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Paint
Filters: Crenlo Saves Nearly $100,000 by Reducing Paint Filter
Waste and Water
Systems for liquid
spray painting booths are designed to safely capture paint
overspray, exhaust solvent vapor and replace the exhausted
air with clean make-up air. Paint that does not adhere to
the parts being painted is captured as overspray by spray
booth paint filters, also called paint arrestors.
The industry standard is disposable dry
filters, pads or rolls of multi-layered disposable paper and
polyester filters. Alternative dry filter media include reusable
fiberglass, metal filters or Styrofoam. Wet or water-wall
paint booths use water with chemical additives to collect
paint overspray.
Base decisions about paint arrestors on
the type and volume of painting done and the waste generated
by the booth itself. Dry filter booths have a lower purchase
cost and generally meet the needs of small- to medium-volume
painting operations. If overall painting volume can justify
the investment, a wet booth may work well with high production
levels.
Dry Paint Filters
Dry filters are either baffle or strainer type filters, or
a combination.
| Baffles |
Strainers |
Less expensive
Lower capture efficiency
Greater potential for blow-by, particle emissions
Last longer |
More expensive
Higher capture efficiency
Lower potential for particle emissions
Tend to clog, shorter life |
Filters must be changed out when they become
clogged with paint and when air flow is restricted. Layering
two paint filters can extend their useful life. The top one
acts as a pre-filter, becoming loaded with overspray sooner
than the bottom/exhaust side filter. Layering can be done
with two filters of either the same or different capture efficiencies.
- Same efficiency. When the top filter
becomes loaded, discard it. The bottom filter replaces the
top one and a new filter is added below.
- Different efficiency. A less expensive
filter on top captures coarse particles. A more expensive
bottom filter captures the finer particles. The top filter
is changed out more frequently than the bottom.
Experimenting will help determine when to
replace filters. Throughput and transfer efficiency affect
changeout frequency. Operators may have hunches based on how
overspray hangs or moves through a booth and based on visual
inspection of the filters. Confirm hunches by weighing the
filters to determine the paint load. With actual data, you
will be able to set the most efficient changeout schedule
and correct problems.
Paint filters in a spray booth may receive
different loading.
- Spraying is often concentrated
close to where parts enter the booth. Filters nearest that
area may become clogged sooner than others near where parts
exit the booth.
- The area where the air flow is greatest
will plug sooner. Paint will then be pulled to other areas
of the filter with less resistance.
- Painting is too close to the filters.
This increases the pull of the air flow on the paint, lowering
transfer efficiency. Filters closest to the air flow clog
sooner.
- Gravity pulls paint down, causing the
lower filters in a side-draft booth to plug sooner than
higher ones.
Variable Frequency
Drives
Fluctuations in air pressure and air flow can affect painting
efficiency and operator safety. Paint rooms need to have a
slightly positive pressure to keep dirt and other contaminants
from entering. In some cases, a slightly negative pressure
within the booth is desirable to keep vapors from exiting.
Paint filters need to fit tightly. Gaps
between the filter and the edge of the spray booth create
a path of least resistance, allowing overspray to escape up
the exhaust stack. As paint filters become plugged, keeping
a constant air flow in the booth becomes difficult, as the
pressure drop across the filter increases. Air pressure in
the booth is also reduced as parts enter and exit.
A variable frequency drive (VFD) system
can compensate for the loss of air pressure by automatically
adjusting the fan’s motor speed. VFDs use less electricity
than other methods of air flow control, like outlet damper
controls, because they decrease the draw on the motor.
Waste Disposal
Filter waste can be disposed of as solid waste if it has been
tested. If a company is spraying paint that contains heavy
metals such as chrome or lead, the filters may have to be
disposed of as hazardous waste. One disadvantage to using
a dry filter booth is waste disposal. Often the majority of
waste is the filter media itself, contaminated by a relatively
small amount of paint. Reusable filters may decrease waste
volume and reduce disposal cost. In some applications, overspray
can be collected for reuse.
A wet booth eliminates the need for disposal
of filter media and reduces waste in weight and volume. The
water is chemically treated to remove the paint’s stickiness.
The paint is then separated from the water by a centrifuge,
cyclone or filter, and dried for disposal. This process can
be automated, eliminating paint-line down time needed to change
out filters.
Crenlo LLC Case Study
Crenlo LLC, in Rochester, fabricates sheet metal products,
including cabs for the agricultural and construction industries
and enclosures for the electronics industry. The company has
over 700 employees. It operates sixteen paint booths. Parts
are coated manually using either high-volume low-pressure
(HVLP) or electrostatic spray guns. Dry filters capture paint
overspray.
Crenlo was changing its paint filters daily
because painters complained about overspray and poor airflow.
Paint was “hanging” in the booth. Test panels located
on the top of the exhaust stacks showed paint, possibly indicating
low paint-capturing efficiency for the filters. And, the back
layer of the filters were discolored by paint.
Filters were costing about $1,000 daily,
exceeding the amount budgeted. Excessive maintenance time
was required for filter changeout. Crenlo requested a MnTAP
intern to help investigate and reduce its paint filter waste
and to decrease its wastewater volume.
The company had started patching metal panels
that allowed air leaks and moved the paint filter grids back
to improve air flow.
Filter Experiments
The intern tested different types of paint arrestors. None
of them performed better than the two different layered grades
of high-efficiency/high-capacity dry filters that Crenlo was
currently using.
The intern also experimented with the frequency
of filter changeout and measured actual paint load. Depending
on the paint line, he was able to extend the use of the top
filters to three or five days without measurable change in
efficiency. The bottom filters could last several days.
Water Booths
Crenlo had converted two water curtain booths to dry filter
booths in order to improve overspray capture efficiency. Initially
dry filters were used as pre-filters to catch the solids in
front of the water curtains. When the company discovered that
the water was not providing extra filtering value it converted
the booths exclusively to dry filters. Water use was reduced
by 354,000 gallons per year and $4,000 in water, sewer, chemical
costs and labor were saved.
Savings
Based on production levels at the time of the intern project,
modifying the paint filter changeout schedule saved Crenlo
$95,600 a year. Filter purchase cost was reduced by $88,200
and waste disposal was reduced by 21 tons a year, saving $7,400.
Over 1,500 hours a year of maintenance labor was no longer
needed to change out filters and could be directed to other
tasks.
For More Information
MnTAP has a variety of technical assistance services available to help Minnesota businesses implement industry-tailored solutions that maximize resource efficiency, prevent pollution, increase energy efficiency, and reduce costs.Our information resources are available online. Or,
call MnTAP at 612.624.1300 or 800.247.0015 from
greater Minnesota for personal assistance.
Additional Resources
This intern project
was conducted in 2003 by MnTAP intern Yafet Girma, a composite
materials engineering senior at Winona State University.
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