The optimum pressure ratio for a CCGT plant

Rufli s calculations (Fig. 7.7a, b), indicated that the optimum pressure ratio for a CCGT plant is relatively low compared with that of a simple gas turbine (CBT) plant. In both cases, the optimum pressure ratio increa.ses with maximum temperature. Davidson and Keeley [6] have given a comparative plot of the efficiencies of the two plants (Fig. 7.9), showing that the optimum pressure ratio for a CCGT plant is about the same as that giving maximum specific work for a CBT plant. 

The rea.son for this choice of low pressure ratio is illustrated by an approximate analysis [12], which extends the graphical method of calculating gas turbine pierformance described in Chapter 3. If the gas turbine higher plant is assumed to ojjerate on an air standard cycle (i.e. the working fluid is a perfect gas with a constant ratio of specific heats, y), then the compressor work, the turbine work, the net work output and the heat supplied may be written as 

Timmermans [13] suggested that the steam turbine work output (per unit gas flow in the higher plant) is given approximately by 

It can be seen from Fig. 7.10 that the eurve for wcv lies above that for wh. As for the gas turbine eyele the pressure ratio for maximum effieiency in the eombined plant may be obtained by drawing a tangent to the work output curve from a point on the x-axis where x= 1 +7 (.(0— 1), i.e. X = 4.6 in the example. The optimum pressure ratio for the eombined plant (r = 18) is less than that for the gas turbine alone (r = 30) although it is still greater than the pres.sure ratio which gives maximum speeific work in the higher plant (r = 11). However, the efficieney tjcp varies little with r about the optimum point. 

It may also be noted that by differentiating Eq. (7.9) with respeet to r (or x), and putting the differential equal to zero for the maximum effieiency, it follows that 

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Horlock, J.H. (1995), The optimum pressure ratio for a CCGT plant, Proc. In.stn. Mech. Engrs. 209, 259-264. 

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