This document describes refrigeration systems and the vapor compression refrigeration cycle. It discusses that refrigeration systems maintain a cold region below its surroundings' temperature. The vapor compression cycle involves: 1) evaporation and cooling, 2) compression, 3) condensation and warming, 4) expansion. Actual systems have lower efficiency than ideal Carnot cycles due to irreversible heat transfer. Common refrigerants are discussed along with their global warming potentials. An example problem calculates the coefficient of performance for an ideal and actual refrigeration system.

1. Refrigeration Systems

2. INTRODUCTION
• The purpose of a refrigeration system is to maintain a cold
region at a temperature below the temperature of its
surroundings.
• It is best known for its use in the air conditioning of
buildings and in the preservation of foods and chilling of
beverages.
• Examples of large-scale commercial processes requiring
refrigeration are the manufacture of ice and solid CO2, the
dehydration and liquefaction of gases, and the separation of
air into oxygen and nitrogen.

3. THE CARNOT REFRIGERATION CYCLE

4. Coefficient of Performance (ω)
• The coefficient of performance of any refrigeration cycle is
the ratio of the refrigeration effect to the net work input
required to achieve that effect.
• The working fluid operates in a cycle for which ΔU is zero.
The first law for the cycle is therefore:

5. Coefficient of Performance (ω)

6. Departures from the Carnot Cycle
Heat transfers between the refrigerant and the two regions (In actual
systems, these heat transfers are not accomplished reversibly as presumed
in carnot cycle).
ω_actual

7. Departures from the Carnot Cycle
• Wet and dry compression
Refrigerant enters the compressor as a two-phase
liquid–vapor mixture. This is commonly referred to as
wet compression. Wet compression is normally avoided
because the presence of liquid droplets in the flowing
liquid–vapor mixture can damage the compressor. In
actual systems, the compressor handles vapor only.
This is known as dry compression.

8. Departures from the Carnot Cycle
•Small Expansion work
The expansion typically produces a relatively small amount of
work compared to the work input in the compression
process. The work developed by an actual turbine would be
smaller yet because turbines operating under these
conditions have low isentropic efficiencies. Accordingly, the
work output of the turbine is normally sacrificed by
substituting a simple throttling valve for the expansion
turbine.

9. Vapor Compression Refrigeration Cycle
• There are four principal
control volumes involving
these components:
• Evaporator
• Compressor
• Condenser
• Expansion valve
• Most common refrigeration cycle in use today
All energy transfers by work and heat are taken as positive in
the directions of the arrows on the schematic and energy
balances are written accordingly.
Two-phase
liquid-vapor mixture

10. The Vapor-Compression
Refrigeration Cycle
Process 4-1: two-phase liquid-vapor
mixture of refrigerant is evaporated
through heat transfer from the
refrigerated space.
Process 1-2: vapor refrigerant is
compressed to a relatively high
temperature and pressure requiring
work input.
Process 2-3: vapor refrigerant
condenses to liquid through heat
transfer to the cooler surroundings.
Process 3-4: liquid refrigerant
expands to the evaporator pressure.
•The processes of this cycle are
Two-phase
liquid-vapor mixture

11. Coefficient of Performance (β)
The Vapor-Compression
Refrigeration Cycle
Performance parameters
Carnot Coefficient of Performance
This equation represents the maximum theoretical
coefficient of performance of any refrigeration cycle
operating between cold and hot regions at TC and TH,
respectively.

12. Features of
Actual Vapor-Compression Cycle
• Heat transfers between refrigerant and cold and warm
regions are not reversible.
• Refrigerant temperature in
evaporator is less than TC.
• Refrigerant temperature in
condenser is greater than
TH.
• Irreversible heat transfers
have negative effect on
performance.

13. Features of
Actual Vapor-Compression Cycle
• The COP decreases – primarily due to increasing compressor
work input – as the
• temperature of the
refrigerant passing through
the evaporator is reduced
relative to the temperature
of the cold region, TC.
• temperature of the
refrigerant passing
through the condenser is increased relative to the
temperature of the warm region, TH.
Trefrigerant ↓
Trefrigerant ↑

14. Features of
Actual Vapor-Compression Cycle
• Irreversibilities during the compression process are
suggested by dashed line from state 1 to state 2.
• An increase in specific entropy
accompanies an adiabatic
irreversible compression
process. The work input for
compression process 1-2 is
greater than for the counterpart
isentropic compression process
1-2s.
• Since process 4-1, and thus the refrigeration capacity,
is the same for cycles 1-2-3-4-1 and 1-2s-3-4-1, cycle
1-2-3-4-1 has the lower COP.

15. Isentropic Compressor Efficiency
• The isentropic compressor efficiency is the ratio of the
minimum theoretical work input to the actual work
input, each per unit of mass flowing:
work required in an actual
compression from compressor
inlet state to exit pressure
work required in an isentropic
compression from compressor inlet
state to the exit pressure

16. p-h Diagram
►The pressure-enthalpy (p-h) diagram is a thermodynamic
property diagram commonly used in the refrigeration field.

17. 17
Other Refrigeration Cycles
Cascade refrigeration systems
Very low temperatures can be achieved by operating two or more vapor-compression
systems in series, called cascading. The COP of a refrigeration system also
increases as a result of cascading.

18. Refrigerant Types and Characteristics
Global Warming Potential (GWP) is a simplified index that estimates the potential
future influence on global warming associated with different gases when released
to the atmosphere.

19. Example
A refrigerated space is maintained at −20°C, and cooling water
is available at 21°C. Refrigeration capacity is 120,000 kJ⋅h−1.
The evaporator and condenser are of sufficient size that a 5°C
minimum-temperature difference for heat transfer can be
realized in each. The refrigerant is 1,1,1,2-tetrafluoroethane
(HFC-134a),
(a) What is the value of ω for a Carnot refrigerator?
(b) Calculate ω and m ∙ for a vapor-compression cycle if the
compressor efficiency is 0.80.

20. Solution
• Allowing 5°C temperature differences, the evaporator temperature is
−25°C = 248.15 K, and the condenser temperature is 26°C = 299.15 K.
(a) Thus for carnot refrigerator:
• (b) The entry at −25°C indicates that HFC-134a vaporizes in the
evaporator at a pressure of 1.064 bar. Its properties as a saturated
vapor at these conditions are: