1. REFRIGERATION
CYCLES
ERT 206 THERMODYNAMICS

2. OBJECTIVE
• Introduce the concepts of refrigerators and heat pumps and the measure of their performance.
• Analyze the ideal vapor-compression refrigeration cycle.
• Analyze the actual vapor-compression refrigeration cycle.
• Review the factors involved in selecting the right
refrigerant for an application.
• Discuss the operation of refrigeration and heat pump systems.
• Evaluate the performance of innovative vapor compression refrigeration systems.
• Analyze gas refrigeration systems.
• Introduce the concepts of absorption-refrigeration
systems.

3. REFRIGERATORS & HEAT PUMPS
OBJECTIVE:
MAINTAIN REFRIGERATED SPACE AT TL BY REMOVING QL
OBJECTIVE:
MAINTAIN HEATED SPACE AT TH BY ABSORBING QH
COOLING CAPACITY OF REFRIGERATORS
Rate of heat removal from refrigerated space

4. THE REVERSED CARNOT CYCLE
Schematic of a Carnot refrigerator and T-s diagram of the reversed Carnot cycle.
The most efficient refrigeration cycle operating between TL and TH
NOT a suitable model for refrigeration cycles since processes 2-3 and 4-1 are not practical
Process 2-3  Compression of a liquid–vapor mixture, requires a compressor that will handle two phases
Process 4-1  Expansion of high-moisture-content refrigerant in a turbine.

5. THE IDEAL VAPOR-COMPRESSION
REFRIGERATION CYCLE
Eliminated the impracticalities of Reversed Carnot:
Refrigerant is vaporized completely before compression
Turbine is replaced by throttling device
Consists of 4 main processes
The ideal vapor-compression refrigeration cycle involves an irreversible (throttling) process to make it a more realistic model for the actual systems, thus it is not internally reversible cycle.
Superheated vapor
1-2
Isentropic compression
3-4’
Isentropic turbine
3-4
Thtrottling
Δh = 0
Schematic and T-s diagram for the ideal
vapor-compressionrefrigeration cycle.

6. All 4 components in vapor-compression refrigeration cycle are steady flow devices.
Steady-flow energy balance on a unit-mass basis:
Condenser & evaporator do not involve any work, compressor can be approxinated as adiabatic.
COPs for refrigerators & heat pumps on vapor-compression refrigeration cycle:
Where
The P-h diagram of an ideal vapor-compression refrigeration cycle.
In P-h diagram,
Process 2-3 & 4-1, constant Pressure
Process 3-4, Thtrottling
Δh = 0

7. EXAMPLE 1
A refrigerator uses R-134a as the working fluid and operates on an ideal vapor-compression refrigeration cycle between 0.14 & 0.8 Mpa. If the mass flow rate of the refrigerant is 0.05 kg/s, determine
the rate of heat removal from the refrigerated space and the power input to the compressor, [ kW, kW]
the rate of heat rejection to the environment [8.996 kW]
COP of the refrigerator [3.9662]
SOLUTION: Rate of refrigeration, power input, rate of heat rejection, COPR
Assumption: Steady operating conditions, PE & KE negligible
Analysis:
Draw the T-s diagram of the system
Determine the enthalpies at all 4 states (1-2 Δs=0, 3-4 Δh=0)
Solve all

8. ACTUAL VAPOR-COMPRESSION
REFRIGERATION CYCLE
Actual vapor-compression refrigeration cycle differs from the ideal one owing
mostly to the irreversibilities that occur in various components  fluid friction and heat transfer to or from the surroundings.
Schematic and T-s diagram for the actual vapor-compression refrigeration cycle.
1-2
Non-isentropic compression,
Superheated vapor at 1, friction in compressor, Δs≠0, Win increase
2-4-5
Sub cooled liquid at condenser exit, P drops

9. SELECTION OF REFRIGERANT
PARAMETERS TO CONSIDER
Temperatures of the 2 media which the refrigerant exchanges heat
Toxicity, flammability, chemical stability
Availability at low cost
For heat pumps: Tmin & Pmin may be considerably higher
CFCs
AMMONIA
HYDROCARBON
CO2, AIR, H2O
Industrial & heavy sectors
ADV low cost, high COP, low E cost, high heat transfer, no effect on ozone layer
DISADV toxicity
Low cost & versatile
R-11: large capacity water chillers
R-12: domestic refrigerators & freezers
R-22: NH3 competitors
R-502: R-115 & R-22 blends, commercial refrigerators (supermarket)
Fully haloganated CFCs damage ozone layer
Developed R-134a chlorine-free

10. HEAT PUMP SYSTEMS
The most common energy source for heat pumps is atmospheric air (air-to- air systems).
Water-source systems usually use well water and ground-source (geothermal) heat pumps use earth as the energy source. They typically have higher COPs but are more complex and more expensive to install.
Both the capacity and the efficiency of a heat pump fall significantly at low temperatures. Therefore, most air-source heat pumps require a supplementary heating system such as electric resistance heaters or a gas furnace.
Heat pumps are most competitive in areas that have a large cooling load during the cooling season and a relatively small heating load during the heating season. In these areas, the heat pump can meet the entire cooling and heating needs of residential or commercial buildings.
A heat pump can be used to heat a house in winter and to cool it in summer by adding a reversed valve

11. INNOVATIVE VAPOR-COMPRESSION REFRIGERATION SYSTEMS
• The simple vapor-compression refrigeration cycle is the most widely used refrigeration cycle, and it is adequate for most refrigeration applications.
• The ordinary vapor-compression refrigeration systems are simple, inexpensive, reliable, and practically maintenance-free.
• However, for large industrial applications efficiency, not simplicity, is the major concern.
• Also, for some applications the simple vapor-compression refrigeration
cycle is inadequate and needs to be modified.
• For moderately and very low temperature applications some innovative refrigeration systems are used. The following cycles will be discussed:
• Cascade refrigeration systems
• Multistage compression refrigeration systems
• Multipurpose refrigeration systems with a single compressor
• Liquefaction of gases

12. CASCADE REFRIGERATION SYSTEMS
Some industrial applications require moderately low temperatures, and the temperature range involve may be too large for a single vapor-compression refrigeration cycle to be practical.
The solution is CASCADING.
Cascading
improves the
COP of a
refrigeration
system.
Some systems
use three or
four stages of
cascading.
A two-stage cascade refrigeration system
with the same refrigerant in both stages.

