The efficiency of an air compressor is most commonly measured in scfm delivered at full-load input power. The scfm measures the volume of air under standard conditions of temperature, pressure and humidity. The density of air reduces with an increase in the air temperature. Thus as the air temperature reduces the mass of air contained in a unit volume i.e. density increases.
Reducing inlet temperature or increasing the air density is equivalent to increasing the mass flow though the volume remains constant. Thus though the input power nearly remains the same, the mass flow from the compressor will increase.
This mass flow increase effect is less pronounced for lubricant-injected, rotary-screw compressors because here the incoming air mixes with the higher temperature lubricant. Conversely, as the temperature of intake air increases, the air density decreases and mass flow and pressure capability of the compressor decreases. The resulting reduction in capacity is often met by operating additional compressors, thereby increasing energy consumption.
Typically the air inside the utility room is warm because of the machine operations. Thus the inlet air to the compressor should be taken from a point cooler point outside the building.
By a rule of thumb, every 5 DegC drop in air temperature at the inlet of an air compressor reduces the energy consumption by 1%. Thus the air inlet for compressors should be taken from a cooler point.
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