Many repairers still consider the cross draft one of the smartest spray booth investments they can make. But this requires the owner to have the proper air makeup. Sadly, the vast majority of owners of this style of booth have no provision for input air to the booth. The result is the all-too-familiar very dirty booth and poor refinishing results no matter how skilled the painter.If you own one of the rare cross draft automotive booths with a compatible gas-fired, heated air replacement unit, then you are a step up on the others. But if you have a cross draft booth with no other accessories and would like to maximize its performance, let's begin by improving the air.To determine fan size, let's establish that the exhaust fan is sized to move a volume of air at a given rate. This is measured in cubic feet per minute, or cfm. The formula for this is quite simple. It is the cross section of the booth times the desired air velocity. The Occupational Safety and Health Administration (OSHA) requires this velocity to be a minimum of 100 feet per minute (fpm), so we'll use this number (See Fig. 3).The example shows that the booth exhausts 12,200 cfm of air. But this is only the half of it. In order for this booth to provide the acceptable minimum velocity of 100 fpm, it must have an equal amount (12,200 cfm) of air available to it.Your booth cannot draw air from your shop unless the facility is large enough. Chances are that it isn't. The accepted figure for any building's natural ability for air leakage is three air changes per hour. Because we are concerned with only one air change, we divide one hour, or 60 minutes, by three, which is 20 minutes. We require 12,200 cfm for our booth. We then multiply the minutes by 20 (for one air change)-12,200 times 20 equals 244,000 cu. ft. Our building must be no smaller in total volume than this. To accommodate this booth with natural building air leakage, the shop has to be about 75 ft. wide by 300 ft. long by 11 ft. high, and it must all be open area.This clearly illustrates the need for heated air makeup to operate the booth properly. All that is required is a general air dump from the makeup air unit located near the booth front. With air drawn through the booth's filtered front and doors and then drawn through the filters and out the exhaust, everything is now perfect. With a vehicle in place in the booth, its displacement automatically elevates the inside air velocity above 100 fpm. Now when you release the trigger on your spray gun, the overspray is whisked away toward the exhaust filters.Take away the heated air makeup, and you are no better off than a do-it-yourselfer painting a car in his or her garage. With a window open, the do-it-yourselfer sprays until he or she can no longer see or breathe.Down Draft Booth Air
This style of booth is a bit different from the cross draft booth in that a heated air replacement unit is a combined part of the booth.In the paint mode of the booth's operation, the air is not recycled. Any time that air is fed into a booth and exhausted by another means, such as the exhaust fan, you must pay careful attention to keep the booth's air in balance. This is a delicate condition because the filters offer resistance to air flow simply by their performance-providing the filters are effective.This gives us two conditions to deal with:1. More air coming in than exhausted. This causes the air to "climb the walls" and recycle within the booth, carrying with it unwanted overspray. The doors have a tendency to blow open.2. Less air coming in than exhausted which causes a negative pressure in the booth. Doors can be difficult to open.You must also know how the exhaust fan is sized because it's different with a down draft booth than with a cross draft.In the illustrations, we see that the cross section is perpendicular to the air flow. In a down draft booth, we turn this cross sectional area 90 degrees. In a typical down draft booth, this would be the entire floor area-14 ft. by 24 ft. or 336 sq. ft. Multiply this by 100 fpm for a result of 33,600 cfm. But wait a minute: If your booth's exhaust fan is rated at 12,000 cfm, you may believe you have a problem. According to the formula 12,000 cfm divided by 336 sq. ft. gives a velocity of about 35.7 fpm. This is a far cry from the required 100 fpm.Here is how it's done with a down draft booth. The area of the pit grating is measured, not the floor area. The vehicle parked over the pit causes a venturi effect to draw off overspray. A typical pit measures 3 ft. by 20 ft., or 60 sq. ft. Thus, 12,000 cfm divided by 60 sq. ft. equals a velocity of about 200 fpm at the grating.This velocity is only felt around the vehicle and not anywhere else in the booth. This means that these booths can only be used to paint a complete vehicle. You shouldn't paint small parts in these booths because if there is no vehicle, there is no venturi effect. Without the venturi effect, the air velocity is unacceptable. Now we are back to the cloud of overspray and poor air flow.This means that you should have a walk-in booth for small parts, as well as the down draft paint and cure booth for complete vehicle finishes. It is also very important to have an HVAC contractor check and adjust your booth periodically for a balance of air flow from air in to the exhausted air out.Booth Filters
