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Monday, 08 March 2010 14:53

The Relationship between your Compressor, your Spray Gun and the Final Appearance of your Paint Finish

Written by Stefan Gesterkamp

After my February column (see related here) on spray-gun choices appeared in Autobody News, I was asked to clarify a point I made on CFM availability in the shop during peak air consumption.

CFM stands for cubic feet per minute and a spray gun’s peak performance is depending on proper air volume. Each spray gun is engineered and tuned, just like a carburetor, for a specific CFM consumption. Some spray guns ask for 8-9 CFM and others want 17 CFM or more for optimum performance. Less CFM consumption doesn’t automatically translate into a better quality spray gun; it simply means that it could be the better choice for your situation. Most manufacturers’ spray guns will consistently perform well and do exactly what they are designed to do, as long as you provide them with their basic pressure and volume requirements.

 

How do you know how much CFM you have available? The following is not a 100% scientific answer to that question, there are simply too many unknown (to me) variables in every shop’s situation, but it is a fairly reliable rule of thumb and it beats buying highly expensive equipment you are likely to use only once. Just look at the tag on your compressor for the necessary information and do the following math.

A standard two-stage piston compressor produces about 4 CFM per HP (horsepower) and a screw drive compressor generates about 4.7 CFM per HP. Multiply your compressor’s HP rating by the appropriate CFM and you get your maximum CFM output. To have all of the potential CFM available to you, the air should be delivered to your work area in a 1½–2 inch pipe. Make sure that the connections from the compressor to any filter or dryer is also properly sized. Whenever possible, the pipe should be a closed loop system. The moment you close the pipes in a looped system, the pipe becomes a very effective storage reservoir. It also evens out the air availability to each work station. All of your quick-disconnect couplers in the paint department should be 3/8 of an inch in diameter. Many shops are still using ¼ inch couplers or wall regulators with too low of a CFM rating and can’t figure out why the paint jobs are not as nice as they hoped for. Leave the rest of the shop on ¼ inch couplers. Nobody in your shop is as dependent on CFM as your painter and he deserves to get preferential treatment when it comes to air supply. Most shops don’t have a dedicated compressor for the paint department. In that case, you must deduct the air consumption of all other air tools that could potentially be used at the same time from the total CFM produces. By deducting all of the potential air tool CFM consumption from the total volume produced, you get a good idea what type of spray-gun you can consistently support in your shop. Be sure to consult the air tool owner’s manual for the actual CFM consumption of each tool.

Following are some general figures for the most frequently used tools in our industry. A standard DA sander consumes about 10–15 CFM and an air buffer 20–25 CFM. A wide-open air blower could use as much as 35–40 CFM. The length of your air hose is also a factor in the calculation; you can lose an additional 1 CFM for each 10 feet of hose over the standard 32-foot length. Depending on the type and the manufacturer of your quick-disconnect couplers, you may lose as much as 7 CFM for each quick-disconnect coupler the air has to travel through.

One last comment about CFM. Your air supply is only as good as the weakest link in the system. If your air volume is restricted anywhere between the compressor and the gun, your available volume can’t be more than the restriction allows to pass through. For example, if the compressor is hooked to the main line with only a ¾ inch pipe, it doesn’t matter what you do after that restriction, you can only access the volume that passes through that pipe. Whether it’s 100 HP or 10 HP compressors wouldn’t matter.

Last but not least, all pressure regulators and filtration systems should be rated for sufficient CFM pass-through.

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