Useful Formulae and Statistics

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Here is a list of rules of thumb and formulae to help you size an air compressor system.

•    The typical body shop/garage uses 2.5 HP per man

•    4 CFM per Horsepower on a 100 PSI air compressor

•    Each PSI pressure drop equals 0.5% in horsepower.

•    7.48 gallons per cubic feet.

•    The heat load of an air cooled compressor is HP X 2545 BTU/HR

•    60% of condensate is removed after the aftercooler

•    96% is removed after the refrigerated air dryer

•    Pressure and flow are inversely proportionate. Ex. Starting with 20CFM at 125 PSI: if flow increases by 20% to 24 CFM, pressure will decrease by 20% to 100 PSI

•    Every 20˚ increase in discharge temperature increases moisture content by 50%

Useful Formulae

•    Compressor RPM =         motor pulley dia (in.) X motor RPM
                                                        Compressor pulley diameter

•    Motor Pulley dia (in.) =     compressor pulley dia (in.) X compressor RPM
                                                                            Motor RPM

•    Compressor Pulley dia (in.) = motor pulley dai (in.) X compressor RPM
                                                                        Compressor RPM

•    Motor RPM =             compressor pulley dia (in.) X compressor RPM
                                                                Motor pulley RPM