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## BASIC COMPRESSED AIR FORMULAS

Electrical Cost  =  HP x .746 x hours x Kw cost  /  motor efficiency
Example: 50 HP air compressor that runs 8 hours a day 5 days a week for a year with a \$.06 Kw electric rate and a 90% efficient electric motor.

50 HP x .746 x 2,080 hours x \$.06  /  .90  =  \$5,172.26 per year

Compressor RPM  =  motor pulley diameter x motor RPM  /  compressor pulley diameter
Motor Pulley Diameter  =  compressor pulley diameter x compressor RPM  /  motor RPM
Compressor Pulley Diameter  =  motor pulley diameter x motor RPM  /  compressor RPM
Motor RPM  =  compressor pulley diameter x compressor RPM  /  motor pulley diameter
Gallons  =  cubic feet  /  .134
Cubic Feet  =  gallons x .134
 Pump Up Time (Minutes) = V (tank size) x (final pressure – initial pressure) 7.48 x atmospheric pressure x pump delivery (CFM)
Example: 7.5 HP compressor rated at 24 CFM with an 80 gallon tank, unit starts at 100 PSI and turns off at 150 PSI
 80 gallons x (150 PSI – 100 PSI) = 4000 = 1.51 minutes 7.48 x 14.7 x 24 CFM 2638
Pressure Drop and Horsepower  =  every 1 PSI of pressure drop equals 0.5% in horsepower
Heat and Horsepower  =  rejected heat from an air-cooled compressor is equal to total machine horsepower x 2,545 BTU per hour

Example:  50 HP compressor with 3 HP fan motor will produce:

53 HP x 2,545 = 134,885 BTU per hour
Note:  This information is provided as a quick reference resource and is not intended to serve as a substitute for qualified engineering assistance.  While every effort has been made to ensure the accuracy of this information, errors can occur.  As such, neither Flodraulic, any of its affiliated companies nor its employees will assume any liability for damage, injury or misapplication as result of using this reference guide.