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Aerospace Article
Upgrades for Older Paint Booths
Richard Thelen, PE, Global Finishing Solutions
This Old Paint Booth
I have always enjoyed the topics and principles of the television show “This Old House”. Its premise is to take an aged and wizened old structure and fix it up while updating its features to more modern materials and technology. Can this principle be applied to a paint booth? We think so, and will endeavor to show some ideas which will make the oldest and crustiest paint booth just like new and for a fraction of the cost of building a new paint booth.
Let’s look at a few of the technologies that make up a modern paint booth. As you read along, mentally check and see if the idea can be used on your booth.
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Table 1. Fluorescent Light Fixture Comparison
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Lamp and Fixture
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Watts
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Total Lumens
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4-tube T-12
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160 watts
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9800 lumens
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4-tube T-8
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128 watts
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12800 lumens
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6-tube T-8
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192 watts
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19200 lumens
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It is easy to see that considerable gains in lighting intensity can be obtained by changing to a higher output fixture. Notice that the T-8 tube requires only 32 watts per tube while the T-12 tube requires 40 watts per tube. The gains in output come with lower energy costs! Re-lamping costs are lower also.
The T-12 lamp will become obsolete in a few years due to its mercury content and will become unavailable. This will force conversion to the more environmentally friendly T-8 lamp. The Universal Waste Rule, a part of the Resource and Recovery Act, governs the handling of these types of waste. The lamps must pass the Toxic Characteristic Leaching Procedure. The Philips ALTO lamps already pass this test and others will follow.
You may choose to replace just the ballasts and the wiring to upgrade the lighting equipment or you may opt to replace the entire fixture. Some sheetmetal work may be required if new fixtures are used since the hold sizes may be different.
At a large Western USA aircraft maintenance hub, the paint booth for medium transports had over 200 fluorescent lights of a 1986 vintage. Lighting levels were poor. The fixtures were changed, and light levels increased dramatically. In addition, the higher power of the new fluorescents made the lighting effective at over 35’ from the floor. The old fixtures were losing light at 18-20 feet and were ineffective at that point.
HID (Metal Halide) Lighting
Pulse start metal halide fixtures have been around since 1998 and have shown to be a very economical investment. Pulse Start metal halide fixtures are the current ‘best technology’ in HID lighting. They offer similar wattage with higher lumen levels than the older standard metal halide lights.
The standard metal halide lights go out slowly. In fact, it can be argued that they stay dim long after their ideal replacement time, causing the operator to think they are still lit so they must be good. In truth, the lumen levels have at this point dropped to less than 20% of their initial level and should be replaced. The new pulse start lamps will not gradually go dim, dim, dimmer forever, but will go out completely indicating it is time to re-lamp. Lamp life, by comparison is 50% greater.
From Table 2 you can see that a gain of 22% in lighting levels is achievable. The energy costs remain the same, but the lamp life goes way up, and it is easy to tell when to change the lamp. |
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Table 2. HID Light Fixture Comparison
Airflow management
If your paint booth is equipped with 3-stage filters, you may have already noticed that the airflow falls off dramatically as the filters clog with paint particulates. The effects of poor airflow become evident in the poor finish quality that is caused by un-captured overspray. Dirt and dust inclusions are present in the finish. When a positive pressure situation develops in the paint booth, eddy currents and turbulence are generated in the airstream. The result is higher re-work and customer dissatisfaction.
The use of a variable speed exhaust fan motor will solve this problem. With paint booths installed before 1998, it was too costly to install variable speed motor controls. Advancements in technology have allowed the use of VFD (variable frequency drives) drives which are all electronic and thus relatively inexpensive.
In the case of forced air systems, the cabin pressure is sensed and the fans are sped up as the exhaust filters clog up. This causes a balance in the airflow that keeps the system at the peak operating conditions for a much longer period. You will find that filter changes will be less frequent because the system is much more balanced.
