a cooling lake or pond will have a
water retention time of 10 days or more. This helps explain the low tempera-
ture approach possible with a cooling lake.
The advantages of cooling lake use are as follows:
1. Except for hot dry climate areas, little or no surface
blowdown stream is generally required, mainly because waste
heat transfer by evaporation is roughly only 30 - 60 percent
in ponds as opposed to more than 70 percent for evaporative
towers.
2. Lower capital costs than for cooling towers
3. Can possibly operate for extended periods without makeup
water
4. Lake serves as settling basin for suspended solids
5. Lake could be useful for other purposes
22
-------
Table 4. Capital Costs of Selected Cooling Ponds^ (c)
PLANT
H. B. ROBINSON
ROXBORO (a)
BRAUNIG
KINCAID
MT. CREEK
NORTH LAKE
BALDWIN (a>
VALLEY
MT. STORM (a>
1970
CAPACITY
(MW)
906
1067
885
1319
990
709
584
725
1140
SURFACE AREA
(ACRES) (b)
2145
3750
1250
2400
2710
800
1000
1120
CAPITAL COST
OF COOLING POND ($)
4,800,000
4,831,000
4,717,000
3,819,000
4,333,000
3,555,000
3,000,000
918,209
6,523,000
UNIT COST
$/mw
5,298
4,528
5,330
2,895
4,377
5,014
5,137
1,266
5,722
$/ACRE
2238
1288
3774
1591
1599
4444
918
5824
COMMENTS
1971 Capacity
a Future expansion at site contemplated.
b One acre equals 4047m2.
c Covers land and land rights costs only.
-------
a. Recreation
b. River flow control to help avoid both flooding and
low flow periods.
6. In many cases, chemical treatment of recirculating water
can be avoided.
Disadvantages of cooling lakes include:
1. Large land areas are required
2. To avoid excessive seepage soil basin must have low
permeability. Seepage into ground water strata has
been calculated at .10 - .25 m3/sec - thousand m^
(4 - 10 ft^/sec - 10^ acres) for three lakes, in
Mississippi, Arizona and Washington
3. Possible fogging and icing problems
4. Could collect wind blown debris
5. Possible weed and algae growth and subsequent maintenance problems.
A thorough survey of cooling lake costs has been made by Sonnichsen et. al.^
Their listing of selected power plant cooling lakes with associated land and
land use costs, edited to include only power plants rated in excess of 500
mw is shown in Table 4.
Several sources have estimated capital and operating costs for cooling lakes.
lakes.4,5,11,20 The concensus is that capital costs are $8 - $ll/kw or
$2,000 - $2,700 per million BTU/hr of heat rejected. Total annual operating
costs and annualized capital charges (at 15 percent/yr) are .15 - .28 mills/
kwh. These data are compared with the costs of other cooling systems in
Table 3.
SPRAY CANALS AND PONDS
This method consists of a pond or lake, usually having a smaller area than
an ordinary cooling lake, using either perforated pipes or some mechanical
means such as floating aerators to form water droplets of .97 - 1.27 cm (.38
.50 inches). These aerators promote air-water contact and thus evaporative
heat transfer by spraying the droplets about 5 meters (15 feet) into the air.
Spray pond design will vary depending mainly on climate conditions, normal
wind speed and land availability.
A few spray ponds are already in operation which have recirculation rates
greater than 51,000 m3/hr.
Their advantages are:
1. Reduced land requirements compared with cooling lakes
2. Less expensive than cooling towers
24
-------
3. Chemical treatment of recirculating water often not required.
The disadvantages of spray ponds and canals are:
1. Increased water losses through drift
2. Fogging and icing can become severe
3. Performance directly proportional to wind speed. A
calm will probably prohibit power plant operation at
full capacity
4. Mechanical equipment may be costly to operate and maintain
5. Slowdown normally necessary
6. More area required than for cooling towers.
The sources who have estimated^ 5 Or installed spray pond and canal costs
conclude that capital costs are $10-17/kw, operating and maintenance costs
are .10 - .12 mills per kwh, and total annual costs are .27 - .41 mills/kwh.
These data are compared to the other commerically available cooling system
costs in Table 3.
NEW COOLING SYSTEM DEVELOPMENTS
Several modifications of existing cooling methods are being developed to
help reduce costs and improve effectiveness. No full-scale units have as
yet been built to our knowledge so economic data are unavailable.
Cooling Towers -
Several cooling tower suppliers are working on various combinations of
hyperbolic and mechanical draft cooling towers. These units would have
smaller stack heights than conventional hyperbolic towers and would be
equipped with either induced draft or forced draft fans.
The horsepower requirements for these fans are lower than that required for
ordinary mechanical draft cooling towers. The size of hyperbolic towers
could thus be reduced while still taking advantage of natural draft charac-
teristics and high plume discharge. Forced draft hyperbolic towers can be
used in areas where prevailing climatic conditions may prohibit the use of
ordinary hyperbolic natural draft towers. Cooling tower manufacturers
estimate that the capital cost of forced draft hyperbolic towers should be
lower than the cost of ordinary hyperbolic towers.
on
Rogers notes the development of an igloo tower, which is similar in con-
cept to the forced draft dyperbolic towers. Forced draft fans are used at
the base of a tall igloo structure. This design might be used where climate
conditions are unfavorable to natural draft towers, but where relatively
high plume discharge is desirable.
25
-------
Cooling Ponds -
Reflective materials to reduce radiation and thus heat input to pond water
are being studied. The effect of pond geometry on heat transfer and pond
performance are also under investigation, as is selective withdrawal from
stratified water bodies.
From an environmental standpoint, Sonnichesen et. al.4 recommend further con-
sideration of cooling lakes for recreation and commercial use. They suggest
including cooling lakes in comprehensive land use plans. In addition, heat-
ed water might be beneficial toward increasing product yields from various
forms of agriculture and aquaculture.
26
-------
REFERENCES
1. Cooling Towers. Power. S 1-24, March 1973.
2. Cooling Tower Fundamentals and Application Principles. Kansas City,
The Marley Company, 1967. 116 p.
3. Woodson, R. D. Cooling Towers. Scientific American. 224: 70-78,
May 1971.
4. Sonnichsen, Jr., J. C., S. L. Engstrom, D. C. Kolesar, and G. C.
Bailey. Cooling Ponds - A Survey of the State of the Art. Hanford
Engineering Development Laboratory. Richland, Washington. AT (45-1)
2170. U.S. Atomic Energy Commission. September 1972. 99 p.
5. A Survey of Alternate Methods for Cooling Condenser Discharge Water:
Large-Scale Heat Rejection Equipment. Dynatech R/D Company.
Washington Project No. 16130 DHS. Environmental Protection Agency,
July 1969. 127 p.
6. Problems in Disposal of Waste Heat From Steam-Electric Power Plants.
Federal Power Commission Bureau of Power. 1969.
7. De Monbrun, J. R. Factors to Consider in Selecting a Cooling Tower.
Chemical Engineering, p. 106-116, September 9, 1968.
8. Feasibility of Alternative Means of Cooling For Thermal Power Plants
Near Lake Michigan. National Thermal Pollution Research Program -
Pacific Northwest Water Laboratory and Great Lakes Regional Office,
U.S. Department of the Interior. Federal Water Quality Administra-
tion. August, 1970. 114 p.
9. Ovard, J. Industrial Cooling Towers - A Use Profile. Pollution
Engineering, p. 20-23. May/June 1971.
10. Dickey, Jr., J. B., and R. E. Gates. Managing Waste Heat with the
Water Cooling Tower. Kansas City, The Marley Company. April 1973.
18 p.
11. Woodson, R. D. Cooling Alternatives For Power Plants. (Presented to
Minnesota Pollution Control Agency. November 30, 1972) 15 p.
12. Cooling Towers. New York. American Institute of Chemical Engineers.
1972. 145 p.
13. Oleson, K. A., and R. R. Boyle. How to Cool Steam - Electric Power
Plants. Chemical Engineering Progress. 67: 70-76. July 1971.
14. Jimeson, R. M., and G. G. Adkins. Waste Heat Disposal in Power Plants.
Chemical Engineering Progress. 67; 64-69. July 1971.
27
-------
15. Kolflat, T. D. How to Beat the Heat in Cooling Water. Electrical
World. October 14, 1968.
16. Woodson, R. D. Cooling Towers For Large Steam-Electric Generating
Stations. In: Electric Power and Thermal Discharges, Eisenbud, M.,
and G. Gleason (eds.), New York, Gordon and Breach, 1969, p. 351-380.
17. Jones, W. J. Natural Draft Cooling Towers. (Presented at the Cooling
Tower Institute Annual Meeting. January 29-31, 1968.) 15 p.
18. Gates, R. E., and J. A. Nelson. Dry Cooling Towers For Large Power
Installations
-------
28. Thackstron, E. L., and F. L. Parker. Geographical Influence on Cooling
Ponds. Journal of the Water Pollution Control Federation. 44; 1334-
1351, July 1972.
29. Brady, D. K., J. C. Geyer, and J. R. Sculley. Analyses of Heat Transfer
at Cooling Lakes, Commercial Power Generation. Chemical Engineering
Progress Symposium Series. ^7 (119): 120-125.
30. Kelley, R. B. Large Scale Water Cooling via Floating Spray Devices. 8 p.
31. Rogers, K. R. Tile Fill Packing. In: Cooling Towers. New York.
American Institute of Chemical Engineers. 1972. p 101-103.
32. Industrial Waste Guide on Thermal Pollution. Federal Water Pollution
Control Administration, Northwest Region, Pacific Northwest Water
Laboratory. Corvallis. September 1968.
33. Campbell, J. C. A New Look At Cooling Towers For the Power Generation
Industry. Ceramic Cooling Tower Company. (Presented at Cooling Tower
Institute Meeting. Houston. January 20-22, 1969.)
29
-------
SECTION IV
THE USE OF CORROSION RESISTANT CONSTRUCTION MATERIALS
INTRODUCTION
The use of corrosion resistant materials is standard practice in the electric
utility industry. Unlike most other industries, power stations are built for
service lives of thirty years or more. Thus, corrosion prevention is a neces-
sary consideration in power plant cooling systems. Although corrosion resis-
tant materials are more costly, their use is justified by less maintenance,
improved heat transfer, and reduced water treatment costs. Corrosion and
scale inhibiting chemical usage can be minimized in large cooling water sys-
tems through the proper selection of construciton materials and protective
coatings. In fact, there are a few power stations where the existence of
corrosion resistant materials plus the use of on-line condenser tube clean-
ing equipment has eliminated the need for chlorine and other biocides, thus
allowing operation without any chemical treatment.
