---
title: "Reheater in steam plant with layout"
subject: "Power Plant Equipment"
chapter: 7
key: "semester_7/power_plant_equipment:steam-reheater-layout"
source: https://www.bctengineer.com/q/semester_7/power_plant_equipment/steam-reheater-layout
site: BCT Engineering
---

# Reheater in steam plant with layout

Power Plant Equipment · Chapter 7: Thermal (Steam) Power Plant

- Asked in: 2073 Shrawan, 8 marks
- Video: [Reheat Cycle](https://www.youtube.com/watch?v=5pub7egimeA) — TutorialsPoint
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_7/power_plant_equipment/steam-reheater-layout
- Whole chapter: https://www.bctengineer.com/md/semester_7/power_plant_equipment/chapter-7

## Question

Explain how reheater increases the output of the steam turbine power plant along with neat sketch. Draw the layout and show the cycle in T-S diagram.

## Short answer

A reheater in the boiler reheats steam from the HP turbine at intermediate pressure before LP expansion. This raises output by increasing work per kg, reducing steam consumption, and raising mean heat addition temperature, while the exhaust becomes drier, reducing blade erosion and improving efficiency.

## Answer

A **reheater** is a heat exchanger in the boiler that reheats steam coming from the HP turbine at constant intermediate pressure before it enters the LP turbine.

#### Layout

```text
               1
  +----------+----->+------+     +------+
  |  Boiler  |      |  HP  |=====|  LP  |===> Generator
  |  + S/H   |  2   | turb |     | turb |
  | Reheater |<-----+------+  +->+------+
  |          |------------3---+     | 4
  +----------+                      v
       ^ 6                    +-----------+
       |      +------+   5    | Condenser |
       +------| Pump |<-------+-----------+
              +------+
```

#### Working

1. Feed pump (5-6) sends water to the boiler where it is heated, boiled and superheated (6-1).
2. Steam expands in the **HP turbine** (1-2) to about 20-25% of boiler pressure.
3. It returns to the **reheater** and is heated at constant pressure to almost the initial temperature (2-3).
4. Reheated steam expands in the **LP turbine** to condenser pressure (3-4); the condensate is pumped back (4-5).

#### How reheating increases output and efficiency

- **Output:** each kg of steam now does work $(h_1-h_2) + (h_3-h_4)$, which is larger than $(h_1 - h_{4'})$ without reheat, because constant-pressure lines diverge on the h-s chart. Steam consumption per kWh falls, so for the same boiler the plant output rises.
- **Efficiency:** heat in the reheater is added at a high mean temperature, so the cycle's mean temperature of heat addition rises (gain about 4-5%).
- **Dryness:** exhaust steam becomes much drier (about 0.9 or more), reducing blade erosion and wetness loss, so turbine internal efficiency improves.
- Allows higher boiler pressures without excess moisture at exhaust.

$$
\eta = \frac{(h_1 - h_2) + (h_3 - h_4) - W_p}{(h_1 - h_6) + (h_3 - h_2)}
$$

#### T-s diagram

```text
  T
  ^                  1          3
  |                 /|         /|
  |                / |        / |
  |     f--------g'  |       /  |
  |    /             2------'   |
  |   /                         |
  |  6                          |
  |  5--------------------------4
  +--------------------------------> s
  5-6 pump, 6-f-g'-1 boiler (heat,
  boil, superheat), 1-2 HP turbine,
  2-3 reheater, 3-4 LP turbine,
  4-5 condenser
```

## Check yourself

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

1. A reheater is:
   A. A heat exchanger in the boiler reheating HP exhaust at constant intermediate pressure
   B. A heater using bled steam
   C. A condenser section
   D. A feed pump stage
   Answer: A. It reheats steam from the HP turbine before the LP turbine. (Key point: What a reheater is)

2. Why does work per kg increase with reheating?
   A. Work (h1-h2)+(h3-h4) exceeds (h1-h4') because constant-pressure lines diverge on the h-s chart
   B. Pump work doubles
   C. Steam flow is increased
   D. Boiler pressure rises
   Answer: A. Reheated steam expands from higher enthalpy, so the total enthalpy drop is larger. (Key point: Output increase)

3. HP turbine expansion ends at about what fraction of boiler pressure?
   A. 20-25%
   B. 90%
   C. 5%
   D. Equal to condenser pressure
   Answer: A. Optimum intermediate pressure is about 20-25% of boiler pressure. (Key point: Reheat pressure)

4. Heat supplied in the reheat cycle is:
   A. (h1 - h6) + (h3 - h2)
   B. (h1 - h2) + (h3 - h4)
   C. (h3 - h4)
   D. (h2 - h3)
   Answer: A. Boiler heat (h1-h6) plus reheater heat (h3-h2). (Key point: Efficiency formula)

## Related in Power Plant Equipment

- [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×
- [Regeneration in steam plant with P-V and T-S](https://www.bctengineer.com/question.md?key=semester_7%2Fpower_plant_equipment%3Asteam-regeneration-pv-ts) — Power Plant Equipment, chapter 7
- [Steam plant components and reheating](https://www.bctengineer.com/question.md?key=semester_7%2Fpower_plant_equipment%3Asteam-components-reheating) — Power Plant Equipment, chapter 7
- [How a regenerator improves gas turbine output](https://www.bctengineer.com/question.md?key=semester_7%2Fpower_plant_equipment%3Agt-regenerator-explain) — 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×

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