Abstract: A control system controls the temperature of a chemical process vessel by recirculating a heat transfer medium through a jacket surrounding the vessel. In order to control the temperature of the recirculating medium, the control system uses an internal cooling heat exchanger connected in series with an internal heating heat exchanger and can also connect the recirculating loop to a central refrigeration system. Operation of the heat exchangers and connections to the central refrigeration system are automatically controlled by a computer which selects the cooling method in order to minimize energy use while maintaining the selected temperature. The computer also monitors the inlet and internal temperatures of the vessel and the outlet temperature of the cooling heat exchanger to prevent thermal shock damage when the system changes between heating and cooling during operation.