Chapter 4 - Refrigeration

Refrigeration Refrigeration SystemsSystems

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ObjectivesObjectives Introduce the concepts of refrigerators and heat

pumps and the measure of their performance. Analyze the ideal and actual vapor-compression

refrigeration cycles. Discuss the operation of refrigeration and heat

pump systems. Evaluate the performance of innovative vapor-

compression refrigeration systems.

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4.1 Introduction4.2 Reversed Heat Engine Cycle4.3 Performance of Refrigeration Cycle and

Heat Pump4.4 The Ideal Vapor-Compression

Refrigeration Cycle4.5 The Practical Refrigeration Cycle4.6 Refrigeration Load4.7 Flash Chamber4.8 Multistage Compression Refrigeration

System4.9 Cascade Refrigeration System4.10 Absorption Refrigeration Systems

TopicsTopics

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Refrigerators And Heat PumpsRefrigerators And Heat PumpsThe transfer of heat from a low-temperature region to a high-temperature one requires special devices called refrigerators.The objective of a refrigerator is to remove heat (QL) from the cold medium; the objective of a heat pump is to supply heat (QH) to a warm medium.Refrigerators and heat pumps are essentially the same devices; they differ in their objectives only.Performance:

• From 1st Law of thermodynamics ∑dQ=∑dW; The net work input to the system,W=W12-W34 The net heat rejected by the system;Q=Q2-Q1

•

• If COPR is positive, then COPHP > 1• The rate of heat removal from a system is called cooling capacity.• Cooling capacity is normally measured in tons of refrigeration• 1 ton = 211 kJ/min

1

1

RHP

in

LHP

in

inL

in

H

inLH

COPCOPWQ

COP

WWQ

WQ

WQQ

Refrigerators And Heat PumpsRefrigerators And Heat Pumps

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• The most efficient heat engine is represented by the Carnot cycle. (Remember that Carnot cycle is reversible)

• A reversed heat engine is represented by Carnot cycle which operates in a reversed direction

• This cycle is called a reversed Carnot cycle

• A refrigerator/heat pump using this cycle is called Carnot refrigerator/Carnot heat pump

• Its function is to remove heat from a low-temperature region to a high-temperature region.

The Reversed Carnot CycleThe Reversed Carnot Cycle

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• (1 – 2)– Wet vapor enters compressor and

is compress (Isentropic)– Temperature is increased

• (2 – 3)– Vapor is condensed at constant

temperature– Heat rejected by refrigerant

• (3 – 4)– Isentropic expansion (Isentropic)– Temperature is reduced

• (4 – 1)– Heat for evaporation process is

supplied from cold source in evaporator at constant temperature.


1

23

41

23

4

8


Schematic of a Carnot refrigerator and T-s diagram of the reversed Carnot cycle.

Both COPs increase as the difference between the two temperatures decreases, i.e. as TL rises or TH falls.

The most efficient refrigeration cycle (the reversed Carnot cycle) operating between TL and TH. But it is impractical model for refrigeration cycles because:

(i)process 1-2 involves compression of a liquid–vapor mixture - requires a compressor that will handle two phases, (ii)process 3-4 involves expansion of high-moisture-content refrigerant in a turbine.Reversed Carnot cycle is only for comparison with the actual refrigeration cycles.

PERFORMANCE OF REFRIGERATION CYCLE AND HEAT PUMP

• From T-s diagram,• TL= T1 = T4 and TH= T2 = T3

• s1 = s2 and s4 = s3

• QL= TL(s1 – s4) and QH= -TH(s3 – s2)

• Win = QH - QL

= -TH(s3 – s2)– T1(s1 – s4) = -T2(s4 – s1) – T1(s1 – s4) = (T2 – T1) (s1 – s4)


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• Similarly for COPHP, we get:

H

LLH

HHP

TTTT

TTT

TCOP

1

1

12

2

1

1

12

1

4112

411

L

HLH

LR

R

in

LR

TTTT

TTT

TCOP

ssTTssTCOP

WQCOP

• So, COPR can be given as follows:


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A steady-flow Carnot refrigeration cycle uses refrigerant-134a as the working fluid. The refrigerant changes from saturated vapor to saturated liquid at 30°C in the condenser as it rejects heat. The evaporator pressure is 160 kPa. Show the cycle on a T-s diagram relative to saturation lines, and determine:

(a)the coefficient of performance, (b)the amount of heat absorbed from the refrigerated space, and (c)the net work input.

