Knowledge Bank

Spray Booths for Liquid Painting

A spray booth (for liquid painting) can be defined as an enclosure equipped with a means of safely capturing overspray paint, diluting and exhausting solvent vapors and replacing the exhausted air with clean makeup air.

Makeup air may be supplied inside the booth by fans and ducts, or plant air may be allowed to replace air exhausted from the spray booth. This definition makes a spray booth sound exceedingly simple. Actually, a spray booth is quite a complex system. A spray booth almost always has openings on both side walls for entry and exit access for conveyed products to be painted. A spray booth without such conveyor openings is sometimes used for low-production batch spraying. Access for product entry and exit is through the wall at the front of the spray booth. The front wall may be fitted with doors.

The need for forced air exhaust and forced air makeup in a spray booth presents an engineering problem. The exhaust must be sufficient to satisfy health and insurance underwriter regulations. Exhaust requirements are greater when humans are in the booth. Air makeup must do two things: replace the exhausted air and supply a slight amount of additional air to maintain a positive pressure, which is necessary to prevent drawing plant air into the booth. The extra amount of air makeup cannot be excessive or some overspray could possibly be forced out of the conveyor openings into the plant instead of into the exhaust system.

Makeup air is drawn into the booth from outside the plant to avoid disturbing the rest of the plant's air balance. If the spray booth air were taken from inside the plant, a negative plant pressure would be created. Plants like to maintain a slight positive pressure to avoid drawing unfiltered and unheated air into the plant. The booth makeup air is filtered to introduce clean air into the booth. In cold climates the air makeup is heated and often humidified during cold weather. During summer temperature extremes, it may be air-conditioned. Sometimes moisture is introduced into dry makeup air to prevent excessively fast evaportion of volatiles from the applied paint film. This extra moisture is needed only in frigid weather because very cold air used for makeup in the spray booth holds just a tiny amount of moisutre.

The air temperature and amount of water vapor in the air have significant effect on the rate at which paint solvents (and water) evaporate from wet paint films. For example, a paint applied in the day time on a hot and dry day say, at 40 deg C, may surface dry in 2 hrs and dry to overcoat in 6 hrs. The same paint applied at night say at 23 deg C would take 3 hrs to surface dry and min 15 hrs for dry to overcoat, and on a cold day or night say, at 15 deg C, the same paint will take 6 hrs to surface dry and 24 hrs for dry to over coat. Moisture levels in the air (humidity) also has similar effects on the drying time of paints.

If the primary movement of air makeup and exhaust is from the booth ceiling to the floor, the booth is termed "downdraft". Fresh makeup air enters through the filters at the ceiling, and overspray is exhausted through the floor grating to a filtering system. If the air movement is mainly horizontal, the booth is termed "sidedraft". In both types the air movement permits a human spray operator to work in a safe environment without a separate fresh-air supply for breathing. However, where toxic chemicals are sprayed, a separate fresh air supply needs to be supplied for human breathing, usually to a hood that fits around the operator's head. In automated booths where humans are not working, the air flow rates may be reduced.

The overspray paint cannot be exhausted directly from a spray booth to outside a plant because of exhaust ductwork fire hazards and the possibility of depositing on cars and buildings in the vicinity of the exhaust outlet. Unfiltered spray booth exhaust sends overspray paint into the exhaust ductwork and onto fan blades, causing a fire hazard. The overspray paint accumulation can unbalance fan blades, causing fan vibration and wear and possible damage to the exhaust housing. Overspray paint exhausting directly to outside the plant can accumulate on nearby buildings and cars, causing untold damage.

The method of filtering the paint overspray from the exhaust is another way of categorizing spray booths. If the overspray is removed by filters, it is termed a "dry-filter booth". If the overspray is removed by a water system, it is called a "water-wash booth" or "wet" booth.

Atomized paint exiting a spray gun consists of various sizes of paint droplets in flight toward and past the vicinity of the part to be painted. As the droplets move through the air, solvent evaporates from them continuously, creating solvent and particulate (resin, pigments and additives) components in the overspray. The evaporated solvent, both from atomized particles and from the deposited paint film, is comprised of extremely tiny molecules and almost totally escapes filtration, moving directly into the exhaust air stream. The remaining overspray paint particulate missing the target are carried in the air exhaust stream, but are mostly captured by a filtering device before they leave the booth.

