A look at crossdraft and downdraft paint booths
By Rich Thelen
It goes without saying that the most important elements of any professional finishing program are the painters themselves. Their skill and touch are often the difference between an indifferent paint job and a high-quality paint job. In the end it is the finish that is the objective, not the airflow patterns in the booth. Having said that, if we took the approach that we had the best painters on earth, what would be the ideal airflow pattern in a paint booth? Would it be cross-draft or downdraft?
Airplanes are difficult items to design a paint booth for. The natural shape and function of an airplane tends to cause disruptions in airflow. Many items that are painted in paint booths like buses and trucks have enough volume that they significantly affect the remaining volume of the booth. That is not true with airplanes. In crossdraft mode the head-on silhouette is a very small percentage of the booth cross sectional area. In the downdraft mode, the plane 'shadow' area is still very low and there are dead spots in the four corners.
There have been many discussions regarding the perfect paint booth for painting aircraft. Many people prefer crossdraft booths while others make an argument for downdraft booths. This article will explore some issues involved with each type of booth.
Crossdraft booths
Crossdraft booths have a filtration section on one end drawing air through the booth. This exhaust filter bank contains three stages of filtration in accordance with NESHAP (National Environmental Standard for Hazardous Air Pollutants). All three stages may be placed in a single filter rack. The exhaust fan is placed on the clean air side of this filter chamber.
The air into the booth may be either forced draft or natural. In the natural draft style, the supply air is drawn through the product filter doors from the hangar interior. The replacement air is discharged into the hangar from a make-up air unit.
In forced draft style, the air is supplied to the booth interior through a duct, usually from an air make-up unit. This requires a plenum in the front of the booth to receive the forced draft air. This plenum (often a plenum door) is usually filtered and allows for very clean supply air to the booth.
Downdraft booths
A downdraft booth allows air to enter the booth from the ceiling through
intake filters and then to flow downward over the plane and into exhaust trenches located in the floor. The main trench in the floor is laid out in the same shape as the airplane's shadow. The first layer of the three-stage filtration is located just under the trench grating. The other stages of filtration are located in the filter house.
The trenches allow the exhaust system to draw air from the floor level to a nearby filter room. Trenches can be a bit difficult to maintain with paint particles penetrating the first stage of filtration and impinging on the floor or other parts in the airstream. For this reason, the use of dampers or other moving parts in the trench is not recommended.
The filter house is built to allow for the trenches and the underfloor ducts to vent the contaminated air into the filter house. The air then expands and moves through the other two stages of filters. Exhaust fans are located on the clean air side of this filter house and discharge the filtered air above the roof line of the facility.
Capital cost
Crossdraft paint booths are the lowest cost method of building a paint booth. The reason why downdraft paint booths may be more expensive is that they require pits to be installed in the concrete floor, which can add costs to the project.
A crossdraft paint booth insert can be easily installed into an existing hangar. The floor is already in place and all that must be added is static grounding lugs. The concrete floor will carry the load of a plane, even fully loaded. On a 'greenfield' site, the building can be designed to support the structural load of the booth, otherwise it can be independently supported from the floor. In either case, both types of inserts may be relocated to another facility.
The most cost-effective way to install a downdraft paint booth for airplanes is to design it into the facility from the start. The shadow trench should be designed to allow for the plane's wheels to have full support while moving into and out of the paint hangar and while being painted. On small planes this is not much of a problem, but with larger planes, it can be a design problem.
Flexibility
Crossdraft booths can accommodate any plane that can fit into the booth.
It will need only minimal clearance around the ends to move people and materials and stands around the ends and around the wingtips.
Downdraft booths are designed for specific planes that were known at the time of the design. The exhaust trench is installed in the shadow of the aircraft. If all aircraft are swept wing (for example) and are nearly the same size, then airflow will be optimal. If the planes are of varied sizes and wing shapes, airflow will be ideal in only a few of the planes.
Airflow
The airflow direction in a crossdraft booth should flow from 'nose to tail.' An airplane has a preferred way of moving through the air and 'nose to tail' is the best and most logical way to design the booth. Eddies are reduced to a minimum when you examine the nose to tail airflow pattern.
Overspray from the paint gun will drop on its journey to the filter wall. In many cases this may cause blemishes if the painter does not cover sections of the aircraft downstream of the spray guns. It is best to work high to low and front to back in crossdraft booths.
