An autoclave apparatus comprising an autoclave forming a product-receiving chamber having an inlet port in one end and an outlet port in the opposite end and being, further, formed with an access opening. An electrically heated heat exchanger is formed with a chamber which contains heat storage means and such chamber has an inlet in one end and an outlet in the opposite end. Conduits connect the inlet to the autoclave with the outlet from the heat exchanger and the outlet of the autoclave with the inlet to the heat exchanger. Means is provided for supplying gas to the system whereby such system may be pressurized with the gas and such gas circulated through the autoclave and in heatexchange relationship with the heat storage means to heat such gas to an elevated temperature thereby providing an atmosphere in the product-receiving chamber of the desired temperature and pressure.