Dry-Filter Booth:

In a dry-filter booth the overspray (evaporated solvent molecules and atomized paint particulates) are drawn by air circulation through a network of air filters into the exhaust duct system. Nearly all dry-filter booths are sidedraft. Dry-filter booths use easily replaceable disposable filters that vary in size, composition and particulate capture efficiency. The filters may be of the strainer type or the baffle type. The baffle type can take heavy loading levels but is not highly efficient at removing all of the overspray particulates. The strainer type is effective at removing most particles, but tends to "blind" (clog) quickly. For some time, manufacturers have been making variable-density filters or dual-material filters. These avoid the heavy face loading of the strainer type but can still achieve high capture efficiencies. A baffle-type filter is often placed ahead of a strainer type to combine the advantages of both.

When filters load up with paint to the point that air flow is impaired, they must be replaced. Disposal of the spent filters is usually straightforward when no toxic substances are present in the paint. The exceptions are paints that can ignite spontaneously when the used filters are stacked together. Stacking does not permit the escape of heat generated by oxygen reacting with the finely divided organic materials on the dirty filters, and spontaneous combustion can result. When this problem is encountered, the filters should be plunged into a drum of water and the filled drums then tightly sealed. Some plants must bake used filters before they are allowed to discard them in conventional ways. Baking expels residual solvents from the trapped overspray on the filters. Getting rid of filters containing toxic paint can be expensive. Depending on the applicable regulations, used filters laden with toxic paint components, such as chromates, may need to be disposed in facilities licensed to handle toxic wastes. Somewhat surprisingly, in at least three states, if water leach tests show that only low amounts of toxics are released, then filters do not require handling and disposal as toxic waste. Most states do not allow materials classified as toxic to be discarded so casually.

Water-Wash Booths:

In water-wash booths the overspray is exhausted through a curtain of water or overlapping pressurized sprays of water that entrap and remove the atomized paint overspray. Water curtains or pressurized sprays are usually located on the side of the booth behind the parts being painted. Some booths have the water-impingement system located below floor grating. Water-wash booths can be of either sidedraft or downdraft design. The water curtain or spray is pumped from a water reservoir tank and returned to the tank. The volume of the reservoir tank depends on the size of the spray booth and may range from several hundred gallons for a small booth to thousands of gallons for a very large booth. A number of booths may use a common reservoir of recirculating booth water, frequently a large "pit" tank. If the collected overspray paint particles in the water circulation system were not chemically treated, they would stick to the sides of the reservoir and clog pumps, headers and nozzles. The chemicals added to prevent this are called detackifying agents. The detackifying process is called "killing" the paint. Chemicals can be used either to float the detackified paint for skimming or to degrade it to a fine sand-like consistency that will sink to the bottom of the reservoir. To "kill" the paint, detackification agents may combine one or more of the following methods:
  • Use absorbent materials such as clay.
  • Use alkaline materials that chemically degrade many paint resins. High alkalinity can "sink" the sludge; lesser amounts float the killed paint, which permits removal by skimming.
  • Use polyelectrolyte materials that chemically encapsulate the paint particles, rendering them nonsticky.

Detackifier formulations are available in solid form or as concentrated liquids. Best results in separating the killed paint from the water are obtained when paint particles are agglomerated by coagulants into large particles, which facilitates floating and settling. Antifoamers or defoamers may be added to the water to prevent foaming onto the booth floor, pump cavitation (pumping air) and impeller erosion. Foam interferes with the paint/water separation, delaying paint settling and producing excessively wet sludge, making disposal costly because of the extra handling weight and transportation expenses. Additional environmental restrictions may also apply. Water containing organic material can be potential food for fungi and bacteria unless antimicrobial agents are used. These prevent slime, odors and corrosion pitting. To minimize these problems, the water is kept mildly alkaline at a pH of about 7.5 to 9.0.

Some paints are easy to kill; others are not. High-solids paints tend to be difficult to detackify. The detackified paint sludge can be collected by skimming floating material, by scraping up sunken material or by filtering. Any toxic materials remaining in the booth water after sludge removal must be eliminated before releasing the water to drain.

