What Is a Jacketed Reactor? Principles, Design & Applications Answering the core question: What is a jacketed reactor, and how does its surrounding jacket deliver precise heating and cooling? A jacketed reactor is a vessel wrapped by an outer shell - the jacket - through which heat-transfer fluid circulates to control the reaction temperature within plus or minus 0.5-2 degree C. Jackets raise the effective heat-transfer area by 2-4 times versus internal coils alone,