Answers: (a) 5.64, (b) 147 kJ/kg, (c) 26.1 kJ/kg

Problem 1Problem 1

Ideal Vapor-compression Refrigeration CycleIdeal Vapor-compression Refrigeration CycleIs the ideal model for refrigeration systems. The refrigerant is vaporized completely before it is compressed and the turbine is replaced with a throttling device.

Schematic and T-s diagram for the ideal vapor-compression refrigeration cycle.

The most widely used cycle for refrigerators, A-C systems, and heat pumps.

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Process4-1 Constant Pressure EvaporationHeat from a cold space is absorbed by the refrigerant. As a result, the refrigerant evaporates at a constant evaporator pressure, from state 4 to become a drier saturated vapor at state 1.

Ideal Vapor-compression Refrigeration CycleIdeal Vapor-compression Refrigeration Cycle

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Process1-2 Isentropic compressionThe saturated vapor is compressed from the evaporator pressure to the condenser pressure, in a reversible adiabatic manner. The refrigerant exits the compressor as a superheated vapor at state 2.


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Process2-3 Constant Pressure CondensationHeat is rejected from the refrigerant to a warm space. As a result, the refrigerant condenses at a constant condenser pressure until it becomes a saturated liquid at state 3.


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3-4 Constant Enthalpy ExpansionThe refrigerant expands through the throttle valve adiabatically. As a result, it’s pressure drops from the condenser to the evaporator pressure. The enthalpy is constant during the process, i.e. h3 = h4.


Note: The expansion process is highly irreversible, thus making the vapor-compression cycle an irreversible cycle.

Process

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An ordinary household refrigerator.

The P-h diagram of an ideal vapor-compression refrigeration cycle.

The ideal vapor-compression refrigeration cycle involves an irreversible (throttling) process to make it a more realistic model for the actual systems.

Steady-flow energy balance

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Refrigeration LoadRefrigeration Load

massunitpereffectingrefrigeratcapacityorrefrigeratm

• Refrigeration Capacity,

– defined as the amount of heat that has to be transferred from a cold space per unit time

– determines the mass flow rate of refrigerant

• 1 ton = 200Btu/min = 211kJ/min = 3.516kW

• ton : “the rate of heat transfer to produce 2000 lb of ice at 0oC (32o)F from liquid water at 0oC (32oF) in 24 hours”

• Mass flow rate of refrigerant

LQ

Solving ProblemSolving Problem• 2 methods can be used for cycle analysis.

– Using property table for refrigerants– Using the P-h diagram

P

h

1

23

4s

cons

tant

v constant

x co

nsta

ntq2 = h2 – h3

q2 = h1 – h4

win = h2 – h1

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P-h D

iagram for R

efrigerant 134a

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A refrigerator uses refrigerant-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 saturation lines. Determine:a) the rate of heat removal from the refrigerated space,b) the power input to the compressor, c) the rate of heat rejection to the environment, and d) the coefficient of performance.

Answers: (a) 7.41 kW, 1.83 kW, (b) 9.23 kW, (c) 4.06

Problem Problem 2 2

Ideal and Actual Vapor-Compression Refrigeration Cycles

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Actual Vapor-Compression Refrigeration CycleActual Vapor-Compression Refrigeration Cycle

Schematic and T-s diagram for the actual vapor-compression refrigeration cycle.

An actual vapor-compression refrigeration cycle involves irreversibilities in various components - mainly due to fluid friction (causes pressure drops) and heat transfer to or from the surroundings. As a result, the COP decreases.

Differences•Non-isentropic compression;•Superheated vapor at evaporator exit;•Sub-cooled liquid at condenser exit;•Pressure drops in condenser and evaporator.

