Abstract: A liquid cryogen vaporizer is devised in which the cryogenic liquid is first partially vaporized in a cryogenic heat exchanger which is provided with heat from nonfired sources. The partially vaporized liquid cryogen is then completely vaporized in a second downstream cryogenic heat exchanger also provided with heat from the nonfired sources. The nonfired sources comprise an internal combustion engine and an ambient air heat exchanger. The internal combustion engine drives a hydraulic circuit which provides a constant load on the engine. A cryogenic pump used to flow the cryogenic liquid through the cryogenic heat exchanger is in turn hydraulically driven from this circuit. Heat is also transferred from the hydraulic circuit into a heat exchanging circuit. The heat exchanging fluid is driven around the heat exchanging circuit by means of a pump driven by the engine through the ambient air heat exchanger, a hydraulic heat exchanger and the first cryogenic heat exchanger.