This discussion covers why it is not good to turn the CAF system compressor on & off in rapid succession. Here, we’re referring to just turning the compressor on/off and not referring to the load/unload, auto and fixed switches of the Auto Sync system.
Now mostly, this is only going to be an issue in training when firefighters are lined up taking turns learning to operate the CAF system. Each time you turn off the compressor, you should wait 2-3 minutes before turning it back on, and here’s why.
The compressor has 2 opposing rotating screws to compress the air. Oil has to move through the compressor to lubricate the screws to keep them from seizing up and destroying the compressor. The moment you turn on the compressor (which might be powered by the transmission PTO, a pump mounted clutch and drive belt, or a belt drive right off the pump drive shaft), the compressor starts compressing air at roughly 30-40 psi that is used solely to push the oil through the compressor.
This is evident by looking at the air pressure gage on the pump panel. It will always show the minimum pressure while the compressor is turning. None of this minimum air is used in firefighting.
When you shut the compressor off, it takes 2-3 minutes for this pressure to bleed down out of the compressor. Immediately turning it back on in rapid succession puts stress and load on the compressor driver (belt drive, clutch, PTO, etc.) and can result in mechanical damage to the CAF system or compressor drive system including belts, drive shafts, transmission, etc.
On most engines, the truck engine is still running when the compressor is shut off so the operator generally cannot hear the compressor bleed down. Click here to view a short CompressedAirFoam.com video {Link} of an auxiliary engine driven CAF system where the engine stops simultaneously with the compressor so you can hear the bleed down.
The best way to prevent possible damage during training is to have the 1st student turn the compressor on and the last student turns it off and all the other students In between can simulate turning the compressor on and off, or just make sure there is about a 3-minute wait between one student turning it off and the next student turning it back on.
For a video on this subject – click on the Videos Page or this specific video – CAF Compressor Bleed Down.
