1. 1 AIR COMPRESSORS Auto Collision Repair/Refinishing Year One

2. 2 HOW COMPRESSORS ARE RATED The following terms are used to measure the performance of a compressor.

3. 3 RATINGS Horsepower Cubic Feet Per Minute Displacement cfm Free Air cfm Pressure

4. 4 Horsepower = hp hp is a measure of work hp is the capacity of the motor that drives the compressor usual hp = 3 to 25 hp the greater the hp, the more powerful the compressor as hp increases, other ratings increase

5. 5 Cubic Feet Per Minute = cfm cfm is the volume of the air being delivered by the compressor to the air tool cfm measures the compressor’s capabilities compressors with higher cfm ratings provide more air through the hose to the tool compressors have two cfm ratings

6. 6 Displacement cfm amount of air the compressor can pump in one minute Displacement cfm = area of piston x stroke x rpm x # of pistons 1728 displacement of a two-stage compressor is always given for that of the first stage cylinder(s) only

7. 7 Free Air cfm actual amount of free air that the compressor can pump in one minute at working pressure free air delivery is the true rating of a compressor It should be the primary cfm rating considered when selecting an air compressor.

8. 8 Volumetric Efficiency ratio of free air delivery to displacement rating Displacement x Volumetric Efficiency = Free Air Delivery Example: A compressor unit for 100 pounds service has a displacement of 8 cfm and its volumetric efficiency is 75%, then its free air delivery is ______. Solution: 8 cfm x 75% = 6 cfm

9. 9 Pressure = psi psi = pounds per square inch the measure of air pressure or force delivered by the compressor to the air tool always purchase a compressor that will handle maximum air consumption