13. MULTISTAGE COMPRESSION REFRIGERATION SYSTEMS
When the fluid used throughout the cascade refrigeration system is the same, the heat exchanger between the stages can be replaced by a mixing chamber (called a flash chamber) since it has better heat transfer characteristics.
A two-stage compression refrigeration
system with a flash chamber.

14. MULTIPURPOSE REFRIGERATION SYSTEMS WITH SINGLE COMPRESSOR
Some applications require refrigeration at more than one temperature. A practical and economical approach is to route all the exit streams from the evaporators to a single compressor and let it handle the compression process for the entire system.
Schematic and T-s diagram for a refrIgerator–freezer unit with one compressor.

15. LIQUEFACTION OF GASES
Many important scientific and engineering processes at cryogenic temperatures (below about -100°C) depend on liquefied gases including the separation of oxygen and nitrogen from air, preparation of liquid propellants for rockets, the study of material properties at low temperatures, and the study of superconductivity.
Linde-Hampson system for liquefying
gases.
The storage (i.e., hydrogen) and transportation of some gases (i.e., natural gas) are done after they are liquefied at very low temperatures. Several innovative cycles are used for the liquefaction of gases.

16. GAS REFRIGERATION CYCLES
Gas Refrigeration Cycle The Reversed Brayton cycle can be used for refrigeration.
Internally reversible, executed in ideal gas refrigeration cycle
T-s diagram
Area under 4-1 = QL
Area enclosed by = Win
Heat transfer not isothermal, low COP
2 desirable characteristics: simpler, lighter component & incorporated with regeneration
Simple gas refrigeration cycle
where

17. EXAMPLE 2
An ideal gas refrigeration cycle using air as the working medium is to maintain a refrigerated space at -18°C while rejecting heat to the surrounding medium at 27°C. The pressure ratio of the compressor is 4, determine
the maximum & minimum T in the cycle [106°C, -71°C]
the coefficient of performance [2.05]
The rate of refrigeration for a mass flow rate of 0.05 kg/s [2.67 kW]
SOLUTION: Tmax & Tmin, COPR, Rate of refrigeration
Assumption: Steady operating conditions, air as ideal gases with variable specific heats, PE & KE negligible
Analysis:
Draw the T-s diagram of the system
Determine the enthalpies at all 4 states (1-2 and 3-4isentropic for ideal gases)
Solve all

18. ABSORPTION REFRIGERATION
SYSTEMS
Absorption refrigeration is economical when there is a source of inexpensive thermal energy at a temperature of 100 to 200°C.
EXAMPLES: Geothermal energy, solar energy, and waste heat from cogeneration or process steam plants, and even natural gas when it is at a relatively low price.
Absorption refrigeration systems (ARS) involve the absorption of a refrigerant by a transport medium. The most widely used system is the ammonia–water system, where ammonia (NH3) serves as the refrigerant and water (H2O) as the transport medium.
Major advantage: A liquid is compressed instead of a vapor and as a result the work input is very small and often neglected in the cycle analysis.
ARS are often classified as heat-driven systems & primarily used in large commercial and industrial installations.
ARS are much more expensive than the vapor-compression refrigeration systems since it is more complex and occupy more space, much less efficient & require much larger cooling towers to reject the waste heat, and they are more difficult to service since they are less common.
Therefore, ARS should be considered only when the unit cost of thermal energy is low and is projected to remain low relative to electricity.

19. PRINCIPLE OF ARS
The system is much like the vapor-compression cycle, except for the compressor which has been replaced by a complex absorption mechanism
Absorption unit  absorber, pump, generator, rectifier, regenerator, valve
PURPOSE increased the Pressure of NH3
MECHANISM:
NH3 vapor leaves evaporator enters the absorber, dissolves & reacts with H2O (exo rxn, heat released)
The NH3+H2O solution pumped to generator, vaporize some solution by heat transfer to the solution
The vapor passes through rectifier to separate the water
High P NH3 vpor enter condenser & the rest of the cycle
The hot NH3+H2O solution passes through the regenerator, transfer some heat to rich solution from the pump
The solution is throttled to the absorber P.

20. Determining the maximum COP of an absorption refrigeration system.
The COP of ARS is defined as
The MAX COP is determined by assuming that the entire cycle is totally reversible.
Heat from the source (Qgen) transferred to Carnot HE & Woutput of this HE is supplied to Carnot Refrigerator
Determining the maximum COP of
an absorption refrigeration system.
The COP of actual absorption refrigeration systems is usually less than 1.

21. THANK YOU

22. TUTORIAL III
A refrigerator uses R-134a as the working fluid and operates on an ideal vapor-compression refrigeration cycle between 0.12 and 0.7 Mpa. The mass flow rate of the refrigerant is 0.05 kg/s. Show the cycle on a T-s diagram with respect to the saturation lines. Determine
The rate of heat removal from the refrigerated space & the power input to the compressor
The rate of heat rejection to the environment
The coefficient of performance