Typical costs of an auto-balance system for two 20 HP exhaust fan motors is $7,100 plus installation. The installation is simple if the VFDs are placed near the original control panel. The starters are abandoned and the existing conductors to the motors are hooked into the VFD system. The pressure sensor is mounted in the cabin with an electrical cable run back to the control box. You should check that the motors are rated for inverter duty, but most motors of this size purchased since 1998 are so rated.
Once installed and balanced, this system will keep your re-work to a minimum and your paint quality high. Your customers will notice the difference in quicker turnaround and a better look.
At an air base in southeast USA, VFDs were added to the exhaust fans of a booth that had serious stratification problems and defects resulting in serious paint quality issues. The addition of the VFDs and pressure controls on the fans eliminated the problem.
Filter Monitor Frames
When multiple stage filters are installed in a filter wall, there is little chance of measuring the pressure drop across each stage of filtration. It is necessary to know pressure drop on each stage with some certainty to know when to change the filters.  The picture at right shows a filter monitor. It fits into a filter frame in the filter wall. The
three gauges display static pressure across the three stages of filtration. Notice the bulkhead air fittings on the top of the box. Tubing to the remotely located 3-gauge display rack connects these fittings.
Convert to forced-draft
Many paint booths are ‘natural draft’ paint booths. This means that they are designed to remove air from the surrounding space. It is necessary for the owner to provide for heated air replacement by installing an air heater to dump heated replacement air into the space. This is a good system, but over time, things change, like owners who add small exhausters that remove the replacement air and thus the control of space temperature suffers. Naturally, operating conditions in the paint booth suffer.
The best way to control the cleanliness of the painter’s environment is to duct the air into the booth. In this manner, the air will not pick-up any additional dust and dirt on its way to the painted part. It is possible to do one or two stages of fresh air filtration at the air handler and then to add a third stage at the very entrance of the paint booth to give the best possible cleanliness.
A natural draft booth usually has an open front or has a filter door. In order to pressurize the booth, it is necessary to add a filter chamber at the front end of the booth for the intake air. There are three ways to do this in crossdraft booths:
• Install a plenum filter door. Such a door is a large plenum mounted on wheels that attaches to the front of the booth. Air is forced into the plenum and enters the booth after the final filtering. This style of filter door is used on large booths such as aircraft and very large vehicle booths where the use of a filter chamber is not possible.
• Create a plenum between the booth entrance and the nearest wall and duct the air into that plenum. Install a filter door at the entrance of the paint booth. This style can be used with paint booths that have filter doors.
• Semi-Downdraft. A plenum is created over the top of the door and a filter rack is placed in the ceiling of the booth. Air is ducted to the plenum under pressure.
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Plenum Doors
Plenum Doors offer the best possible intake air conditions for aircraft and parts that do not fill the cross-sectional area of the paint booth. The door is powered by an electric motor. It has a self-leveling “air-ride” suspension that rides smoothly over uneven terrain. It will act as both the interior and the exterior door.
Figure 1. Plenum Filter Door on
Aircraft Booth with Intake Ducts
 It has great air distribution characteristics. In a recent test, we discovered that the air velocities coming off the filters of a plenum door varied only 10-20 fpm across the entire face of the door. This is due to the use of high diffusion filters. These filters require a higher pressure drop but the result is very even air flow. It is not possible to get as equal an air distribution pattern with any other air entry system.
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Create Plenum with Filter Door
This method creates a plenum between the outer wall and the face of the paint booth. About 4 feet of space is required to get a good plenum. Air is ducted into the plenum created in this manner, and the air enters the paint booth through a filter door.
This method gives very clean air to the painting process. It suffers only if maintenance suffers. If the outer door leaks air in the winter, then the resulting cooler air enters the booth and warm air goes out the cracks in the outer door. If the booth is to be air conditioned, special care must be taken to insulate the plenum panels that surround the new plenum. Air distribution is not as easy to control as with the plenum filter door.
