United States
Environmental Protection
Agency
Industrial Environmental Research
Laboratory
Cincinnati OH 45268
Research and Development
EPA-600/S7-83-037 Oct. 1983
Project Summary
Environmental and Energy
Benefits of Microprocessor
Control of Oven Airflows from
Metal Painting Operations
In most industrial operations the
percentage of the lower explosive limit
(LEL) of VOC emissions typically remain
at below 5 to 10 percent LEL at most
metal coating operations. Although
monitors are available to determine
solvent concentrations, they require
manual corrective action to the pro-
cess when potentially explosive oven
atmospheres are generated. This proj-
ect demonstrated the capability of
microprocessors to control and allow
ovens to operate close to 50 percent
LEL while not exceeding that level.
With instantaneous process control
allowing operation closer to 50 per-
cent of the LEL dilution air and energy
requirements for the process and for
incineration will be reduced. Success-
ful demonstration of the use of micro-
processors resulted in a control option
that is both efficient and economical,
and make the use of incineration with
heat recovery much more efficient and
attractive.
Included in the final report is a tech-
nical design manual on the use and
implementation of microprocessor tech-
nology into paint baking ovens for
energy conservation and VOC pollu-
tion reduction from metal coating
processes.
This Project Summary was developed
by EPA '.i Industrial Environmental Re-
search Laboratory. Cincinnati, OH. to
announce key findings of the research
project that is fully documented in a
separate report of the same title (see
Project Report ordering information at
back).
Introduction
Industrial coating curing ovens require
ventilation air to maintain the solvent con-
centration below its lower explosive limit
(LEL). However, higher-than-required ven-
tilation airflow rates drastically increase
the oven exhaust rate and result in higher
energy and environmental compliance
costs. The increased cost of environmen-
tal compliance results from larger incin-
erator capital and operating costs. The
added energy costs are due to the heating
requirements for the ventilation air and the
added fuel for the incinerator.
Prior to this project automatic control of
ventilation airflow rates based on con-
tinuous solvent concentration measure-
ments in curing ovens and microcomputer
technology were not used in the American
coating industry. There were several rea-
sons for this:
• Only recently have fuel costs and
environmental compliance require-
ments made improved control a
major economic factor.
• There was considerable industrial
skepticism about the accuracy of
instruments for continuous measure-
ment of solvent concentrations.
• The complexities of oven dynamics
preclude simple analog control and
only recently has the microprocessor
brought digital control within an
economic range for most oven
systems.
Because of the large number of paint
curing ovens in the United States, auto-
matic control can have a major impact on
the cost of environmental compliance and
energy costs.
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The U.S. Environmental Protection
Agency, the Department of Energy, and the
Chemical Coaters Association cooperative-
ly sponsored a program to evaluate, design,
and demonstrate automatic ventilation air-
flow rate control through direct measure-
ment and control of the concentration of
the solvent in the oven. The specific objec-
tive of this effort was to demonstrate that
ventilation airflow rates could be safely
reduced, thus reducing the energy costs
for curing and incineration of solvents
being vented from the curing oven.
Industrial Survey
Prior to developing a control system, an
industrial survey was undertaken to deter-
mine the potential benefits of this concept.
A survey of 29 sites indicated that the
average solvent concentration in the ex-
haust from curing ovens was about 7
percent of the LEL In comparison, the
National Fire Protection Association (NFPA)
requires only that the percentage of the
LEL be kept below 25 percent unless
solvent indicators, alarms and shutdown
of oven heating systems are provided. If
this equipment is installed, the maximum
solvent concentration should not exceed
50 percent of the LEL.
The lower-than-required solvent con-
centration in actual operation is usually a
result of compounding safety factors built
into the oven design, starting with the
user's specifications and continuing with
the oven manufacturer. The end result is
excessively high curing oven ventilation
airflow rates in most plants.
Based on the evaluation of curing oven
systems during Task 1, it was concluded
that automatic control could significantly
reduce operating costs in both ovens and
incinerators as well as lower investment
costs for VOC control systems.
The sponsoring agencies stipulated that
this concept be proven in the miscella-
neous metal products industry and that its
conclusions and benefits be applicable for
other coaters using coatings containing
volatile organic compounds. This could
include such industries as:
Coil
Fabric
Can
Automobiles and Light Duty Trucks
Printing
Appliance
Demonstration Site
Based on established criteria for selec-
tion of a host site, Mack Trucks, Inc.,
agreed to participate. The Building 10
Assembly Plant in Allentown, Pennsylvania,
which had multiple curing ovens and one
catalytic incinerator, was chosen as the
demonstration site. The curing ovens are
typical of the systems used throughout the
miscellaneous metal products coating
industry.
Figure 1 shows the basic design func-
tions of a control system. After a detailed
evaluation, an Intel 8086-based controller
and an analyzer using the flame ionization
detection (FID) principle of operation were
selected as the key hardware components.
Solvent concentration was monitored at
four points inside the controlled oven.
The system started up on October 1,
1981 and immediately reduced airflows
from 1.6 mVs to 0.23 mVs (3400
stdftVmin to 490 stdftVmin), a reduc-
tion of 86 percent This resulted in an
energy usage reduction for air heating of
55 percent in the curing oven and 13
percent in the catalytic incinerator. VOC
destruction efficiency showed an improve-
ment of 84 percent to 94 percent At this
point most oven control dampers were
closed and inflow from oven openings
became the only source of ventilation
airflow.
System Design
The control system at Mack Trucks wa
designed to control the dip oven at th
minimum ventilation airflow rate whic
would either control the solvent concer
tration at 35 percent of the LEL or mair
tain a slightly negative pressure inside th
oven. The control point became pressure
the maximum solvent concentration reache
was 12 percent of the LEL The minimur
ventilation airflow rate was 0.23 m3/
(490 stdftVmin) which was 1.37 m3/
(2910 stdftVmin) lower than the rat
before the control system was installed.
