Designing a Spray Booth System that Works.

Carefully integrating the various parts of a finishing system will

result in a safe, efficient environment that's a pleasure to use.

A SPRAY BOOTH isn't a finishing system. However, a finishing

system generally includes a spray booth, which is a large part of a

finishing system.

Without intending to sound confusing, it certainly can be. To

compound the problem, there is little documentation on how to put a

finishing system together.

In nearly 25 years of spray booth and finishing systems design, I

have noticed that nearly 95 percent of the existing ones I've seen

have had problems. What cause these problems? The answer is

misapplication. The solution is application.

An Application Example

The parts of a finishing system are simple enough, with the largest

being the spray booth itself. However, it's the application of the

parts that makes the finishing system work.

Let’s look at an example of a company that needs to finish

dining room tables that are 3 1/2 feet wide by 6 feet long.

By allowing 3 feet of working space on all sides of the table, the

booth’s dimensions would need to be at least 9 1/2 feet wide by 12

feet deep. The booth selected, shown in Sketch “A”, has

work-section dimensions of 10 feet wide by 12 feet deep by 7 feet high.

To determine the size of the exhaust fan, use booth mathematics.

The following calculations are by far the most important concerning the

selected booth size.

Doing the Math

OSHA’s requirements concern the flow of air in the booth when

a person is working inside it. The main requirement is to have a minimum

of 100 feet per minute (Thin) air flow velocity through the booth past

the breathing zone of a person in the booth’s working section.

To achieve that requirement, the formula is simply a result of the

air velocity in feet per minute through the cross-sectional area of the

booth in square feet and including a safety factor of 125 fpm. This

results in a movement of air rated in a volume of cubic feet per minute.

In the above example, the formula looks like this:

cfm = area x velocity

cfm = (10 ft. x 7 ft.) x 125

cfm = 8,750

Thus, the volume of air required for the booth in the example is

8,750 cfm. Exhaust fans are rated in cfm. The volume of air they're

able to move.

Finishing System Components

Air input components consist of heated outside air supplied by an

air make-up unit and fan-fed input air supplied by a filtered air input


Spray booth components include an inclosure, lighting and

filtration. Other components are an exhaust fan, exhaust stack and

safety devices.

Each of these components is much like a link in a chain. Eliminate

or weaken any link and the chain no longer functions.

Air Input Considerations

Having air available for the finishing system’s exhaust fan is

by far the most critical part of a finishing system. When it's not

present, there will be lingering overspray in the booth, overspray on

the floor and a lingering odour of finishing materials left in the booth.

The best source for input air is a gas-fired, heated-outside-air,

feed-air make-up unit. It should be sized to the same cfm capacity as

the exhaust fan so that there is an even balance of good, filtered air

that's fed to the booth area. This can consist of a general air dump in

front of an open booth or a filtered booth front. It can also be fed

into an input air plenum, as shown in Sketch “B”.

Drawing the necessary input air from the building itself rarely

works. The main reason is that the building is usually too small to

provide air without opening windows or doors.

The booth given in the example requires 8,500 cfm, which would

require a building with a volume of 175,000 cubic feet —. It'd

have to be an open building. If the air had to be drawn room from room,

there would be a great turbulence in all the rooms and areas, In a

furniture shop, the goal is to keep the air in all areas as quiet as


Designing an Enclosure

Home-built inclosures fabricated from common building supplies may

or may not be proper, adequate or subject to codes. So why take chances?

Booths manufactured from 18-gauge galvanized steel panels are

probably the best that can be found. they're approved and safe. In

addition, if the booth needs to be moved, the panels can be unbolted and

reassembled in the new location.

All of the necessary accessories are either included with a booth

or are available as options. Additional accessories, such as a filtered

booth front, access doors and a filtered air input plenum for totally

enclosed booths, make designing a booth quite easy.

The exhaust chamber or plenum is the best standard feature in a

manufactured booth. it's compatible with standard filters and with the

exhaust fan and stack.

Air Filtration

Most companies don’t take filtration seriously. Filters and

filtration are really a science. Filter types range from fiberglass

furnace filters —. Which don’t really work well —. To exotic HEPA filters that are found in “clean rooms”. In the electronics


Another thing to be aware of is the difference between filters and

baffles. Anything that's large holes or openings that allow the

passage of air is a baffle. The trouble with baffles is that particles

from overspray also pass through because there is nothing to capture and

hold the overspray particles. that's what a filter does —. It captures

and holds particulate matter in the air that passes through it.

