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New corrosion control aircraft paint hangar
New Corrosion Control Aircraft Paint Hangar by Ron Joseph, Organic Coatings Editor E-mail, [email protected]
ecently, I had the opportunity to visit Nellis Air Force Base outside Las Vegas where a new aircraft paint hangar was recently installed. Over the past several years I have been through many such paint hangars, but this one, designed by JBI of Osseo, Wis., has bells and whistles that I thought would be of interest to our readers. If you have never been inside a paint hangar, you will be amazed at its vastness, because an entire aircraft must fit comfortably inside and still have sufficient space for other activities to be carried out. For instance, at an Air Force base, most aircraft have already been painted at least once before they come back for rework or for a completely new paint job; therefore, the hangar must have sufficient space for operators to scuff down the aircraft, use water hoses to thoroughly wash down the surfaces, mask appropriate areas, and then apply the primer and topcoat.
R
PAlNT NANGAN DIMENSIONS
For the operators to be able to work on all areas of the aircraft, large movable stands and platforms are required. But wing stands and tail stands have quite large footprints and operators must be able to store them on the sides of the booth when they are not being used. Add to that an area to store lo-gal pressure pots, hose reels, a gun washer, paint storage cabinets, a containment area to store hazardous waste drums, and suddenly you have used up a large area for nonpainting activities. Taking all these factors into account the Nellis
Figure 1. View inside the large paint hangar booth at Nellis Air Force Base.
8
booth is 80 R long x 73 ft wide x 12 R high. In the section of the ceiling that is under the aircraft wing, the booth is 22 ft high, as can be seen in Figure 1. Like many similar hangars, this one has a cross-draft design, with air moving from the air supply system, parallel to the floor, and exhausting through the dry filters at the other end. To maintain an average air velocity of 100 fpm throughout the hangar, JBI designed for airflow of 84,000 chn, calculated as follows: Airflow (cfm) or (ft3/min) = 70 ft wide x 12 ft high x 100 ftimin = 84,000 cfm or (ft3/min) SWING-OUT PLENUM DOORS
One of the unique design features is that the supply air enters through filters in the product access doors at the back of the booth. Two separate air supply systems, each delivering 42,000 cfm, have been provided, one for each door. Although this was not the first time I had seen supply air entering through the plenum doors, what impressed me was the ease with which these humongous and heavy doors could be swung open and closed. As the facility ages, the concrete slab outside the hangar will not always be smooth and flat, and so the doors use an air-ride suspension system, each operated by a 5-hp motor. In fact, the slab was designed to slope away from the hangar so that rainwater cannot enter the hangar. Another intriguing design feature is the manner in which the supply air enters the plenum doors by means of large 44-in. diameter ducts that seal against the doors when they are closed. (Fig. 2) A personnel access panel door leads into each plenum door so that you can stand inside and walk along the full width of the plenum. Special air supply filters, with a relatively high pressure differential of 0.25 in. W.C. have been installed to satisfy two functions: filter out dust that might enter the hangar from the outside and even out the airflow pattern within the hangar. By selecting appropriate air supply filters the air moves in a laminar motion toward the exhaust section at the other end. Apparently, conventional air supply filters that have a low pressure drop (DP) make it more diffiMetal Finishing
tions call for 100 foot-candle&t2 the lighting in this facility produces 139 foot candles/ft2. The lights have been positioned to prevent or limit shadows.
Figure 2. Open door with two large air ducts.
cult for the booth designer to balance the airflow through the length of the spray booth.