In addition, corrosion resistant materials generally help prevent water
pollution. By minimizing corrosion and scale inhibiting chemical usage,
the amounts of toxic materials such as zinc and chromate discharged into
lakes and streams are reduced. Corrosion products in blowdown streams can
also be maintained at low levels.
CORROSION RESISTANT MATERIALS
IN COOLING TOWERS
Although redwood or Douglas fir are still the normal structural elements
used in mechanical draft cooling towers, concrete towers are now being
built. All the hyperbolic natural draft cooling towers built in the United
States to date have been of concrete construction. Concrete towers should
last longer, are structurally superior to wood and can save utility compa-
nies on fire insurance premiums. A cooling tower manufacturer recently
mentioned that a client could save $100,000/yr on insurance premiums by
using concrete induced draft cooling towers instead of wood.
Type 2 concrete is normally used for cooling towers, but where waters con-
taining 1,000 ppm or more of sulfate ion (S04=) are encountered such as in
the West and Southwest, Type 5 must be specified.
Concrete mechanical draft cooling towers are more expensive than conventional
wood towers by 60-75 percent. For a 1,000 mw power plant, the specification
of a concrete tower will add roughly $2 million to the investment cost.
The use of wood in the internals of large cooling towers is also diminishing.
Plastics such as polyvinyl chloride (PVC) are now being used for splash-
type tower fill, drift eliminators, and fill hangers. Film-type fill is
normally made from asbestos concrete board (ACB). Air louvers and some
drift eliminators are also made of ACB. When ACB is used in a tower, it
is important to maintain cooling system pH at more than 6.0 to avoid
deterioration.
30
-------
Hardware used in cooling tower construction is normally stainless steel,
although hot dipped galvanized steel, naval brass, copper alloys and silicon-
bronze can also be used.
Stacks in mechanical draft towers are generally made from fiberglass rein-
forced plastic (FRP).
CORROSION RESISTANT MATERIALS
IN RECIRCULATING SYSTEMS
Several materials are used in recirculating systems. Prestressed reinforced
concrete piping (Type 2 or Type 5) is commonly used as is vinyl painted car-
bon steel. Redwood stave piping is being used less frequently since red-
wood is becoming more scarce. Risers and headers, which are large pipes
located at the cooling towers, are made of concrete or redwood staves, and
less frequently of vinyl painted carbon steel. Where brackish or salt water
is used for tower makeup, 316 stainless steel and coated carbon steel are
commonly specified for piping systems.
Concrete and stainless steel cost 2-3 times more than carbon steel. Cost
differences between stainless steel and prestressed reinforced concrete
will depend on differences in material grade, freight charges and instal-
lation costs.
Most power companies do not make detailed cost comparisons between piping
materials, but one Ohio utility determined that for a new power station,
concrete recirculation lines would cost about $300,000 more than coated car-
bon steel.
CORROSION - RESISTANT MATERIALS
IN CONDENSERS
The use of corrosion resistant materials in steam condenser tubes is stan-
dard practice. A wide variety of materials are in use or are available
including admiralty, 304 & 316 stainless steel and copper-nickel alloys.
Cold water boxes are normally made of carbon steel. Occasionally, phenolic
or epoxy coatings are applied.
CORROSION RESISTANT MATERIALS
IN SPRAY COOLERS
Spray cooling apparatus is normally constructed with 304 or 316 stainless
steel. Cathodic protection devices have been installed on some floating
spray aerator-coolers.
RESISTANT PRETREATMENTS AND COATINGS
Redwood and Douglas Fir construction materials used in mechanical draft cool-
ing towers are always pressure treated with either acid copper chromate or
chromated copper arsenate to prevent fungus attack. Tower suppliers have
indicated that this pretreatment is effective and that leaching of the treat-
ment chemical does not occur after the initial few weeks of tower operation.
31
-------
Since chemical pretreatment of cooling tower lumber is a necessary process,
its cost has not been separated from the total purchased cost of tower
systems.
Carbon steel pipe and hardware are sometimes coated with epoxy, phenolic, or
vinyl paints to reduce corrosion rates. Steel piping is occasionally given
a coal-tar bitumastic coating on the inside to prevent corrosion. Bitumas-
tic coatings may also be applied to the outside of pipes to prevent leakage
and corrosion. These coatings normally cost $20 - $AO/ft of pipe length.
32
-------
SECTION V
MAKEUP AND RECIRCULATING WATER TREATMENT
INTRODUCTION
An increasing number of power plant installations have adopted the water
usage system as shown in Figure 3, where water is cycled from the conden-
ser to evaporative towers, lakes or spray ponds and returned again to the
condenser. Only a small portion, usually about 1-3 percent, is discharged
from the recirculating system as blowdown.
Recirculating water in evaporative cooling systems causes any dissolved and
suspended materials in the water to become more concentrated. These solids,
which for the most part'were brought into the system with the makeup stream,
are left behind when part of the cooling water evaporates to atmosphere.
In some cases, dissolved solids can concentrate until they reach their
saturation limit in water. A portion of the solids will then start to
precipitate.
The increase in solids levels as a result of recirculating the cooling water
and reducing the blowdown stream flow rate is referred to as increasing the
cycles of concentration. To determine the cycles of concentration, recir-
culating cooling water system analyses should be made for a material present
in the feed water but not added to a significant extent through chemical
treatment. An ion that is commonly monitored for this purpose is chloride
(Cl~). Cycles of concentration are calculated by dividing the recircula-
ting water chloride ion concentration by the concentration of chlorides in
the makeup stream. Cycles can usually run between 1.5 - 10, depending on
the cooling system characteristics.
There are several reasons for thus increasing the cycles of concentration:
1. The need for water conservation may exist. Table 5 shows
how makeup rates are affected by cycles of concentration
for a 1,000 mw power plant (40 percent thermal efficiency)
having a cooling tower with an 11.1°C (20.0°F) temperature
range.
2. Since lower flow rates are discharged to receiving streams
when recirculating water systems are implemented, the amount
of heat fed to the streams will be proportionally reduced.
Table 6 shows how the quantity of heat discharged to a
stream is reduced by increasing cycles of concentrations
for various temperature differences between the blowdown
stream and the receiving stream.
3. If the cooling system makeup source is not the same as the
receiving stream, less dissolved solids will be discharged
to the receiving body in the blowdown if the cooling water
is recirculated.
33
-------
Table 5. Effect of Cycles of Concentration
on Cooling System Makeup Rate
For a 1,000 mw Power Station
Cycles of Evaporation
1.5
2.0
3.0
4.0
6.0
8.0
10.0
1230
1230
1230
1230
1230
1230
1230
Drift, Leakage,
and Slowdown, m3/bl
2460
1230
615
410
245
175
135
Makeup
r^c' Required J
m3/hr
3690
2460
1845
1640
1475
1405
1365
Percent Reduc-
tion Over Once
Through System
95.7
97.1
97.9
98.1
98.3
98.4
98.4
Power plant thermal efficiency assumed to be 40 percent.
(a) Recirculation rate is 86,260 m3/hr. Heat duty is 3.81 billion BTU/hr and
temperature range is 11.1°C (20.0°F).
(b) Assumed heat transfer is 75 percent evaporative, 25 percent sensible
(c) nH/hr x 4.4 equals gpm.
34
-------
Table 6. Effect of Cycles of Concentration
on Heat Discharge Rate
From a 1,000 raw Power Station
Cycles of Slowdown
Heat Discharge for Temperature
Differences Between Receiving
and Slowdown,million BTU/hr
Percent Reduction Over
Helper Tower Assisted
Concentration^3^
1.5
2.0
3.0
4.0
6.0
8.0
10.0
Rate, m3/hr(b,c)
2460
1230
615
410
246
176
137
AT=1°C
9.37
4.87
2.43
1.62
0.97
0.70
0.54
AT=5°C
48.66
24.33
12.16
8.11
4.87
3.48
2.70
AT=10°C
97.33
48.66
24.33
16.22
9.73
6.95
5.41
Once-Through System^-"'
96.2
98.1
99.0
99.3
99.6
99.7
99.8
(a) Power plant thermal efficiency assumed to be 40 percent. Heat duty
is 3.81 billion BTU/hr and cooling tower temperature range is 11.1°C (20°F)
(b) Assumed heat transfer is 75 percent evaporative
(c) m-Vhr x 4.4 = gpm
(d) Based on the operation of a 65,150 mVhr "helper tower" in which warm water
from the condenser(s) passes through the cooling tower and then directly to
a receiving body.
35
-------
4. Increasing cycles of concentration lowers the cost of any
blowdown treatment or evaporation that may be required.
There are several factors which can limit the cycles of concentration attain-
able in a cooling water system if treatment processes are not implemented.
The more prevalent factors are as follows:
1. Alkalinity and sulfate ion concentrations are the most
common limitations to cycles of concentration. These
ions in the presence of water hardness (calcium and
magnesium ions) can exceed their solubility limits in
water and precipitate as calcium carbonate and calcium
sulfate scale. These scales are hard materials that
attach to condenser tubing and pipe surfaces, restric-
ting flow rates and, more importantly, heat transfer.
Blowdown rates are normally controlled to stay below
the solubility limits of calcium sulfate and calcium
carbonate which will vary depending on the other im-
purities present, treatment chemicals used, water and
wall temperatures. Guidelines for the proper cycles
of concentration for each individual cooling system
can be established by consultants or most water treat-
ment chemical suppliers. In addition, some cooling
tower system operators have established rules-of-thumb
guides to regulating the blowdown rate. For example,
several operators hold the sum of the calcium ion con-
centration plus the sulfate ion concentration below
1500 mg/1. Calcium carbonate scaling can usually be
avoided by monitoring either the Langelier or Ryznar
stability index.
2. Where the makeup water source is muddy or silty, suspended
solids concentrations can limit the cycles. These solids
can plug condenser tubes or adhere to tube walls, restric-
ting heat transfer. Most operators try to maintain less
than 100 mg/1 of total suspended solids in the recircula-
ting water.
3. The presence of silica (Si02) in the recirculating water, as
in some areas of the country, can restrict the cycles of
concentration. Several water treatment chemical suppliers
have developed proprietary guidelines to measure silica
deposition tendencies.
4. Iron can be found in recirculating water systems which use
water contaminated with acid mine drainage for makeup.
Iron oxides can form sludges which plug condenser tubes or
hard scale which retards heat transfer.
5. Cycles of concentration are sometimes limited by the need of
water elsewhere in the plant. In fossil fueled plants, for
example, cooling system blowdown can be used for ash sluicing
or stack gas scrubbing.
