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The heat transfer coefficient has SI units in watts per square meter per kelvin (W/(m 2 K)). The overall heat transfer rate for combined modes is usually expressed in terms of an overall conductance or heat transfer coefficient, U. In that case, the heat transfer rate is: ˙ = where (in SI units):
The rate of heat flow is the amount of heat that is transferred per unit of time in some material, usually measured in watts (joules per second). Heat is the flow of thermal energy driven by thermal non-equilibrium, so the term 'heat flow' is a redundancy (i.e. a pleonasm). Heat must not be confused with stored thermal energy, and moving a hot ...
The number of transfer units (NTU) method is used to calculate the rate of heat transfer in heat exchangers (especially parallel flow, counter current, and cross-flow exchangers) when there is insufficient information to calculate the log mean temperature difference (LMTD). Alternatively, this method is useful for determining the expected heat ...
The statement of Newton's law used in the heat transfer literature puts into mathematics the idea that the rate of heat loss of a body is proportional to the difference in temperatures between the body and its surroundings. For a temperature-independent heat transfer coefficient, the statement is:
The heat transfer rate can be written using Newton's law of cooling as = (), where h is the heat transfer coefficient and A is the heat transfer surface area. Because heat transfer at the surface is by conduction, the same quantity can be expressed in terms of the thermal conductivity k:
Q is the exchanged heat duty , U is the heat transfer coefficient (watts per kelvin per square meter), A is the exchange area. Note that estimating the heat transfer coefficient may be quite complicated. This holds both for cocurrent flow, where the streams enter from the same end, and for countercurrent flow, where they enter from different ends.
A hot fluid's heat capacity rate can be much greater than, equal to, or much less than the heat capacity rate of the same fluid when cold. In practice, it is most important in specifying heat-exchanger systems, wherein one fluid usually of dissimilar nature is used to cool another fluid such as the hot gases or steam cooled in a power plant by a heat sink from a water source—a case of ...
This equation shows that the temperature decreases exponentially over time, with the rate governed by the properties of the material and the heat transfer coefficient. [7] The heat transfer coefficient, h, is measured in , and represents the transfer of heat at an interface between two materials. This value is different at every interface and ...