Exhaust and supply fans are 60 hp each and the exhaust fans are regulated by means of variable frequency drives (VFD). They function by monitoring the pressure differential between the interior of the hangar and the outside, hence insuring that the hangar always operates at a slightly negative pressure of 0.03 in. W.C. When the DP falls outside the desired range, the VFDs increase or decrease the rpm of the fans to bring the airflow back into specification. By operating at a slight negative pressure very little dust and dirt will be drawn into the spray area from the outside. Some spray booth specifiers prefer their booths to operate at a slight positive pressure (+0.03 in. to +0.07 in. W.C.) to keep the dirt out, but in the Air Force there is often a concern that the solvent vapors and polydiisocyanate monomers from the polyurethane coatings might escape into other work areas of the facility. Hence, while a slight positive pressure might result in a cleaner-looking paint job, health and safety of the personnel takes precedence, and to the extent possible, vapors are prevented from entering work spaces outside the hangar.
Whenever I interview aircraft painters one of their most common complaints regards poor lighting. I have watched painters apply coatings to the underside of the belly or wing of an aircraft and the lighting has been so poor that they were literally painting “in the dark.” To overcome this potential problem JBI designed the hangar with 80 light fixtures located in the ceiling and walls. Each light fixture comprises four tubes, 32 W high vision Phillips white 3,000 lumens/tube. While most spray booth specifica10
MAJORNOISE RRDUCTION If you’ve ever spent much time in a spray booth you will probably have been irritated at the noise level of the fans. In some facilities, painters are required to wear ear protection, such as the common sponge ear plugs, to protect them from hearing loss. I have had the misfortune of working in spray booths in which the noise level was so high that I could not easily communicate with the painters. In the Nellis AFB hangar this is surprisingly not so. Despite the large blowers that have been installed you can easily talk to someone without raising your voice. Most specifications call for a maximum noise level of 73 db, but at Nellis the noise level was not measurable. This is not magic. For an extra cost the exhaust and supply blowers are located outside the building and not immediately above the ceiling of the facility. The low noise level alone makes it much more comfortable for the operators to work. (cowTRoLLpDENVIRONMENT Las Vegas gets hot in summer.. .very hot, so much so that gamblers who visit the city prefer to spend their days inside the casinos rather than walk the streets at 115”F! On the other hand, the city lies in the high desert where for much of the year the relative humidity can be as low as 15%. The coatings used on the aircraft painted at Nellis are sensitive to humidity. Applied at conditions that are too dry, the paint dries before it hits the plane and does not cure correctly; whereas for optimum performance the coatings should not be applied when the humidity is too high. With this in mind, the Air Force engineers specified a spraying environment inside the hangar of 72°F and a relative humidity of 50%. One can control temperature with a refrigeration system and humidity with a humidification system, both of which are costly. To keep the energy costs down JBI devised an intriguing design. The entire air handling system comprises the following elements (see Fig. 3). Step #l: Warm outsidedesert air passes through an airtoair plateheat exchangerwhere the air losesheat to cooler air being exhausted from the hangar The relative humidityof the air that leavesthe heat exchangerincreases slightly on account of the drop in temperature.This increasecan be determinedby referringto a standardpsychometric chart that you can obtain from engineering handbooksor perhapseven horn the Internet. Step #2: The cooler air now passes through an Metal Finishing
Figure 3. General concept for the air handling system.
evaporative cooler, which comprises a stream of city water falling over corrugated cardboard media. When the air emerges on the other side of the media the relative humidity has increased, and because evaporation is a cooling process, or so we learned at school, the temperature is lower. Step #3: The air will now pass through refrigeration cooling coils followed by a natural gas heater, only one of which will be operating at any one time. For instance, if the temperature of the air leaving the evaporative cooler is higher than 72°F (our goal) the refrigeration coils will be turned on, but the gas heater will be off. By the same token if the air temperature is lower than 72”F, such as during winter months, the refrigeration system will be turned off, but the gas heater will be on. By the time the air comes to the large fan it should be at the desired temperature and humidity. But what if the humidity is off specification? Let’s go back to the diagram. Step #4: If the relative humidity (RH) of the incoming desert air is already too high (>50%), such as on rainy days, the evaporative cooler can be turned off. If the air is slightly colder than 72°F and the RH too high, the evaporative cooler can remain off, but an electrical resistance heating element in the air duct will be turned on so that a slight increase in the air temperature will lower the relative humidity. Again, you can see how temperature changes affect RH by referring to a psychometric chart. If you study the various options (see Fig. 5-7) you will see that the design of the air handling system allows the operators to control a wide combination of desert air temperature/relative humidity conditions. So where are the advantages of this apparently complex system? The primary energy guzzler is the refrigeration system, while the natural gas heater and the resistance element heater also consume energy. The cost to operate the air-to-air plate heat exchanger and the evaporative cooler is relatively 12
Figure 4. Normal dry summer conditions.
