EN-202

Fall 2024
INNOVATIVE VAPOR-COMPRESSION
REFRIGERATION SYSTEMS

Dr. Farrukh Khalid

School of Energy Science and Engineering
IITG
 Objectives
• 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.
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REFRIGERATORS AND
HEAT PUMPS
The transfer of heat from a low-temperature
region to a high-temperature one requires
special devices called refrigerators.
Another device that transfers heat from a
low-temperature medium to a high-
temperature one is the heat pump.
Refrigerators and heat pumps are essentially
the same devices; they differ in their
objectives only.

for fixed values

of QL and QH
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THE REVERSED CARNOT CYCLE
The reversed Carnot cycle is the most efficient refrig. cycle operating between TL and TH.
It is not a suitable model for refrigeration cycles since processes 2-3 and 4-1 are not practical
because Process 2-3 involves the compression of a liquid–vapor mixture, which requires a
compressor that will handle two phases, and process 4-1 involves the expansion of high-
moisture-content refrigerant in a turbine.

Both COPs increase as the

difference between the two
temperatures decreases,
that is, as TL rises or TH
falls.

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THE IDEAL VAPOR-COMPRESSION
REFRIGERATION CYCLE
The vapor-compression refrigeration cycle is the
ideal model for refrigeration systems. Unlike the
reversed Carnot cycle, the refrigerant is vaporized
completely before it is compressed and the turbine is
replaced with a throttling device.

This is the most

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

Schematic and T-s

diagram for the ideal
vapor-compression
refrigeration cycle. 5
The ideal vapor-compression refrigeration cycle involves an irreversible (throttling)
process to make it a more realistic model for the actual systems.
Replacing the expansion valve by a turbine is not practical since the added
benefits cannot justify the added cost and complexity.
Steady-flow
energy balance

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 ACTUAL VAPOR-COMPRESSION
REFRIGERATION CYCLE
An actual vapor-compression refrigeration cycle differs
from the ideal one owing mostly to the irreversibilities
that occur in various components, mainly due to fluid
friction (causes pressure drops) and heat transfer to or
from the surroundings.
DIFFERENCES
Non-isentropic compression
Superheated vapor at evaporator exit
Subcooled liquid at condenser exit
Pressure drops in condenser and evaporator

The COP
decreases as a
result of
irreversibilities.

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SECOND-LAW ANALYSIS OF VAPOR-
COMPRESSION REFRIGERATION CYCLE
The maximum COP of a refrigeration cycle operating
between temperature limits of TL and TH

Actual refrigeration cycles are not as efficient as ideal ones like the Carnot cycle
because of the irreversibilities involved. But the conclusion we can draw from Eq.
11–9 that the COP is inversely proportional to the temperature difference TH - TL
is equally valid for actual refrigeration cycles.
The goal of a second-law or exergy analysis of a refrigeration system is to
determine the components that can benefit the most by improvements.
This is done identifying the locations of greatest exergy destruction and the
components with the lowest exergy or second-law efficiency.
Exergy destruction in a component can be determined directly from an exergy
balance or by using

8
Note that when TH = T0, which
is often the case for
refrigerators, II,cond = 0 since
there is no recoverable exergy
in this case. 9
 The exergy rate associated
with the withdrawal of heat
from the low-temperature
medium at TL at a rate of QL
This is equivalent to the power that can be
produced by a Carnot heat engine receiving heat
from the environment at T0 and rejecting heat to
the low temperature medium at TL at a rate of QL.

Note that when TL = T0, which is often the case for heat pumps,
II,evap = 0 since there is no recoverable exergy in this case.
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 Total exergy
destruction

Second-law (exergy) efficiency

T0 = TH for a
refrigeration cycle

This second-law efficiency definition accounts for all

irreversibilities associated within the refrigerator, including the
heat transfers with the refrigerated space and the environment. 11
 SELECTING 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 (replaced R-12, which damages ozone layer) 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. Fully halogenated CFCs (such as R-11, R-12, and R-115) do the
most damage to the ozone layer. Refrigerants that are friendly to the ozone layer
have been developed.
• Two important parameters that need to be considered in the selection of a
refrigerant are the temperatures of the two media (the refrigerated space and the
environment) with which the refrigerant exchanges heat. 12
 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.

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14
 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

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Cascade Some industrial applications require moderately low temperatures, and the
Refrigeration temperature range they involve may be too large for a single vapor-
Systems 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.

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Multistage Compression When the fluid used throughout the cascade
Refrigeration Systems 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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Multipurpose Refrigeration Systems with a 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.

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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.

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.

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GAS REFRIGERATION CYCLES
The reversed Brayton cycle (the gas refrigeration
cycle) can be used for refrigeration.

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The gas refrigeration cycles have lower COPs relative to the vapor-
compression refrigeration cycles or the reversed Carnot cycle.
The reversed Carnot cycle consumes a fraction of the net work (area 1A3B)
but produces a greater amount of refrigeration (triangular area under B1).

Despite their relatively low COPs, the gas

refrigeration cycles involve simple, lighter
components, which make them suitable for
aircraft cooling, and they can incorporate
regeneration 21
Without regeneration, the lowest turbine inlet temperature is T0, the
temperature of the surroundings or any other cooling medium.
With regeneration, the high-pressure gas is further cooled to T4 before
expanding in the turbine.
Lowering the turbine inlet temperature automatically lowers the turbine
exit temperature, which is the minimum temperature in the cycle.
Extremely low temperatures can be achieved
by repeating regeneration process.

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ABSORPTION REFRIGERATION SYSTEMS
Absorption
refrigeration is
economic 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 often classified as heat-driven systems.
• 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.
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The COP of actual absorption refrigeration
systems is usually less than 1.
Air-conditioning systems based on
absorption refrigeration, called absorption
chillers, perform best when the heat source
can supply heat at a high temperature with
little temperature drop.

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 Summary
• Refrigerators and Heat Pumps
• The Reversed Carnot Cycle
• The Ideal Vapor-Compression Refrigeration Cycle
• Actual Vapor-Compression Refrigeration Cycle
• Second-law Analysis of Vapor-Compression
Refrigeration Cycle
• Selecting the Right Refrigerant
• Heat Pump Systems
• Innovative Vapor-Compression Refrigeration
Systems
• Gas Refrigeration Cycles
• Absorption Refrigeration Systems
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