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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 ri, outer radius r0), the overall heat transfer coefficient is obtained as:


where hi, and ho 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 Ai  will be equal to U o Ao ; or,  Ui ri =U o ro .

 

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 hsi and hso 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 Ti, and To 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 |


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