36
-------
Where impurities limit the cycles of concentration in recirculating cooling
water systems, either the makeup or recirculating water can be treated
physically or chemically to reduce or eliminate the effects of the impurities.
In most cases, several treatment methods are used.
INFLUENT TREATMENT METHODS
Screening Equipment -
If debris is contained in the makeup water supply, bar screens with mesh as
small as 1/4", located at the makeup water intake pipe are generally in-
stalled. More sophisticated equipment such as traveling screens and grit
collectors are occasionally used, expecially on cooling ponds. Negligible
cost is involved to install bar screens. They are normally cleaned manually
as needed.
Traveling screens and grit collectors normally cost less than $50,000, and
their power consumption and operating costs are usually negligible.
Settling Ponds and Clarifiers -
Ponds or clarifiers are frequently used throughout the country to settle out
suspended solids from makeup water. Ponds are normally sized for 1-2 days
water retention time and are 3-5 meters (10-16 feet) deep. Installation
costs should run $1,000 - $4,000/acre, depending on land availability.
Clarifiers are also used for solids sedimentation, especially where coagula-
ting aids such as alum, ferric chloride or sodium aluminate must be added
to enhance sedimentation. Clarifiers are normally sized for water retention
times of 1-3 hrs. and liquid depths of 4 1/2 - 6 meters (15-20 feet).
Clarifier effluent quality usually ranges between 10-50 mg/1 SS according
to the majority of the operators surveyed. Coagulant dosages, which must
be determined from laboratory settling tests and actual operating experiences,
usually run about 10 ppm. Installed clarification equipment generally costs
$290 - $420 per m3/hr of capacity ($66 - $96/gpm). Coagulating aids are
normally inexpensive and should cost less than $10,000/yr for a typical
1,000 mw system.
Sediment from the clarifiers is either pumped back to the makeup water source
or to a holding lagoon.
Chlorinators -
Liquid chlorine is sometimes added to relatively polluted makeup water to
effect partial disinfection. Chlorination equipment generally costs less
than $5,000. Liquid chlorine itself costs 13-15
-------
Cold Lime Softeners -
Cold lime softeners are used to treat makeup water for several large cooling
systems. These systems can reduce hardness (calcium and magnesium) to roughly
20-50 mg/1 as CaC03, phosphate to about 1 mg/1, silica by a significant per-
centage, and suspended solids to less than 30 mg/1 from makeup water and allow
increased cycles of concentration in the recirculating system;
Cold lime softeners consist of a steel clarifier system with lime feeding
equipment. Other chemicals such as soda ash, alum or anionic polyelectro-
lytes are also added at some locations to increase softening capabilities
or improve floe sedimentation.
Lime sludge pumped from the bottom of softeners is usually fed to holding
ponds. Supernatant water then overflows to the receiving stream or to
evaporation ponds.
Softeners are usually designed for liquid loadings of 1.7 - 2.4 m/hr (0.7 -
1.0 gpm/ft^). Recent installed equipment costs have been $330 per m^/hr
($75/gpm) of capacity, and chemicals costs have run $13 - $19/thousand cubic
meters (5.7
-------
1. Corrosion
2. Scaling
3. Biological growth
Several books are available which discuss cooling water treatment in
-1 _0 -IT °
general. J» ±t
The electric utility industry, for the most part, uses fewer water treatment
chemicals than other industries, mainly because the systems are constructed
of corrosion-resistant materials and the recirculating water is controlled
at relatively low cycles of concentration. Typically, cycles are between
1.5 and 3.0. .Most of the large power plant cooling tower systems use either
acid or lime pH control, and chlorine for biological organism control.
Most power plant cooling lakes and spray canals require only chlorine as
shown in Table 7, which is taken from Sonnichsen, et. al.^5 A more detailed
account of large cooling system recirculating water chemical treatment follows,
:pH Adjustment -
In most areas of the country, cooling system makeup water which is taken
from wells, rivers and lakes is alkaline, having a pH greater than 7.0;
that is, it contains carbonate, bicarbonate and possibly hydroxide ions.
This alkalinity is usually removed by the addition of sulfuric acid. In
certain areas, where acid mine drainage contributes to the cooling water
makeup stream, the pH is below 7.0, indicating that the water contains acid.
This excess acidity is neutralized with lime. The pH is usually controlled
so that the Langelier Saturation Index is maintained near zero.
The Langelier Saturation Index (LSI) is a quantitative measure of the cor-
rosive or scaling tendencies of a water system. A negative LSI implies that
water is corrosive, while a positive index indicates that scaling problems
can occur. The magnitude of the LSI shows the severity of the scaling or
corrosive tendencies. For example, the water stream having an index of
+6.0 is much more likely to cause scaling than a water with an LSI of +1.0.
The Langelier Saturation Index is calculated by subtracting the pH of
saturation of calcium carbonate (CaC03) from the actual water pH. The pH
of saturation (pHg) is a function of total dissolved solids concentration,
temperature, calcium and alkalinity concentration. Tables or graphs, to
allow rapid calculation of the LSI, are commonly available.
Due to occasional discrepancies encountered between LSI values and scaling/
corrosive tendencies, a slightly different saturation index was formulated
by Dr. John W. Ryznar. This index, called the Ryznar Stability Index is
calculated as follows:
Stability Index = 2pHs - pH
where pHs = pH of saturation
& pH = actual water pH.
39
-------
Table 7. Annual Chemical Use By Selected Steam Electric Power Stations
Operating Cooling Ponds and
Plant
Flow
(cfs)
Chlorine
Phosphate
Ub)
Alum
(lb)
Caustic
Soda
(lb)
Sulfuric
Acid
(lb)
Lime
(lb)
Cholla
4-Corners
H.F. Lee
Asheville
Roxboro
Coffeen
Coughlin
Braunig
Kincaid
Dallas
Mountain Crk
Xorth Lake
Parish
Baldwin
Fox Lake
Montrose
Comal
Sin Gideon
Aurora
Delta
Rex Brown
Horseshoe Lake
Cameo
Valmont
Ft. Churchill
Canadys
Knox Lee
Lieherman
Lone Star
Wilkes
Eagle Mt.
Graham
Hand ley
Morgan Creek
Lake Creek
Lake Pauline
Oak Creek
Paint Creek
San Angel o
180
1082
346
200
870
325
445
535
980
900
1660
720
765
200
500
186
308
354
1175
75
467
103
600
435
416
122
490
323
753
755
276
556
90
183
174
76,700
108,000
62,000
6,000*
24,000
112,000
6,600
145,500
86,826
30,000
46,000
4,000
320,000
50,000
2,950
65,900
54
32,000
9 ,230
8,000
2,000
30,250
892*
3,783
3,650
120,000
3,800
600
8,000
16,000
34,000
57,808
74,000
66,000
20,000
12,000
4,400
6,000
13,000
7,300
14,300
500
33,000
150
30,000
3 , 581 , 380
600
* gal Sodium Hypochlorite
** Cameo Plant is essentially once-thru 90 percent of the time
40
-------
These stability index numbers are always positive. An index of 6.0 is
considered neutral. Values smaller than six indicate that water is scale
forming, while numbers greater than six show that water is corrosive.
By maintaining a neutral stability index in their cooling water, power plant
operators can minimize or avoid the use of corrosion inhibitors and antifou-
lants.
Sulfuric acid and lime both cost about 4 l/2c/kg (2c/lb). Their use will
depend on the initial pH and alkalinity of the makeup water and the cycles
of concentration maintained, but will usually cost roughly 0.66c/m3 of
makeup (2.5c/l()3 gal.)-
Corrosion Prevention -
Corrosion is the electrochemical deterioration of metal. In power plant
cooling tower systems, the condenser is the most vulnerable piece of equip-
ment because of its high tube wall temperatures and must be protected to
avoid loss of heat transfer and possible replacement costs. As mentioned
above, in many cases simple pH adjustment is adequate to control corrosion
rates below 3 mills per year. However, due to water chemistry and occasion-
ally the need to maximize cycles of concentration, a chemical corrosion
inhibitor in some cases must be used.
Large cooling systems in services outside the power industry tend to use
chromate, zinc-chromate, or phosphate-phosphonate-acrylic corrosion
inhibitors.
Zinc-chromate-phosphate corrosion inhibitor formulations have been a water
treatment standard for many years because of their high effectiveness and
relatively low costs. Unfortunately however, these materials are environ-
mentally undesirable. Zinc and chromate are toxic to certain forms of
aquatic life at concentrations under 1 ppm, while phosphate contributes to
algae growth and enhances eutrophication. This treatment will cost $1.10 -
$4.40/thousand m3 (.50 - $2.0/million pounds of blowdown).
One large electric power plant cooling system uses a chromate inhibitor (at
a cost of less than $l/million pounds of blowdown) mainly because of high
chloride (Cl~) ion concentrations in the recirculating water.
A few major power plant towers use sodium hexametaphosphate, maintaining
a 2-5 mg/1 phosphate residual. This treatment affords marginal corrosion
protection, but it is a good dispersant at that level. Recirculating water
is controlled in the scaling region when sodium hexametaphosphate is used.
The corresponding pH levels are normally 6.8-7.5. This treatment costs
$0.30 - $1.75/thousand m3 ($.15 - $.80/million pounds of blowdown).
Scale Prevention -
The concept of maintaining scaling water (thus reducing acid usage) and
adding antifoulants to avoid depositon is becoming increasingly popular,
mainly because of the pollution problems caused by corrosion inhibitors
which are currently on the market.
41
-------
Although this treatment method is significantly more expensive in terms of
chemical costs than the use of conventional corrosion inhibitors, the dis-
persants and antifoulants currently in use seem to be compatible with the
environment.
A class of dispersant compounds being used in large cooling systems at the
present time is the phosphonates. This material is a very good dispersant
according to plant experience, and has corrosion inhibiting properties when
used with small doses of zinc salts. In one power plant system, a proprie-
tary formulation of polyacrylamide polymer and phosphonate is added to pro-
vide increased dispersant quality. This program costs $4.40 - $30.80/
thousand m^ ($2 - $14/million pounds of blowdown).
Other types of antifoulants and corrosion inhibitors are commercially
available for use but have not been applied to large cooling systems.^
1. Cationic high molecular weight polymers have been effective
in smaller systems, but they cost even more than phosphonate
blends. As noted earlier, in some cases, these polymers are
blended with phosphonates. These polymers form light floe
which is carried out of the cooling system with the blowdown
stream. These products are non-toxic and biodegradable.
2. Low molecular weight anionic polymers change the crystal
structure of precipitating scale products, dispersing them
in a colloid form which does not adhere to metal surfaces.
These polymers are also reported as non-toxic and biode-
gradable. Polymer programs can cost up to $33 per thousand
m3 ($15/million pounds of blowdown).