Disposal of wet paint sludge has become an increasingly expensive problem, especially if toxic components are present. In light of so many health hazards being traceable to chemicals that were improperly disposed of, many areas are exceedingly reluctant to allow dumping of any manufacturing wastes in their communities, much less their burial or incineration. Many regions either have laws or are considering laws to ban hazardous chemicals disposal except on the plant site, and then only by approved methods. Some efforts have been made to recycle paint sludge, but it is usually less costly simply to dispose of it and buy new paint. If no toxics are present, sludge can be pressed into briquettes and burned for their heat value. Pressing to remove water is necessary because wet sludge burns poorly and may require almost as much heat to equipment to heat and dry paint sludge and then crush it to a free-flowing powder

High-Solids Overspray Recovery:
Recovering and recycling high-solids overspray is increasing in popularity. High-solids overspray remains tacky or sticky because of the absence of crosslinking and due to the low amount of solvent that can evaporate. This trait makes recovery and recycling fairly simple. Vertical baffles are stacked behind the parts being painted in a side-draft booth. A large portion of the overspray is drawn toward the flat overlapping or spiral channeled baffles, collecting on them and gradually flowing downward into a trough. The trough slopes into a collection container, recovering much of the overspray paint that would have been caught up either by dry filters or by waterwash systems. Augers are also used to move the collected overspray. The collected paint needs to be filtered and adjusted to viscosity (by solvent addition) before it can be reused. Occasionally a slight color correction may be required. High-solids overspray recovery produces great cost reductions by allowing paint purchasing to be reduced and by lowering dry filter replacement or sludge treatment requirements. Low-solids solventborne paint overspray cannot be recovered because it dries so quickly. The same type of problem exists with recovering waterborne coating overspray.

Dry or Water-Wash Booths?
Whether to install a dry-filter or water-wash booth depends largely on the amount of painting to be done. Equipment and operational costs of a dry-filter booth tend to be lower than for a water-wash booth until at least 300 to 400 Ltrs of paint are applied each day. Having to stop painting to replace filters more than 2 or 3 times per shift can be avoided by switching to a wet type spray booth. The particular paint being sprayed has an effect on the relative costs between wet or dry capture as well. How readily the paint is chemically detackified, the cost to dispose of wet sludge and the method of paint application are also important factors in booth selection.

VOC Removal:
Because VOC readily escapes through dry filters and water-wash booths, VOC emission regulations for industrial painting are structured to require low-VOC coatings to be used. For example, a category of finishing (automotive, appliance, etc.) may require that coatings contain no more than 3.5 pounds of VOC per gallon. As long as this requirement is met, the VOC given off during coating application is within compliance. However, the regulations require that if coatings are used that exceed the VOC maximum, "add-on" VOC treatment methods must be used, such as incineration or carbon adsorption. In the incineration of VOC, the fumes are burned at about 1400 deg F to convert the VOC to carbon dioxide and water vapor. The incineration process is inherently energy-inefficient unless the hot exhaust can be used elsewhere in the finishing line or in the plant. Often the exhaust is passed through heat exchangers to preheat air makeup for ovens and other burners and to provide heat for dry-off ovens and for washer baths. Attempts are made to get the incinerator exhaust temperature as low as possible before it leaves the plant. Incinerators are used extensively on coil coating lines, where low-solids coatings are often used and where the VOC emissions from coatings application and bake ovens are high. The VOC concentration often is high enough to serve as fuel for the incinerator, allowing a cutback in natural gas use. Incinerator use on coil coating lines is very cost-efficient. Incinerator use is impractical where VOC emissions are low. As a general rule of thumb, incinerator use is limited to destroying VOC from bake ovens where fume concentration is fairly high and where the VOC temperature is already raised 300 deg F or so above ambient, requiring that much less heat input to reach incinerating temperature (approximately 650 degC). Incineration use is impractical except in a few special instances as a means of destroying VOC from a spray booth because the high air makeup and exhaust requirements for spray booths provide an excessive amount of diluted VOC air to handle. Finishers with modest production loads should comply with the VOC pounds/gallon coating limit and not consider incineration. Activated carbon-bed adsorption units, while totally effective in cleaning solvent vapors from booth exhaust air, tend to be prohibitively expensive except for large painting operations. Huge beds of carbon are needed, plus facilities for periodically stripping solvent-saturated carbon beds with a flushing material. In most cases live steam is used for regenerating the carbon. Equipment to separate solvents from the condensed mixture of steam and solvents adds to the high cost of adsorption systems. Several newer designs continuously strip and concentrate the booth air/solvent stream ahead of incineration. A rotating carbon-fiber wheel cleans the exhaust air of all solvents. Part of the hot incineration exhaust back-flushes the adsorbed solvents off the carbon wheel and concentrates them by a factor of 5- to 20-fold. The smaller, richer solvent and air mixture can be incinerated less expensively.