Refrigeration Cycle

Reading Problems
11.8 → 11.13 11.85, 11.86, 11.89, 11.95

Definitions

• refrigeration cycles may be classified as

– vapour compression
– gas compression

• refrigerators and heat pumps have a great deal in common. The primary difference is in the
manner in which heat is utilized.

– Refrigerator ↓C → H
|{z}
| {z }
takes heat f rom transf ers to

– Heat Pump C
|{z} → H ↑
| {z }
takes heat f rom transf ers to

• the Carnot cycle can serve as the initial model of the ideal refrigeration cycle

QL = TL (s3 − s2 )

QH = TH (s4 − s1 )

Win = Qnet = QH − QL

= (TH − TL )(s3 − s2 )

The coefficient of performance (β) is given by

benef it
β=
cost

1
 TL
βref rig =
TH − TL

TH
βheat pump =
TH − TL

Vapour Compression Refrigeration Cycle

Room Air

QH
superheated
sat. liquid
Condenser vapour

compressor
Expansion
Valve h4 = h 3 gas

Evaporator
2 phase sat. vapour
QL

Food

2
Refrigeration Process

Process Description

1-2s: A reversible, adiabatic (isentropic) compression of the refrigerant.

The saturated vapour at state 1 is superheated to state 2.
⇒ wc = h2s − h1

2s-3: An internally, reversible, constant pressure heat rejection

in which the working substance is desuperheated and then condensed
to a saturated liquid at 3. During his process, the working substance
rejects most of its energy to the condenser cooling water.
⇒ qH = h2s − h3

3-4 An irreversible throttling process in which the temperature and

pressure decrease at constant enthalpy.
⇒ h3 = h4

4-1 An internally, reversible, constant pressure heat interaction

in which the working fluid is evaporated to a saturated vapour
at state point 1. The latent enthalpy necessary for evaporation
is supplied by the refrigerated space surrounding the evaporator.
The amount of heat transferred to the working fluid in the evaporator
is called the refrigeration load.
⇒ qL = h1 − h4

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The thermal efficiency of the cycle can be calculated as

qevap h1 − h4
η= =
wcomp h2s − h1

Common Refrigerants
There are several fluorocarbon refrigerants that have been developed for use in VCRC.

R11

R12 CCl2 F2 dichlorodifluoromethane

- used for refrigeration systems at higher
temperature levels
- typically, water chillers and air conditioning

R22 CHClF2 has less chlorine, a little better for the

environment than R12
- used for lower temperature applications

R134a CF H2 CF 3 tetrafluorethane - no chlorine

- went into production in 1991
- replacement for R12

R141b C2 H3 F Cl2 dichlorofluoroethane

Ammonia N H3 corrosive and toxic

- used in absorption systems

R744 CO2 behaves in the supercritical region

- low efficiency

R290 propane combustible

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How to Choose a Refrigerant
Many factors need to be considered

• ozone depletion potential

• global warming potential
• combustibility
• thermal factors

Ozone Depletion Potential

• chlorinated and brominated refrigerants act as catalysts to destroy ozone molecules

• reduces the natural shielding effect from incoming ultra violet B radiation

Global Warming Potential

• gases that absorb infrared energy

• gases with a high number of carbon-fluorine bonds
• generally have a long atmospheric lifetime

Combustibility

• all hydro-carbon fuels, such as propane

Thermal Factors

• the heat of vaporization of the refrigerant should be high. The higher hf g , the greater the
refrigerating effect per kg of fluid circulated

• the specific heat of the refrigerant should be low. The lower the specific heat, the less heat
it will pick up for a given change in temperature during the throttling or in flow through the
piping, and consequently the greater the refrigerating effect per kg of refrigerant

• the specific volume of the refrigerant should be low to minimize the work required per kg
of refrigerant circulated

• since evaporation and condenser temperatures are fixed by the temperatures of the surround-
ings - selection is based on operating pressures in the evaporator and the condenser

