Calculating Free Air Discharge of Air Compressor at shop floor…

Calculating free air discharge at shop floor is very easy, we need to just isolate Air compressor from normal operation and require one air receiver for getting free air discharge (FAD) of air compressor.

Step by step process is explained below:-

Instrument required: Pressure gauge (1 Nos), Timer (1 Nos), Volume of vessel and line holdup.

Step -1: Prepare compressor for measurement, Isolate Air receiver and hookup with the compressor you want to measure air discharge.

Step -2: Measure holdup volume(V) of air compressor and pipeline .

Step -3:  Drain moisture if available in recover and pipeline and then close all drains.

Step -4:  Note initial pressure (P1) at air compressor. it is recommended to keep initial vessel pressure above atmospheric pressure for at least 5min, this will ensure leakage in the system of available.

Step -5:  Start compressor and timer while raising the compressor pressure to its design pressure (P2). Note time taken (T ) in minutes for air compressor to reach pressure P2 from initial pressure P1

Step -6:  After Getting P1, P2, V, T Use below Formulae

Q = (P1-P2)/P0 X (V / T)


 Q = Flow in Nm3/Min

P1 = Initial Pressure in Kg/cm2 (a)

P2 = Final Pressure in Kg/cm2 (a)

P0 = Atmospheric Pressure in Kg/cm2 (a)

V = Volume of receiver and pipeline

T = Time taken to build-up pressure P2 from P1.

The above equation is relevant where the compressed air temperature is same as the ambient air temperature, i.e., isothermal compression. In case the actual compressed air temperature at discharge, say t2 deg C is higher than ambient air temperature say t1 Deg C (as is usual case), the

FAD is to be corrected by a factor (273 + t1) / (273 + t2)

Example :

Air Compressor FAD trail was taken of a compressor for 550 Nm3/Hr Capacity compressor and

Receiver Volume 6 m3, Line holdup = .25 m3

Initial pressure P1 = 1.5 Kg/cm2(a), Final pressure P2= 8 Kg/cm2(a),

Amb Pressure P0=1.026Kg/cm2(a)

Time Taken For pressure rise T=5.2 min

Air Inlet Temp = 25 Deg C, Air Outlet Temp 40 Deg C

Lets us find FAD of compressor

Free Air Discharge = (8-1.5)/1.026 X (6.35/5.2) = 8.2 Nm3/min

= 492Nm3/hr

FAD after Temp Correction = 492X (273 +25)/(273+40)

= 468.4 Nm3/hr

Capacity Reduction of compressor = (550-468.4)/650 = 14.8%  ( Need to be investigated)

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