Abstract: A processing vessel in which a temperature control element defining a helical flow passage, disposed on an outer surface of an inner barrel within the processing vessel. The barrel contains a processing liquid and a heating or cooling temperature control medium is caused to flow through the flow passage. The inner barrel and temperature control element are disposed in a vessel body to define a chamber between the temperature control element and an inner surface of the vessel body. The chamber defined between the vessel body and the temperature control element is a closed chamber. Preferably, a system for generally equalizing the pressures in the inner barrel and the closed chamber is provided. The temperature control element can be assembled in the vessel body after being manufactured outside the vessel body. Therefore, the processing vessel can be manufactured with improved efficiency and can be serviced by ordinary in-vessel maintenance operations.