833 Heat Exchanger Effectiveness

The effectiveness of a heat exchanger is defined as a ratio of the actual heat transferred to the maximum possible heat transfer considering the temperatures of two streams entering the heat exchanger. For a given flow arrangement, the effectiveness of a heat exchanger is directly proportional to the surface area that separates the heated and cooled fluids. The effectiveness of typical heat exchangers is given in Figure 8.16 in terms of the parameter AU/Cmin where A is the effective heat-transfer area, U the effective overall heat conductance, and Cmin the mass flow rate times the specific heat of the fluid with minimum mc . The conductance is the heat rate per unit area per unit temperature difference. Note that as AU/Cmin increases, a linear relation exists with the effectiveness until the value of AU/Cmin approaches 1.0. At this point the curve begins to knee over and the increase in effectiveness with AU is drastically reduced. Thus one sees a relatively early onset of the law of diminishing returns for heat-exchanger design. It is implied that one pays heavily for exchangers with high effectiveness.

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