When it comes to filters, repairers must fully understand the difference between a filter and a baffle. A baffle is not a filter. If you do not understand this, you are likely to buy on price alone. Unfortunately, you get what you pay for. To make sure you're paying for what you need, consider a few factors.A true filter is any device that captures and holds dust and dirt particles, as in the case of air filters, or atomized paint overspray particles, as in the case of paint filters. The ability to capture and hold is very important in how a filter performs. The more a filter can capture and retain, the better it is.The best filter media to come along in the last decade or so is the non-woven polyester, or synthetic, style. It looks like upholstery material and is usually white in color. It can be found in pink, yellow or blue or any combination of these colors.There are five methods of capture for all filtration, from the least of filters to the most sophisticated HEPA filters used in hospitals and clean rooms. These methods include:impingement;straining;diffusion;inertia (browning effect);electrostatic.For spray booth purposes, you want filtration that offers at least the first two of these methods or, better yet, the first three. The fourth and fifth are too exotic and costly to be effective in spray booth filtration.The two most common types of paint overspray filters available are fiberglass and synthetic. Fiberglass is used as a filter, but it really is an insulation media. It's one method of capture is impingement. Paint overspray is supposed to stick to the rods of glass, which are coated with a sticky coating-a flypaper effect. This filter's ability to hold captured material is very poor. What doesn't stick, blows on through, and dried overspray falls off when the filter is touched.A synthetic filter has two or three methods of capture with straining being the most effective. This media has a graduated density as part of its makeup. This allows paint to enter the media and stay there.Here are some more points to consider:Q: What is a baffle?A: A baffle offers a flat surface for paint overspray to adhere to. The main problem is that if the overspray is dry, there is nothing to capture it and it is allowed to pass on through. This is called blow-by. This results in a very dirty booth with dried paint, dust and dirt covering the floor.Q: Why is filtration so important?A: For safety reasons, specifically to prevent fire. Dried paint residue can fuel a booth fire rather easily. Worse yet, dried paint residue can start a booth fire. It doesn't require an external spark to ignite a fire-paint residue can ignite by itself. It is called composting. This is especially true if the residue is made up of different types of paint. The layers of paint can generate heat, much like a pile of cut grass can. Booth fires are not pretty, nor are they easy to extinguish. Fire departments do not like to deal with them because they are not easy to get at when they are inside the stack.OSHA doesn't have a lot to say regarding spray booths, but what it does say is very important. In the Code of Federal Regulations 1910.107 (g) (2), the organization clearly states that "spraying areas (including exhaust chamber, exhaust fan and stack) shall be kept free of residue with cleaning to be conducted daily if necessary." Nobody has the manpower or budget to clean all of this daily. But with correct and effective filtration, accumulation can be kept to a minimum.The above applies equally to down draft booths. The floor pit and the tunnel connecting the pit to the metal exhaust stack are all considered stack items. This, in effect, poses an even greater problem in that the walls of the tunnel and pit are porous concrete. Paint overspray filtration in the pit area is very critical. This is one area where improvement over the manufacturer's provided filtration is a must.The wire support for roll media can easily be converted to 20 in. by 20 in. holding frames to allow the installation of pocket, bag or cube style polyester paint overspray filters. For added filtration, a polyester roll media can be laid over the pocket or cube filters and can function as a roughing filter. This roll media is inexpensive and will add to the life of the more expensive filters. If you are serious about keeping the internal exhaust portions of your booth clean and performing well, this method works well.Take for example one industrial application where heavy concentrated spraying is done all day long in a very small booth. The exhaust chamber is only 5 ft. wide by 7 ft. high, and it has nine filter frames that are 20 in. by 20 in. Each frame holds a two-pocket polyester bag filter that is 15 in. deep and has a 20 in. by 20 in. polyester pad in front as a roughing filter.About five years ago when the booth was converted to frames, the exhaust chamber was thoroughly cleaned inside, and a new stack and new exhaust fan were installed. Today, when the painter changes the bag filters, the inside of the chamber and stack are still shiny new silver metal. In all this time, the owner has never had to clean the chamber or stack. The filters have done that automatically, and the cost of these filters has saved the owner a lot of time and labor by not having to clean the inside of the booth.Q: When is it time to change a filter?A: When a filter is working correctly, it captures and holds paint overspray or dirt particles. When the filter is loaded sufficiently, it offers resistance to the air flow in the booth. This resistance causes the air flow inside of the booth to decrease. This is when it's time to change the filter.Poor or cheap filters do not hold much. They unload or allow overspray and particles to pass. These filters never offer a resistance to air flow, which is why you don't want to use them. Good filters can be monitored by a pressure differential gauge, either a manometer or a magnehelic gauge. Either one will measure the pressure differential before and after the filter.By following the manufacturer's instructions, the proper pressure reading will be your indicator that the filter needs to be replaced. Remember, no matter how dirty a filter may look, it is still in working condition until the pressure differential says it is spent.Lighting
