Paint Spray Guns

Non-Electrostatic

SPRAY GUNS: AIR VOLUME

Q: I would like to know more about the pressures and volumes used in HVLP spray equipment because I am getting lots of orange peel.  I can always maintain the air pressure that is required, but I don't get proper atomization when my system is fully charged with air.  As I use my compressed air, the compressor can't quite keep up and gets loaded down.  When the compressor is "behind" the gun sprays and atomizes much better.  Can you tell me what is going on?  Can I have different air volumes while the pressure remains the same?  I suspect that the lines in my shop are undersized.  Could that cause this problem?  Please advise, or tell me somewhere to go to find the answers.

G.C.

A: Two comments that arouse immediate suspicion are; “I can always maintain the air pressure that is required ...”, and “Can I have different air volumes while the pressure remains the same?”

For the same spray gun and air cap the air pressure entering the gun and the air volume passing through it are inter-dependant.  As the pressure to the handle of the gun increases, the air volume also increases, and visa versa. That is why painters who experience orange peel often increase the pressure to the gun, because this is immediately accompanied by an increase in air volume.  Therefore, I question the two statements given above, because they contradict each other. I believe that G.C. does experience changes in air volume, but doubt that he is making a note of the air pressure at the handle of the gun.

Perhaps G.C. is not referring to the pressure on the handle of the gun, but rather to the pressure on the regulator at the wall of the spray booth. The same argument applies.  If the spray gun and the air hose leading to the gun remain the same, then any change in the air volume passing through the hose and gun will be a result of changing air pressure.  The two go hand-in-hand.  The only way the air volume between the regulator and the gun can change while the pressure on the regulator remains constant, is if more than one painter intermittently draws air from the compressor.  The take-off for the second gun would need be downstream of the regulator.

Generally, orange peel can be reduced or perhaps even eliminated by increasing the volume of air leading to the HVLP gun, and in reality this is done by increasing the pressure to the gun.  Unfortunately, many states regulate the maximum air pressure at the CAP of the gun to no more than 10 psig. and this can prevent one from satisfactorily solving the orange peel problem.

In any case, it does sound as if the compressor is unable to maintain the volume of air that is needed to properly supply the HVLP spray gun.  Remember, HVLP stands for High Volume (air) Low Pressure (air), and in all probability the compressor is too small to maintain the needed air volume.

An obvious solution is to purchase a larger compressor, but before doing so take a look at the air hose leading to the gun.  Many paint shops are still using 1/4" diameter hoses, because that was the standard for conventional air atomizing guns.  Because HVLP guns need a greater volume, a larger diameter hose is recommended, either 5/16" or better still 3/8".  This is particularly important if the hose is long, such as 50 ft or more.  The pressure drop in a small diameter hose will tend to starve the gun of air.

Another hint is to eliminate as many restrictions in the hose as possible.  Replace redundant quick disconnects with ball valves.  For instance, if you don’t need to frequently remove the hose from the regulator on the wall of the booth then a ball valve will suffice.  Clearly, a quick disconnect (QD) is most convenient to attach the air hose to the handle of the spray gun.  One last hint.  Make sure that the QD has as wide a diameter as possible.  You will be surprised how many paint shops use narrow bore QDs, even when attaching a large diameter hose to the gun.

When considering a compressor, look at the total CFM of air, not just the horse power (HP) of the motor. Some HVLP spray guns demand in excess of 22 cfm in order to function properly.  Have a closer look at the compressor’s air supply, and compare it with the spray gun manual. If more than one painter or someone using a pneumatic tool will be drawing air at the same time, then the compressor must be sized accordingly.

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