A gas compressor comprising a first piston moving reciprocally within a compression chamber being driven by a second piston moving reciprocally within a cylinder and being drivable by a crankshaft connected to a motor; a plenum chamber sandwiched between and connecting the compression chamber and the cylinder, the plenum chamber having an inlet and an outlet for water circulation, and inlet and outlet valves controlling water flow through the plenum chamber from a water tank, the water tank being configured to provide cooling water into the plenum chamber while the gas is being compressed and receive water from the plenum chamber while the gas is drawn into the compression chamber; a compressed gas collection chamber connected to the compression chamber for storage and discharge of compressed gas; a gas inlet on the wall of the compression chamber between the first piston and the collection chamber; and a non-return valve between the compression chamber and the compressed gas collection chamber.