3. Chelating agents such as EDTA and NTA can form soluble com-
plexes with scaling products. Their cost for large systems
is prohibitive.
4. Proprietary formulations of polymer and silicates have been
developed and have performed relatively well in some plant
scale tests.11 Although these materials are environmentally
safe, their use would be quite expensive for large systems.
In addition, these formulations are unusually pH sensitive.
It is estimated that a silicate-based treatment program would
cost $44 - $55 per thousand m3 ($20 - $25/million pounds of
blowdown).
5. Proprietary non-zinc, non—chromate formulations are being
developed by several water treatment chemical manufacturers.
Many of these materials reportedly have low toxicity in
addition to being satisfactory corrosion or scale inhibitors.
6. Blends which can contain two or more materials such as zinc,
chromate, phosphate, phosphonate, cationic polymer, anionic
polymers, and silicates are available from the major water
treatment chemical suppliers for combined corrosion and scale.
inhibition. Most of these blends operate in relatively high
pH ranges between 7.0 and 9.0. Their costs will be highly
variable.
42
-------
Biological GrowthInhibition -
The vast majority of large cooling water systems, including towers, ponds
and spray canals, use chlorine to control the growth of algae, fungi, and
bacteria. These organisms can foul condenser tubes (retarding heat transfer
and cooling water flow) and other metal surfaces. Some types of bacterial
fouling can contribute to corrosion. Cooling tower lumber can also be dam-
aged through biological attack.
In most cases, liquid chlorine stored in one-ton cylinders is fed to the
cooling tower basin or the recirculating pump sump through automated commer-
cially available chlorination equipment which usually includes flow controllers
shut-off devices operated by timers, chlorine-water mixing devices, and
diffusers.
Experience with chlorine use varies considerably. At most locations, chlorine
is slug fed for 0.5-2.0 hours, and residual free chlorine levels of 0.2-1.0
ppm are normally attained. However, the necessary frequency of chlorination
varies between 4 times per day to once every two weeks. Chlorine usage will
also vary considerably depending on the climate, makeup, water source, etc.
Data we have received indicates chlorine consumption can range from 2,700 -
91,000 kilograms (kg) (6,000 - 200,000 pounds/month). The cost of this
treatment is $360 - $14,000/month.
/
One disadvantage of chlorine is that higher residuals are required when the
recirculating water pH increases above 7.0, as shown in Figure 8. Other
disadvantages are that chlorine is hazardous to handle and will react with
any ammonia dissolved in cooling water to form chloramines which have rela-
tively high toxicities to aquatic life.
Some installations supplement chlorine with a non-oxidizing biocide like
sodium pentachlorophenate, slug-fed 1-4 times/month, particularly during
warm weather. This intermittent treatment can cost $500 - $5,000/month.
The only chemicals used in place of chlorine for microbiological control in
large cooling towers are dodecylguanidine salts. Recent field tests by
Hopkins and BaldwinlS showed that these materials have been effective sub-
stitutes for chlorine at three Pennsylvania power stations. Dodecylguanidine
salts are available from at least two water treatment chemical suppliers.
Some of the advantages of dodecylguanidine cited by the authors are as
follows:
1. It is an easily dispersed pourable liquid
2. It affords broad spectrum protection
3. It is biodegradable at discharge concentrations
4. It is effective up to a pH of 10
5. It has a relatively low toxicity to humans, most animals,
and fish.
43
-------
Figure 8. Disinfecting Efficiency of Chlorine}4 (Graph provided by Betz
Laboratories, Inc.; Trevose, Pennsylvania)
1.0
.01
.001
10
12
44
-------
Dodecylguanidine is now used full-time in the three power stations in Pennsyl-
vania with satisfactory results. Treatment costs are $1,000 - $2,000/month.
Several other biocides are commercially available, and many are used in
smaller cooling systems. A list of some of the more well-known biocides
are shown below:
1. Thiocyanates
2. Acrolein
3. Organotin-Quaternary Amine combinations
4. Organosulfur combinations
5. Dinitrochlorobenzene
6. Copper salts.
PHYSICAL TREATMENT OF CONDENSER TUBES
On-line physical cleaning of condenser tubes was developed in Europe during
the mid-19501s. The concept was soon adopted in this country and has been
growing at an increasing rate. Over 150 power stations, either in service
or on the drawing boards, are being supplied with on-line tube cleaning
equipment. These systems are used in stations as large as 1171 megawatts.22
There are basically two tube cleaning systems that are commercially avail-
able. 23 xhe most extensively used system, called Amertap, involves recir-
culating sponge rubber balls through the condenser tubes. The second method
which has the tradename American M.A.N., consists of plastic brushes which
normally rest in open plastic baskets mounted at the ends of each condenser
tube. The brushes are forced through the tubes when the cooling water flow
through the condenser is reversed.24
Both of these systems have been reported to maintain condenser tubes in a
clean condition. This provides the following advantages and potential for
cost savings:
1. Clean condenser tubes assure maximum heat transfer rates
2. Reductions in chemicals for pH adjustment, scale preven-
tion and biological control are usually possible. The
Amertap Corporation has reported that 28 power stations
have been built with sponge rubber ball systems in lieu
of chlorination equipment. We assume that slime problems
on towers and in transfer lines are insignificant in these
applications. In addition, three existing stations elimi-
nated the use of chlorine after mechanical tube cleaning
systems were installed.
3. Elimination of tube scale reduces the tendency for pitting
corrosion.
45
-------
The sponge rubber ball tube cleaning process is shown in Figure 9. Elastic
sponge rubber balls, which are oversized in comparison with the diameter of
the condenser.tube, are injected into the cooling water system upstream of
the condenser. The pressure drop across the condenser tubes forces the balls
through the tubes where they wipe the tube walls clean. A screening device is
located in the condenser discharge line to collect the balls and route them
to a recirculating pump which returns the balls back to the condenser inlet
line.
About one ball is used for every 10 tubes in the condenser. It is estimated
that each tube is cleaned by a ball every five minutes. Balls are usually
replaced once per month. If left in service for longer periods they can
wear and pass through the screen collector, sometimes plugging hot water
distribution nozzles on top of cooling towers.
Balls coated with an abrasive material are sometimes used to clean up an
already scaled condenser.
Costs associated with typical sponge rubber ball tube cleaning systems are
shown in Table 8. Estimated equipment costs, as provided by Amertap, are
$243 - $585/mw. Total capital and costs are shown in Table 9.
The on-line tube cleaning system, using plastic brushes, is illustrated in
Figure 10. Each condenser tube is fitted with an open plastic cage on each
end which catches and holds the brush. By reversing the cooling water flow
in the condenser, the brushes are forced from one end of the tube to the
other, thus cleaning the metal surface. After 30-80 seconds, the flow
pattern is returned to normal. Brush cleaning is usually practiced every
eight hours, but the frequency can be varied depending on tube fouling rates.
Condenser flow reversals are normally done with a single four-way valve which
may be pneumatically operated on an electrically timed cycle.
The investment cost for a brush system per the American M.A.N. Corporation
is roughly $150 - $500/mw. Total annual costs, including amortized capital,
operating and maintenance expenses are 0.004 - 0.012 mills/kwh. These costs
are shown in Table 9.
46
-------
Figure 9. Sponge Rubber Ball Tube Cleaning Process. (Sketch Courtesy of
Amertap Corporation)
Strainer
Section
• ;nt
•«•(?•
A
( i.
\>
c. •
V
xi^Di
Distributor
Pump Collector
47
-------
Figure 10. On-Line Condenser Tube Cleaning with Plastic Brushes. (Sketch.
Courtesy of the Taulman Company - Distributors for American
M.A.N. Corporation)
Water flow in normal direction
-\ main
J** valve
cooling water inlet
cooling water discharge
Cooling water flow reversed
-------
Table 8: Typical Sponge Rubber Ball Tube Cleaning System Costs
Unit Capaci
mw
900
1,190
680
950
ty f Coolinp Water
Recirculation Rate
gpm
405,
440,
220,
882,
000
000
000
000
Equi
$
$229,
312,
165,
556,
.pmenl
000
000
000
000
b Cost v"'
$/mw
$254
284
243
585
Additional Power
Required to Operate
System, kw
70
76
46
166
Replacement Ball
Costs ,$/yr
$6,
15,
6,
11,
400
000
000
000
Maintenance
Labor Rqd ,
hrs/wk
1
1
1
2
(a) Data provided by Mr. W. I. Kern, Amertap Corporation
(b) Estimated total installed cost of system, including capital equipment for new installation
2 x equipment cost.
-------
Table 9. Capital and Operating Costs for On-Line Tube Cleaning Equipment
System
Capital Costs
$/106 BTUyhr $/kw
Rejected
Annualized Capital
Mills/kwh(b)
Operating and Maintenance
$10b BTU/hr. Costs
Rejected Mills/kwh
Total Annual
Costs
Mills/kwh
Recirculating 120-290 0.48-1.16
Sponge Balls
.008-.020
2.85-5.15 .0013-.0024
(a)
.O09-.022
Plastic Brushes 38-125 0.15-.50
.003-.009
3.07-6.12 .0014-.0028
(c)
.004-.012
a. Power costs estimated at 4 mills/kwh. Maintenance labor estimated at $7.00/hr
b. Based on 15% per year
c. Includes allowance for replacing brushes every five years.
-------
REFERENCES
1. Betz Handbook of Industrial Water Conditioning. 6th edition. Trevose,
Betz Laboratories, Inc., July 1967. 427 pages.
2. Cooling Water Treatment Manual. Houston, National Association of
Corrosion Engineers, 1971. 35 p.
3. System Design Manual - Water Conditioning. Syracuse, Carrier Air
Conditioning Company, 1968. 52 p.
4. Curtis, S. D., and R. M. Silverstein. Corrosion and Fouling Control
of Cooling Waters. Chemical Engineering Progress. 67; 39-44,
July 1971.
5. Bischof, A. E., and P. Goldstein. Chemical Treatment for Cooling
Tower and Related Systems. Materials Protection and Performance
JLO(2): 26-28, December 1971.
6. Cone, C. S. A Guide for Selection of Cooling Water Corrosion Inhibitors.
Materials Protection and Performance, _9: 32-34, July 1970.
7. Hwa, C. M. Use of Phosphonates for Treating Cooling Water Systems.
Materials Protection and Performance. 10; 24-25, December 1971.
8. Silverstein, R. M. and S. D. Curtis. Cooling Water. Chemical
Engineering. Pages 84-94, August 9, 1971.
9. Schweitzer, G. W. How Phosphonates Control Scale and Corrosion in
Cooling Water Systems. Heating/Piping/Air Conditioning. 43: 78-82,
May 1971.
