It is not a simple task to design a paint booth for an airplane, let alone multiple planes — an undertaking that Global Finishing Solutions (GFS) does time and again for military, commercial and business aircraft of all sizes.
Aircraft paint booths must be designed to accommodate the unique shape and sheer size of aircraft in order to reduce the painter’s exposure to the hazards of the process, lower energy costs, improve paint finish quality and increase production. The single biggest factor in accomplishing this is providing airflow where it is needed most — at the surface of the aircraft.
The two most common airflow styles for aircraft paint booths are crossdraft and downdraft. Crossdraft spray booths utilize a horizontal airflow pattern that is parallel to the body of the aircraft or fuselage. This configuration reduces turbulence in the airstream, as air flows with the natural contours of the aircraft. Downdraft booths, on the other hand, pull air vertically from the ceiling, down over the aircraft and into a filtered pit in the floor; so air runs perpendicular to the body of the aircraft. They are often the most efficient in terms of particulate removal and reducing exposure to painters.
Both airflow styles have their own advantages and disadvantages. Here are seven considerations when choosing between crossdraft and downdraft airflow for an aircraft paint booth:
1. Type(s) of Aircraft
How many different aircraft types and sizes are you planning to process through your booth?
Crossdraft spray booths provide the most flexibility and configuration options for processing multiple types and sizes of aircraft in a single booth. Smaller aircraft in a booth designed for large aircraft can also be positioned closer to the exhaust system to optimize airflow around the aircraft. The airflow pattern in a crossdraft booth is less affected by the cross section of the aircraft.
Downdraft spray booths, though less flexible at accommodating varying aircraft sizes, are very efficient when processing a single aircraft model or multiple models with similar wing and fuselage profiles. Downdraft designs are also commonly incorporated into new construction projects, as the work to accommodate the in-ground pit and ducting systems can be accounted for within new building designs.
2. Control of Overspray
Controlling overspray within the paint process is not only critical to the finish quality of your product but also plays a big part in ensuring the safety of the painters and efficiency of the process.
Crossdraft spray booths typically require less air volume than equivalent-sized downdraft booths; however, the velocity of that air is maintained at a higher speed to ensure overspray and volatile organic compounds (VOCs) are carried to the exhaust filtration system. A well-designed airflow gradient around the aircraft helps ensure overspray is carried to the filters and not dropped on the floor or parts of the aircraft downstream of the painter, while not being so high that the velocity negatively affects the transfer efficiency of the paint.
Downdraft spray booths use a higher volume of air but can be maintained at a slower velocity through the booth, with overspray and VOCs being heavier than air naturally falling, and therefore can be more easily controlled. This provides painters optimal transfer efficiency and limits overspray from collecting on parts. Additionally, downdraft airflow helps reduce painters’ exposure to overspray. Floor protection, such as Booth Shield® from GFS, also can be used to prevent overspray from accumulating on the floor of the aircraft booth and makes for easier cleanup.
3. Ease of Changing Filters
Changing filters in an aircraft paint booth can be time consuming and costly, with most aircraft booths requiring three-stage NESHAP-compliant exhaust filtration. The sheer volume of air processed by the filtration systems require a large amount of filtration area and quantity of filters.
Crossdraft spray booths allow for filter change outs entirely from inside the booth, with just a lift needed to reach the highest filters. The first-stage exhaust filter is typically a roll media and easily accessible for quick change outs. The first-stage filters should be changed more frequently to prevent premature loading of the more expensive second- and third-stage filters.
First-stage roll filters in downdraft spray booths are located in the floor, beneath the pit grating, requiring the grating to be lifted to change them. Design considerations are made to provide a grating system capable of supporting the aircraft in the area of the wheel travel, with only lighter top grating needed to be removed for change outs. In addition, the second- and third-stage filters are located in an exhaust filter plenum alongside the paint booth, with direct access to the filters, similar to that of the crossdraft design.
