-United States
Environmental Protection
Agency
Air and Energy Engineering
Research Laboratory
Research Triangle Park NC 27711
Research and Development
EPA/600/S2-85/107 Nov. 1985
Project Summary
Transfer Efficiency of Improperly
Maintained or Operated Spray
Painting Equipment Sensitivity
Studies
K. C. Kennedy
The U.S. Environmental Protection
Agency initiated a research program to
investigate the impact of common in-
dustrial operating and maintenance
practices on the efficiency of spray
painting systems.
Centec Consultants Inc. was con-
tracted to conduct the research pro-
gram. Assistance was also sought and
received from representatives of the
spray painting equipment manufactur-
ing industry and users of spray painting
equipment.
The results indicate strong directional
responses in painting efficiency to cer-
tain common painting practices.
This Project Summary was developed
by EPA's Air and Energy Engineering
Research Laboratory, Research Triangle
Park, NC, to announce key findings of
the research project that is fully docu-
mented in a separate report of the same
title (see Project Report ordering infor-
mation at back).
Introduction
Spray painting transfer efficiency (TE)
is the percentage of paint solids deposited
on a substrate compared to the total
amount of paint solids sprayed at the
substrate. Past research conducted by
EPA has shown that certain operating
and maintenance practices can drastically
affect TE. This test program is designed to
show the effects of defined operating and
maintenance (O&M) parameters on TE.
Operating and maintenance param-
eters were selected for testing from
information obtained through literature
searches, industry contacts, service man-
uals, and technical bulletins.
Some of the identified parameters were
considered qualitative; their value could
be neither identified nor controlled quan-
titatively for a full range of values. Factors
such as booth air velocity, fan air (shaping
air), and electrode position were consid-
ered qualitative for the purposes of the
test program. Qualitative factors were
tested at a minimum of two levels. The
remaining factors, considered quantita-
tive, were tested at five levels. Examples
of quantitative factors selected are:
• restricted air lines (measured by air
pressure at gun),
• restricted paint lines (measured by
paint pressure at gun),
• gun cleanliness (measured by percent
of paint/air holes in air cap plugged),
and
• voltage.
Using the selected O&M parameters,
an experimental design was completed. A
separate experimental design was devel-
oped for each type-of spray equipment
tested. The equipment tested included air
atomized conventional, airless conven-
tional, air atomized electrostatic, and
airless electrostatic.
The test program was conducted in a
fully equipped industrial spray painting
laboratory. Graco, Inc., of Minneapolis,
MN, provided their research laboratory
for the testing phase of the program.
The data analysis techniques used
explored first- and secpnd-order system
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responses, as well as interaction between
parameters. Regressions were developed
to describe the response of TE to O&M
factors for each equipment type. An error
analysis was performed to determine the
"lack of fit" of the models. Equations
describing the system response for each
of the four spray gun types with each of
the two target types were developed using
the Statistical Analysis System (SAS) of
the SAS Institute, Inc.
Operating and Maintenance
Variables
During the initial phase of the project,
industry representatives, consumers, and
manufacturers of spray painting equip-
ment identified 17 operating and mainte-
nance variables considered important in
achieving optimum operating efficiency
for spray equipment. These variables are
listed in Table 1.
Table 1. Operating and Maintenance
Variables for Air Atomized
Conventional Spray Equipment
• gun cleanliness,
• restricted paint lines, and
• fan (or horn) air.
Experimental Design
An experimental design was developed
to address the effects of each variable as
completely as possible.
The major testing limitation was the
number and types of simulation levels for
each variable. Only two levels of linear air
velocity (booth air rate) were possible in
the test laboratory, three levels of fan air
(sometimes called horn air or shaping air)
were achievable, and five or more levels
of the other variables could be simulated.
Table 2 presents the type of variable
(quantitative/qualitative) and levels ac-
commodated in the experimental design.
A central composite experimental de-
sign was selected as the most thorough
way to examine the effects of these
factors with the fewest test runs. The
experimental design was characterized
by combining a fractional factorial design
portion with a "star" portion, augmented
by replicates.
Conclusions
Air Atomized Conventional System
(AAC) TE was most influenced by re-
stricted air lines. Although the nature of
the restriction for any given operation
cannot be defined, the results do indicate
the importance of maintenance for all
paint lines to ensure the free flow of paint
to the spray gun. The booth air rate was
also determined to have a significant
effect on painting efficiency, although not
as strongly as restricted air lines.
Air Atomized Electrostatic System
(AAE) TE was affected by the most
variables. The most prominent effect was
voltage, followed by restricted air lines
and restricted paint lines. Booth air, gun
cleanliness, fan air, and electrode position
also had significant effects.
Airless Conventional System (ALC) TE
was determined to be overwhelmingly
affected by tip erosion.
Airless Electrostatic System (ALE) TE
effects are similar to both the AAE and
ALC systems: voltage and electrode posi-
tion had the greatest effects, but effects
of other factors were contingent on target
configuration.
Atomizing air
Booth air rate
Booth configuration
Cure schedule (time, temperature}
Paint discharge technique
Equipment design
Flash off
Gun cleanliness
Gun condition
Gun-to-target distance
Operator error
Paint mass flow rate
Paint characteristics
Restricted air supply
Restricted paint supply
Shaping air (fan air)
Target configuration
Table 2. Experimental Variables Selected for Testing Air Atomized Conventional Spray
Systems
Factor
ID
A
B
C
D
E
Factor Description
Restricted atomizing air lines
Booth air rate (linear velocity)
Gun cleanliness
Restricted paint lines
Fan air (sometimes called horn
Quant./
Qual.
Quant.
Quant.
Qual.
Quant.
Qual.
Number of
Test Levels
5
2
5
5
3
air or shaping air)
Five of the 17 variables were selected
for testing on the basis of the number of
times they were identified by different
sources, the anticipated size of their effect
on TE, and the ability to simulate them
within the prescribed test methodology.
The five selected test variables were:
• restricted air lines,
• booth air rate.
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K. C. Kennedy is with Centec Corporation, 11260 Roger Bacon Drive, Reston, VA
22090-5281.
Charles H. Dan/in is the EPA Project Officer (see below).
The complete report, entitled "Transfer Efficiency of Improperly Maintained or
Operated Spray Painting Equipment Sensitivity Studies. "(Order No. PB 86-108
271/AS; Cost: $16.95, 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:
Air and Energy Engineering Research Laboratory
U.S. Environmental Protection Agency
Research Triangle Park NC 27711
United States
Environmental Protection
Agency
Center for Environmental Research
Information
Cincinnati OH 45268
Official Business
Penalty for Private Use $300
EPA/600/S2-85/107
OC00329 PS
4GENCY
I
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