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 Designation Chemical Ozone Depletion Global Warming
Formula Potential1 Potential2
Ozone Depleting & Global Warming Chemicals
CFC-11 CCl3 F 1 3,400
CFC-12 CCl2 F2 0.89 7,100
CFC-13 CClF3 13,000
CFC-113 C2 F3 Cl3 0.81 4,500
CFC-114 C2 F4 Cl2 0.69 7,000
CFC-115 C2 F5 Cl1 0.32 7,000
Halon-1211 CF2 ClBr 2.2-3.5
Halon-1301 CF3 Br 8-16 4,900
Halon-2402 C2 F4 Br2 5-6.2
carbon tetrachloride CCl4 1.13 1,300
methyl chloroform CH3 CCl3 0.14
nitrous oxide N2 O 270
Ozone Depleting & Global Warming Chemicals - Class 2
HCFC-22 CHF2 Cl 0.048 1,600
HCFC-123 C2 HF3 Cl2 0.017 90
HCFC-124 C2 HF4 Cl 0.019 440
HCFC-125 C2 HF5 0.000 3,400
HCFC-141b C2 H3 F Cl2 0.090 580
HCFC-142b C2 H3 F2 Cl 0.054 1800
Global Warming, non-Ozone Depleting Chemicals
carbon dioxide CO2 0 1
methane CH4 0 11
HFC-125 CHF2 CF3 0 90
HFC-134a CF H2 CF3 0 1,000
HFC-152a CH3 CHF2 0 2,400
perfluorobutane C4 F10 0 5,500
perfluoropentane C5 F12 0 5,500
perfluorohexane C6 F14 0 5,100
perfluorotributylamine N (C4 F9 )3 0 4,300

1 - relative to R11
2 - relative to CO2

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Cascade Refrigeration System

• two or more vapour compression refrigeration cycles are combined

• used where a very wide range of temperature between TL and TH is required

• the condenser for the low temperature refrigerator is used as the evaporator for the high
temperature refrigerator

Advantages

• the refrigerants can be selected to have reasonable evaporator and condenser pressures in the
two or more temperature ranges

QL (↑)
β= overall(↑)
Wnet (↓)

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Absorption Refrigeration System
Differences between an absorption refrigeration system and a VCRC

VCRC Absorption RS
• vapour is compressed • the refrigerant is absorbed by
between the evaporator and an absorbent material to form a
the condenser liquid solution
• process is driven by work • heat is added to the process
to retrieve the refrigerant vapour
from the liquid solution
• process is driven by heat

Common Refrigerant/Absorber Combinations

Refrigerant Absorber

1. ammonia water

2. water lithium bromide

lithium chloride

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Process

Room Air Source of

Heat

liquid
QH Q*H
ammonia
ammonia vapour only
Condenser Generator

weak ammonia cold

Expansion water solution
Valve regenerator

Evaporator Absorber
2 phase dry vapour
QL
Q*L
Food strong ammonia pump
cold water solution
sink

• the compressor is replaced by an absorber, pump, generator, regenerator and a valve

• in the absorber, ammonia vapour is absorbed by liquid water

– the process is exothermic (gives off heat)

– ammonia vapour is absorbed into the water at low T and P maintained by means of
Q∗L
– absorption is proportional to 1/T ⇒ the cooler the better

• in the generator, ammonia is driven out of the solution by the addition of Q∗H ,
(endothermic reaction)

• a regenerator is used to recoup some of the energy from the weak ammonia water solution
passed back to the absorber. This energy is transferred to the solution pumped to the genera-
tor. This reduces the Q∗H required to vapourize the solution in the generator. It also reduces
the amount of Q∗L that needs to be removed from the solution in the absorber.

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PROBLEM STATEMENT:

A computer facility in the Sahara Desert [T0 = 40◦ C] is to be maintained at 15◦ C by a vapour-
compression refrigeration system that uses water as the refrigerant. The water leaves the evaporator
as a saturated vapour at 10◦ C. The compressor is reversible and adiabatic. The pressure in the
condenser is 0.01 M P a and the water is saturated liquid as it leaves the condenser.

i) determine the coefficient of performance for the cycle

ii) determine the second law efficiency of the system
iii) briefly explain what factors in this system lead to the destruction of exergy?

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