Designing a Spray Booth System that Works.

Carefully integrating the various parts of a finishing system will result in a safe, efficient environment that is a pleasure to use.

A SPRAY BOOTH is not 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 have 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 is 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, or the volume of air they are 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 plenum.

Spray booth components include an enclosure, 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 is not present, there will be lingering overspray in the booth, overspray on the floor and a lingering odor 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 is 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 -- and it would 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 possible.

Designing an Enclosure

Home-built enclosures 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 are 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 is 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 industry.

Another thing to be aware of is the difference between filters and baffles. Anything that has 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 is what a filter does -- it captures and holds particulate matter in the air that passes through it.

The best filters that I have 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, or ring panel, is growing in popularity. It is a similar to a cut pad, except that it has 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 is 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 should not be reluctant to request additional or higher-quality lighting in the ceiling and side walls.

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 color rendering than standard, cool-white tubes. Balancing white light, maintaining lumens and high color rendering are but a few of the buzzwords that apply to booth lighting. It is 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, or 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 booth.

* 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 is not necessary as the motor is mounted outside of the exhaust flow.

Attic fans, ventilator fans, duct fans and the like do not belong in a spray booth. A correct and proper fan is paramount to booth performance. Do not 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 have 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 is 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 snow.

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 have 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 is only human nature that when a spray booth is set up correctly, everyone is eager to show it off. I have 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 is a shame. Everyone wants a favorable return on their investment, whether the dollar amount is large or small. One of the best ways to ensure 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,