low. According to the JBI engineers the bottom line is that on the days when the refrigeration system is operating (most of the time) the energy load is reduced from 660 tons to 120 tons, a savings of 440 tons. In terms of dollars and cents, we have used current energy costs in Las Vegas, which is near Hoover Dam, to arrive at the following: Current electric costs in Las Vegas are 8.48 cents/Kwh. Operating costs if there were no evaporative cooler = 660 Hp x 0.746 Kw/Hp = 492 Kw x $O.O848lKwh
= $41.90/mOperating costs incorporating the evaporative cooler = 120 Hp x .746 Kw&Ip = 89.8 Kw x $O.O848lKwb
= $7.62lhr Energy savings on the days that the refrigeration unit is operating = ($41.90 - $7.62& = $34.28ihr
Like many other Air Force installations, the exhaust plenum of the spray booth is fitted with a threestage paint particulate filtration system. The first stage is a roll or blanket filter, followed by a second Metal Finishing
Figure 5. Normal rainy summer
conditions.
stage panel filter, followed by a third stage pocket filter. To insure compliance with the Aerospace NESHAP all filters have been certified by the respective vendor to pass EPA Method 319. You will recall that this method is used to qualify the capture efficiency of the filter media. Each stage must capture at least a minimum percentage of dust particles within a range of micron sizes, specified by the EPA. The overall capture efficiency for all three stages must be 98% of particles 2 microns and larger. In the 1990s Edwards AFB, also in the high desert, voluntarily installed a carbon bed that comprises a wall of 3-in.-thick pelletized filters that sits immediately behind the third stage of pocket filters. Nellis AFB followed suit so that all solvent laden air passing through the spray booth automatically also passes through the carbon bed, where the solvents WOC) are adsorbed (see Fig. 7). When the carbon is saturated it is sent off site for regeneration. Apparently, the carbon can be used for 6 to 9 months before it needs to be regenerated. Many facilities, including Edwards AFB, have installed a flame ionization detector to monitor that the solvents don’t penetrate the carbon bed. The Aerospace NESHAP calls for continuous monitoring of VOC emissions and the regulation defines
Figure 7. Filters are shown at left and carbon beds on the right.
“continuous” as once per shift. For nonregenerative carbon adsorbers owners may use a portable monitoring device to monitor total HAP or VOC concentrations and record the measurements in a journal or log to comply with the intent of the NESHAI? The carbon filter manufacturer supplies stick-on paper monitors that change color at breakthrough. The color is recorded at the beginning/end of each shift. CONCLUSIONS
The new aircraft paint hangar at Nellis AFB is a state-of-the-art facility. What makes this facility different from most others of its kind are the uniquely designed plenum doors mounted on air-ride suspension and the manner in which the supply air enters through ducts that seal against the doors when they are closed. The use of high-diffusion filters allow for easy balancing of airflow over the entire surface of the door. Where it was thought necessary to provide complex balancing dampers and plates, these were found to be unnecessary. Other major features that are notable include the entire air handling system, incorporating the refrigeration system and the evaporative cooler. The use of white painted walls and fluorescent lighting allow for a much more natural daylight style of lighting with fewer shadows. There is little or no glare, making the operation of painting today’s most lethal and modern airplanes a pleasure.
