Air Compressor Capacity

The normal capaity of an air compressor is 1 HP=4CFM at 100 PSI.

You could see how long it takes to fill a receiver tank of known volume to a predetermined pressure fo a very close figure.

BudT,

Is it applicable to different types of compressors (say- screw,reciprocating,etc)

Thnx

Simple method of Capacity Assessment in Shop floor

Isolate the compressor along with its individual receiver being taken for test from main compressed air system by tightly closing the isolation valve or blanking it, thus closing the receiver outlet.

Open water drain valve and drain out water fully and empty the receiver and the pipe line. Make sure that water trap line is tightly closed once again to start the test.

Start the compressor and activate the stopwatch.

Note the time taken to attain the normal operational pressure P₂ (in the receiver) from initial pressure P₁.

Calculate the capacity as per the formulae given below :

Actual Free air discharge

Q = [{P₂ - P₁} x V] / {P₀ x T } Normal M³ / Min.

Where

P₂ = Final pressure after filling (kg/cm² a)

P₁ = Initial pressure (kg/cm²a) after bleeding

P₀ = Atmospheric Pressure (kg/cm² a)

V = Storage volume in m³ which includes receiver, after cooler, and delivery piping

T = Time take to build up pressure to P₂ in minutes

The above equation is relevant where the compressed air temperature is same as the ambient air temperature

A lot depends on what you want the info for.

Apart from rigging up a flow meter, if you know the motor size, then as already stated - 4 cfm per hp at 100 psig is about right - but if you are a contender for a Nobel Prize in the subject - then you need a formula that is a bit more sophisticated.

Because of volumetric efficiency, a compressor delivers more cfm at low pressure (on start-up with an empty tank) than it does at working pressure with a full tank (when you need the air).

So the formula given by Raviamdekar would give a more likely figure if you timed the compressor between two levels of working pressure. Usually this is the time when the compressor starts pumping when it drops to minimum working pressure and the time it takes to stop pumping when reaching maximum working pressure.

If more accuracy is required you must also correct for ambient intake pressure, the pressure drop across the inlet filter, the ambient temperature, the inlet temperature, the discharge temperature, and the tank temperature. Also add the volume of all the pipes and valves to the volume of the tank, plus the increase of volume due to temperature and pressure (how much air does a balloon hold?) - and so on...

However, after all this, it is still a long long way to a Nobel Prize.

Indeed. What horace-40 says is absolutely right.

What I have suggested is the simplest way that you can find the capacity with some reasonable quantum of accuracy. However, as has been rightly pointed out, other parameters do play a substantial role. e.g: Prefilter, cooler, piping volume, temperature etc.

If you really need to do a `NOBEL PRIZE' equivalent job, then obviously you will need to have more sophisticated and accurate and perfectly calibrated equipment and again, consequently a much more complicated formula.

I believe that the original query was by way of a `THUMB SOLUTION' but at the same time a bit more accurate than `1 HP = 4 CFM'.

I would request the original poster of this query to confirm if I am right.

Thanks

Dear Sirs,

Thanks. I got my answer, I do not want Nobel Prize but formula also to be know in case of some accuracy.

Thanks again to all who participated and given their valuable suggestions.

If my / our contribution has resulted in your problem being sorted out, I think it has served its purpose.

You are welcome again!