Heat transfer system for obtaining essentially constant temperature control over a long vertical heat transfer surface which includes a series of shallow depth, open-topped jacket segments on the inside diameter of an intermediate refrigeration casing between an inner casing and an outer casing. The jackets are filled with liquid refrigerant and are provided with overflow pipes so that refrigerant liquid can flow down through the entire series of jackets and keep each one full. Each of the jackets communicates with a common vapor space or annulus. As heat is absorbed the liquid boils and vaporizes and passes into and upwardly through the vapor annulus. By maintaining an essentially constant pressure in the vapor space an essentially constant boiling temperature is maintained in each segment and thus the heat transfer surface to which the jacket segments are fastened is also maintained at that constant temperature. The vapor is condensed at the well head and returned to the system as liquid.

Thawing of permafrost surrounding a surface casing of a wellbore is minimized during production of hot oil by injecting liquid refrigerant in an annulus between the surface casing and the production casing, allowing heat from the hot oil to vaporize the refrigerant as it descends in said annulus, and removing the resulting vaporized refrigerant at the well-head, thereby maintaining the surface casing at a temperature of 32.degree. F. or lower.

Thawing of the permafrost around a well transporting hot fluids through a permafrost zone is prevented by a cyclic refrigeration operation in which the well is refrigerated by natural convection refrigeration operated only when the ambient conditions are suitable for the transfer of heat from the well to the atmosphere. The refrigeration system is a closed system comprised of one or more insulated conduits for transporting liquid refrigerant downwardly through the refrigerated zone, an annulus in the well for conducting the liquid refrigerant to the surface, a heat exchanger located at the surface to cool the refrigerant by heat exchange with ambient air, a refrigerant reservoir to maintain a supply of refrigerant in the system, and a temperature responsive shutoff device to stop the circulation of refrigerant during periods of high ambient temperature. The difference in density of the liquid refrigerant in the insulated conduit and the annulus induces circulation of refrigerant through the system.