Abstract: This invention relates to an improved isentropic process for liquefying a gas, preferably a cryogenic fluid, which comprises forming a critical fluid, expanding the critical fluid through a Pelton Wheel to a pressure below the critical pressure forming a liquefied gas and removing work. The improvement for enhancing the efficiency of the Pelton Wheel in removing work comprises:withdrawing the liquefied gas from the Pelton Wheel at its saturation temperature or at a temperature below its saturation temperature, andestablishing and maintaining a vapor environment about the Pelton Wheel by the method selected from the group consisting of injecting a second gas into the Pelton Wheel, said second gas being introduced into the Pelton Wheel at the saturation temperature or at a temperature slightly above the saturation temperature of the liquefied gas; and raising the enthalpy of the liquefied gas after it has been withdrawn from the Pelton Wheel to an enthalpy above the enthalpy of saturated liquid.