Reheating in the upper gas turbine cycle

Rice [15] made a comprehensive study of the reheated gas turbine eombined plant. He first analysed the higher (gas turbine) plant with reheat, obtaining (t o)h turbine exit temperature, and power turbine expansion ratio, all as funetions of plant overall pressure ratio and firing temperatures in the main and reheat burners. (The optimum power turbine expansion ratio is little different from the square root of the overall pressure ratio.) He then pre-seleeted the steam eyele eonditions rather than undertaking a full optimisation. 

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 

Rice found high CCGT efficiencies with gas turbine reheat at optimum pressure ratios even higher than those discussed above. 

An extension of the approximate analysis of Section 7.6 suggests that the pressure ratio for both the combined and higher level plants, for the example given there, should be about 48 which is higher than that used in the ABB plant (about 30). 

Most modem CCGT plants use open air cooling in the front part of the gas turbine. An exception is the GE MS9001H plant which utilises the existence of the lower steam plant to introduce steam cooling of the gas turbine. This reduces the difference between the combustion temperature T ot and the rotor inlet temperature The effect of this on the overall combined plant efficiency is discussed in Ref. [1] where it is suggested that any advantage is small. 

---