Most gas lift, flash gas, and vapor recovery compressors require a recycle valve because of the unsteady and sometimes unpredictable nature of the flow rate. Indeed there may be periods of time when there is
no flow at all to the compressor.
At a constant speed, a constant volume of gas (at suction conditions of pressure and temperature) will be drawn into the cylinder. As the flow rate to the compressor decreases, the suction pressure decreases until the gas available expands to satisfy the actual volume required by the cylinder. When the suction pressure decreases, the ratio per stage increases and therefore the discharge temperature increases. In order to keep from having too high a discharge temperature, the recycle valve opens to help fill the compressor cylinder volume and maintain a minimum suction pressure.