Simple analyses of EGT type plants

A discu.ssiim of dry recupemlive plant. with ideal heat e.xehanger.s 

Before eonsidering the effects of water injection in an EGT type plant, it is worthwhile to refer to the earlier studies on the performanee of some dry recuperative cycles. Fig. 6.6 shows the T..s diagram of a [CBT i X r cyele, with a heat exchanger effectiveness of unity. It is implied that the surface area for heat transfer is very large, so that the outlet temperature on the cold side is the same as the inlet temperature on the hot side. However, due to the higher specific heat of the hot gas, its outlet temperature is higher than the inlet temperature of the cold air. 

In their original air standard cycle analysis, using constant specific heats, Hawthorne and Davis 4 considered the dry [CBTIiXr cycle. They assumed a perfect heat exchanger, with the specific heats of gas and air constant and identical, so that Ty becomes equal to Tj in Fig. 6.6. From their examination of the enthalpy-entropy diagram of this 

Note also that the heat rejected is equal to the compressor work in this case. For this air standard cycle with constant specific heats, Eq. (6.17) reduces simply to 

Consider next a similar recuperative cycle, but one in which the compression process approximates to isothermal rather than isentropic, with the work input equal to the heat rejected (this may be achieved in a series of small compressions of polytropic efficiency Tjp, followed by a series of constant pressure heat rejections). It may then be shown that the thermal efficiency of this cycle is given by 

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