---
title: "Feed water heating combined circuit and reasons"
subject: "Power Plant Equipment"
chapter: 8
key: "semester_7/power_plant_equipment:combined-feedwater-reasons"
source: https://www.bctengineer.com/q/semester_7/power_plant_equipment/combined-feedwater-reasons
site: BCT Engineering
---

# Feed water heating combined circuit and reasons

Power Plant Equipment · Chapter 8: Combined Power Plant

- Asked in: 2072 Kartik, 6 marks
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_7/power_plant_equipment/combined-feedwater-reasons
- Whole chapter: https://www.bctengineer.com/md/semester_7/power_plant_equipment/chapter-8

## Question

In a combined gas and steam turbine power plant, exhaust gases of regenerative gas turbine is used to heat feed water for the boiler of a steam power plant. According to this concept draw a complete circuit of a combined power plant showing "Heating feed water with exhaust gases". What are the reasons that inspire you to construct a combined cycle power plant?

## Short answer

Circuit: gas turbine with regenerator preheating compressed air; remaining exhaust heats feed water in a gas-to-water heater before the fired steam boiler, replacing bled-steam heaters. Reasons: exhaust carries 60-70% of fuel energy, giving 50-60% overall efficiency, lower fuel cost and emissions, lower capital cost, fast start, less cooling water, and repowering old steam plants.

## Answer

#### Circuit: heating feed water with exhaust gases

The gas turbine is regenerative: its exhaust first preheats the compressed air in a regenerator, and the remaining exhaust heat heats the boiler feed water.

```text
  air 1                fuel
   v                    v
  [C]-2->[ Regen ]-x->[ CC ]-3->[GT]==> G1
           ^   |                  | 4
           +---|------------------+
               v y exhaust
   stack <-- +------------------+
             | Feed water heater|<--d--+
             +------------------+      |
                   | e hot water   +------+
                   v               | Pump |
  fuel --> +--------------+   a    +------+
           | Steam boiler |--+       ^ c
           +--------------+  v       |
                           [ST]==>G2 |
                             | b     |
                             v       |
                        [Condenser]--+
```

1. Compressed air (2) is preheated by the turbine exhaust in the regenerator (x), heated in the combustion chamber (3) and expanded in the gas turbine (4).
2. Exhaust leaving the regenerator (y), still at about 300-400 C, flows through the feed water heater and then to the stack.
3. Condensate pumped from the condenser (d) is heated by this gas to state e and enters the fired steam boiler, which produces superheated steam (a) for the steam turbine.
4. Since bled-steam heaters are no longer needed, more steam expands fully in the turbine, increasing steam plant output with the same boiler fuel.

#### Reasons for building a combined cycle plant

- Gas turbine exhaust (450-600 C) carries 60-70% of fuel energy; using it gives **overall efficiency of 50-60%**.
- **Lower fuel cost and CO2 per kWh**; lower emissions with natural gas.
- **Lower capital cost and shorter construction time** than coal or nuclear plants; phased installation possible.
- **Fast start and load-following**, useful for varying demand.
- **Less cooling water and space** than a steam plant of the same output.
- Old steam plants can be **repowered** by adding a gas turbine.

## Check yourself

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

1. In the regenerative GT feed-water-heating circuit, what does the exhaust do first?
   A. Preheats compressed air in the regenerator, then heats feed water
   B. Goes straight to the stack
   C. Heats the feed water, then the air
   D. Drives the steam turbine directly
   Answer: A. Exhaust first preheats compressed air; the remaining heat (about 300-400 C) heats the feed water. (Key point: Regenerator then feed water heater)

2. Why does steam plant output rise when exhaust heats the feed water?
   A. The boiler uses no fuel
   B. Condenser pressure increases
   C. Bled-steam heaters are not needed, so more steam expands fully in the turbine
   D. The pump is removed
   Answer: C. Steam no longer extracted for feed heating expands through the whole turbine, giving more work for the same boiler fuel. (Key point: Benefit of removing bled-steam heaters)

3. What fraction of fuel energy does gas turbine exhaust typically carry, inspiring the combined cycle?
   A. 5-10%
   B. 20-30%
   C. 100%
   D. 60-70%
   Answer: D. Exhaust at 450-600 C carries 60-70% of fuel energy; using it gives 50-60% overall efficiency. (Key point: Reasons for combined cycle)

## Related in Power Plant Equipment

- [Combined plant heating feed water with exhaust](https://www.bctengineer.com/question.md?key=semester_7%2Fpower_plant_equipment%3Acombined-feedwater-layout) — Power Plant Equipment, chapter 8, asked 2×
- [Waste heat methods and feed water heating layout](https://www.bctengineer.com/question.md?key=semester_7%2Fpower_plant_equipment%3Acombined-waste-heat-feedwater) — Power Plant Equipment, chapter 8, asked 2×
- [Regenerative scheme with open feed water heater](https://www.bctengineer.com/question.md?key=semester_7%2Fpower_plant_equipment%3Aregenerative-open-heater-layout) — Power Plant Equipment, chapter 7
- [Working of combined gas-steam plant](https://www.bctengineer.com/question.md?key=semester_7%2Fpower_plant_equipment%3Acombined-gas-steam-working) — Power Plant Equipment, chapter 8
- [Regenerative gas turbine: 1 to 6.9 bar](https://www.bctengineer.com/question.md?key=semester_7%2Fpower_plant_equipment%3Agt-regenerative-6-9-bar) — Power Plant Equipment, chapter 6
- [Reheat gas turbine power plant](https://www.bctengineer.com/question.md?key=semester_7%2Fpower_plant_equipment%3Agt-reheat-plant) — Power Plant Equipment, chapter 6

## Related in other subjects

- [Combined cycle gas turbine versus hydro](https://www.bctengineer.com/question.md?key=semester_8%2Fpower_plant_design%3Accgt-vs-hydro) — Power Plant Design, chapter 1
- [Boiler trial heat balance sheet](https://www.bctengineer.com/question.md?key=semester_3%2Fapplied_thermodynamics%3Aboiler-heat-balance-sheet) — Applied Thermodynamics, chapter 1
- [Boiler efficiency with superheated steam](https://www.bctengineer.com/question.md?key=semester_3%2Fapplied_thermodynamics%3Aboiler-efficiency-superheated-steam) — Applied Thermodynamics, chapter 1
- [Gas turbine 60 MW: heat loss and areas](https://www.bctengineer.com/question.md?key=semester_2%2Ffundamental_of_thermodynamics_and_heat_transfer%3Agas-turbine-60mw-heat-loss-areas) — Fundamental of Thermodynamics & Heat Transfer, chapter 4