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T

s

1

2

4

3’Cooling water temperature

4’

Undercooling (Subcooling) And Its EffectsUndercooling (Subcooling) And Its Effects

In the condenser, the vapor can be further cooled at constant pressure to a temperature that is lower than temperature in condenser

3

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• Undercooling (subcooling) increases the refrigerating effect (h1 – h4) > (h1 – h4’) where h4 is enthalpy with undercooling (subcooling) and h4’ is

initial enthalpy

• Undercooling (subcooling) is limited by temperature of cooling water and temperature difference of cycle

3

Undercooling (Subcooling) And Its EffectsUndercooling (Subcooling) And Its Effects

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Refrigerant-134a enters the compressor of a refrigerator as superheated vapor at 0.14 MPa and - 10°C at a rate of 0.12 kg/s, and it leaves at 0.7 MPa and 50°C. The refrigerant is cooled in the condenser to 24°C and 0.65 MPa, and it is throttled to 0.15 MPa. Disregarding any heat transfer and pressure drops in the connecting lines between the components, show the cycle on a T-s diagram with respect to saturation lines, and determine:

a)the rate of heat removal from the refrigerated space, b)the power input to the compressor, c)the isentropic efficiency of the compressor, and d)the COP of the refrigerator.

Answers: (a) 19.4 kW, 5.06 kW, (b) 82.5 percent, (c) 3.83

Ideal and Actual Vapor-Compression Refrigeration Cycles

Problem 4Problem 4

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Selecting the Right RefrigerantSelecting the Right Refrigerant Several refrigerants may be used in refrigeration systems such as chlorofluorocarbons

(CFCs), ammonia, hydrocarbons (propane, ethane, ethylene, etc.), carbon dioxide, air (in the air-conditioning of aircraft), and even water (in applications above the freezing point).

R-11, R-12, R-22, R-134a, and R-502 account for over 90 percent of the market. The industrial and heavy-commercial sectors use ammonia (it is toxic). R-11 is used in large-capacity water chillers serving A-C systems in buildings. R-134a is used in domestic refrigerators and freezers, as well as automotive air conditioners. R-22 is used in window air conditioners, heat pumps, air conditioners of commercial

buildings, and large industrial refrigeration systems, and offers strong competition to ammonia.

R-502 (a blend of R-115 and R-22) is the dominant refrigerant used in commercial refrigeration systems such as those in supermarkets.

CFCs allow more ultraviolet radiation into the earth’s atmosphere by destroying the protective ozone layer and thus contributing to the greenhouse effect that causes global warming. Refrigerants that are friendly to the ozone layer have been developed.

Two important parameters to be considered - the temperatures of the refrigerated space and the environment with which the refrigerant exchanges heat.

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Innovative Vapor-compression Innovative Vapor-compression Refrigeration SystemsRefrigeration Systems

The simple vapor-compression refrigeration cycle is the most widely used refrigeration cycle, and 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.

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

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Cascade Refrigeration SystemsCascade Refrigeration Systems•Some industrial applications require moderately low

temperatures, and the temperature range they involve may be too large for a single vapor compression refrigeration cycle to be practical.

•A large temperature range also means a large pressure range in the cycle and a poor performance for a reciprocating compressor.

•One way of dealing with such situations is to perform the refrigeration process in stages, that is, to have two or more refrigeration cycles that operate in series.

•Such refrigeration cycles are called cascade refrigeration cycles.

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Cascade Refrigeration SystemsCascade Refrigeration Systems

A two-stage compression refrigeration system with a flash chamber.

A two-stage cascade refrigeration cycle is shown. The two cycles are connected through the heat exchanger in the middle, which serves as the evaporator for the topping cycle and the condenser for the bottoming cycle.

A two-stage cascade refrigeration system with the same refrigerant in both stages.

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Cascade Refrigeration SystemsCascade Refrigeration Systems• Assuming the heat exchanger is well insulated and the

kinetic and potential energies are negligible, the heat transfer from the fluid in the bottoming cycle should be equal to the heat transfer to the fluid in the topping cycle.

• Thus, the ratio of mass flow rates through each cycle should be

• The coefficient of performance of the cascade system is

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11–41 Consider a two-stage cascade refrigeration system operating between pressure limits of 0.8 and 0.14 MPa. Each stage operates on the ideal vapor-compression refrigeration cycle with refrigerant-134a as the working fluid. Heat rejection from the lower cycle to the upper cycle takes place in an adiabatic counter-flow heat exchanger where both streams enter at about 0.4 MPa. If the mass flow rate of the refrigerant through the upper cycle is 0.24 kg/s, determine the: a)mass flow rate of the refrigerant through the lower cycle, b)rate of heat removal from the refrigerated space, c)power input to the compressor, and d)coefficient of performance of this cascade refrigerator.