10. Marshall, W. L. Cooling Water Treatment in Power Plants. Industrial
Water Engineering. Pages 38-42, February/March 1972.
11. Puckorius, P. R., and L. D. Lindemuth. Cooling Water Performance with
the New Polymer/Silicate Program. Zimmite Corporation. (Presented
at the International Water Conference of the Engineer's Society of
Western Pennsylvania. Pittsburgh. October 26, 1972.) 15 p.
12. Briggs, J. L. Literature Survey of Corrosion Inhibitors for Re—
circulating Cooling Water Corrosion Inhibition. Dow Chemical
U.S.A. Springfield, Virginia. Contract AT (29-1) - 1106. U.S.
Atomic Energy Commission. July 1972. 14 p.
13. Beochek, F. J. Internal Treatment for the Control of Corrosion and
Deposits - Inhibitors, Antifouling Agents, Scale Suppressants.
Dearborn Chemical Division - CHEMED Corporation. (Presented at
Water Conditioning University Extension, The University of Wisconsin
Madison. May 2-3, 1972.) 13 p.
51
-------
14. Donohue, J. M. and C. V. Sarno. Pollution Abatement Pressures Influence
Cooling Water Conditioning. Materials Protection and Performance.
10: 19-21, December 1971.
15. Zecher, D. C. Problems in Replacing Chromate as a Corrosion Inhibitor
for Open Recirculating Cooling Waters. In: Industrial Process Design
for Water Pollution Control.. New York, American Institute of Chemical
Engineers, 1970. p. 89-92.
16. Boies, D. B. Study of Chemicals Currently Being Used for Open Recir-
culating Cooling Water Systems. Proprietary WAPORA report 1-26.
May 1972.
17. Bregman, J. I. Corrosion Inhibitors. New York, The Macmillian
Company, 1963. p. 70-125.
18. Hopkins, R. D., and L. V. Baldwin. Selection and Use of Biocides
in Open Circulating Cooling Tower Systems. April, 1973.
19. Shair, S. Microbiocide Treatment of Recirculating Cooling Water.
Industrial Water Engineering. Pages 26-30, January 1971.
20. Yost, W. H. Microbiolgocial Control in Recirculating Water Systems
Avoids Fouling. Oil and Gas Journal. April 16, 1973.
21. Lanford, C. Effect of Trace Metals on Stream Ecology. Petro-Tex
Chemical Corporation. (Presented at the Cooling Tower Institute
Meeting January 20, 1969.) 16 p.
22. Amertap Reference List for Condenser Tube Cleaning Systems. New York.
January 31, 1973.
23. Mawer, J. R. The Use of Stainless Steel Tubing in Condenser and
Related Power Plant Equipment - A Progress Report. Combustion.
July 1967.
24. McAllister, R. A. On-Stream Cleaning of Heat Exchange Tubes -
Fouling Prevented by Regular Brushing. New York. Report No.
68-PET-12. Publication of the Americal Society of Mechanical
Engineers. June 1968. 5 p.
25. Sonnichsen, Jr., J. C., S. L. Engstrom, D. C. Kolesar, and G. C.
Bailey. Cooling Ponds - A Survey of the State of the Art. Hanford
Engineering Development Laboratory. Richland, Washington. AT (45-1) -
2170. U.S. Atomic Energy Commission. September 1972. 99 p.
26. Process Design Manual For Suspended Solids Removal. Burns & Roe, Inc.
Oradell, New Jersey. Contract No. 14-12-930. Environmental Protection
Agency-Technology Transfer. October 1971. p 11-2.
52
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SECTION VI
PROCESS CONTROL
INTRODUCTION
Practically all large scale cooling systems have instrumentation and equip-
ment to automatically control many of the system variables. This saves man-
power expense, helps to minimize water and chemical usage, and can help pre-
vent downtime and system damage in case of upsets.
The most common automatic control applications are for both makeup and blow-
down flow rates, and pH adjustment. Where applicable, feed rates for corro-
sion and scale inhibitor chemicals are also automatically controlled. In
addition, instrumentation has been developed to automatically control chlorine
addition based on residual concentrations and corrosion inhibitor based on
instantaneous corrosion rates. However, these two systems have not yet been
used in the larger cooling systems.
MAKEUP CONTROL
Basin Level Control -
~*~~~^—~^*~~~~^-^^^^^—^*,i^f~m~.^r^f*^-v—~ /
Cooling tower water recirculation rates are usually fixed so any imbalance
between makeup and total system losses are reflected in changes of tower
basin liquid level. Thus, the most direct means of controlling makeup flow
is by level control at the base of the tower.
Basic costs for sensing and control equipment are about $10,000. Allowances
for installation and other engineering contingencies should be made on the
basis of individual plant experience. Usually, total installed costs for
instrumentation amount to 1.5 - 2.5 times the basic equipment cost.
Condensate Flow Rate -
Steam condensate flow rate from the condenser system also serves as a means
of controlling makeup rates, although this method is infrequently used.
Assuming the blowdown rate is constant, the steam condensate flow back to
the boilers will be proportional to the evaporative loss. Thus, the con-
densate flow rate can be used as a signal to control cooling system makeup,
which is equal to the sum of tower blowdown and evaporation. It is also
possible to use steam condensate flow as a means of controlling blowdown
rates, if the makeup rate is controlled by level.
Blowdown Control -
The most common control scheme for blowdown rate consists of automatically
monitoring total dissolved solids (TDS) levels and actuating a blowdown
flow valve accordingly. A convenient measure of TDS is electrical conduc-
tivity, and most blowdown control schemes utilize the conductivity signal
as a means of control.-'-"^ An estimate for blowdown control equipment costs
is as follows:
53
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Typical Automatic Slowdown Control Equipment Costs
Item Cost
Control Valve
Conductivity Cell
Controller
Recording Instrument
Total Equipment $4,100
Installation 3.200
Total Cost $7,300
pH ADJUSTMENT
Control of pH in cooling water is one of the most critical parameters to
consider. If pH levels are too high, scaling potential is increased signi-
ficantly, and likewise, if they are too low, the corrosion rates become
excessive. Further, the acceptable limits for pH are usually very narrow.4
The difficulties with a pH control system stem primarily from the normal
sensing elements used which are subject to drift and the need for routine
preventive maintenance.^*^ Although more sophisticated sensing elements
are available, as discussed by Feitler,4 these elements require a controller
device with memory capability. The installation of conventional pH control-
ling equipment costs about $3,000.10
RESIDUAL CHLORINE
Although reliable continuous analyzers for residual chlorine are not a
recent innovation, they have yet to be tested on large systems. The prime
purposes for automatic control of residual chlorine is reduction in chemical
costs and environmental effects.
Several amperometric devices for residual chlorine are currently on the
market.8,9 Total residual chlorine includes some chlorine that may have
reacted but can easily be freed for biocidal purposes (e.g., tied up as a
chloramine). If this measurement is desired, sample pH adjustment to 4.0
is required and no chromate can be present (chromates interfere with the
analysis). A device is also available for free residual chlorine.^ This
instrument has the advantage of not requiring pH adjustment and is not
affected by the presence of chromates, phosphates and defoaming agents.
Approximately $10,000 should cover equipment and installation costs (in-
cluding safety devices) of adding residual chlorine control.I1
Nelsonl2 has published a mathematical model which predicts residual chlorine
levels in cooling tower blowdown. The analysis of the model suggests poten-
tial methods of reducing residual chlorine levels. In addition to installa-
tion of controls as discussed above and reducing the level and duration of
54
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chlorination as much as possible, these methods include:
1. Chlorination of only a portion of the system at a time, so
that chlorinated effluent may be reacted with and diluted
by unchlorinated effluent.
2. Increasing the water volume of the tower system.
3. Stopping blowdown during chlorination.
TEMPERATURE CONTROL
Automatic control based on cooling water temperature is not practiced. How-
ever, it is good practice to monitor both warm water and cold water tempera-
tures and set high temperature control points beyond which an alarm would
sound. This is useful to detect possible emergency situations and alert
plant personnel accordingly.
Standard thermocouple instrumentation is appropriate since cooling water is
neither exceptionally high in temperature nor contaminant content.
CORROSION DETECTION
Most methods of corrosion detection are manual in nature. Coupons are in-
serted in appropriate areas of the cooling system and removed from time to
time to examine their weight loss. There are at least two devices^ for the
continuous monitoring of corrosion, and as such could be used to control
chemical inhibitor feed rates. Such a system would probably cost about
$5,000.11
CHEMICAL ADDITIONS
Chemicals are normally fed to the tower system by means of a variable-rate
pump. The feed rate is either set manually and adjusted as indicated by
periodic analysis, or may be automatically controlled. Automatic control
of chemical addition is most commonly derived from make-up rate when a spe-
cific control measurement is not possible.
55
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REFERENCES
1. Dearborn Chemical Feeding Equipment, Equipment Bulletin,
1007. 7.1, Model 780 Automatic Feed and Bleed Controller
for Cooling Water.
2. Hercules Environmental Services Bulletin, ESD-108 A, Tower
Controller II, Wilmington, Delaware.
3. Feitler H., and C. R. Townsend, Novel Cooling Tower Control
System, Magna Corporation, (Presented at Bi-Annual Meeting,
The Cooling Tower Institute, Los Angeles, June 1968.)
4. Schieber, J. R., Cooling Water Control, Technical Paper 218,
Universal Interloc, Inc., Santa Ana, California.
5. Whitcomb, C. G., Cooling Water Treatment Automation Saves
Money Resources, Cuts Down Time. Heating, Piping and Air
Conditioning, Vol. 42, October 1970.
6. Baker, R. J., Instrumentation of Residual Chlorine Measurement,
In: Proceedings of the 29th International Water Conference,
Pittsburgh, Engineers Society of Western Pennsylvania, November
19-21, 1968. p. 65-67.
7. Wallace & Tiernan, Residual Chlorine Analyzer for Cooling
Water, Series 50-236, Belleville, New Jersey, Cat. File 50.236.
8. Ibid, Residual Chlorine Analyzer for Wastewater, Series A-702,
A-767, A-780, Belleville, New Jersey, Cat. File 50.215.
9. Fisher and Porter, Anachlor Residual Chlorine Controller,
Specification 17S2210, Warminster, Pa.
10. Weirbach, S. Ibid. Telephone conversation on August 29, 1973.
11. Petrey, E. Q. Petrolite, Inc. St. Louis, Missouri. Telephone
conversation on September 6, 1973.
