Recently a paint and body shop with two 7.5 hp screw compressors purchased another one with the same capacity. They’d added a blast cabinet and said the system could not keep up. Blast cabinets can certainly be air hogs, so depending on the nozzle size and amount of use, it’s not unreasonable to need more air if you add one to your shop. But…
During the visit to install the new unit, our service technician performed some routine maintenance that had been deferred for well over two years. He immediately noticed that the fluid/lubricant level was less than 25% of what it should be and looked terrible:
At first the tech was surprised that the unit had not overheated. The mystery was solved when he changed the inlet filter and separator cartridge:
On the lower right you can see what a clean filter looks like, as opposed to the others that look more like chocolate cake.
They were so clogged that the compressor could not take in enough air to compress. Remember Charles’ Law from high school science class, which taught us that temperature increases with pressure. Compressors generate heat when the energy from the motor is transferred to the compression chamber. Squeezing the air excites air molecules, and as they bump into each other they give off heat. What happens if the compressor is starved for air due to clogged filters? Well, not much compression and not much heat.
This sheds light on an odd statement the customer made to the sales rep some weeks after the installation. He happily reported that since then the shop runs very well. He has plenty of air and the older units don’t seem to come on much at all. So it is quite possible that the existing system was fully capable of supplying all the air he needed even with the blast cabinet. Had the existing units been periodically inspected and maintained, he may not have needed to spend the money on a new compressor, installation, and the additional PM that new unit will require going forward.
Before you presume to blame the sales rep, keep in mind they were not fully aware of the state of the equipment, and the addition of a blast cabinet was a very plausible reason for needing more volume. Still, we can turn this hindsight into foresight.
Before you invest in additional compressors, check to see that the existing compressors are running well and making the air they should be. This applies to any size system.