When a fluid exchanges heat in a heat exchanger typically it has to experience three conductive resistances, thin stagnant films of viscous liquid adhering to either side of a metal plate, the mode of heat transfer through these films is conduction, and the metal's thermal conductivity, Heat is transferred from the hot fluid to the wall via convection, then through the wall via conduction, and finally to the cold fluid via convection. Typically, a heat exchanger consists of two flowing fluids separated by a solid wall. When there are external resistances like the metal wall between the hot and cold fluid in a heat exchanger with boundary layers, the equation gets modified to, Q= UAdT, U -factor is called the overall heat transfer coefficient. For a fluid flow, when there is no external resistance to heat flow, only Cp offers the internal resistance to heat flow. Cp takes away Q /mass- degc and supplies it for molecular vibrations, the equation is Q = mCpdT. Heat flow when there is no external resistance In liquids, a fluid encounters three conductive resistances: one by the metal wall and two by films adhering to either side of the metal wall. The magnitude of the individual coefficients is determined by the nature of the heat transfer process, fluid physical properties, fluid flow rates, and the physical layout of the heat transfer surface. The fouling factor is the additional thermal resistance caused by fluid impurities, rust formation, or other fluid-wall reactions. The deposition of a film or scale on the heat transfer surface reduces the rate of heat transfer significantly. The overall heat transfer coefficient may also take fouling into account in the heat transfer process. The reciprocal of the overall resistance to heat transfer, which is the sum of the individual resistances, is the overall heat transfer coefficient. All these make the transfer rate slower than the theoretical rate, and dependent on the resistance to the flow of heat within each of the substances involved in the transfer.The heat transfer coefficient takes conductive and convective resistance into account between two fluids separated by a solid wall. The heat being removed from the fluid creates the internal gradient, because there is a resistance to the flow of heat within the fluid - just as there is within the air doing the cooling, while the metal separating the fluids also has a gradient across its cross sectional area. In the case of a radiator, for example, there is a thermal gradient within the internal fluid, within the metal separating the fluid from the cooling air, and within the air in contact with the metal thus the two substances exchanging heat are not in direct contact. As a practical matter, no such substances exist all substances present resistance to the flow of heat. One of the keys to remember is that the absolute law relates to two substances, each directly in contact with each other, and with no resistance to the flow of heat within them. Finally, in the case of heat transfer by thermal radiation, Newton's law of cooling is not true. This condition is generally true in thermal conduction (where it is guaranteed by Fourier's law), but it is often only approximately true in conditions of convective heat transfer, where a number of physical processes make effective heat transfer coefficients somewhat dependent on temperature differences. Newton's law of cooling states that the rate of heat loss of a body is proportional to the difference in temperatures between the body and its surroundings. As such, it is equivalent to a statement that the heat transfer coefficient, which mediates between heat losses and temperature differences, is a constant. One complication to the heat transfer process is Newton's Law of Cooling, although it does not apply to all three methods of heat transfer outlined above: Heat transfer will occur in a direction that increases the entropy of the collection of systems. Heat transfer changes the internal energy of the systems from which and to which the energy is transferred. The direction of heat transfer is from a region of high temperature to another region of lower temperature, and is governed by the Second Law of Thermodynamics. Radiation, which is how the heat of the Sun gets to us through the vacuum of space. Heat is transferred by one or more of three processes.Ĭonduction, an example of which is having the objects in physical contact.Ĭonvection, in which the heat is transferred though a medium, such as air, so it's a slower, much less efficient process. Longer more detailed answer, based on Wikipedia Heat and Temperature The greater the temperature difference, the greater the rate at which heat transfers.
0 Comments
Leave a Reply. |
AuthorWrite something about yourself. No need to be fancy, just an overview. ArchivesCategories |