12. Nelson, Guy R., Predicting and Controlling Residual Chlorine
in Cooling Tower Slowdown. Pacific Northwest Environmental
Research Laboratory, U.S. Environmental Protection Agency.
Corvallis, Oregon.
56
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SECTION VII
BLOWDOWN TREATMENT AND REUSE
INTRODUCTION
Slowdown from large cooling towers can be handled in the following ways:
1. Discharged directly to receiving water
2. Treated and then discharged
3. Evaporated or treated for reuse (zero discharge)
All three of these methods are being practiced in various parts of the
country. In addition, more sophisticated blowdown treatment techniques
are being developed for large power plant cooling systems.
Both existing handling methods and new developments are discussed below.
DISCHARGE METHODS
Direct Discharge -
Where either: (a) effluent limitations and water quality standards allow,
(b) chemical treatment is minimal, or (c) the temperature difference between
the discharge stream and the receiving water is small, direct discharge of
cooling tower blowdown to the river is practiced. An open-ended discharge
pipe is usually located at the surface of the receiving water body. Natural-
ly, the cost of this type of discharge system is minimal, usually less than
$1 million, depending on plant site characteristics.
Diffusion Systems -
Diffusion systems are used in some steam-electric plant once-through cool-
ing systems to distribute the heated effluent into the receiving water, thus
minimizing .the temperature rise of the water near the discharge point.
Diffusers are also applied, though much less frequently, to cooling tower
blowdown streams in order to either minimize temperature increases near the
discharge point or to minimize the color added from cooling water treated
with either chromates or lignosulfonates. It should be noted that while
diffusers can help reduce temperature increases, they cannot reduce the
amount of heat added to the receiving water.
Several types of diffusion systems are used. The first is a multiport
diffuser which basically consists of two or more discharge headers contain-
ing multiple discharge nozzles designed for an exit velocity of 3-5 meters/
sec (10-15 ft/sec). Figure 11, taken from Brodfeld's presentation to the
Pacific Coast Electric Association,9 shows a multiport diffusion system
installed in a large lake. The design discharge flow rate for this system
is 84,150 m3/hr (825 ft3/sec). High velocity jet diffusers positioned be-
low the surface of the receiving water can also be applied.
57
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-PORTS 16" TO 24" DIA
IN ALTERNATE DIRECTIONS
DISCHARGE LINE
(12' DIA)
•JETTIES
POWER
PLANT
SCREENWELL
INTAKE
CANAL
PLAN
DIFFUSER PORT
CONCRETE DIFFUSER PIPE
OIFFUSER PORT
CONCRETE
OIFFUSER PIPE
CONCRETE
.^AROUND PORTS
SAND FILL
GRAVEL BED
TYPICAL LONGITUDINAL SECTION THROUGH DIFFUSER
TYPICAL SECTION THROUGH DIFFUSER PORT
Figure 11. Multiport Diffusion System
-------
Table 10. Effect of Cycles of Concentration
on Cooling Tower Effluent
Diffusion System Costs
Cycles of
Concentration
Capital Costs, $/kw
Dilution
Multiport Pumping
Operating & Maintenance
Costs, mills/thousand kwh
, . Dilution, , v
MultiportUJ Pumping' ;
1.5
2.0
3.0
4.0
0.
0.
0.
0.
15-0.
10-0.
06-0.
05-0.
79
52
35
27
0.
0.
0.
0.
39-1.
26-1.
17-0.
13-0.
57
04
69
53
0.
0.
0.
0.
34-1.
23-1.
14-0.
11-0.
80
19
80
62
2.
1.
0.
0.
67-10.74
57- 8.14
96- 5*47
89- 3.36
(a) Includes an annual maintenance allowance equal to two percent of
the capital investment.
(b) Assumes design operation 8760 hrs/yr.
59
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Dilution pumping can also help to minimize temperature increases in receiving
waters. In this process, cold water from the receiving body is pumped into a
dilution pond or channel where it mixes with cooling water from the condenser.
Brodfeld has estimated the incremental cost of diffusion systems over conven-
tional surface discharge. His estimates were based on the requirements of a
1,000 raw nuclear power station, once-through system, with a condenser tempera-
ture increase of 11°C (20°F). Cost estimates he derived for fossil-fueled
plants have been manipulated in an effort to correlate diffuser costs with
cycles of concentration. These costs are given in Table 10. Costs may be
even higher where exceptionally long discharge header pipes are required.
TREATMENT METHODS
Sedimentation Ponds -
Sedimentation is by far the most commonly used blowdown treatment method.
Either earthen ponds, built specifically for the blowdown stream, or ash
holding ponds are generally used to hold blowdown for periods ranging from
ten minutes to twenty-four hours or more. Steel clarifiers with mechanical
rakes, which are sometimes used to remove suspended solids from cooling sys-
tem makeup water, have not been applied to the blowdown streams of large
cooling towers.
Blowdown sedimentation ponds usually accomplish three purposes:
1. Suspended solids removal - effluent from ponds normally contain
less than 35 ppm of suspended matter.
2. Chlorine residual dissipation - where chlorine is used as a
biocide in a tower, the pond normally provides sufficient
holding time to allow the residual free chlorine to dissipate.
Approximately 30 minutes is required for this purpose.
3. Final cooling - allows blowdown to more closely approach the
receiving water temperature, depending on the pond retention
time and climatic factors.
Sedimentation ponds are usually 10-15 feet deep and have a clay lining.
Apparently plastic liners have not been widely accepted as yet. Their
cost will depend mainly on land values and the desired retention time for
the blowdown. The estimated range is $3 - $6 per m3/hr ($0.68 - $1.36/gpm)
of blowdown as shown in Table 11. The costs provided in Table 11 are based
on the blowdown volume, and not on power station electrical generation.
Maintenance and operating costs will be dependent on whether the pond liner
requires repair, but should be in the order of $0.01 - $0.02 per thousand cubic
meters ($0.38 - $0.76/million gallons blowdown). Total costs, assuming an
amortization rate of 15% per year are $0.06 - $0.12 per thousand cubic meter
($0.23 - $0.46/million gallons) of blowdown.
Cooling Towers -
Mechanical draft evaporative cooling towers have found use in cooling two
. types of effluent streams from steam-electric power plants. As shown in
60
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Figure 2, they handle cooling water from the condenser just prior to discharge
in once-through cooling systems. But in addition, mechanical draft towers are
also used to further cool the blowdown from other cooling towers. This dissi-
pates additional heat from the blowdown and decreases the temperature differ-
ence between the receiving water and the blowdown stream. One such system
was recently built in Pennsylvania.
Although the use of cooling towers allows positive control of blowdown stream
temperature, towers may have to be treated occasionally with biocides for bio-
logical organism control. These biocides may cause harm to aquatic life in
the receiving stream if addition rates and biocide residuals are not properly
controlled. In many recirculating cooling systems, the blowdown stream is
shut off during the chlorination (biocide addition) cycle to prevent large
doses of free chlorine from reaching the receiving waters. However, in a
blowdown cooling process, recirculation of the water is not possible, so
either low chlorine dosages must be applied or relatively low toxicity bio-
cides must be used.
Mechanical draft cooling towers in blowdown cooling service should cost
$20 - $30 per m3/hr ($5 - $7/gpm) of blowdown feed. Operating and mainten-
ance costs should be in the range of $2.20 - $2.75 per thousand m3 of blow-
down fed ($8.30 - $10.40 per million gallons), and total costs, including
a capital cost amortization of 15 percent/yr should run $2.54 - $3.26/
1,000 m3 ($9.60 - $12.30 per million gallons).
ZERO BLOWDOWN TREATMENT AND REUSE
Evaporation Ponds -
Blowdown streams are disposed of in evaporation ponds where warm dry
climatic conditions exist and where land costs are relatively low. Evapo-
ration ponds are normally lined with clay to prevent leakage, and the ground-
water tables are frequently monitored to assure that there is no infiltration
of blowdown water. In at least one case, however, an evaporation pond has
been built over an underground brackish water table. Over 185,000 cubic
meters per year (6 1/2 million cubic feet/yr) will seep into this water sys-
tem, and the remainder of the blowdown water will evaporate. It should be
noted that EPA does not consider infiltration as a means for achieving zero
discharge.
Evaporation rates in the Southwest are 0.11 - 0.22 m/yr (2.5 - 5.0 gpm/acre).
Based on recent cost analyses by a California utility company, the expected
installed costs for evaporation ponds are $10,000 - $30,000 per m3/hr ($2,300
$6,900/gpm). Operating and maintenance costs will depend on the frequency
of pond lining repair, but should average $100 - $150 per 1,000 m3 (38c -
57
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Table 11. Slowdown Treatment System Costs
System
cr.
Installation Annualized ., . Operating and . .Total Costs Principal System
Costs Capital Costs' Maintenance Costs3' Characteristics
$/m3/hr fed •$'/!.OOP m3 fed $/1.00Q m3 fed $71.000 m3 fed :
Slowdown Treatment
Sedimettation Pond
Mechanical Draft
Evaporative
Cooling Tower
Chromate Recovery
Blowdown Disposal
Mechanical
Evaporator
3-6
20-30
8400
38,000
0.05^0.10
0.34-0.51
144
650
0.01-0.02
(c)
2.20-2.75
37
230
0.06-0.12
2.54-3.26
181
880
1. Provides solids settling,
chlorine dissipation, and
usually some cooling.
2. Costs dependent on land
values and climate.
1. Allows positive control of
blowdown temperature.
2. May require biocide treatment.
1. Can provide zero discharge'*5)
2. Concentrated brine must be
disposed of: evaporation costs
estimated at $.13-.66/m3 of
blowdown fed.
Evaporation Pond 10,000-30,000 171-514
100-150
(d)
271-664
Successful use depends on
arid climate.
(a) Based on 15 percent/yr
(b) No credit taken for reusable water.
(c) Maintenance estimated at 3 percent/yr of capital investment
(d) A California utility noted extensive problems with evaporation pond
linings. Their actual maintenance costs have been used to calculate these
cost estimates.
-------
pond with secondary treated sewage for Irrigation purposes." Another farmer
has used unblended cooling tower blowdown water for irrigation over the past
eight years. Some problems with crop seed germination have been encountered
when the irrigation water total dissolved solids concentrations were high.
Blowdown Recycle to Lime-Soda Softener -
Fowlkes^ has reported that blowdown from the K-33 cooling tower at the Atomic
Energy Commission's Oak Ridge Gaseous Diffusion Plant is recycled back to the
cold lime-softeners which pretreat the tower makeup. This closed 1°°P
system is possible mainly because the concentrations of sodium, chloride,
and sulfate ions in the makeup stream from the Clinch River is less than 10
ppm. Sludge from the lime softeners is stored in holding lagoons.
