---
title: "Gas turbine improvement methods and reheating"
subject: "Power Plant Equipment"
chapter: 6
key: "semester_7/power_plant_equipment:gt-methods-reheat"
source: https://www.bctengineer.com/q/semester_7/power_plant_equipment/gt-methods-reheat
site: BCT Engineering
---

# Gas turbine improvement methods and reheating

Power Plant Equipment · Chapter 6: Gas Turbine Power Plant

- Asked in: 2071 Chaitra, 8 marks
- Video: [Reheating | Gas turbine power plant | SPPU](https://www.youtube.com/watch?v=j3vR49e_FIw) — Professor Nair
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_7/power_plant_equipment/gt-methods-reheat
- Whole chapter: https://www.bctengineer.com/md/semester_7/power_plant_equipment/chapter-6

## Question

List the common methods used for the performance improvement of the gas turbine power plants. Explain how reheater increases net work output of the plant.

## Short answer

Methods: regeneration, intercooling, reheating, higher temperatures, combined cycles. Reheating splits expansion into stages with reheating between turbines; the second stage temperature drop is larger, increasing total turbine work and net work output without changing compressor work.

## Answer

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

```text
 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.

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. In a reheat gas turbine, the gas after the HP turbine is:
   A. Cooled in an intercooler
   B. Reheated in a second combustion chamber to about T3 before the LP turbine
   C. Exhausted to atmosphere
   D. Fed back to the compressor
   Answer: B. Excess air allows more fuel to be burnt, restoring temperature T5 close to T3. (Key point: Staged expansion)

2. Total turbine work with reheat is:
   A. cp(T3 - T6)
   B. cp(T3 - T4) + cp(T5 - T6)
   C. cp(T3 - T4) - cp(T5 - T6)
   D. cp(T5 - T4)
   Answer: B. Work from each turbine is added: HP drop T3 to T4 and LP drop T5 to T6. (Key point: Total turbine work)

3. Why is the LP turbine drop T5 - T6 larger than the drop without reheat?
   A. Compressor work is smaller
   B. Constant-pressure lines diverge on the T-s diagram at higher temperature
   C. Mass flow doubles
   D. Pressure ratio is increased
   Answer: B. Because constant-pressure lines diverge, expansion from a higher temperature gives a bigger temperature drop, hence more turbine work. (Key point: Why work rises)

4. What is the effect of reheat alone on efficiency?
   A. It always strongly increases efficiency
   B. It may slightly lower efficiency; with regeneration both output and efficiency improve
   C. It gives zero net work
   D. It halves work ratio
   Answer: B. Extra fuel is burnt and exhaust is hotter, so efficiency may fall slightly unless regeneration recovers that heat. (Key point: Effect on efficiency)

## Related in Power Plant Equipment

- [Steam plant improvement methods and reheating](https://www.bctengineer.com/question.md?key=semester_7%2Fpower_plant_equipment%3Asteam-methods-reheating) — Power Plant Equipment, chapter 7, asked 4×
- [Gas turbine improvement methods and regeneration](https://www.bctengineer.com/question.md?key=semester_7%2Fpower_plant_equipment%3Agt-methods-regeneration) — Power Plant Equipment, chapter 6
- [Steam improvement methods, reheating, layout and T-S](https://www.bctengineer.com/question.md?key=semester_7%2Fpower_plant_equipment%3Asteam-methods-reheat-layout) — Power Plant Equipment, chapter 7, asked 4×
- [Gas turbine 5:1: power, efficiency, work ratio](https://www.bctengineer.com/question.md?key=semester_7%2Fpower_plant_equipment%3Agt-5-1-work-ratio) — Power Plant Equipment, chapter 6
- [Gas turbine 6:1: power output and efficiency](https://www.bctengineer.com/question.md?key=semester_7%2Fpower_plant_equipment%3Agt-6-1-power-efficiency) — Power Plant Equipment, chapter 6, asked 2×
- [Gas turbine 6:1: generator power output](https://www.bctengineer.com/question.md?key=semester_7%2Fpower_plant_equipment%3Agt-6-1-power-output) — Power Plant Equipment, chapter 6, asked 2×

## Related in other subjects

- [Ideal gas turbine 100 kW: pressure ratio, work, efficiency](https://www.bctengineer.com/question.md?key=semester_2%2Ffundamental_of_thermodynamics_and_heat_transfer%3Aideal-gas-turbine-100kw-pressure-ratio) — Fundamental of Thermodynamics & Heat Transfer, chapter 6
- [Three-process gas cycle: net work, heat, type](https://www.bctengineer.com/question.md?key=semester_2%2Ffundamental_of_thermodynamics_and_heat_transfer%3Athree-process-cycle-pv-sketch) — Fundamental of Thermodynamics & Heat Transfer, chapter 4, asked 2×
- [Three-process gas cycle: net work and heat](https://www.bctengineer.com/question.md?key=semester_2%2Ffundamental_of_thermodynamics_and_heat_transfer%3Athree-process-cycle-pv-const-800kpa) — Fundamental of Thermodynamics & Heat Transfer, chapter 4
- [Gas turbine power from isentropic efficiency](https://www.bctengineer.com/question.md?key=semester_7%2Fturbo_machines%3Agas-turbine-power-isentropic-efficiency) — Turbo Machines, chapter 1