Ron Joseph is an independent coating consultant in San Jose, Calif.
rlgure 0. lvormal wlnrer conomons.
October 2002
13
ecently, I had the opportunity to visit Nellis Air Force Base outside Las Vegas where a new aircraft paint hangar was recently installed. Over the past several years I have been through many such paint hangars, but this one, designed by JBI of Osseo, Wis., has bells and whistles that I thought would be of interest to our readers. If you have never been inside a paint hangar, you will be amazed at its vastness, because an entire aircraft must fit comfortably inside and still have sufficient space for other activities to be carried out. For instance, at an Air Force base, most aircraft have already been painted at least once before they come back for rework or for a completely new paint job; therefore, the hangar must have sufficient space for operators to scuff down the aircraft, use water hoses to thoroughly wash down the surfaces, mask appropriate areas, and then apply the primer and topcoat.
R
PAlNT NANGAN DIMENSIONS
For the operators to be able to work on all areas of the aircraft, large movable stands and platforms are required. But wing stands and tail stands have quite large footprints and operators must be able to store them on the sides of the booth when they are not being used. Add to that an area to store lo-gal pressure pots, hose reels, a gun washer, paint storage cabinets, a containment area to store hazardous waste drums, and suddenly you have used up a large area for nonpainting activities. Taking all these factors into account the Nellis
Figure 1. View inside the large paint hangar booth at Nellis Air Force Base.
8
booth is 80 R long x 73 ft wide x 12 R high. In the section of the ceiling that is under the aircraft wing, the booth is 22 ft high, as can be seen in Figure 1. Like many similar hangars, this one has a cross-draft design, with air moving from the air supply system, parallel to the floor, and exhausting through the dry filters at the other end. To maintain an average air velocity of 100 fpm throughout the hangar, JBI designed for airflow of 84,000 chn, calculated as follows: Airflow (cfm) or (ft3/min) = 70 ft wide x 12 ft high x 100 ftimin = 84,000 cfm or (ft3/min) SWING-OUT PLENUM DOORS
One of the unique design features is that the supply air enters through filters in the product access doors at the back of the booth. Two separate air supply systems, each delivering 42,000 cfm, have been provided, one for each door. Although this was not the first time I had seen supply air entering through the plenum doors, what impressed me was the ease with which these humongous and heavy doors could be swung open and closed. As the facility ages, the concrete slab outside the hangar will not always be smooth and flat, and so the doors use an air-ride suspension system, each operated by a 5-hp motor. In fact, the slab was designed to slope away from the hangar so that rainwater cannot enter the hangar. Another intriguing design feature is the manner in which the supply air enters the plenum doors by means of large 44-in. diameter ducts that seal against the doors when they are closed. (Fig. 2) A personnel access panel door leads into each plenum door so that you can stand inside and walk along the full width of the plenum. Special air supply filters, with a relatively high pressure differential of 0.25 in. W.C. have been installed to satisfy two functions: filter out dust that might enter the hangar from the outside and even out the airflow pattern within the hangar. By selecting appropriate air supply filters the air moves in a laminar motion toward the exhaust section at the other end. Apparently, conventional air supply filters that have a low pressure drop (DP) make it more diffiMetal Finishing
tions call for 100 foot-candle&t2 the lighting in this facility produces 139 foot candles/ft2. The lights have been positioned to prevent or limit shadows.
Figure 2. Open door with two large air ducts.
cult for the booth designer to balance the airflow through the length of the spray booth.
Exhaust and supply fans are 60 hp each and the exhaust fans are regulated by means of variable frequency drives (VFD). They function by monitoring the pressure differential between the interior of the hangar and the outside, hence insuring that the hangar always operates at a slightly negative pressure of 0.03 in. W.C. When the DP falls outside the desired range, the VFDs increase or decrease the rpm of the fans to bring the airflow back into specification. By operating at a slight negative pressure very little dust and dirt will be drawn into the spray area from the outside. Some spray booth specifiers prefer their booths to operate at a slight positive pressure (+0.03 in. to +0.07 in. W.C.) to keep the dirt out, but in the Air Force there is often a concern that the solvent vapors and polydiisocyanate monomers from the polyurethane coatings might escape into other work areas of the facility. Hence, while a slight positive pressure might result in a cleaner-looking paint job, health and safety of the personnel takes precedence, and to the extent possible, vapors are prevented from entering work spaces outside the hangar.
