---
title: "Ideal versus actual vapour compression cycle"
subject: "Applied Thermodynamics"
chapter: 3
key: "semester_3/applied_thermodynamics:ideal-vs-actual-vcr-cycle"
source: https://www.bctengineer.com/q/semester_3/applied_thermodynamics/ideal-vs-actual-vcr-cycle
site: BCT Engineering
---

# Ideal versus actual vapour compression cycle

Applied Thermodynamics · Chapter 3: Refrigeration

- Asked in: year not given, 6 marks
- Video: [Ideal and Actual Vapor-compression Refrigeration Cycles](https://www.youtube.com/watch?v=6F100qUXHlw) — Dr Kidd Engineering
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_3/applied_thermodynamics/ideal-vs-actual-vcr-cycle
- Whole chapter: https://www.bctengineer.com/md/semester_3/applied_thermodynamics/chapter-3

## Question

Explain why the actual vapour compression refrigeration cycle differs from the ideal cycle. Differentiate between the ideal and actual cycles and show the actual cycle on a T-s diagram.

## Short answer

Actual vapour compression cycle differs due to superheating at compressor inlet, subcooling at condenser exit, pressure drops in evaporator and condenser, and irreversibilities in compression. The actual cycle has lower COP than the ideal because of increased compressor work and non-isentropic processes.

## Answer

The ideal cycle assumes isentropic compression, no pressure drops and exact saturation at the compressor inlet and condenser outlet. A real plant departs from this because of friction, heat transfer across a temperature difference and practical safety requirements.

| Point | Ideal cycle | Actual cycle |
|---|---|---|
| Compressor inlet | Dry saturated vapour | Slightly superheated (to protect against liquid carry-over) |
| Compression | Isentropic, reversible | Polytropic with friction; entropy increases; needs more work |
| Pressure in evaporator and condenser | Constant | Pressure drops due to friction and bends |
| Condenser exit | Saturated liquid | Subcooled liquid |
| Suction and delivery lines | Insulated, no loss | Heat gain in suction line; heat loss from delivery line |
| Valves | No loss | Pressure drop through suction and delivery valves |
| COP | Higher | Lower |

#### Causes of deviation in more detail
1. **Superheating** in the evaporator exit and suction line increases specific volume and compressor work slightly, but ensures dry vapour.
2. **Subcooling** in the condenser (and liquid line) gives extra refrigerating effect.
3. **Pressure drops** in the evaporator, condenser and lines require the compressor to start at a lower suction pressure and deliver at a higher pressure.
4. **Compressor irreversibility:** isentropic efficiency of 70 to 85 %; part of the work appears as extra heat in the vapour.

```text
 T
 |                 2  actual
 |                2s  ideal
 |   3'--3-----2'  |  |
 |    |  |         |  |
 |    4---------1--1'
 +------------------------- s
```

1-1' superheating; 1'-2s ideal compression; 1'-2 actual compression (entropy rises); 2-2'-3-3' desuperheating, condensation and subcooling; 3'-4 throttling.

## Check yourself

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

1. In the actual cycle the vapour at compressor inlet is
   A. Wet vapour
   B. Subcooled liquid
   C. Dry saturated exactly
   D. slightly superheated, to protect against liquid carry-over
   Answer: D. Superheat ensures dry vapour enters the compressor. (Key point: Compressor inlet)

2. Actual compression compared with ideal is
   A. polytropic with friction, entropy rises and more work is needed
   B. isentropic, with less work
   C. isothermal
   D. isenthalpic
   Answer: A. Irreversibility adds heat and work (isentropic efficiency 70-85 %). (Key point: Compression)

3. In the actual cycle the liquid leaving the condenser is
   A. a wet mixture
   B. subcooled
   C. superheated vapour
   D. exactly saturated vapour
   Answer: B. Subcooling gives extra refrigerating effect. (Key point: Condenser exit)

4. The COP of an actual cycle compared with the ideal cycle is
   A. higher
   B. identical
   C. lower
   D. infinite
   Answer: C. Pressure drops, friction and heat transfer losses lower the COP. (Key point: COP)

## Related in Applied Thermodynamics

- [Ideal vapour compression cycle for 5 TR](https://www.bctengineer.com/question.md?key=semester_3%2Fapplied_thermodynamics%3Avcr-ideal-cycle-numerical) — Applied Thermodynamics, chapter 3
- [Simple vapour compression cycle](https://www.bctengineer.com/question.md?key=semester_3%2Fapplied_thermodynamics%3Asimple-vcr-cycle-ts-ph-diagrams) — Applied Thermodynamics, chapter 3
- [Actual VCR cycle with compressor efficiency](https://www.bctengineer.com/question.md?key=semester_3%2Fapplied_thermodynamics%3Avcr-actual-compressor-efficiency-numerical) — Applied Thermodynamics, chapter 3

## Related in other subjects

- [Vapour compression refrigeration cycle](https://www.bctengineer.com/question.md?key=semester_6%2Findustrial_power_distribution_and_illumination%3Avapour-compression-refrigeration) — Industrial Power Distribution and Illumination, chapter 5
- [Vapor compression refrigeration cycle p-h and T-s](https://www.bctengineer.com/question.md?key=semester_2%2Ffundamental_of_thermodynamics_and_heat_transfer%3Avapor-compression-refrigeration-ph-ts) — Fundamental of Thermodynamics & Heat Transfer, chapter 6
- [First law for cyclic process; power and refrigeration cycles](https://www.bctengineer.com/question.md?key=semester_2%2Ffundamental_of_thermodynamics_and_heat_transfer%3Afirst-law-cyclic-power-refrigeration) — Fundamental of Thermodynamics & Heat Transfer, chapter 4
- [First law for control mass; cycle; power and refrigeration](https://www.bctengineer.com/question.md?key=semester_2%2Ffundamental_of_thermodynamics_and_heat_transfer%3Afirst-law-control-mass-cycle) — Fundamental of Thermodynamics & Heat Transfer, chapter 4