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This equation uses the overall heat transfer coefficient of an unfouled heat exchanger and the fouling resistance to calculate the overall heat transfer coefficient of a fouled heat exchanger. The equation takes into account that the perimeter of the heat exchanger is different on the hot and cold sides.
Here, is the overall heat transfer coefficient, is the total heat transfer area, and is the minimum heat capacity rate. To better understand where this definition of NTU comes from, consider the following heat transfer energy balance, which is an extension of the energy balance above:
The equations for the use of the data retrieved from these tables are very simple. Q= heat gain, usually heat gain per unit time A= surface area. U= Overall heat transfer coefficient. CLTD= cooling load temperature difference SCL= solar cooling load factor CLF= cooling load factor SC= shading coefficient
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.
The total rate of heat transfer between the hot and cold fluids passing through a plate heat exchanger may be expressed as: Q = UA∆Tm where U is the Overall heat transfer coefficient, A is the total plate area, and ∆Tm is the Log mean temperature difference. U is dependent upon the heat transfer coefficients in the hot and cold streams. [2]
Tubular heat exchanger Partial view into inlet plenum of shell and tube heat exchanger of a refrigerant based chiller for providing air-conditioning to a building. A heat exchanger is a system used to transfer heat between a source and a working fluid. Heat exchangers are used in both cooling and heating processes. [1]
Q is the heat transfer Rate U is the overall heat transfer coefficient A is the overall heat transfer area T lm is the temperature difference or log mean temperature difference. For a general shell and tube heat exchanger, U is given by the equation [3]
In convective heat transfer, the Churchill–Bernstein equation is used to estimate the surface averaged Nusselt number for a cylinder in cross flow at various velocities. [1] The need for the equation arises from the inability to solve the Navier–Stokes equations in the turbulent flow regime, even for a Newtonian fluid .