The expected control set point of 3!
percent of the LEL was not reached fortw
reasons. First due to the depressed true
market the production level through th
dip oven dropped to about one-third of th
normal level which, in turn, reduced th
solvent load. Secondly, although dat
received from Mack Trucks that repr(
sented normal production levels indicate
that the solvent loading would give a
average exhaust concentration of appro)
imately 6 percent of the LEL at 1.6 m3/
(3400 stdftVmin) ventilation airflow rate
Oven &
Incinerator
Hydrocarbon
Analyzer
Solvent Concentration
Calibration & Safety Checks
Micro Computer
CRT & Printer
T
Alarms
Figure 1. Basic design features of control system.
Alarms
Printed Reports
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measurements taken at the plant just prior
to installation of the control system showed
that the solvent concentration was less
than 1 percent of the LEL With these very
low solvent loadings and frequent line
changeovers when no parts were cured, it
was not possible to reach 35 percent of
the LEL since the required airflow would
be impractically low— well below normal in
leakage. At an airflow rate of 0.23 mVs
(490 stdftVmin), the highest average
solvent concentration approached 7 per-
cent of the LEL. During brief runs at higher
coating and production rates, the solvent
concentration increased to 12 percent of
the LEL
Although the maximum solvent level of
35 percent of the LEL was not achieved.
the primary objective of airflow reduction
combined with proof of concept for all
safety, operating control loops, and report
generation was accomplished. The result-
ing energy savings in both the oven and
incinerator were nearly equal to the pro-
jected savings. The airflow rate reduction
was equal to the original projection of 1.3
m3/s (2900 stdftVmin).
System Reliability
Table! is a summary of oven conditions
immediately before and after the system
was placed in automatic control.
The system reliability appeared good,
but decreasing production with numerous
shutdowns because of low sales at Mack
Trucks prevented long-term evaluation of
Table 1. Mack Trucks' Oven and Incinerator Conditions
Oven
Exhaust airflow, stdft?/min
Solvent rate, Ib/hr
Exhaust solvent concentration, % LEL
Exhaust air temperature, °F
Energy consumption, million Btu/hr
ncineratorf
Total airflow, stdff/min
Operating temperature, °F
VOC destruction efficiency, %
Energy consumption, million Btu/hr
Before Automatic
Control
3400
5.7
1
252
1.74
9240
975
84
4.7
After Automatic
Control
490
5.7
7*
282
0.77
6330
1014
94
4.1
continuous mechanical reliability. As ex-
pected, the only major systems requiring
significant maintenance attention were
the hydrocarbon analyzers.
With the first prototype testing com-
plete, the expected benefits to other curing
ovens can be calculated from a knowledge
of oven flow rates and the average percent
LEL maintained before control. Table 2
summarizes the projected industrial fuel
benefits as a function of oven parameters
for the various applications.
This project has shown that reduction of
airflow in curing ovens via microprocessor
control of solvent concentration is a prac-
tical, workable technology. Although the
prove-out of the Mack Trucks parts line
does not allow the automatic extension of
the technology to ovens with higher and
more variable solvent loadings and tem-
peratures, that prove-out on these other
applications will most likely occur through
normal commercial evolution.
*Maximum concentration in the oven was 12 percent of the LEL.
t Includes three ovens plus fume tunnel exhausts. Temperature increased due to minimum
catalyst reactivity temperature requirement and the higher solvent concentration.
NOTE Data supplied by Mack Trucks, October, 1981; incinerator fuel demands were variable due
to cycling of the temperature controller.
Table 2. Projected Energy Benefits from Oven/Incinerator Control for Various Applications
Initial exhaust airflow
rate, Stdff/min
Controlled exhaust airflow
rate, Stdtf/min
Solvent removal rate, gph
Energy consumption of oven
wo/control, MM Btu/hr
Energy consumption of oven
w/control, MM Btu/hr
Energy consumption of oven
ft incinerator wo/control.
MM Btu/hr
Energy consumption of oven
& incinerator w/control,
MM Btu/hr
Miscellaneous Metal
Products Coating
2,000-10,000
1,000-3,000
5-20
1.0-4.0
0.5-1.5
3.0-15.0
1.0-2.5
Fabric and
Paper Coating
10,000-17,000
3,000-4,500
20-33
4.0-6.5
1.5-2.0
15.0-25.0
2.5-3.0
Coil
Coating
17,000-40,000
4,500-8,000
33-75
6.5-15.0
2.0-3.2
25.0-60.0
3.0-4.5
Basis:
Solvent is Toluene.
Initial solvent concentration is 7% of LEL.
Oven exhaust temperature is 350°F.
Thermal incinerator is operated at 1400°F, no heat recovery.
Ranges below are approximations.
Air heating demand only.
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This Project Summary was prepared by staff of Chemical Coalers Association,
Wheaton. IL 60187.
Charles Dan/in is the EPA Project Officer (see below).
The complete report, entitled "Environmental and Energy Benefits of Micro-
processor Control of Oven Airflows from Metal Painting Operations," fOrder
No. PB 83-225 250; Cost: $25.00, subject to change) will be available only
from:
National Technical Information Service
5285 Port Royal Road
Springfield, VA 22161
Telephone: 703-487-4650
The EPA Project Officer can be contacted at:
Industrial Environmental Research Laboratory
U.S. Environmental Protection Agency
Cincinnati, OH 45268
*U,S. GOVERNMENT PRINTING OFFICE: 1983-659-017/7210
United States
Environmental Protection
Agency
Center for Environmental Research
Information
Cincinnati OH 45268
Official Business
Penalty for Private Use $300
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