---
title: "Conditions favouring Francis over Pelton"
subject: "Hydropower Engineering"
chapter: 7
key: "semester_7/hydropower_engineering:francis-preferable-over-pelton"
source: https://www.bctengineer.com/q/semester_7/hydropower_engineering/francis-preferable-over-pelton
site: BCT Engineering
---

# Conditions favouring Francis over Pelton

Hydropower Engineering · Chapter 7: Hydro-electric Machines

- Asked in: 2070 Chaitra, 4 marks
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_7/hydropower_engineering/francis-preferable-over-pelton
- Whole chapter: https://www.bctengineer.com/md/semester_7/hydropower_engineering/chapter-7

## Question

What are the conditions under which Francis turbines are preferable to Pelton turbines?

## Short answer

Francis is preferred for medium heads (30–500 m) and large discharges, giving higher specific speed, smaller faster runner, direct generator coupling, and a draft tube to recover tailwater head. It offers higher peak efficiency (~95%) and compact powerhouse, but needs clean water and steady load.

## Answer

Francis turbines are preferred to Pelton turbines in the following conditions:

1. **Medium head range:** about 30 to 500 m, where the Francis has a higher specific speed and a smaller, faster runner. (The Pelton is preferred above about 300 to 500 m.)
2. **Large discharge:** for a given power at medium head, a Francis handles a large flow in a compact runner; a Pelton would need many jets and a very large wheel.
3. **High specific speed needed:** a Francis runs faster, so it can be directly coupled to a smaller, cheaper generator.
4. **Compactness and lower civil cost:** a smaller powerhouse, with the unit on a vertical shaft close to tailwater, and a lower cost per kW at moderate heads.
5. **Utilisation of the full head:** the draft tube lets the turbine use the head between the runner and the tailwater, which matters when this height is a large part of the total head (low and medium heads). A Pelton wheel must stay above tailwater and loses this head.
6. **Higher peak efficiency:** up to about 95 % at the design load, which is useful when the plant runs at a nearly constant load.
7. **Clean water and steady load:** where the sediment load is small (otherwise runner erosion is severe) and the load varies little.
8. **Size of the machine:** at medium heads a Pelton wheel would be large and slow-running, needing a bigger, costlier unit and generator, whereas the Francis stays compact.

In summary, choose a Francis for medium head, large flow and steady load; choose a Pelton for high head, low flow, silty water or widely varying load.

## Check yourself

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

1. In which head range is Francis preferred?
   A. Above 1000 m only
   B. Below 5 m
   C. Heads over 800 m with low flow
   D. Medium heads, about 30 to 500 m
   Answer: D. Pelton is for higher heads. (Key point: Head range)

2. Why is Francis better for large discharge at medium head?
   A. Compact runner handles large flow, where Pelton needs many jets and a very large wheel
   B. Francis uses jets
   C. Pelton is submerged
   D. Francis has no losses
   Answer: A. Large flow suits a reaction runner. (Key point: Discharge)

3. What head advantage does a Francis have through the draft tube?
   A. It avoids penstock
   B. It uses the head between runner and tailwater, which Pelton loses
   C. It increases gross head
   D. It needs no tailrace
   Answer: B. Pelton must stay above tailwater. (Key point: Utilisation of head)

4. Which conditions favour Francis?
   A. Silty water and widely varying load
   B. Very high head
   C. Clean water and steady load
   D. Very small flow
   Answer: C. Silt erodes the runner; part load lowers efficiency. (Key point: Water condition)

## Related in Hydropower Engineering

- [Pelton vs Francis performance; Francis setting](https://www.bctengineer.com/question.md?key=semester_7%2Fhydropower_engineering%3Apelton-francis-performance-curves-setting) — Hydropower Engineering, chapter 7
- [Francis runner design (40 m head, 10 m³/s)](https://www.bctengineer.com/question.md?key=semester_7%2Fhydropower_engineering%3Afrancis-turbine-velocity-triangle-40m) — Hydropower Engineering, chapter 7
- [Francis design (90 m, 60 m³/s, 94%)](https://www.bctengineer.com/question.md?key=semester_7%2Fhydropower_engineering%3Afrancis-design-90m-60cumec-94) — Hydropower Engineering, chapter 7
- [Number of Francis/Kaplan units (300 m³/s, 30 m)](https://www.bctengineer.com/question.md?key=semester_7%2Fhydropower_engineering%3Aturbine-units-number-francis-kaplan-300cumec) — Hydropower Engineering, chapter 7
- [Pelton unit speed, discharge, power](https://www.bctengineer.com/question.md?key=semester_7%2Fhydropower_engineering%3Apelton-unit-quantities-190rpm) — Hydropower Engineering, chapter 7
- [Pelton wheel 70 kW: flow rate and diameter](https://www.bctengineer.com/question.md?key=semester_7%2Fhydropower_engineering%3Apelton-wheel-70kw-100m-400rpm) — Hydropower Engineering, chapter 7

## Related in other subjects

- [Pelton versus Francis performance characteristics](https://www.bctengineer.com/question.md?key=semester_6%2Fhydro_power%3Apelton-vs-francis-performance) — Hydro Power, chapter 7
- [Conductor tension under varying conditions](https://www.bctengineer.com/question.md?key=semester_8%2Ftransmission_and_distribution_system_design%3Atension-varying-conditions-equation) — Transmission and Distribution System Design, chapter 4
- [Under, over and balanced sections](https://www.bctengineer.com/question.md?key=semester_7%2Fdesign_of_rcc_structure%3Aunder-over-and-balanced-sections) — Design of RCC Structure, chapter 4, asked 6×
- [Normal: 31% under 45, 8% over 64](https://www.bctengineer.com/question.md?key=semester_5%2Fprobability_and_statistics%3Anormal-31-percent-under-45) — Probability and Statistics, chapter 3