Intake filters, which require less frequent change outs, should also be considered. Intake filters in crossdraft spray booths are located in the booth wall opposite the exhaust filters, while intake filters in downdraft spray booths are housed in the ceiling.
4. Paint Finish Quality
Without question, paint booths offer a more effective environment for painting aircraft, resulting in a better paint job compared to open paint hangars. Controlling the airflow and cleanliness of the environment is essential to obtaining a blemish-free appearance.
Downdraft spray booths are recognized as the top-of-the-line solution for high-quality finishing applications, even with a product as large as an airplane. This is primarily because downdraft booths use gravity to control overspray and reduce exposure of unpainted areas of the aircraft to overspray.
High-quality finishes also can be achieved in crossdraft spray booths, which provide the simplest, most cost-effective configuration; however, additional skill and attention to detail are required to minimize rework and achieve the desired finish.
5. Intensity & Placement of Lighting
Lighting is a crucial element in achieving the best possible finish. Lights should be strategically positioned throughout the paint booth to illuminate every part of the plane, minimizing shadows and glare. GFS engineers can perform light calculations to determine the appropriate placement and quantity of lighting for optimal working conditions.
Fluorescent fixtures have historically been most commonly used in aircraft paint booths. LED lighting is now becoming more widely used, thanks to advances in technology to improve the quality of the lights themselves and make them more affordable. GFS ensures light fixtures are code compliant by using a Class I, Division 2 fixture and monitoring each lens for proper installation. Additionally, most aircraft paint booths feature precoated white interior panels for increased reflectivity, as well as easier overspray cleanup.
6. Temperature & Humidity Control
Temperature and humidity requirements in paint booths vary widely, depending on the coating being applied. Some spray booths may require only a minimum temperature, while others may require a more tightly defined temperature and humidity window during the paint application process.
More accurate temperature and humidity control is critical for the adherence of the low observable, thermal barriers and metalized performance coatings used in defense applications. This decreases drying times and improves quality, while reducing rework. Elevated temperatures may also be used to accelerate the curing process and improve throughput.
Regardless of the airflow style, design considerations in the air velocity and filtration layout should be made to prevent stratification — warm air rising — in the booth and promote even air distribution throughout the booth.
7. Energy Costs
Large aircraft booths generally require a dedicated source of outside (fresh) air that must be conditioned to the requirements of the finishing process. It is expensive to heat replacement air in a paint booth, since so much air is exhausted out of the stack, along with the cost to condition it. Adding the requirement for cooling and humidity control within the environment can cause costs to skyrocket and prompt greater attention to energy conservation.
The first step in conserving energy is to minimize the amount of air required in the paint booth. Conformal booth designs should closely match the size and shape of the aircraft being painted, reducing the overall area of the booth, as well as the overall volume of air required.
Additionally, energy recovery can also be added to mechanical systems to help recoup the energy used to condition the incoming air as it is exhausted from the system. When designing energy recovery systems for paint booths, special consideration must be made to ensure that particulate and VOCs from the process are not allowed to enter back through the intake system. It is not uncommon to expect an energy recovery system to achieve a minimum of 40 percent energy savings. The savings may even be higher, depending on geographic location, booth conditions and type of recovery system.
Air recirculation has also become widely accepted as a means of reducing energy costs and for mechanical equipment requirements for aircraft paint booths. Recirculation can reduce outside air requirements of the booth system by as much as 80 percent, while maintaining an equivalent air velocity in the booth. Careful consideration should be taken to ensure safe operating conditions in the aircraft booth at all times.
What Are the Next Steps?
Once you have considered these factors, aircraft paint booth engineers can help you design the ideal paint finishing environment for your budget, production and quality needs. GFS has a complete team of electrical engineers, licensed structural engineers, airflow specialists, code compliance experts, mechanical engineers, systems designers and project managers who will work with you to build equipment that meets your unique requirements.