The single jacket design consists of an outer jacket which surrounds the vessel. Heat transfer fluid flows around the jacket and is injected at high velocity via nozzles. The temperature in the jacket is regulated to control heating or cooling. The single jacket is probably the oldest design of external cooling jacket.The half coil jacket is made by welding a half pipe around the outside of the vessel to create a semi circular flow channel. The heat transfer fluid passes through the channel. A large reactor may use several coils to deliver the heat transfer fluid. It mostly used for heating or cooling. It is mostly preferred when thermic fluid heating is done instead of steam heating.