Chapter: Mechanical : Heat and Mass Transfer : Phase Change Heat Transfer and Heat Exchangers

The Overall Heat Transfer Coefficient

The determination of the overall heat transfer coefficient is an essential, and often the most uncertain, part of any heat exchanger analysis.

The Overall Heat Transfer Coefficient

The determination of the overall heat transfer coefficient is an essential, and often the most uncertain, part of any heat exchanger analysis. We have seen that if the two fluids are separated by a plane composite wall the overall heat transfer coefficient is given by:

If the two fluids are separated by a cylindrical tube (inner radius r_i, outer radius r₀), the overall heat transfer coefficient is obtained as:

where h_i, and h_o are the convective heat transfer coefficients at the inside and outside surfaces and V, is the overall heat transfer coefficient based on the inside surface area. Similarly, for the outer surface area, we have:

and U _i A_i will be equal to U _o A_o ; or, U_i r_i =U _o r_o .

The effect of scale formation on the inside and outside surfaces of the tubes of a heat exchanger would be to introduce two additional thermal resistances to the heat flow path. If h_si and h_so are the two heat transfer coefficients due to scale formation on the inside and outside surface of the inner pipe, the rate of heat transfer is given by

where T_i, and T_o are the temperature of the fluid at the inside and outside of the tube. Thus, the overall heat transfer coeffiCIent based on the inside and outside surface area of the tube would be:

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Mechanical : Heat and Mass Transfer : Phase Change Heat Transfer and Heat Exchangers : The Overall Heat Transfer Coefficient |