Turbine entry temperature

Thus the cooled reversible cycle [CHT]rci with a first rotor inlet temperature, Tj, will have an internal thermal efficiency exactly the same as that of the uncooled cycle [CHTJru with a higher turbine entry temperature 3 = Tr, and the same pressure ratio. There is no penalty on efficiency in cooling the turbine gases at entry but note that the specific work output, w = (wj — wc)/CpT = [(0 /x) — 11(j — 1), is reduced, since 0 < 0. 

For two step cooling, now with irreversible compression and expansion, Fig. 4.7 shows that the turbine entry temperature is reduced from Ti. to by mixing with the cooling air i/ H taken from the compressor exit, at state 2, pressure p2, temperature T2 (Fig. 4.7a). After expansion to temperature Tg, the turbine gas flow (1 + lp ) is mixed with compressor air at state 7 (mass flow i/h.) at temperature Tg. This gas is then expanded to temperature T g. 

Even for this simplest CCGT plant, iterations on such a calculation are required, with various values of p, in order to meet the requirements set on T, the steam turbine entry temperature, and 7s (the calculated value of 7s has to be such that the dewpoint temperature of the gas (7jp) is below the economiser water entry temperature (7b) and that may not be achievable). But with the ratio /i satisfactorily determined, the work output from the lower cycle Wl can be estimated and the combined plant efficiency obtained from... 

This approaeh was well illustrated by Brieseh et al. f 14], who showed separate plots of ( o)h. ( o)l and (t o)cp against pressure ratio for a given T ax and r ,in (Fig. 7.11), illustrating the validity of Eq. (7.35). But note that the limiting allowable steam turbine entry temperature also influences the ehoiee of pressure ratio in the gas turbine eyele. 

Rice argued that a high temperature at entry to the HRSG (resulting from reheat in the gas turbine plant) leads via the pineh point restrietion to a lower exhaust staek temperature and heat loss , in eomparison with an HRSG reeeiving gas at a lower temperature from... 

A reversible cycle with turbine expansion split into two steps (high pressure, HP, and low pressure, LP) is illustrated in the T, s diagram of Fig. 4.3. The mass flow through the heater is still unity and the temperature rises from T2 to Tt, = Tq hence the heat supplied (3b is unchanged, as is the overall isentropic temperature ratio (x). But cooling air of mass flow i//H is used at entry to the first HP turbine (of isentropic temperature ratio. xh) and additional cooling of mass flow is introduced subsequently into the LP turbine (of isentropic temperature ratio Xl)- The total cooling flow is then i/( = i/ h + >h.-... 

The results of calculations for the cooling air flow fractions in the first (nozzle guide vane) row of the turbine, ba.sed on the assumptions outlined in Section 5.2 for film cooled blading, are illustrated in Fig. 5.1. The entry gas temperature Tgi was taken as the combustion temperature Tcoi = Ty and the cooling air temperature as the compressor delivery temperature T2. The cooling air required is. shown here as a fraction of the exhaust gas flow, i.e. as ip/( 1 + ip), plotted against compressor pressure ratio and combustion temperature for an allowable blade metal temperature, Tpi = 800°C. Also shown are... 

Of course, there is no methane at exit from the PO reactor, and no oxygen. The hydrogen content is quite high, over 15% and comparable to that in Lloyd s example of the steam/TCR cycle, but the CO content is also nearly 8%. It is interesting to note that the calculated equilibrium concentrations of these combustible products from the reactor are reduced through the PO turbine (because of the fall in temperature) before they are supplied to the gas turbine combustor where they are fully combusted, but it is more likely that the concentrations would be frozen near the entry values. 

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