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The three basic modes of heat transfer: conduction, convection, and radiation. The process by which heat is transferred from the parts of an object that are at higher temperatures to those at lower temperatures, or to another object at a lower temperature that is in contact with it, is called heat conduction. In a pure heat conduction process, there is no relative movement between the different parts of the object. Convection refers to the process of heat transfer that occurs as a result of relative movement of particles within a fluid. Therefore, convection can only take place within fluids. In chemical manufacturing, it is common for heat to be transferred from the fluid to a solid surface, or from the solid surface into the surrounding fluid, when the fluid flows over that surface. This process is known as convective heat transfer. If mechanical energy is used (such as stirring the fluid or pumping it through pipes) to induce convection and thus facilitate heat transfer, it is called forced convective heat transfer. If the fluid was originally at rest but experiences local changes in density due to heating, which in turn leads to convection and heat transfer, this is known as natural convective heat transfer. In all cases of convective heat transfer, the amount of heat transferred per unit time is calculated using Newton’s cooling law: when the fluid is heated, Q = XA(tw – t); when the fluid is cooled, Q = XA(T – tw). In these formulas, tw represents the wall temperature, while T and t represent the average temperatures of the hot and cold fluids respectively. A is the heat transfer area, and the coefficient X is known as the convective heat transfer coefficient. Radiation is a phenomenon in which energy is transmitted through electromagnetic waves. Objects emit radiant energy for various reasons, and the process of emitting radiant energy due to heat is called thermal radiation. When an object releases heat, thermal energy is converted into radiant energy, which is emitted in the form of electromagnetic waves and travels through space. When it encounters another object, this radiation is partially or entirely absorbed, thereby being converted back into thermal energy. Thus, radiation involves not only the transfer of energy but also a change in its form; this is what distinguishes thermal radiation from heat conduction and convection. As a result, radiant energy can travel through a vacuum without the need for any medium. Although objects can transfer heat through radiation, it is only at high temperatures that radiation becomes the primary method of heat transfer.