Answers: (a) 0.195 kg/s, (b) 34.2 kW, 7.63 kW, (c) 4.49

Problem 5 Problem 5 Cascade Refrigeration Systems

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Multistage Compression Refrigeration SystemsMultistage Compression Refrigeration Systems

A two-stage compression refrigeration system with a flash chamber.

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.

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FLASH CHAMBER

• Flash chamber is used in a multi-staging refrigeration system• It separates vapor and liquid refrigerant during the throttling

process• The purpose is to avoid vapor refrigerants from entering

evaporator• The vapor developed during throttling (flash vapor) is bled out of

the throttling device and fed back to the compressor


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• A multistage compression refrigeration system is one example of a system that uses a flash chamber

• It can be carried out with the use of one or more compressors


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• The T-s diagram representing the cycle of a two-stage vapor-compression cycle

1

2

39

4

5

67

8

s

T


39

Two-stage refrigeration cycle represented by the p-h diagram

• The P-h diagram is a more convenient representation of the cycle because it can easily be compared to the plant layout


Flash Chamber

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• 1kg refrigerant moves through condenser• 1kg liquid enters 1st throttle valve• 1kg (mostly liquid) enters flash chamber and

starts to evaporate and becomes mixture of gas (x)kg and liquid (1–x)kg

• (x) moves towards 2nd stage compressor at Pi • (1–x)kg liquid make its way through the 2nd

throttle valve into the evaporator• (1–x)kg vapor enters the 1st stage compressor

where it is compressed to Pi

• At Pi (state 3) (1-x)kg vapor mixes with (x)kg vapor adiabatically and becomes 1kg vapor

• 1kg vapor is compressed in 2nd stage compressor

• 1kg vapor enters condenser to be condensed and becomes 1kg liquid

P

h

1

29

45

67

8

3

Condenser

Evaporator


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ANALYSIS• Fraction of refrigerant which evaporates in the flash chamber can be given as

follows.

• Refrigerating Effect, QL= (1 – x)(h1 – h8)

• Total work input, ∑Win = W12 + W94

= (1 – x)(h2 – h1) + (h4 – h9)

• Heat rejected in condenserQH = (h4 – h5)

i

i

fg

f

hhh

x

6


42

• Coefficient of Performance

9412

81

11

hhhhxhhx

WQCOP

in

LR


43

11–44 A two-stage compression refrigeration system operates with refrigerant-134a between the pressure limits of 1 and 0.14 MPa. The refrigerant leaves the condenser as a saturated liquid and is throttled to a flash chamber operating at 0.5 MPa. The refrigerant leaving the low-pressure compressor at 0.5 MPa is also routed to the flash chamber. The vapor in the flash chamber is then compressed to the condenser pressure by the high-pressure compressor, and the liquid is throttled to the evaporator pressure. Assuming the refrigerant leaves the evaporator as saturated vapor at a rate of 0.25 kg/s and that both compressors are isentropic, determine the: a)fraction of the refrigerant that evaporates in the flash chamber, b)rate of heat removed from the refrigerated space, and c)coefficient of performance.

Problem7 Problem7 Two-Stage Compression Refrigeration Systems

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Absorption Refrigeration SystemsAbsorption Refrigeration Systems

Ammonia absorption refrigeration cycle.

Absorption refrigeration is when there is a source of inexpensive thermal energy at a temperature of 100 to 200°C. Some examples include 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.

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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.

Other systems include water–lithium bromide and water–lithium chloride systems, where water serves as the refrigerant. These systems are limited to applications such as A-C where the minimum temperature is above the freezing point of water.

Compared with vapor-compression systems, ARS have one major advantage: A liquid is compressed instead of a vapor and as a result the work input is very small (on the order of one percent of the heat supplied to the generator) and often neglected in the cycle analysis.

ARS are much more expensive than the vapor-compression refrigeration systems. They are more complex and occupy more space, they are much less efficient thus requiring 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.

ARS are primarily used in large commercial and industrial installations.

47The maximum COP of an absorption refrigeration system.

The COP of actual absorption refri-geration systems is usually less than unity.Air-conditioning systems based on absorption refrigeration, called the absorption chillers, perform best when the heat source can supply heat at a high temperature with little temperature drop.

Chapter 4 - Refrigeration

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