When it comes to lighting in standard booths, there never seems to be enough. This is a shame because a low cost standard booth does little good if the painter has difficulty seeing what he or she is doing due to a lack of lighting.Side wall lighting is what is needed, not ceiling or hip lighting. Light is needed at the side and rocker panel area of the vehicle. If your booth is a non-insulated, single skin type, solid panels can be replaced with panels suited for mounting fluorescent light fixtures. Horizontal lighting is more beneficial than vertical lighting fixtures.If your booth is insulated, side wall lighting is not very easy or practical. One solution is to replace the lamps with brighter ones. There are 40-watt or 32-watt, 4-ft. dual pin lamps that are up to 100 percent brighter in foot candles of light produced.If your light fixtures have the old style "hammered wire" pebble finish glass on them, replace it with clear tempered glass. This will improve the output of light by a noticeable amount.Be weary of adding fixtures inside the spray booth. First, they will obstruct the air flow. Second, any fixture installed inside the booth must be explosion proof. Third, any explosion-proof fixture mounted inside the booth must also have explosion proof conduits, fittings and wires. This is not an inexpensive undertaking.One way to brighten the booth is to paint it with flat or semi-gloss white. A lot of strippable booth coatings are available for this. There is also clear to coat the glass, which can easily be removed by pealing it away. Be careful, though, of strippable coatings if your booth is a paint-and-cure type. Some strippable coatings harden or bond to the walls when subjected to heat, and they can't be pealed off.Another method to brighten a booth interior is to attach white booth paper to the walls with magnets. This may seem a bit crude, but it does work and it is easy to change. It also offers the painter a target to test and adjust his or her spray gun.Maintenance
The key word for booth maintenance is "clean." Keeping your booth as clean as possible is the best thing that can be done to improve your booth's performance. How is this best done? The following contribute to clean spray booths:Avoid storing anything in the booth. Nothing should be in the booth except for the vehicle being painted, the painter, a spray gun and air hoses.Vacuum the booth floor after each paint job. Don't clean the floor with water and a mop or squeegee. When the water dries, the dust will still be there. Besides, water mixed with dried paint overspray equals contaminated water, and you can't put this down a sewer. If you have a central vacuum system, install a vacuum droop into your booth.Keep all air hoses to a minimum length. Long air hoses that drag on the floor spread dust and dirt. The best way to keep hose lengths to a minimum is to install an air drop on each side wall of the booth with a quick disconnect, filter and regulator. When the painter moves to the opposite side of the vehicle, he or she simply disconnects the air hose and re-connects on the other side. You should be able to get by with a 10 ft. or 12 ft. length of hose this way.Keep all filter monitoring devices clean and in good repair.Hang a strip of booth paper from the wall using magnets. This enables your painter to check and adjust his or her spray gun when necessary.Provide and require your painters to wear paper paint suits. Lint from clothing is a major contributor to finish flaws.Remove all duster guns from the booth. Better yet, don't allow any duster guns in your shop at all. When you need to remove dust, use a tack rag or a vacuum. Your goal is to remove dust, not put it back into the air.To ensure cleanliness in a down draft booth with floor grating, you may want to do either of the following: Remove any grating that sits on solid cement because it isn't needed, or if the grating can't be removed, cover it with sheets of fiberglass reinforced plastic or metal deck plate.Grating on the floor is common in above-ground down draft booths. It provides hundreds of dirt catching pockets in the booth floor. It is a necessary evil, but it does little to enhance booth cleanliness. Much of the floor grating for these booths is necessary to gain access to louvers required to balance the air draw. Because these booths are about 12 in. above the floor, they require a wider pit to compensate for the depth they lack. This requires a larger grating area and creates floors that are very difficult to keep clean.Understanding the principles of air flow, filtration, lighting and maintenance is a necessary part of spray booth operation. Taking the factors illustrated above, shop operators can take stock of their existing booth and improve its performance or make an educated decision to purchase a new booth. Whatever the method, better refinish quality will be the end result.
