How does temperature affect compressor performance?
Theoretically the increase in volume is directly proportionate to the ratio of the absolute temperatures. So with the Rankin scale, 68F = 527.4R, therefore a 5F reduction in temp would increase the volume 1% (0.95% to be exact). The rule of thumb of 1% increase in volume for a 5F decrease is close because we are dividing by 500. Now the practical part – the oil coolant circuit with a contact cooled rotary is designed and controlled to maintain the coolant temperature above 180F (depends on the machine, manufacturer preference, pressure rating, and tolerance of the oil mixing thermo valve). Consequently, the air-end casting is normally operating at a temperature of 180 or higher. When the air enters the air-end, the temperature of the air is quickly elevated because the heat capacity of air is so low. As a result, the air entering the intake plenum of the element before the rotors turn to an intake closed position is not the same as the ambient air. That is why on a steady state application you do not see any significant change is compressor performance between day and night when the air temperature drops. Summer to winter will have a difference but it is also influenced by the cooler operating temperature of the compressor.
In summary, the change in volume associated with temperature assumes no external influences. To be accurate you would need to measure the change in air temperature at the intake plenum. It makes a difference but also depends on the compressor design, installation and temperature ranges.
There are so many standards out there today on the definition of SCFM. SCFM is defined today by CAGI (Compressed Air and Gas Institute) as 14.5 psia, 68° F with 0% RH. Some additional standards on the market today are … Continue reading