Calculating CFM for a Rotating Compressor

Easy!

Just calculate the volume of the swept piston space, that gives your cubic feet.

Then calculate how many times a minute it sweeps that volume...

divide / multiply one into the other and you get cubic feet per minute!!

Its easy because the compressor manufacturers state the flow rate at free air pressure so you don't need to use Boyle's / Charle's law...

John.

Your caption says, 'Rotary Compressor' but content asks for, 'Reciprocating Compressor'. Which one is of your interest?

For a Reciprocating Compressor, piston attached to conrod, (like in any automobile engine) type are Single-Acting. For such compressors, one can calculate the Piston Displacement (PD) from the Piston Diameter and Crankshaft Stroke multiplied by machine's intended running speed in revolutions per minute (RPM).

For smaller compressors, normally PD. is used, for its capacity of delivery in CFM (cubic feet per minute). here the machine can be a single-stage or multi-stage. as a practice for multistage machine only, the first stage piston diameter and crankshaft stroke is considered, when calculating the PD. value for the machine. the Actual CFM is somewhat less, because of the machine efficiency etc.

such practice is followed for those machines having construction as given above and falls in the range of up to 25 to 30 HP. these are of air cooled type having cooling fins casted on the periphery of the cylinders, having somewhat higher running speeds (approx up to -1000 rpm plus). this can be less up to a point, when it start to effect the lubrication of pistons when splashing method is employed.

For Crosshead-type Compressors normally, the PD. is calculated in the similar manner but do not publish its delivery in terms of PD.value. the manufacturers usually give actual CFM at a given pressure and temperature. these machines are larger compared to earlier ones as above.

Such machines can be Single-Stage or Multi-stage, in Vertical or of Horizontal construction, Water-Cooled cylinders and having somewhat slower speeds ( up to 500 to 750 rpm max.) depending upon machine's stroke size etc. These machines can be Single-Acting or Double-Acting type and are having 30 HP to 1000 HP plus range.

Please folks.... if you're going to copy and paste please give the source of the information...

Its not only polite to credit the author but it could be useful for others to know where it came from.

John.

For john's information, I would agree in full, for his good suggestion, about giving reference for the article material from where it was taken. firstly, to do justice to its author and secondly, for the reason of, one can get further details from article in question etc. (if desired).

In the case of my answer, to question raised originally, there is no reference, but it is basis my personal experience over years in the field of compressors.

I do endorse fully with your suggestion, as being a good professional practice, that everyone should follow this without fail. thanks.

Sorry if my post seemed like I was suggesting you had copied and pasted - it was meant as a general comment...

John.

Hi Ahsan

Commercial compressors are closely related to the horse power of the driving motor.

In many cases a rotary compressor will deliver between 4 to 5 SCFM (Free Air) at 100 psig discharge pressure. To be conservative we use 4 SCFM as a rule of thumb when the manufacturers specifications are not available. Not many will approach 5 SCFM per horse power.

The speed of rotation of the compressor is faster at a lower discharge pressure because the resistance or back pressure is reduced.

The speed of rotation of the compressor is faster with a larger electric motor (to a point) and the discharge flow will be greater.

For example in some cases the same basic compressor may be fitted with either a 5 or a 10 horse power electric motor. The larger motor, with more horse power will drive the compressor (pump) faster. You would achieve maximum flow if the compressor were driven at the free rotation speed of the motor.

Many rotary compressors are sized for 50% duty cycle with a reservoir that is 5 to 10 gallons times horse power of the compressor motor and a high/low pressure switch to start and stop the motor.

The discharge air from rotary compressors will become very hot at 100 psig or more and over 50% duty cycle. The reservoir aids in heat exchange to ambient as well as reducing pressure fluctuation.