Whenever I interview aircraft painters one of their most common complaints regards poor lighting. I have watched painters apply coatings to the underside of the belly or wing of an aircraft and the lighting has been so poor that they were literally painting “in the dark.” To overcome this potential problem JBI designed the hangar with 80 light fixtures located in the ceiling and walls. Each light fixture comprises four tubes, 32 W high vision Phillips white 3,000 lumens/tube. While most spray booth specifica10
MAJORNOISE RRDUCTION If you’ve ever spent much time in a spray booth you will probably have been irritated at the noise level of the fans. In some facilities, painters are required to wear ear protection, such as the common sponge ear plugs, to protect them from hearing loss. I have had the misfortune of working in spray booths in which the noise level was so high that I could not easily communicate with the painters. In the Nellis AFB hangar this is surprisingly not so. Despite the large blowers that have been installed you can easily talk to someone without raising your voice. Most specifications call for a maximum noise level of 73 db, but at Nellis the noise level was not measurable. This is not magic. For an extra cost the exhaust and supply blowers are located outside the building and not immediately above the ceiling of the facility. The low noise level alone makes it much more comfortable for the operators to work. (cowTRoLLpDENVIRONMENT Las Vegas gets hot in summer.. .very hot, so much so that gamblers who visit the city prefer to spend their days inside the casinos rather than walk the streets at 115”F! On the other hand, the city lies in the high desert where for much of the year the relative humidity can be as low as 15%. The coatings used on the aircraft painted at Nellis are sensitive to humidity. Applied at conditions that are too dry, the paint dries before it hits the plane and does not cure correctly; whereas for optimum performance the coatings should not be applied when the humidity is too high. With this in mind, the Air Force engineers specified a spraying environment inside the hangar of 72°F and a relative humidity of 50%. One can control temperature with a refrigeration system and humidity with a humidification system, both of which are costly. To keep the energy costs down JBI devised an intriguing design. The entire air handling system comprises the following elements (see Fig. 3). Step #l: Warm outsidedesert air passes through an airtoair plateheat exchangerwhere the air losesheat to cooler air being exhausted from the hangar The relative humidityof the air that leavesthe heat exchangerincreases slightly on account of the drop in temperature.This increasecan be determinedby referringto a standardpsychometric chart that you can obtain from engineering handbooksor perhapseven horn the Internet. Step #2: The cooler air now passes through an Metal Finishing
Figure 3. General concept for the air handling system.
evaporative cooler, which comprises a stream of city water falling over corrugated cardboard media. When the air emerges on the other side of the media the relative humidity has increased, and because evaporation is a cooling process, or so we learned at school, the temperature is lower. Step #3: The air will now pass through refrigeration cooling coils followed by a natural gas heater, only one of which will be operating at any one time. For instance, if the temperature of the air leaving the evaporative cooler is higher than 72°F (our goal) the refrigeration coils will be turned on, but the gas heater will be off. By the same token if the air temperature is lower than 72”F, such as during winter months, the refrigeration system will be turned off, but the gas heater will be on. By the time the air comes to the large fan it should be at the desired temperature and humidity. But what if the humidity is off specification? Let’s go back to the diagram. Step #4: If the relative humidity (RH) of the incoming desert air is already too high (>50%), such as on rainy days, the evaporative cooler can be turned off. If the air is slightly colder than 72°F and the RH too high, the evaporative cooler can remain off, but an electrical resistance heating element in the air duct will be turned on so that a slight increase in the air temperature will lower the relative humidity. Again, you can see how temperature changes affect RH by referring to a psychometric chart. If you study the various options (see Fig. 5-7) you will see that the design of the air handling system allows the operators to control a wide combination of desert air temperature/relative humidity conditions. So where are the advantages of this apparently complex system? The primary energy guzzler is the refrigeration system, while the natural gas heater and the resistance element heater also consume energy. The cost to operate the air-to-air plate heat exchanger and the evaporative cooler is relatively 12
Figure 4. Normal dry summer conditions.