DEVELOPMENTS IN BLOWDOWN HANDLING TECHNOLOGY
Chromate Removal Systems -
Few large cooling systems are treated with chromates for corrosion protection,
although chromate removal systems are available should the need for them arise.
Two types of chromate removal systems have been developed. In the first sys-
tem, blowdown is filtered for suspended solids removal, adjusted to a pH of
less than 5.0 with sulfuric acid, and then fed to an ion exchange unit where
chromate ions are selectively removed by strong base anion exchange resin.
This process is in use in a number of smaller cooling systems. One of the
disadvantages of this system is that it can add significant amounts of dis-
solved solids to the blowdown stream.
Chromate can be collected in a fairly concentrated form from the ion exchange
resin bed by intermittently backwashing with caustic solution. This backwash
water may be added back to the cooling water system as usable corrosion
inhibitor.
The second chromate removal system is less commonly used and does not allow
reuse of collected chromate. Instead, the blowdown stream is adjusted to a
pH of less than 5.0 with sulfuric acid, then treated with a reducing agent
such as sulfur dioxide, ferrous sulfate or sodium bisulfite.-5 The pH is then
raised above 8.0 where the chromate precipitates and settles as chromic hydro-
xide. The sludge collected must then be discarded.
Estimated costs associated with an ion exchange chromate removal system as
derived from Nalco Chemical Company economic data are shown in Table 11.
Installed costs are roughly $8,400 per m^/hr ($l,900/gpm) of blowdown fed.
Operating and maintenance costs are approximately $37 per thousand cubic
meters (14£/thousand gallons), and total costs equal $181 per 1,000 m3
(69c/thousand gallons). Chromate removal to below 1 mg/1 is claimed by
the manufacturer.
fil
-------
Combined Use of Sewage and Slowdown -
Consideration has been given to the combined treatment of cooling tower blow-
down and sewage.5,7 Although large treatment systems have not been designed
for this purpose, there are several benefits that could be derived from this
combined treatment concept, as cited by Young:7
1. Waste heat from a power plant would be effectively utilized
to treat sewage.
2. A large saving of horsepower could result from the automatic
aeration of sewage that takes place during the cooling of the
blowdown-sewage mixture.
3. Capital investments could be substantially reduced over
separate treatment systems (cooling and sewage treatment).
4. The use of clean water from ground water tables or other
relatively pure sources could be reduced or eliminated.
Further discussion of a plant utilizing sewage plant effluents as cooling
tower makeup, is given in the Section VIII.
Sulfite Treatment for Chlorine -
Aside from retention in sedimentation ponds, one of the methods for chlorine
dissipation in blowdown streams is chemical reduction of the free chlorine
using gaseous sulfur dioxide or bisulfite. Up to now, only one major North-
west power station is implementing this method. However, the adoption of
effluent standards pertaining to chlorine will probably result in greater
interest for this method.
Blowdown Evaporation -
Although not in full-scale use, one of the methods that will be tested at
several of the new major power stations is cooling tower blowdown evapora-
tion. This process, if implemented for the entire blowdown stream, could
produce a high quality water stream of near-boiler feedwater quality and a
concentrated brine stream that is roughly 1-5 percent by volume of the ori-
ginal blowdown stream.^ The major advantage of this system is that it pro-
vides zero discharge.
A sketch of the blowdown evaporation process is shown in Figure 12. The
blowdown stream is pretreated with sulfuric acid, heated with warm product
water to roughly 98°C (200°F) and then deaerated to remove oxygen, carbon
dioxide and any other non-condensible gases that may be present. The blow-
down stream is then fed to the evaporator where it is pumped to the top of
the heat transfer section. The water falls as a film over the outside of
heat transfer panels and some of the water is evaporated as steam. This
steam is collected and compressed in a compressor, and then fed to the in-
side of the heat transfer sections where it is condensed as clean water
while evaporating the blowdown-brine stream. This product stream which
should contain less than 10 mg/1 of total dissolved solids, is cooled and
then pumped away for reuse.
64
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PRETREATMENT SECTION
ACID
FEED
SCAVENGE
COMPRESSOR
TRIM
HEAT
EXCHANGER
STEAM
COMPRESSOR
PRODUCT^-
SCAVENGE
HEAT
EXCHANGER
Figure 12. Slowdown Evaporation System Flow Diagram
(Sketch provided by Resources Conservation Co.)
VENT
EVAPORATOR1
SECTION
-
WASTE
BRINE DISCHARGE
WASTE
PUMP
SEED
RECYCLE
&
SLOWDOWN
SYSTEM
SEED SEPARATORS
-------
The blowdown and brine that does not evaporate falls to the bottom of the
evaporator and is recycled to the top for additional passes across the heat
transfer section. Fouling and scaling are avoided in the heat transfer sec-
tions by recirculating crystals of calcium sulfate with the brine. These
particles are preferential sites for crystal growth. A relatively small
portion of brine is withdrawn from the evaporation system for further pro-
cessing in dryers.
Resources Conservation Co. has recently estimated the capital and operating
costs of a blowdown evaporation system. These data are presented in Table
10. The estimated installed cost for titanium evaporator units is $38,000
per m3/hr ($8,600 per gpm) of blowdown fed. Operating and maintenance costs
are 23c/m3 (87
-------
REFERENCES
1. Young, K. G., and T. M. Fosberg. The Treatment and Reuse of Power
Generation Waste Waters. Resources Conservation Company, Renton,
Washington. 1973.
2. Fosberg, T. M. Industrial Waste Water Reclamation. (Presented at
the 74th National Meeting of the American Institute of Chemical
Engineers, New Orleans, March 11-15, 1973). 13 p.
3. Ladd, K., and S. L. Terry. City Waste Water Reused for Power Plant
Cooling and Boiler Makeup. Southwestern Public Service Company.
Amarillo, Texas.
4. Fowlkes, C. C. Softening of Cooling Tower Slowdown Water for Reuse.
Union Carbide Corporation. Oak Ridge, Tennessee. Report No. K-P 4023.
United States Atomic Energy Commission. January 5, 1973. 22 p.
5. Reuse of Chemical Fiber Plant Wastewater and Cooling Tower Slowdown
Fiber Industries, Inc., and Davis & Floyd Engineers, Inc. Wasington.
Report NO. 12090 EUX. United States Environmental Protection Agency,
October 1970. 66 p.
6. Kelly, B. J. Removing Chromates. Nalco Chemical Company. Chicago.
Reprint No. 174. September 1968. 4 p.
7. Young, R. S. Combined Treatment Answers Two Problems. Pollution
Engineering, p. 28-29. July 1972.
8. Goldman, E. and P. J. Kelleher. Water Reuse in Fossil-fueled
Power Stations. Bechtel Corporation. (Presented at the Conference on
Complete Water Reuse, Sponsored by AIChE and EPA. Washington, April
23-27, 1973.) 29 p.
9. Brodfeld, B. Thermal Discharge Regulations - Engineering and Cost
Considerations. Stone & Webster Engineering Corporation. (Presented
at the Pacific Coast Electric Association Meeting. Los Angeles.
March 22, 1973.) 18 p.
67
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SECTION VIII
EUROPEAN PRACTICES
COOLING TOWER TYPES
In Europe fuel costs are higher than in the United States, so that the use
of hyperbolic natural draft cooling towers rather than the power consuming
mechanical-draft towers has been common for several decades. Also, because
of the more crowded conditions and the lack of large inland rivers, the need
for cooling towers has been felt earlier and to a greater extent. These con-
siderations have resulted in many more natural draft towers cooling electri-
cal power generating stations and a considerably longer operating experience.
It is because of this that a survey of practices in England, Germany, and
France was included in this program. The following sections discuss the
results of this survey.
FEED WATER PRETREATMENT
In general, feed-water treatment is not practiced. This is especially true
in England, where the power plants are located near rivers with adequate
water supply or in coastal areas. Only occassionally is settling necessary
to remove suspended solids; normally, power plants are shut down every 12 to
18 months for boiler inspection, and the tower basin will be cleaned at those
times.
In Germany there are areas which are highly developed industrially, where a
combination of scarce water and discharge restrictions have resulted in the
necessity of extensive pretreatment. At Immeburen, the feed-water for a
100 mw unit is deionized by a two-bed ion exchange system, at a cost of approx-
imately $0.9-0.15/m3 ($0.35-0.55/1,000 gal.), resulting in a minimum makeup and
zero blowdown. The high water cost at this plant provided the economic justifi-
cation for the dry tower discussed later.
Where pretreatment to remove suspended solids is practiced, the high cost
of land must be considered. The use of sand filters and chemical flocculants
is often justified to reduce the land requirement.
IN-TOWER TREATMENT
Treatment with chlorine to control microbiological growth in the system is
universal, normally on an intermittent basis, to a 0.5 ppm free chlorine
residual. In England, no other treatment is normally practiced, and the
number of cycles of concentration is limited to 1.5 to 3, usually 2.0 to
2.5, in order to control scale. Calcium carbonate scale desposition is
the normal controlling factor limiting the concentration cycle. In general,
it is considered more economical to install the extra pumping capacity and
water supply to limit the concentration than to feed acid to control scale.
68
-------
In Germany, for the reasons discussed earlier, higher cycles of concentrations
are normally employed, with 4-7 concentrations being common, Limiting factors
are usually total dissolved solids of 5,000 ppm, sulfate of 5-600 ppm, and
hardness of 90-120 ppm. if the hardness goes much above 100 ppm as CaC03,
sulfuric acid is added to control the pH to the 6.8-7.2 range. In addition,
the use of commercial dispersants to control deposition is common above 2-?3
concentrations. The use of sidestream filters for 2-5 percent of the re-
circulating water is common on smaller units operating at higher concentra-
tions, but we were not able to find any such installations in units of the
size being considered here.
Chlorination is the only microbiocidal treatment used, normally 0.1 to 0.3
ppm free Cl2» controlled only by dosage, not directly.
The Balke-Durr Company, which supplies cooling towers and consultation on
treatment, stated that they recommend the control of acid addition by pH
measurement. Other chemical additives are controlled by pulses from a flow-
meter on the makeup water.
SLOWDOWN TREATMENT
No power plant installations were found which practice any blowdown treat-
ment before discharge except thermal. Where possible, hot water is used for
blowdown, but normally thermal requirements necessitate blowdown from the
cool water side. At least one plant in England has had to install a small
mechanical-draft tower to further cool the blowdown before discharge.
