---
title: "Steam improvement methods, reheating, layout and T-S"
subject: "Power Plant Equipment"
chapter: 7
key: "semester_7/power_plant_equipment:steam-methods-reheat-layout"
source: https://www.bctengineer.com/q/semester_7/power_plant_equipment/steam-methods-reheat-layout
site: BCT Engineering
---

# Steam improvement methods, reheating, layout and T-S

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

- Asked in: 2079 Baisakh, 2+4+2 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-methods-reheat-layout
- Whole chapter: https://www.bctengineer.com/md/semester_7/power_plant_equipment/chapter-7

## Question

List the common method used for the performance improvement of the steam turbine power plant. Explain how reheating increases efficiency of the plant. Draw the layout and show the cycle in T-S diagram.

## Short answer

Common methods: raising boiler pressure, superheating, reducing condenser pressure, reheating, and regeneration. Reheating partially expands steam, then reheats it to near initial temperature before final expansion, raising the mean temperature of heat addition and reducing exhaust moisture, thus improving efficiency and work output.

## Answer

#### Common methods for performance improvement

The efficiency of a Rankine (steam) cycle is $\eta = 1 - \dfrac{T_{\text{low}}}{T_{m,\text{in}}}$ roughly, so it improves by **raising the mean temperature of heat addition** $T_{m}$ or **lowering the temperature of heat rejection**. Common methods:

1. **Increasing boiler (steam) pressure** - raises the saturation temperature at which heat is added.
2. **Superheating the steam** (higher turbine inlet temperature) - raises mean temperature of heat addition and dryness at exhaust.
3. **Reducing condenser pressure** (better vacuum) - lowers heat rejection temperature.
4. **Reheating** - steam partly expanded in HP turbine is reheated in the boiler and expanded in LP turbine.
5. **Regenerative feed water heating** - steam bled from the turbine heats the feed water in feed water heaters.
6. **Using economiser and air preheater** - recover heat from flue gases.
7. **Supercritical / binary vapour and combined cycles** - use very high pressure, or a topping cycle (gas turbine) whose waste heat runs the steam cycle.
8. Reducing losses: better turbine blading, insulation, reduced throttling.

#### How reheating increases efficiency

In a reheat cycle the steam leaving the boiler (state 1) expands in the **HP turbine** only up to an intermediate pressure (state 2). It is taken back to the **reheater** in the boiler and heated at constant pressure to nearly the original temperature (state 3). It then expands in the **LP turbine** to condenser pressure (state 4).

How reheating increases efficiency and output:

- **Higher mean temperature of heat addition:** heat added in reheating (2-3) is at a high temperature. If the reheat pressure is chosen properly (about 20-25% of boiler pressure), the average temperature at which heat is supplied rises, so $\eta$ rises by about 4-5%.
- **More work per kg of steam:** the expansion 3-4 lies to the right of the original expansion line, so the specific work output increases. The steam consumption (kg/kWh) falls and the plant output for the same boiler size increases.
- **Drier exhaust steam:** without reheat, expansion from high pressure ends in very wet steam (dryness below 0.85). Reheat gives exhaust dryness about 0.9 or more, which reduces blade erosion and friction loss, so the **turbine internal efficiency** improves.
- It makes very high boiler pressures usable, which is itself a means of raising efficiency.

Cycle efficiency with reheat:

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

Limitations: extra cost of reheater, piping and controls; useful only for large units (above about 100 MW) and high pressures.

#### Layout and T-s diagram

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

```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. Which method improves performance by lowering the heat rejection temperature?
   A. Superheating
   B. Reducing condenser pressure
   C. Reheating
   D. Raising boiler pressure
   Answer: B. Better vacuum lowers the condensing temperature, so heat is rejected at a lower temperature. (Key point: Heat rejection temperature)

2. What is the correct order of steam flow in a reheat cycle?
   A. Boiler, HP turbine, reheater, LP turbine, condenser
   B. Boiler, LP turbine, reheater, HP turbine, condenser
   C. Boiler, HP turbine, condenser, reheater
   D. Boiler, reheater, condenser, turbine
   Answer: A. Steam expands in the HP turbine, returns to the reheater, then expands in the LP turbine to the condenser. (Key point: Reheat flow path)

3. Why does reheat increase specific work?
   A. Expansion 3-4 lies at higher enthalpy, to the right of the original expansion line, so total drop is larger
   B. The pump work is increased
   C. Steam mass flow is doubled
   D. Condenser pressure rises
   Answer: A. Constant-pressure lines diverge, so the enthalpy drop (h1-h2)+(h3-h4) exceeds the no-reheat drop. (Key point: More work per kg)

4. Why is reheat needed with very high boiler pressure?
   A. Expansion without reheat ends in very wet steam (dryness below 0.85), eroding blades
   B. The steam would be too dry
   C. The pump would fail
   D. The condenser would freeze
   Answer: A. Reheat keeps exhaust dryness around 0.9 or more, allowing high boiler pressures. (Key point: Wetness problem)

## Close variants set in other exams

- [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×

## Related in Power Plant Equipment

- [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
- [Steam plant improvement methods and regeneration](https://www.bctengineer.com/question.md?key=semester_7%2Fpower_plant_equipment%3Asteam-methods-regeneration) — Power Plant Equipment, chapter 7, asked 2×
- [Gas turbine improvement methods and reheating](https://www.bctengineer.com/question.md?key=semester_7%2Fpower_plant_equipment%3Agt-methods-reheat) — Power Plant Equipment, chapter 6
- [Steam plant components and performance improvement](https://www.bctengineer.com/question.md?key=semester_7%2Fpower_plant_equipment%3Asteam-components-performance) — Power Plant Equipment, chapter 7
- [Reheater in steam plant with layout](https://www.bctengineer.com/question.md?key=semester_7%2Fpower_plant_equipment%3Asteam-reheater-layout) — Power Plant Equipment, chapter 7
- [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

## Related in other subjects

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