Gas turbine improvement methods and reheating

Common methods to improve gas turbine performance

1. Reheating – expansion in stages with reheating between turbines.
2. Intercooling – compression in stages with cooling between stages.
3. Regeneration – exhaust gas preheats compressed air.
4. Combination of the above (approaches the Ericsson cycle).
5. Higher turbine inlet temperature using better materials and blade cooling.
6. Optimum pressure ratio, high component efficiencies, low pressure losses.
7. Combined cycle / cogeneration with waste-heat recovery.
8. Water/steam injection, inlet air cooling.

How reheating increases net work output

      fuel            fuel
       v               v
 1->[C]-2->[CC1]-3->[HPT]-4->[CC2]-5->[LPT]-6-> exh
     ||              ||               ||
     ++=====shaft====++=====shaft=====++==> Gen

 T
 |          3       5
 |         *       *
 |        /|      /|
 |       / |     / |
 |      /  *----'  * 6
 |   2 *   4       |
 |     |          /
 |     *--------'
 |     1
 +---------------------- s

1. Gas from the combustion chamber at $T_3$ expands in the HP turbine only to an intermediate pressure, temperature falling to $T_4$.
2. It enters a reheater (second combustion chamber) where more fuel is burnt in the excess air, raising its temperature back to $T_5 \approx T_3$.
3. It then expands in the LP turbine to atmospheric pressure ($T_6$).
4. Total turbine work:

$$W_t = c_p(T_3 - T_4) + c_p(T_5 - T_6)$$

On the T–s diagram, constant-pressure lines diverge as entropy increases, so the temperature drop $T_5 - T_6$ is larger than the drop the gas would have had from $T_4$ without reheat. Hence turbine work increases, while compressor work is unchanged, so net work output and work ratio increase. 5. Maximum work is obtained when both turbines have equal pressure ratios: $p_4 = \sqrt{p_3 p_6}$, with $T_5 = T_3$.

Effect on efficiency: extra fuel is burnt and the exhaust is hotter, so reheat alone may slightly lower efficiency; combined with regeneration, the hot exhaust preheats the air and both output and efficiency improve. Reheating is therefore used where high specific output and a compact plant are needed.