Reheat gas turbine power plant

In a reheat gas turbine plant, expansion is done in two turbines (HP and LP). After partial expansion in the HP turbine, the gas is heated again in a reheat combustion chamber to nearly the maximum temperature, and then expanded in the LP turbine. Because gas turbine exhaust contains a lot of excess air (A/F ratio 60–100), extra fuel can be burnt directly in the reheater.

Sketch

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

T–s diagram

 T
 |          3       5
 |         *       *
 |        /|      /|
 |       / |     / |
 |      /  *----'  * 6
 |   2 *   4       |  (6' without
 |     |          /    reheat is
 |     *--------'      lower)
 |     1
 +---------------------- s

1–2 compression, 2–3 combustion, 3–4 HP expansion, 4–5 reheating at intermediate pressure, 5–6 LP expansion, 6–1 exhaust.

Working and effect

• Turbine work per kg is proportional to inlet temperature. Raising the gas from $T_4$ back to $T_5 \approx T_3$ makes the LP turbine produce more work than a single turbine would from 4 onward. Total turbine work $W_t = c_p(T_3 - T_4) + c_p(T_5 - T_6)$ increases.
• Compressor work is unchanged, so net work output and work ratio increase. Maximum work occurs when the pressure ratios of the two turbines are equal: $p_4 = \sqrt{p_3 p_6}$.
• Heat supplied also increases and exhaust temperature $T_6$ is higher, so reheat alone gives little or no gain in efficiency; with a regenerator, the hot exhaust is used to preheat the air and efficiency also rises.

Applications / advantages of reheat over the simple open cycle

• Where high specific output is needed from a compact, light plant: peak-load and standby power stations, mobile/emergency sets.
• Aircraft jet engines (afterburning is a form of reheat to increase thrust for take-off and combat) and marine propulsion.
• Combined-cycle plants: the hotter exhaust from a reheat turbine gives more heat to the HRSG, increasing steam-cycle output (e.g. sequential-combustion turbines).
• Plants combined with regeneration (and intercooling) for high efficiency in base-load service.
• Reduces the size of plant (air flow) for a given output, so lower capital cost per kW.