Abstract: An apparatus for compressing gaseous fluid including a reciprocating compressor and a receiver or tank connected with the compressor for receiving the compressed gaseous fluid. The compressor includes one or more cylinders supported on a crankcase having a rotating crankshaft therein with throws connected by connecting rods with pistons reciprocating in the cylinders. Discharge valves are reciprocably mounted at ends of the cylinders and are of a size substantially that of the cross-section of the cylinder bore and opening in response to pressure in the respective cylinders.