Thermodynamics

520 | ThermodynamicsT 6 1300 K S h 6 1395.97 kJ>kgP r 6 330.9P r7 P 7P 6P r 6 1 28 1330.92 117.0 S T 7 1006.4 KThenw comp,in 2 1w comp,in,I 2 2 1h 2 h 1 2 2 1404.31 300.192 208.24 kJ>kgw turb,out 2 1w turb,out,I 2 2 1h 6 h 7 2 2 11395.97 1053.332 685.28 kJ>kgThus,andA comparison of these results with those obtained in Example 9–5 (singlestagecompression and expansion) reveals that multistage compression withintercooling and multistage expansion with reheating improve the back workratio (it drops from 40.3 to 30.4 percent) but hurt the thermal efficiency (itdrops from 42.6 to 35.8 percent). Therefore, intercooling and reheating arenot recommended in gas-turbine power plants unless they are accompaniedby regeneration.(b) The addition of an ideal regenerator (no pressure drops, 100 percent effectiveness)does not affect the compressor work and the turbine work. Therefore,the net work output and the back work ratio of an ideal gas-turbinecycle are identical whether there is a regenerator or not. A regenerator, however,reduces the heat input requirements by preheating the air leaving thecompressor, using the hot exhaust gases. In an ideal regenerator, the compressedair is heated to the turbine exit temperature T 9 before it enters thecombustion chamber. Thus, under the air-standard assumptions, h 5 h 7 h 9 .The heat input and the thermal efficiency in this case areandw net w turb,out w comp,in 685.28 208.24 477.04 kJ>kgq in q primary q reheat 1h 6 h 4 2 1h 8 h 7 2 11395.97 404.312 11395.97 1053.332 1334.30 kJ>kgr bw w comp,in 208.24 kJ>kg 0.304 or 30.4%w turb,out 685.28 kJ>kgh th w netq inq in q primary q reheat 1h 6 h 5 2 1h 8 h 7 2 11395.97 1053.332 11395.97 1053.332 685.28 kJ>kgDiscussion Note that the thermal efficiency almost doubles as a result ofregeneration compared to the no-regeneration case. The overall effect of twostagecompression and expansion with intercooling, reheating, and regenerahth w netq inh 7 1053.33 kJ>kg477.04 kJ>kg 0.358 or 35.8%1334.30 kJ>kg477.04 kJ>kg 0.696 or 69.6%685.28 kJ>kg