The movement of thermal energy through a body or across a space as a result of the difference in temperature. The three modes of heat transfer are conduction, convection, and radiation. Heat is generated in chemical processes from a number of sources, including exothermic reactions, mechanical agitation, and adiabatic processes. Heat is removed by cooling water through cooling coils, external cooling jackets, and heat exchangers. Heat may need to be added to endothermic reactions, which require heat to proceed. For example, high-temperature heat is supplied directly to cracking processes by the combustion of fuel in the form of thermal radiation to heat tubes within which the endothermic reaction takes place, such as in the cracking of dichloroethane to form vinyl chloride monomer and hydrogen chloride.