low. According to the JBI engineers the bottom line is that on the days when the refrigeration system is operating (most of the time) the energy load is reduced from 660 tons to 120 tons, a savings of 440 tons. In terms of dollars and cents, we have used current energy costs in Las Vegas, which is near Hoover Dam, to arrive at the following: Current electric costs in Las Vegas are 8.48 cents/Kwh. Operating costs if there were no evaporative cooler = 660 Hp x 0.746 Kw/Hp = 492 Kw x $O.O848lKwh
= $41.90/mOperating costs incorporating the evaporative cooler = 120 Hp x .746 Kw&Ip = 89.8 Kw x $O.O848lKwb
= $7.62lhr Energy savings on the days that the refrigeration unit is operating = ($41.90 - $7.62& = $34.28ihr
Like many other Air Force installations, the exhaust plenum of the spray booth is fitted with a threestage paint particulate filtration system. The first stage is a roll or blanket filter, followed by a second Metal Finishing
Figure 5. Normal rainy summer
conditions.
stage panel filter, followed by a third stage pocket filter. To insure compliance with the Aerospace NESHAP all filters have been certified by the respective vendor to pass EPA Method 319. You will recall that this method is used to qualify the capture efficiency of the filter media. Each stage must capture at least a minimum percentage of dust particles within a range of micron sizes, specified by the EPA. The overall capture efficiency for all three stages must be 98% of particles 2 microns and larger. In the 1990s Edwards AFB, also in the high desert, voluntarily installed a carbon bed that comprises a wall of 3-in.-thick pelletized filters that sits immediately behind the third stage of pocket filters. Nellis AFB followed suit so that all solvent laden air passing through the spray booth automatically also passes through the carbon bed, where the solvents WOC) are adsorbed (see Fig. 7). When the carbon is saturated it is sent off site for regeneration. Apparently, the carbon can be used for 6 to 9 months before it needs to be regenerated. Many facilities, including Edwards AFB, have installed a flame ionization detector to monitor that the solvents don’t penetrate the carbon bed. The Aerospace NESHAP calls for continuous monitoring of VOC emissions and the regulation defines
Figure 7. Filters are shown at left and carbon beds on the right.
“continuous” as once per shift. For nonregenerative carbon adsorbers owners may use a portable monitoring device to monitor total HAP or VOC concentrations and record the measurements in a journal or log to comply with the intent of the NESHAI? The carbon filter manufacturer supplies stick-on paper monitors that change color at breakthrough. The color is recorded at the beginning/end of each shift. CONCLUSIONS
The new aircraft paint hangar at Nellis AFB is a state-of-the-art facility. What makes this facility different from most others of its kind are the uniquely designed plenum doors mounted on air-ride suspension and the manner in which the supply air enters through ducts that seal against the doors when they are closed. The use of high-diffusion filters allow for easy balancing of airflow over the entire surface of the door. Where it was thought necessary to provide complex balancing dampers and plates, these were found to be unnecessary. Other major features that are notable include the entire air handling system, incorporating the refrigeration system and the evaporative cooler. The use of white painted walls and fluorescent lighting allow for a much more natural daylight style of lighting with fewer shadows. There is little or no glare, making the operation of painting today’s most lethal and modern airplanes a pleasure.
Ron Joseph is an independent coating consultant in San Jose, Calif.
rlgure 0. lvormal wlnrer conomons.
October 2002
13