An apparatus or system for cooling the interior of an autoclave furnace vessel after it has been pressurized with a gas and heated. The system comprises a heat conducting conduit opening at one end to the furnace interior and at the other end to a shut-off valve exterior to the vessel. Intermediate the two ends of the conduit is a coil portion comprising a heat exchanger positioned at a location in the furnace where hot vessel gases may be forced through or past the coil. Furnace gases are forced into the conduit and therefore through the coiled portion where they are heated prior to being exhausted. According to a preferred embodiment of this invention, there is a Joule-Thomson valve before the coiled portion of the conduit to reduce the temperature of the exhausting gases prior to arriving at the coiled portion.