10.3-Capacity Assessment of Air Compressor

Rating of Air Compressors and Air Equipment

Common terms rating air flow capacity are-

1.      CFM

• CFM (Cubic Feet per Minute) is the imperial method of describing the volume flow rate of compressed air. It must be defined further to take account of pressure, temperature and relative humidity - see below.

2.      ICFM

• ICFM (Inlet CFM) rating is used to measure air flow in CFM (ft³/min) as it enters the air compressor intake.

3.      ACFM

• ACFM (Actual CFM) rating is used to measure air flow in CFM at some reference point at local conditions. This is the actual volume flow rate in the pipe work after the compressor.

4.      FAD

• FAD (Free Air Delivery) is the actual quantity of compressed air converted back to the inlet conditions of the compressor. The units for FAD are CFM in the imperial system and l/min in the SI system. The units are in general measured according the ambient inlet standard conditions ISO 1217: Ambient temperature = 20^oC, Ambient pressure = 1 bar abs, Relative humidity = 0%, Cooling water/air = 20^oC and Effective working pressure at discharge valve = 7 bar abs.
• 1 m³/min (FAD) = 1000 liter/min (FAD)

= 1000 dm³/min (FAD)

= 16.7 l/s (FAD)

= 16.7 dm³/s (FAD)

= 35.26 ft³/min (FAD)

5.      ANR

• ANR (Atmosphere Normale de Reference) is quantity of air at conditions 1.01325 bar absolute, 20^oC and 65% RH.

6.      SCFM

• SCFM (Standard CFM) is the flow in CFM measured at some reference point but converted back to standard air conditions (Standard Reference Atmosphere) 14.696 psia, 60^oF.

7.      nl/min

• nl/min is the flow in l/min measured at some reference point but converted to standard air conditions 1.01325 bar absolute, 0^oC and 0% RH .

8.      ISO 1217

• standard reference ambient conditions - temperature 20^oC, pressure 1 bar abs, relative humidity 0%, cooling air/water 20^oC, and working pressure at outlet 7 bar absolute.    

     Example - Rating a Compressor

A typical rating of a compressor may look like this

FAD (CFM)	Description
11.1 cfm @ 7.5 Bar	The Free Air Delivery from the compressor is 11.1 CFM at 7.5 Bar
8.2 cfm @ 10 Bar	The Free Air Delivery from the compressor is 8.2 CFM at 10 Bar

 Example:-

Capacity Assessment of Compressor
Procedure
• Isolate the compressor along with its individual receiver being taken for test.
• Open drain valve and empty out the receiver and pipelines.
• Start the Compressor and activate the stopwatch.
• Note the time taken to attain the normal operational pressure P2 (in the receiver) from the initial pressuer P1.
	PARAMETERS	UOM	VALUE
QD	Design Free Air Delivery	Nm3/Min	14.75
P2	Final Pressure	Kg/cm2	7.03
P1	Initial Pressure	Kg/cm2	0.5
P0	Atmospheric Pressure	Kg/cm2	1.026
V	Storage Volume of receiver including after cooler and pipes	M3	8.322
T	Time taken to build up pressure P2	Min	4.021
t1	Atmospheric Temp.	° C	35
t2	Actual Compressed Air Temp.	° C	45
Calculate the capacity as per the given formula.
QA	Actual Free Air Delivery	Nm3/Min	{(P2 - P1 )/ P0} x (V/T)		13.17
f	Correction Factor		(273 +t1)/ (273 + t2)		0.97
QC	Corrected Free Air Delivery	Nm3/Min	QA x f		12.76
	Deviation in Capacity	%	(QD - QC) x 100/QD		13.50

 

Reference: - engineeringtoolbox.com/rating-air-compressors