One interesting cooling system was inspected at the Aquitane refinery at
Pau, France. This system consists of seven cooling towers with an aggre-
gate recirculation rate of 1,150 m^/min (292,000 gpm). The makeup water
has the following analysis:
pH 8.0
Hardness 120 mg/1 as CaC03
Alkalinity 80-100 mg/1 as CaCOs
Sulfate 15 mg/1 as 804
Chloride 4 mg/1 as Cl
Silica 5-10 mg/1 as Si02
The towers operate on an average of 13°C drop across the tower.
Makeup water is filtered through a 0.8 m thick bed of sand with particle
size 0.8 - 2.6 mm. In addition, pressure side-stream filters are used
on each tower operated at a rate such that the tower volume is circulated
through the filter once in 24 hours.
The system is primarily constructed of mild steel. Corrosion is controlled
by use of a chromate-zinc treatment maintained at 15 ppm Cr04. Scale and
deposition are controlled by automatic pH adjustment to 6.4-7.0 with sul-
furic acid and a polyacrylic dispersant. They have tried the
circulating ball system to clean condenser tubes, and found it to be
effective with brass tubes but not steel, because of corrosion. Total
chemical treatment cost is about $50,000/year.
69
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The feed of makeup water is operated by level controls. Slowdown is inter-
mittent, being started when total hardness reaches 8-900 ppm as CaCC>3 and stopped
when hardness reaches 600 ppm. This limitation on concentration is to pre-
vent calcium sulfate scale. The blowdown is treated with ferrous sulfate
solution to reduce chromate, and then mixed with other refinery wastes in a
settling/oil-removal basin. The FeS04 performs two functions; it is utilized
to reduce chromate, and also then provides flocculation in the final treat-
ment basin.
The company is utilizing direct air-cooling more and more in new refinery
processes, and estimate up to 90 percent reduction in water use can be made
in this fashion. However, air-cooling is not feasible in certain processes
where limitations on process temperatures requires water-cooling.
OTHER PROBLEMS
Leaching -
The leaching of wood preservatives is not a problem. Baltic redwood is
universally used for wood-fill towers in England and Germany, and is treated
with a copper-chrome-arsenate (CCA) treatment by means of a vacuum process
to 1.25 pounds per ft^. This gives a life expectancy of 30 years, and no
chemical leaching has been encountered. The general feeling was that Baltic
redwood was better than American in terms of lower resin content and hence,
will take a larger concentration of the CCA preservative.
The English power companies have noticed some leaching of calcium from
cement-board filled towers, but no problems have been encountered. They
have not encountered any problems with copper in the effluent.
Fogging, Drift -
No fogginR or drift problems from natural draft towers have been encountered
from plumes in either Germany or England. In England, one station encount-
ered some icing problems due to drift from the bottom of the tower during high
winds. The various concerns over drift and cloud formation have proved to be un-
founded. Towers of earlier vintage have been back-fitted with drift eliminators,
and modern towers are commonly built to have 0.05 percent drift loss or less.
Visual Impact -
The Central Electric Generating Board of England stated that since most of
the environmental concerns associated with the blowdown and plume had been
unfounded or corrected by proper design, they do not encounter public oppo-
sition concerning the location of new power plants for any reason other than
that of the visual aesthetics, or lack thereof, of the large natural-draft
towers. For this reason they have done some experimenting with arrangements
of multiple towers. For example, on 2,000 mw plants requiring eight 100-
meter towers, they have tried circular, rectangular, and staggered lay-outs,
and are now constructing plants with two or three taller (120-150 meter)
towers. They have also tried the use of various amenities to make generating
stations more acceptable to the public, including nature trails, boating
facilities, and the use of tower basins for fish culture.
70
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UNIQUE FACILITIES
Heller-System Dry Towers -
Two plants in Europe utilize dry towers based on the system developed by
Professor Heller of Hungary. These towers are natural draft hyperbolic
units, with finned radiator sections arranged around the base. The water
from the tower, which is of condensate quality, is pumped to a contact con-
denser instead of the normal tube-type condenser, where it is sprayed into
contact with steam from the turbines to provide the condensing action. A
portion of the water leaving the condenser is sent to the boiler as feed,
and the balance is returned through the tower. This is a completely closed
system and there is no makeup required except for leaks. A system of louvers
around the outside of the tower is automatically opened and closed to control
the temperature of the water leaving the contact condenser. A temperature of
maximum efficiency is set by the fact that as the temperature is lowered, the
back pressure to the turbines is lowered, which is desirable, but at the same
time more fuel is required to heat the water in the boiler. For example, for
the plant at Immeburen, Germany, this temperature is 21°C and it is not desir-
able to cool to below this point.
The primary operating problems which have been encountered involve the radia-
tor sections, which are aluminum fins. At the German plant, dirt collects in
the passages between these fins to the point where air flow is restricted, and
the fins must be cleaned by blowing with compressed air. At the plant at
Rudgely, England, a corrosion problem was encountered at the crevices where
the aluminum fins joined the tube.
Dry tower systems are considerably more expensive than wet towers. For ex-
ample, estimates for a 1,000 mw nuclear plant place the entire cost of the
cooling system as $18 million for once-through cooling, $23.7 million for
a system using wet cooling towers, and $32.4 million for a dry-tower system.
The additional power cost was estimated at 0.54 mills/kwh for the wet tower
system as compared to once-through cooling, with an additional 0.46 mills/kwh
when a dry tower is used. The point is raised that with the dry tower, advan-
tage can be taken of a central location because water supply is not critical,
and that there are no plume problems. The economic break-even point for the
wet-dry tower choice was put at $0.12-0.27/m3 ($0.45 - $1.00 per 1000 gallons)
water cost, with current average water costs estimated at $0.03-0.06/m^ ($0.12-
$0.22 per 1000 gallons).
Cable Cooling Towers -
A novel cooling tower is being constructed under financing of the West German
government. This tower is shown in Figure 13, and consists of a center mast
575 ft tall, with an upper ring 290 ft in diameter and a lower ring 490 ft in
diameter. These rings are connected by cables to give a hyperbolic shape as
shown, with a minimum diameter of 260 ft. The network of cables will be
covered with aluminum panels to complete the tower. It is hoped that this
method of construction will eliminate the size constraints imposed by the
present reinforced-concrete construction. The first tower is being designed
and constructed by two large German cooling tower firms, Balke-Durr and GEA,
working together. The unit will be at the Schmehausen station, cooling a
300 nw unit, but will be designed to accomadate a 500 mw unit.
71
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. Y W
\ '• W
' ! ' \ \ : <
/ A : * X ?
^^
11111\
Figure 13. Cable Tower
_ 72
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Use of Sewage Plant Effluent as Makeup -
At the Croyden D station in England sewage plant effluent is used as makeup
for a 210 mw unit. The sewage treatment plant is an activated sludge secon-
dary treatment plant, and the effluent is sand filtered before use. Consi-
derable problems with calcium phosphate deposition on the tubes were encount-
ered when the plant was first put on line 15 years ago. The pH of the sewage
effluent is in the 7.0-7.2 range, and has a bicarbonate alkalinity of 250 ppm
with 40 ppm free C02. In use, C02 was released, the pH rose to about 8.4,
with scaling of the tubes by a hard scale composed of about equal parts of
Ca(H2P04)2 and CaHP04. Initial efforts to remove the phosphate from the tower
feed by lime treatment were expensive because of the large amounts of acid
needed to reduce the pH. The addition of acid to keep the tower in the pH
7.0 range was also uneconomical due to the high alkalinity. The final solu-
tion was to innoculate with nitrifying bacteria, which converted the 40 ppm
of ammonia in the feed to nitric acid, which neutralized almost all of the
alkalinity, so that the tower operated at a lower pH. It was found that
each ppm of NH3 when converted to nitric acid neutralized 6 ppm of alkalinity
and so the ammonia neutralized 80 percent of the alkalinity present. (Nlfy^SO
is now fed to control the pH to the desired value by supplying the extra NH3
needed. They control by alkalinity, using the following equations:
Ca x P = 103'85 + 10ll-55(H+) to determine the pH necessary to
prevent phosphate precipitation, .where Ca is the ppm of CaCC>3 and
P is the ppm of phosphate as
pH = 5.56 + log alkalinity, to determine the alkalinity to give
the necessary pH.
As an added benefit, the BOD5 value of the tower feed is reduced in the system
by 60-80 percent, so that the tower acts as a tertiary treatment system. The
filtered sludge is returned to the sewage treatment plant. There is a slight
fouling of the tubes, and since chlorination cannot be used because it is re-
duced by the ammonia present, and would interfere with the nitrifying bacteria
population, the fouling is removed periodically by blowing with air with hot
water on the steam side of the tubes. The slime drys and cracks off easily.
The system is so successful that they are planning to add another 1,000 mw
unit to utilize the balance of the effluent available from the sewage treat-
ment plant.
4U.S. GOVERNMENT PRINTING OFF ICE:1974 546-315/2^7 1-3
73
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SELECTED WATER
RESOURCES ABSTRACTS
INPUT TRANSACTION FORM
3. Accession Wo.
4. Title
Technical and Economic Evaluation of Cooling System Blowdown
Control Techniques
7, Authoi(s)
D. B. Boies, J. E. Levin, D. Baratz
9. Organization
Wapora Inc.
6900 Wisconsin Ave. NW
-Washington, DC-20015—
tt. Sponsoring Organization
IS. Su
i'Organiisattoo
"
10. Project No. PE 0B12036
ROAP 16ACQ Task 11
11. Contract/Gram ffo.
68-03-0233
13. Type cf Repot: jnd
Period Covered 4*73
Final 9*73
Environmental Protection Agency ateport number,
EPA-660/2-73-026. November 1973.
16. A hstract
This report presents descriptions of methods which are either currently applied or
commercially available to reduce the pollution Impact of blowdown from large
cooling systems (recirculating rates > 500 cfs). Treatment equipment descriptions,
capabilities and compatabilities are discussed. Where appropriate, broad ranges
of both capital costs and operating expenses are provided.
The described methods Include (a) the application and design of closed-cycle cooling
systems, (b) makeup water treatment, (c) recirculating water treatment (d) mechanical
treatment, and (e) blowdown treatment and/or disposal.
17a. Descriptors
Cooling systems, Cooling towers*, cooling water, Control systems*, water
treatment*
17b. Identifiers
Automatic control, control, instrumentation, water management, water pollution
treatment*
J7c. COWRR Field& Group 05G, 05F
IS. Availability
Release to Public
•, -••• ,,:••
20. S&uiritf Class.
22, •
Send To:
WATER RESOURCES SCIENTIFIC INFORMATION CENTER
U.S. DEPARTMENT OF THE INTERIOfl
WASHINGTON, D. C. 2O24O
Abstractor
I Institution
WHS 1C 102 (REV. JUNE 197 I)
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