Abstract: A waste gas recovery system employs a compressor which takes in raw waste gas from an inlet knock-out drum and passes compressed gas through a heat exchanger to an outlet knock-out drum. The temperature at the outlet of the compressor is sensed by a device which operates valves to inject liquid coolant into the compressor inlet and to re-circulate gas back from the outlet of the outlet knock-out drum to inhibit an excessive temperature rise. A pressure-sensing device senses the pressure of the gas passing into the compressor and controls both the speed of the compressor and an adjustable throttle valve to regulate the gas flow. The throttle valve is closed automatically should there be a fall in the pressure of the gas at the inlet below a safe level. In this event, further pressure-sensing devices act additionally to close the recirculating gas valve and a further valve in the main inlet flow path to reliably isolate the compressor.