Figure 2 – Create Plenum with
Filter Door
 At a large western US air base where Chuck Yeager broke the sound barrier in 1947, many types of special aircraft are maintained and repainted. It was decided to go to a pressurized system to protect the painter’s environment against incoming dust. They elected to create a plenum with their existing filter door and the hangar’s exterior bottom rolling hangar door. It created a dramatic change and allowed for them to introduce cooled air and create a better environment for painting and for the painters.
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Semi-Downdraft Style
In this style, a plenum is created in the front top of the paint booth. Filter racks and filters are added to the ceiling of the paint booth in the plenum area. This takes some considerable modifications to the paint booth.
The airflow is poor and hard to control, and changes with the seasons. It must change direction immediately upon entry into the booth from a downward travel to a horizontal travel. It stratifies easily and the operator sees warmer air at the top of the booth and cooler air at the floor. This is the least desirable method of converting to forced draft air entry for a paint booth.
Figure 3 – Semi-Downdraft Style
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Fall protection
Many of the processes of painting an aircraft require that the painter walk on the top surfaces of the aircraft. Many of these operations are inherently unsafe and accidents are just a matter of “when” not “if”. The use of “alodine” coating for the aluminum skin of aircraft is especially dangerous, since all traction is removed by the alodine coating and the slippery nature of the operation is especially dangerous.
When guardrails are not possible options, fall-arrest methodology must be used. Fall- arrest systems must be designed to be fail-safe. A well-designed system will allow the worker to travel freely without having to disconnect from the system.
It is possible to add fall protections to most paint spray booths at a reasonable cost. Careful attention must be paid to the engineering concerns of column and beam loading, since the forces of a fall can be very great. But, if the paint booth can withstand these forces, the installation of a fall protection system is relatively simple.
Fall protection systems require a cable or track system with a lanyard to connect to a user’s harness. The cable system requires careful load calculations on the paint booth connections. The rail style unit gets by with spreading the loads out over a greater area and is many times the only way to install fall protection in an existing booth.
Climate Control
The use of exotic paints and water-borne paints, which must be applied under precisely controlled environmental conditions, is becoming more frequent. These paints are a newer technology that is driving the use of climate controlled air replacements systems in paint booths.
A good rule of thumb on refrigeration tonnage for make-up air units is 250 cfm/Ton. This will vary from location to location, but will serve for this illustration. Taking the example of a booth handling 84,000 cfm of air, the refrigeration required is approximately 336 TR. At one horsepower per ton, that is a significant mechanical plant. It is beyond the capability of a single package unit, so multiple package units must be used or it will be necessary to install a physical plant that would include water chillers, cooling towers and an electrical substation.
One way to reduce the cost of operation for such a paint booth is to use exhaust recirculation. In this scheme, 80% of the exhaust air is recirculated and 20% of the air is brought in as fresh air. This reduces the refrigeration load to a fourth of the original. So in the example above, only 84TR is required to cool the paint booth in the summer months. This is a much more attractive approach but it carries some warnings.
Exhaust recirculation causes the concentration of VOCs in the paint booth to rise. This can be dangerous to painters who are not in full personnel protective gear, including breathing air. This is already a standard practice for many paint shops even using 100% exhaust.
Exhaust recirculation also raises the risk of fire and additional precautions must be taken to detect the build-up of flammables and to stop and purge the booth. The use of gas detection equipment to warn when volatile levels are increasing is sound and prudent engineering.
Summary
There are many ways to upgrade the performance and extend the life of paint booths. These methods are usually very cost effective with operating costs decreasing (lights and filter costs) and paint performance increasing. It is prudent to look at older booths with an eye toward improving the operation of the booths.
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Global Finishing Solutions
12731 Norway Road • Osseo, WI • 54758
P800-848-8738 • F715-597-2193 • info@globalfinishing.com
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