Automobile Paint Shop - Air to the Booth

[Tom Bender]

Having spent a little time building and revising automobile paint shops was a great adventure for an engineer and woodworker. I can say with confidence that it would be a rare item indeed that gets a better finish than a new car from a major manufacturer. The cost of a new paint shop is in the neighborhood of $1 billion USD. From that experience I can offer some details that may help painters in a smaller situation.

Waterborne and other low VOC finishes are very sensitive to the air in the space where they are sprayed and begin to skin over. To get the air to suitable condition, one paint booth air supply I worked on did the following;

1. outside air at any condition (actually all conditions since it ran 24/365) was drawn in thru filters

2. filtered air was heated to around 120 F so it would absorb humidity

3. heated air was humidified to around 90% (100% was not needed or practical)

4. humid air was cooled to around 40 F to wring out the excess moisture

5. cooled air was reheated to around 70 F which was perfect for the paint

6. perfect air was filtered again

7. perfect filtered air was then sucked thru the fan and ducted to the booth

After step 3 the air was not consistent owing to the difficulty of accurate humidifying and any deviation that produced suboptimal paint jobs was hugely expensive
The fan is not first here because it is difficult to then distribute the air across the filters, coils etc.
Due to constantly changing regulations and chemical advances finishing materials change often so paint shop designs change. Major manufacturers can afford to produce the best finish on their highest volume products.

[Ron Selzer]

interesting that the fan is last, I am used to fan after the filters before the coils in commercial buildings. How bad is the air stratification thru the system or is this one of the reasons the fan is last? Also don't understand heating the air to humidify then immediately dehumidifying, what is the net gain of doing this? Very interesting to me and maybe I can learn some more to help me in my job of building HVAC maintenance.
Thank you
Ron

[Tom Bender]

Hi Ron
The economic factors are interesting

The cost of the facility is high but when a new shop is built, no cost is too great, the money is committed and spent in the name of quality. You may remember incidents where paint flaked off the roof of certain cars due to cost savings. It's been a few years. Nobody in the industry wants to see things like that happen again

The cost of paint is high but not a huge factor

The cost of energy is high but also not a game changer

The cost of productive labor is not so high since there are not a lot of people doing it, but a lot of pressure is placed on it

The cost of maintenance is high since there is so much equipment and the cost of downtime is so high. A short stop in the paint shop is generally contained there but an hour or more will put the rest of the factory at a stop. So there is a repair substantial crew available. And cleaning is a big deal

The cost of defects that need to be repaired in the paint shop is significant but not huge

The risk of a quality spill (defective paint jobs getting delivered to customers) adds an undertone of paranoia.

Adding the requirements for safety, quality, cost control, thruput, peace with the union, 24/7 operation and trying to live a normal life, makes an automotive paint shop a very high pressure place to work.

[Tom Bender]

Stratification is less critical in building HVAC but still can play some tricks on you. Consider a heating and air conditioning air house; these are made at minimum cost so there are plenty of borderline conditions, not just stratification. Here are a few that have plagued me.

Outside air (OA) and Return Air (RA) not well mixed can freeze steam or chilled water coils. Also it can cause poor heat transfer since part of the coil is faced with more temp diff than it is designed for while the rest of the coil is loafing along on RA. Same for filters, the OA portion gets plugged faster.

Coils, filters, dampers etc designed for 500 feet per minute (fpm) but due to poor distribution, much higher air velocity in the center. Poor heat transfer, blown out filters, frozen coils, sticking dampers, condensate blown off the coils. Condensate blown off goes on the air house floor. From there it can go onto the roof or thru the ceiling into the building, both bad. It can also go thru the fan, also unpleasant for you.

Condensate drains are particularly annoying. To start with they are built just barely deep enough for the application, this saves on the cost of pipe, and they are usually inaccessible. If there are drains from the intake mixing area and after the cooling coil, they must have separate traps.

Some air houses have the fan last but it can send a lot of noise thru also. If the fan were a little larger and slower it would be quieter but cost a few dollars more. Anytime you have the opportunity to select a fan go larger and slower than the minimum. Unfortunately there is a customer belief that a louder fan is a more powerful fan.

[Tom Bender]

Back to preheating, when the air contains very little water vapor (desert or winter) it may be below the dew point for the coil and so be too dry for the booth.

[Ron Selzer]

thanks Tom
now I need to think on this and digest the info.
Have measured stratification of over 50 degf in an AHU, interesting how air will not mix in the mixed air chamber if the wrong dampers are used or specified to be installed in bad positions to each other. Have even measured stratification after the fan, still don't completely understand how that happens as I would think the fan would mix it up. Used to do DDC controls only now I do general building maintenance. Have seen the ubends freeze and bust yet not trip the low limit, investigating that the first time got me started on tracking stratification particularly on larger coils as in 8' x 10 to 20' x 35' approx measurements. Also having to move discharge air temp sensors to find a average temp not one of the extremes off of the coil.
Thanks for the info
Ron