Energy dissipation in countercurrent and cocurrent heat exchangers

The total rate of entropy change for the hot and cold streams is 

If the pressure changes of the streams are negligible, and the heat capacities of both streams are constant and equal to Cp, then we have total entropy change 

Applying Eqs. (4.187) and (4.189) for cocurrent and countercurrent operations, we find Cocurrent heat exchanger I  

This ratio shows that the rate of energy dissipated in the cocurrent heat exchanger is almost twice the dissipation in the countercurrent heat exchanger. Although the heat exchanged between the hot and cold streams is the same, the countercurrent operation is thermodynamically more efficient. 

Steam power plants produce electricity with rather low thermal efficiency. An increase in efficiency leads to savings in fuel costs and minimizes environmental effects. The two basic approaches in increasing the thermal efficiency of a cycle are (i) design a process that transfers heat to the working fluid at high temperature in the boiler and (ii) design a process that transfers heat to the working fluid at low temperature in the condenser. These may decrease the temperature differences, and hence the level of irreversibility. 

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Example 4.11 Energy dissipation in countercurrent and cocurrent heat exchangers The two most commonly used heat exchangers are countercurrent and cocurrent at steady-state flow conditions as shown in Figure 4.17. Estimate the energy dissipated from these heat exchangers if the surroundings are at 290 K. Consider the data below ... 

Example 4.12 Energy dissipation in countercurrent and cocurrent heat exchangers... 

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