Exthalpy—Flow Exergy in Open Systems

As we learned in Chapter 2, for open systems, we must account for the mass and the flow work of streams flowing into and out of the system. The flow work required to push a stream 1 into the system is PiVi. However, the environment provides a part of this work from the ambient pressure, PqVi. Hence, it does not cost us anything. Therefore, we add the difference in these two terms, (Pi — Po)vi, to the exergy to account for flow work. 

This analysis leads to the definition of exthalpy of state 1 (also called flow exergy), defined as follows  

Calculation of Internal Exergy Loss in a Heat Exchanger 

A shell and tube heat exchanger is designed to warm air from the environment by condensing steam that is passed through the tubes on the other side. The maximum air flow is 30 k min, and the air is inlet at the temperature and pressure of the environment, 285 K and 1 bar, respectively. The steam flows through the system at a pressure of 10 bar and a flow rate of 3 k min. The quality of the steam at the inlet is 0.9, and at the exit is 0.2. 

First we need to find the oudet temperature of the air. To do this we perform an energy balance  

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