The best filters that I've found are made from a synthetic,

non-woven polyester media. This filter media has been designed to

incorporate at least two or three capture methods. This allows the

filter to both capture and hold the particulate matter from overspray.

Filters using this type of media are available in many styles,

including cut pads, blankets, rolls, panels and bags. The panel. Ring

panel, is growing in popularity. it's a similar to a cut pad, except

that it's an internal wire frame. This gives it rigidity when placed

in a filter holder. It also self-seals, which helps keep the inside of

the exhaust chamber clean.

Lighting a Spray Booth

The standard spray booth packages offered by booth manufacturers

all seem to lack sufficient lighting. The reason is that a standard

booth is designed to compete with every other booth manufacturer’s

product. Therefore, everything that's vital is kept to an absolute

minimum for cost-savings purposes. Unfortunately, booth lighting is the

costliest expendable in a booth.

However, in the furniture refinishing and woodworking industries,

light is an absolute essential. Next to air, light is the most important

item that a booth can have. Because booth manufacturers can provide just

about anything a customer requests, buyers shouldn't be reluctant to

request additional or higher-quality lighting in the ceiling and side


There are some terrific new fluorescent light tubes available in

today’s market. Some are up to 2-1/2 times brighter than standard

tubes. Some also provide better colour rendering than standard,

cool-white tubes. Balancing white light, maintaining lumens and high

colour rendering are but a few of the buzzwords that apply to booth

lighting. it's a long way from simply buying a booth with one,

four-tube fluorescent fixture in the ceiling.

Exhaust Fans

When buying a booth without a fan and planning to buy the fan

separately. When replacing a booth fan, keep in mind the following:

* An axial or tube axial fan is the correct type of fan for a spray


* A belt-driven fan is a better choice than a direct-drive fan.

* Fans are sized by cfm.

* An open, drip-proof (ODP) motor with a 1.25 service factor is the

best choice.

* A totally enclosed, fan-cooled (TEFC) with a 1.0 service factor

is acceptable.

* An explosion-proof motor isn't necessary as the motor is mounted

outside of the exhaust flow.

Attic fans, ventilator fans, duct fans and the like don't belong

in a spray booth. A correct and proper fan is paramount to booth

performance. don't take short cuts with the fan —. You may be placing

yourself and others at great risk.

The Exhaust Stack

The exhaust stack is round and should be the same diameter as the

exhaust fan. Although that may seem simple, I've personally seen many

questionable and some astounding practices taken on exhaust stacks by

sheet-metal workers.

Without going into a lot of technical detail, the diameter of the

stack is critical to the overall flow of air in and through the spray

booth. A certain “stack velocity”. Must be maintained for the

fan to deliver its proper volume of air in cfm.

If the direction of the elbows needs to be changed, try to use

full-throat elbows. A full-throat elbow has the same radius as its bend

(usually 90 degrees) and is equal in length to the diameter of the

stack. If it's not possible to use a full-throat elbow due to space,

then use a half-throat elbow, which is tighter turning than a

full-throat elbow.

Try to have the stack clear the roof by at least 6 feet. Top the

stack with a stackhead with an automatic damper to keep out rain and


Safety Devices

The most important safety device in any spray booth is a fire

suppression system. Sprinkler systems that use water are becoming rare

and are being replaced with dry chemical systems. Dry chemical systems

offer the advantage of being a part of the booth itself and can be moved

along with the booth should the need arise.

When it comes to electricity, never place any electrical

connections inside the booth work section, On the outside of the booth,

all normal electrical connections or devices should be kept at least

live feet from any booth opening. These guidelines are taken from

Bulletin 33, published by the National Fire Protection Assn. Most

municipalities adhere to NFPA codes when they've no specific

regulations of their own.

Final Thoughts

A spray booth is a valuable tool in a finishing system. Any attempt

to weaken or lessen any of the items discussed in this article will

render the finishing system less than desirable.

On the other hand, when all items are addressed and properly

functioning, a finishing system is a pleasure to use. it's only human

nature that when a spray booth is set up correctly, everyone is eager to

show it off. I've witnessed this many times in my travels, both in

large companies and small shops.

When the cost of a system is the only concern, critical items will

often be downgraded or eliminated to keep the overall cost down. that's

a shame. Everyone wants a favourable return on their investment, whether

the dollar amount is large or small. One of the best ways to insure that

return on investment is to seek the advice of a knowledgeable consultant

when considering a finishing system.

Ken Green is a finishing consultant, specializing in spray booths

and related equipment. His company, West brook Enterprises, is located

in Cleveland,