The Law of Energy Conservation -

Heat is a form of energy known as thermal energy. Thermal energy can be either stored in a substance or transferred from one substance to another. An important law of energy is called the Law of Energy Conservation. This law states that energy can be neither created nor destroyed, only altered in form. Basically, this law means that all energy has to be accounted for. There are a couple of ways the Law of Energy Conservation works. As an example, fuels contain chemical energy. When fuel is burned during combustion, heat, or thermal energy, is generated. So the process of burning fuel alters the form of energy from chemical to thermal. As another example, heat from combustion is used to heat a process fluid flowing inside furnace tubes. Much of the heat, or thermal energy, is transferred to the process fluid. However, since no furnace operates at 100% efficiency, not all of the heat is transferred to the process fluid. Some of the heat may go up the stack, and some of it may be absorbed by furnace equipment, but all of it goes somewhere. In other words, all of the thermal energy is accounted for; none of it is destroyed.