Useful Formulas

KW to HP

KILOWATTS per HP = .746 watts  
          25HP compressor is 18KW
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Conversion of scfm to acfm


ACFM = SCFM x x Ps – (RHs x PVs) Where: Pb – (RHa x PVa) Ta Ts x Pb Pa

Where:

Ps = Standard pressure (PSIA)
Pb = Atmospheric pressure – barometer (PSIA)
Pa = Actual pressure (PSIA)
RHs = Standard relative humidity
RHa = Actual relative humidity
PVs = Saturated vapor pressure of water at standard temperature (PSI)*
PVa = Saturated vapor pressure of water at actual temperature (PSI)*
Ts = Standard temperature (°R) NOTE: °R = °F+460
Ta = Actual temperature (°R)

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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

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Pressure Drop and Horsepower

every 1 PSI of pressure drop equals 0.5% in horsepower
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Compressor 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

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