The simple EGT plant with water injection

The discussion of the last section is then useful in considering the evaporative cycles. We shall see that the effect of water injection downstream of the compressor (and possibly in the cold side of the heat exchanger) may lead towards the [CBTJiXr type of plant, with increased cold side effective specific heat and hence increased heat exchanger effectiveness. Water injection in the compressor may lead to a plant with isothermal compression. 

However, the turbine work has been increased because of the extra water vapour flow through the turbine, while the compressor work is unchanged. Thus Eq. (6.17), which is still valid, with turbine work equal to the heat supplied, shows that the thermal efficiency increases compared with the dry cycle. It is important to realise that this efficiency is increased not because of a reduction in the heat rejected (Qa) but because of the increa.se in Wi- The heat rejected is still equal to the compressor work. 

as suggested in Section 6.2.1, the turbine work is increa.sed by a factor (1 + 2S), where S is the water vapour flow, then the dry and wet efficiencies may be written as 

The same expression applies for some of the other variations of the EGT cycle considered below (e.g. the recuperative water injection (RWI) plant with intercooling). 

A further variation of the El-Masri EGT cycle is one in which the evaporation takes place both in an aftercooler and within the cold side of the heat exchanger (Fig. 6.8c). Eq. (6.17) is still valid, but the efficiency is increased because more water can be injected and the turbine work increased further. 

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