 |
| Painter in full personnel protective equipment
|
Spray booths that use robots or automatically reciprocating paint application
equipment are unoccupied during the paint process. For many of these unmanned
spray operations, the time has passed for simple open-faced spray booths with
all of the airflow exhausting out of the back filter bank plenum. During the
last 15 years, spray booth exhaust has been recirculated in unmanned spray booths
in more and more paint finishing systems. Now, unmanned spray booths are commonly
equipped with recirculation fans, automatically advancing roll-up filters and
ultra-fine return air filters. Recirculation has been done primarily to reduce
the capital equipment and operating costs when air pollution control equipment
for Volatile Organic Compounds (VOC) is required.
Case studies have shown examples of cost effective air pollution controls on
recirculated spray booths where the Regenerative Thermal Oxidizer (RTO) or other
control device is one-fourth the size and cost required without recirculation.
When the savings from reducing heated air make-up costs are factored in, recirculation
of spray booth exhaust can actually pay for the operating or running cost of
air pollution control.
Similar savings can be obtained by recirculating spray booth exhaust in manned
spray booths such as manual touch-up booths and large-part spray booths. However,
return of air with VOC’s to an occupied space invokes important safety
questions. Paint line supervisors may ask if painters will have to wear respirators
or even fresh air masks. Plant management may wonder if this meets OSHA and
fire safety codes. Here are some suggestions for the practical design and safe
operation of manned spray booths with recirculated exhaust.
 |
| Recirculating spray booth system
|
Fire Safety of spray booths with exhaust recirculation has been addressed by
the National Fire Protection Association (NFPA) in its national consensus code
entitled NFPA 33 – 2000 “Spray Application Using Flammable or Combustible
Materials,” NFPA 33 Section 5.5.1 limits recirculation to when the actual
VOC concentration in the return air is kept to less than 25% of the Lower Explosive
Limit (LEL), and an LEL probe is interlocked to the spray operation. LEL’s
typically run in the 5-10% range for recirculated spray booths.
Use of exhaust recirculation does not change the need for fire detection or
fire suppression systems. But if some type of fire detection or fire suppression
system is required, then these systems must be properly designed for the recirculating
spray booth. For example, additional fire sprinkler heads in the return air
plenum may be needed to effectively protect a recirculating booth.
Painter Health and Safety requirements are spelled out in OSHA’s respiratory
protection standard (29 CFR 1910.134). This standard requires the use of approved
respirators or other Personal Protective Equipment (PPE) when workers are exposed
to actual paint solvent or VOC concentrations that exceed the OSHA Permissible
Exposure Limit (PEL). PEL’s for commonly used paint solvents range from
50 ppm for toluene to 500 ppm for acetone. Since paint formulations contain
a mixture of solvents, a Hazard Ratio must be calculated for each solvent by
dividing the actual concentration of the specific solvent by its respective
PEL. If any solvent Hazard Ratio is greater than 1.0, then a respiratory protection
plan with PPE is required.
| TABLE
1: Cost Effective Pollution Control Equipment |
| |
Standard
Design
|
Recirculated
Booth Design
|
| Automatic Booths
(4) |
60,000
cfm
|
16,000
cfm
|
| Manual Booths (4) |
48,000
cfm
|
0
|
| Flash-off Areas |
0
|
3,000
cfm
|
| Cure Oven |
4,000
cfm
|
4,000
cfm
|
| Total Exhaust Flow |
112,000
cfm
|
23,000
cfm
|
| Annualized Operating
Cost |
$176,000/yr.
|
$21,600/yr.
|
| Annualized Heated
Air Make-up Cost Savings with Reduced Airflow |
-
|
($37,000/yr.)
|
| Based
on $0.06/KW, $4.00 per MM Btu, 100 lb/hr. VOC, Heated air
make-up required 60% of the year. |
Keep in mind that some catalyzed paints have components with a very low PEL
(0.02 ppm for methyl isocyanate). These may require use of PPE such as supplied
breathing air for spray operations even if recirculation is not used. Also,
it is very possible that adding recirculation to an existing spray booth may
increase the actual exposure level by concentrating the solvents to a level
above the permissible PEL. In this case, PPE must be used.
| TABLE
2: Effect of Spray Booth Exhaust Recirculation |
| |
No
Recirculation
|
Partial
Recirculation
|
Full
Recirculation
|
| Spray Booth Exhaust
Rate (cfm) |
10,000
|
3,333
|
2,000
|
| Paint Use (gal./hr.) |
5
|
5
|
5
|
| Solvent Exposure
(ppm) |
30
|
90
|
150
|
| Solvent PEL (ppm) |
100
|
100
|
100
|
| Hazard Ratio |
0.3
|
0.9
|
1.5
|
| PPE Required |
No
|
No
|
Yes
|
Once the need to use PPE has been established, the next step is to select the
proper type of PPE. PPE is rated by protection factors and each PPE manufacturer
has approved products at various protection factor ratings. Generally, a half-mask
respirator with particulate and organic cartridges has a protection factor of
10X. Full-face respirators with eye protection have protection factors to 50X,
and supplied breathing air systems have protection factors to 1000X. At a minimum,
the protection factor required for the PPE to be used must exceed the highest
Hazard Ratio.
In situations where the PPE protection factor is many times higher than the
highest Hazard Ratio, painter exposure to solvent in the recirculated booth
may be lower than it would be without PPE and recirculation. Also, some painters
prefer the use of more comfortable, battery-operated self-purging face masks
(protection factor of 25X) over half mask respirators when the lower protection
factor is still adequate for health and safety.
A health and safety professional should be consulted with when setting up any
respiratory protection plan. This could be a large company’s corporate
or plant level industrial hygienist. Smaller companies can use a representative
from an established supplier of PPE as a resource. These professionals can help
write the respiratory protection plan, provide training and medical referrals
if needed, document fit testing and implement safeguards to assure PPE is always
in good working condition.
Questions such as what is the permissible PEL, what is the actual exposure,
and what is the best PPE may seem complicated. The important point is that there
are scientific methods and OSHA procedures available to answer all of these
questions. When required, PPE can be combined with properly designed fire safety
features such as LEL interlocks to provide a safe manned spray booth with recirculating
exhaust. As VOC pollution control becomes more widespread and applicable to
smaller VOC emission sources, one is likely to see more recirculated manned
spray booths.
