---
title: "Turbomachines versus positive displacement machines"
subject: "Turbo Machines"
chapter: 1
key: "semester_7/turbo_machines:turbomachine-vs-positive-displacement"
source: https://www.bctengineer.com/q/semester_7/turbo_machines/turbomachine-vs-positive-displacement
site: BCT Engineering
---

# Turbomachines versus positive displacement machines

Turbo Machines · Chapter 1: Introduction

- Asked in: year not given, 5 marks
- Video: [1.4 Difference between Positive Displacement Machines and Turbomachines | ME603(A) |](https://www.youtube.com/watch?v=xOa8iPrLuPs) — RGPV Official
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_7/turbo_machines/turbomachine-vs-positive-displacement
- Whole chapter: https://www.bctengineer.com/md/semester_7/turbo_machines/chapter-1

## Question

Differentiate between turbomachines and positive displacement machines. Give two examples of each.

## Short answer

Turbomachines use dynamic blade action for continuous, steady flow, high speed, and large clearances; examples: centrifugal pump, gas turbine. Positive displacement machines trap fluid in a closed volume, produce pulsating flow, run at low speed with tight seals; examples: gear pump, reciprocating compressor.

## Answer

Both types transfer energy between a mechanical element and a fluid. They differ in the way the energy transfer takes place.

| Point | Turbomachine | Positive displacement machine |
|---|---|---|
| Principle | Dynamic action of rotating blades changes the momentum of the fluid (Euler equation) | Fluid is trapped in a closed volume that is changed by a piston, vane, gear or screw |
| Flow | Continuous and steady | Intermittent or pulsating |
| Working element motion | Pure rotation | Reciprocating, or rotary with sealed chambers |
| Clearances | Large running clearance, no sealing contact | Close clearances or sliding seals needed |
| Speed | High (thousands of rpm) | Low to moderate |
| Capacity and pressure | Large flow, moderate pressure ratio per stage | Small flow, high pressure possible |
| Size and weight per kW | Small, light | Large and heavy |
| Vibration, balancing | Easy to balance, smooth running | Unbalanced forces, more vibration |
| Efficiency curve | Peaks at the design point, falls off-design | Almost constant over the range |
| Viscous fluids | Poor | Good |

**Examples**
- Turbomachines: centrifugal pump, axial-flow gas turbine, steam turbine, centrifugal compressor.
- Positive displacement machines: reciprocating compressor, gear pump, reciprocating IC engine, Roots blower.

## Check yourself

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

1. How does a turbomachine transfer energy to or from the fluid?
   A. By trapping fluid in a closed volume changed by a piston
   B. By dynamic action of rotating blades changing fluid momentum
   C. By sliding seals and gear meshing
   D. By heating the fluid at constant volume
   Answer: B. Turbomachines work by the dynamic action of blades on the fluid (Euler equation), not by volume change. (Key point: Principle of energy transfer)

2. Which statement about flow is correct?
   A. Turbomachine flow is pulsating; positive displacement flow is steady
   B. Both give perfectly steady flow
   C. Turbomachine flow is continuous and steady; positive displacement flow is intermittent or pulsating
   D. Both give pulsating flow
   Answer: C. Turbomachines deliver continuous steady flow while positive displacement machines deliver in discrete volumes. (Key point: Flow nature)

3. Which machine suits small flow at very high pressure and viscous fluids?
   A. Positive displacement machine
   B. Turbomachine
   C. Either equally
   D. Neither
   Answer: A. Positive displacement machines give high pressure at small flow and handle viscous fluids well; turbomachines handle large flow at moderate pressure. (Key point: Capacity and pressure)

4. Which list contains only positive displacement machines?
   A. Centrifugal pump, steam turbine
   B. Gear pump, Roots blower
   C. Axial gas turbine, gear pump
   D. Centrifugal compressor, reciprocating compressor
   Answer: B. Gear pumps and Roots blowers trap fluid in chambers; the others in the distractors include turbomachines. (Key point: Examples)

## Related in Turbo Machines

- [Thermodynamic laws and efficiencies of turbomachines](https://www.bctengineer.com/question.md?key=semester_7%2Fturbo_machines%3Alaws-of-thermodynamics-efficiencies) — Turbo Machines, chapter 1
- [Euler work equation for turbomachines](https://www.bctengineer.com/question.md?key=semester_7%2Fturbo_machines%3Aeuler-turbine-equation-derivation) — Turbo Machines, chapter 2
- [Turbomachine: definition, parts and classification](https://www.bctengineer.com/question.md?key=semester_7%2Fturbo_machines%3Aturbomachine-definition-classification) — Turbo Machines, chapter 1
- [Dimensionless parameters and their significance](https://www.bctengineer.com/question.md?key=semester_7%2Fturbo_machines%3Adimensionless-parameters-turbomachines) — Turbo Machines, chapter 1

## Related in other subjects

- [Turbomachines versus positive displacement machines](https://www.bctengineer.com/question.md?key=semester_5%2Ffluid_machines%3Aturbomachine-vs-positive-displacement) — Fluid Machines, chapter 1