What Is a Jacket Reactor? Heat Transfer & Applications Answering the core question: What is a jacket reactor, and how does its heat-transfer jacket govern reaction temperature? A jacket reactor is defined by the annular or coil subsystem surrounding the vessel shell that circulates heating or cooling fluid to set the wall temperature within plus or minus 0.5-2 degree C. Jacket design determines the overall coefficient U (50 W/m2K plain up to 500 W/m2K half-pipe coil) and the