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

# Steam plant improvement methods and reheating

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

- Asked in: 2081 Baisakh, 8 marks; 2072 Kartik, 8 marks; 2070 Chaitra, 8 marks (asked 3 times)
- 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-reheating
- Whole chapter: https://www.bctengineer.com/md/semester_7/power_plant_equipment/chapter-7

## Question

List the common method used for performance improvement of a steam turbine power plant. Explain how reheating increases the efficiency of the plant.

## Short answer

Common methods: increasing boiler pressure, superheating, reducing condenser pressure, reheating, regenerative feedwater heating, using economizer/air preheater. Reheating reheats partially expanded steam to nearly original temperature, raising the mean temperature of heat addition, increasing net work per kg, and reducing exhaust wetness, improving efficiency by 4–5%.

## Answer

#### Methods of improving performance of a steam power plant

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

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

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.

```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. Rankine efficiency can be improved by:
   A. Lowering mean temperature of heat addition
   B. Raising mean temperature of heat addition or lowering heat rejection temperature
   C. Raising condenser pressure
   D. Removing superheating
   Answer: B. Efficiency roughly equals 1 - T_low/T_m, so a higher T_m or lower T_low helps. (Key point: Principle of improving Rankine efficiency)

2. Which is NOT a method to improve steam plant performance?
   A. Regenerative feed heating
   B. Superheating
   C. Reducing condenser vacuum (raising condenser pressure)
   D. Reheating
   Answer: C. A better vacuum (lower condenser pressure) improves efficiency; raising condenser pressure does the opposite. (Key point: Which are improvement methods)

3. In a reheat cycle, steam is:
   A. Expanded fully in one turbine
   B. Partly expanded in the HP turbine, returned to the boiler reheater at constant pressure, then expanded in the LP turbine
   C. Cooled and compressed between turbines
   D. Bled to a feed heater
   Answer: B. Steam expands to an intermediate pressure, is reheated to nearly its original temperature and then expands to condenser pressure. (Key point: Reheat process)

4. Which is a benefit of reheating?
   A. Wetter exhaust steam
   B. Higher mean heat-addition temperature, more work per kg, and drier exhaust (about 0.9 or more) reducing blade erosion
   C. Lower steam consumption is impossible
   D. No extra equipment is required
   Answer: B. Reheat raises efficiency by about 4-5 %, increases specific work and keeps the exhaust dryness high. (Key point: Benefit of reheating)

## Close variants set in other exams

- [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×

## Related in Power Plant Equipment

- [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
- [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
- [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
- [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

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