Air Compressor Rules of Thumb

As pressure increases from 100 psig to 150 psig at sea level, BHP increases at about 1/2 hp per psi increase. That pressure range covers most American compressor installations for "shop air".

Thanks JohnB

How about going the other way? I assume it is not linear (or it would require 50 hp to make 100psi)

We get a lot of demand profiles, for example, shift 1 uses 300 cfm, shift 2 uses 350 cfm and 3rd shift only uses 100cfm. When dealing with a VFD or multiple sequenced compressors is there a "rule of thumb" way to estimate (conservatively) the power draw during each shift?

Then we also get energy savings claims because the reduced line pressure from 100psi to 90psi. How will that effect power draw?

Thanks everyone for your help.

The energy consumptions are actually very different depending on the type of air compressor you have....single stage, two stage, load/unload, inlet modulation, VSD controlled.

Air compressors are often operating at part loads or cycling between the load and unload. These aspects are crucial to an evaluation of the energy consumption of the machine.

If you are referring to vendors "claims" of energy savings...be careful.

I'd recommend getting familiar with DOE AirMaster+. Its a free software program put out be the Department of Energy to evaluate energy consumption of compressed air systems. If a vendor has done some monitoring, I would ask for the monitoring data and input it into AirMaster. Alternatively, ask for their calculations or AirMaster runs.

http://www.truetex.com/aircompressors.htm

Thanks Guest, amazingly I had actually found that link already (its a small www!) when I was looking to buy a compressor to paint my car with. It is a very discouraging link for those of us on small home project budgets. But I'm glad someone is trying to put the truth out theree! it is a great link.

Using the horse power name-plate rating of a motor to estimate the output of a compressor is useful if the motor has been exactly matched to the compressor. This is usually the case with modern industrial compressors.

Then at 100 psig you will get about 4 scfm (and perhaps a little more) per HP for the smaller fast running compressors and maybe (if you are lucky) up to 5 scfm for large slow running compressors.

you will be lucky to get more than 3.5 scfm per hp from a low cost domestic compressor.

I think it's applicable to Screw type air-compressor.