It has been said that a downdraft booth is the best booth in which to paint cars, trucks, buses, railcars, and farm equipment; in short anything you don't have to paint the bottom of. Planes have vast areas that are on the underside (about 50 percent of the plane) and painting these areas positions the painter between the paint gun and the floor. Painters with a small amount of experience will find ways to avoid overspray that is directed toward them. But, paint overspray is always pulled away from the plane toward the floor filters in a downdraft booth.
In any case, the eddies set up by air rolling over the wing and the fuselage may cause problems in controlling overspray. It is best to work high to low when using a downdraft booth.
Filter replacement
There is a dramatic difference in the ability to maintain the filters
of the two booth types. The crossdraft booth allows for all filter stages to be in one location and you can change the filters from outside the filter house. The only tools needed are a rolling platform to reach overhead filters.
In a downdraft booth the first stage of filters are located in the floor trench under the grating. If the grating is a man load rated grating, then the grates must be lifted off so the filters can be accessed. If the grating is rated for the load of the airplane, it may be necessary to use a hoist to remove the grating because of its weight.
The use of an automatic filter changing system which uses rollers and a support bed can ease the pain of filter change-out as well. These have been used with good success as long as the filters are changed often enough so that the accumulation of paint residue is minimized.
The filters in the filter house (two stages) are handled the same way as the filters in a crossdraft filter house.
Paint quality
Downdraft paint booths are recognized as the state of the art in high-quality finishing applications. Even for airplane painting, downdraft gives a very high quality paint job. But crossdraft booths can also provide quality finishes.
It is apparent that a paint booth will give a better paint job than painting a plane in a hangar with the door open. Controlling the airflow, and the cleanliness of the air is essential to getting a blemish-free appearance.
Lighting
Lighting is a critical element in obtaining the finest possible finish.
The use of white painted wall and ceiling panels will improve reflectivity to get that high-quality finish.
Fluorescent and high intensity discharge (HID) lighting fixtures are typically used in aircraft refinishing booths. Fluorescent lighting fixtures and lamps have adapted well into the finishing market by providing a variety of high-quality lamps typically used when color rendition is of great importance. The HID suppliers are now playing 'catch-up' and are continually coming out with new products that will compete with the fluorescents. Lights should be mounted flush in the sidewalls and in the ceiling and this can be accomplished with either style of booth.
Humidity-controlled booths
The cost of heating the replacement air for a paint booth is expensive, since so much air is exhausted and the energy wasted. If you add the requirement for a humidity-controlled environment, then the cost starts to skyrocket and great attention must be paid to energy conservation.
The first step in conserving energy is to minimize the amount of exhaust air that is required for the paint booth. Airflow is reduced to the minimum that can be afforded and still be both safe and healthful. It is possible to install energy recovery to reclaim cooling from the exhaust air and transfer it to the incoming air. This has been successfully done in a number of installations. The airflow patterns of a humidity-controlled paint booth must be carefully analyzed to promote complete mixing of incoming air.
A crossdraft booth will have a tendency to stratify by temperature during its course from the nose to the tail. Warm air rises and cool air descends. However, good airflow design can minimize this effect. With a downdraft booth, air stratification is minimal. Air is drawn like a magnet to the trenches. The air temperature will be most consistent through the booth if the dead spots are correctly addressed.
Nevertheless the cost of the humidity-controlled air is so high it would be sad to see the effort degraded due to stratification.
Fire protection
Fire protection is an important element of the aircraft paint booth. The
value of the object being painted far exceeds any object painted in similar paint booths, even Mercedes cars. The act that is being performed in the paint booth (painting) is by its nature a dangerous thing. It is wise to spend money to protect that investment.
Fire protection supplies a few challenges to the downdraft booth designer. The floor trenches must be designed to handle a large amount of water during a fire. The addition of drains and sewerage system is very important. Crossdraft booths do not have that problem as the water just drains out under the door.
Conclusion
We have discussed some issues inherent to both styles of paint booths.
Both styles have their pros and cons. It shall be the task of the designer to weigh the importance of all of the issues and make an informed judgment. Is cost more important than paint quality? Is humidity control being used? Is it strip and repaint or scuff-sand and overcoat? What types of planes are to be painted? There are no universal rules for answering these questions that will meet the needs of all users. But this article can serve as a roadmap for identifying some of the major issues.
