---
title: "Combined plant heating feed water with exhaust"
subject: "Power Plant Equipment"
chapter: 8
key: "semester_7/power_plant_equipment:combined-feedwater-layout"
source: https://www.bctengineer.com/q/semester_7/power_plant_equipment/combined-feedwater-layout
site: BCT Engineering
---

# Combined plant heating feed water with exhaust

Power Plant Equipment · Chapter 8: Combined Power Plant

- Asked in: 2072 Chaitra, 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-layout
- Whole chapter: https://www.bctengineer.com/md/semester_7/power_plant_equipment/chapter-8

## Question

Sketch the layout for a combined power plant in which waste heat of gas turbine outlet is used to heat feed water. Explain its working with processes on T-S diagram.

## Short answer

Layout: gas turbine exhaust passes through a feed water heater before stack; condensate is pumped, heated by exhaust, then enters fired steam boiler; steam drives turbine and condenses. On T-S: gas cycle 1-2-3-4, exhaust 4-5 heats feed water d-e; steam cycle c-d pump, d-e heating, e-a boiler, a-b turbine, b-c condenser. Gain about 3-5% points, used for repowering.

## Answer

In this scheme a gas turbine is added to a conventional steam plant and its exhaust is used to heat the boiler feed water, replacing some or all regenerative (bled-steam) feed water heaters.

#### Layout

```text
  air       fuel
   | 1        |
   v          v
  [C]--2-->[ CC ]--3-->[GT]==> G1
   ||                   |  4  exhaust gas
   ++=====shaft=========+  v
          +--------------------------+
 stack <-5| Feed water heater (gas   |
          | to water heat exchanger) |
          +--------------------------+
             ^ d             | e  hot feed water
             |               v
          +------+     +----------+  a  +----+
          | Pump |     |  Steam   |---->| ST |==> G2
          +------+     |  boiler  |     +----+
             ^ c       +----------+       | b
             |              ^ fuel        v
             |           +-----------------+
             +-----------|    Condenser    |
                         +-----------------+
```

Working:

1. Air is compressed (1-2), burnt with fuel in the combustion chamber (2-3) and expands in the gas turbine (3-4), driving the compressor and generator G1.
2. The hot exhaust (about 450-550 C) passes through the **feed water heater** (an economiser-type gas-to-water heat exchanger) and leaves to the stack at 5.
3. Condensate from the steam plant is pumped (c-d) and heated by the exhaust gas (d-e) nearly up to the boiling temperature. This replaces the bled-steam feed heaters, so steam that would have been extracted now expands fully in the turbine and produces extra power.
4. The hot feed water enters the conventional **fired steam boiler**, where fuel supplies the latent heat and superheat (e-a).
5. Steam expands in the steam turbine (a-b) driving G2, is condensed (b-c) and the cycle repeats.

The steam boiler is still fully fired, so this scheme gives a modest gain (about 3-5% points) and is often used to **repower** an existing steam plant by adding a gas turbine.

#### T-s diagram

```text
  T
  ^          3
  |         /|
  |        / |     gas cycle 1-2-3-4,
  |       2  |     exhaust 4-5 heats
  |       |  4     feed water d-e
  |       | /
  |       |5
  |       1            a
  |       f---------g /|
  |      e             |
  |     /              |
  |    d               |
  |    c---------------b
  +---------------------------> s
  c-d pump, d-e heating by GT exhaust,
  e-f-g-a heating in fired boiler,
  a-b steam turbine, b-c condenser
  (gas and steam drawn per kg of
   their own fluid, schematic only)
```

The heat given by the gas, $\dot m_g c_p (T_4 - T_5)$, equals the heat taken by the water, $\dot m_w (h_e - h_d)$.

## Check yourself

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

1. In the feed-water heating scheme, what happens in process d-e?
   A. Feed water is heated by GT exhaust nearly to boiling temperature
   B. Steam expands in the turbine
   C. Water is pumped
   D. Steam is condensed
   Answer: A. Pumped condensate (c-d) is heated by exhaust gas in the gas-to-water heater (d-e). (Key point: Process d-e)

2. What does the fired steam boiler supply in this scheme (e-a)?
   A. Sensible heat only for the compressor
   B. Only superheat from exhaust
   C. Nothing, the exhaust does all heating
   D. Latent heat and superheat using fuel
   Answer: D. The boiler is still fully fired and provides evaporation and superheating. (Key point: Boiler duty)

3. What is the typical efficiency gain and use of this scheme?
   A. Modest gain of about 3-5 % points; used to repower existing steam plants
   B. Gain over 20 % points; used only for new plants
   C. No gain; used for cogeneration
   D. Gain of 50 % points
   Answer: A. Because the boiler is fully fired the gain is small, but adding a gas turbine easily repowers an old steam plant. (Key point: Gain and application)

## Close variants set in other exams

- [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×

## Related in Power Plant Equipment

- [Feed water heating combined circuit and reasons](https://www.bctengineer.com/question.md?key=semester_7%2Fpower_plant_equipment%3Acombined-feedwater-reasons) — Power Plant Equipment, chapter 8
- [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
- [Combined cycle components and waste heat use](https://www.bctengineer.com/question.md?key=semester_7%2Fpower_plant_equipment%3Acombined-waste-heat-utilization) — Power Plant Equipment, chapter 8
- [Methods to utilize waste heat in combined plants](https://www.bctengineer.com/question.md?key=semester_7%2Fpower_plant_equipment%3Acombined-waste-heat-methods) — 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
- [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