A PROJECT REPORT
ON THE
PERFORMANCE ANALYSIS OF REFRIGERATION CYCLE USING
CO2 (R744) AND OTHER NATURAL REFRIGERANTS
BY
GBADEGESIN, MALIK ADEWALE
AK17/ENG/MAE/024
IN PARTIAL FULFILMENT OF THE REQUIREMENT FOR THE
AWARD OF BACHELOR IN ENGINEERING (B. ENG) IN MARINE
ENGINEERING
FEBRUARY, 2023
A PROJECT REPORT
ON THE
PERFORMANCE ANALYSIS OF REFRIGERATION CYCLE USING
CO2 (R744) AND OTHER NATURAL REFRIGERANTS
BY
GBADEGESIN, MALIK ADEWALE
AK17/ENG/MAE/024
A RESEARCH PROPOSAL SUBMITTED TO THE DEPARTMENT OF
MARINE ENGINEERING, FACULTY OF ENGINEERING, AKWA
IBOM STATE UNIVERSITY, IKOT AKPADEN, MKPAT ENIN LOCAL
GOVERNMENT AREA, AKWA IBOM STATE, NIGERIA
IN PARTIAL FULFILMENT OF THE REQUIREMENT FOR THE
AWARD OF BACHELOR IN ENGINEERING (B. ENG) IN MARINE
ENGINEERING.
SUPERVISOR: PROF. OGBONNAYA EZINNWA
CO-SUPERVISOR: MR. MAURICE JOSEPH
FEBRUARY, 2023
TABLE OF CONTENT
Title Page
Contents
i
ii
List of Figures
List of Tables
iii
iv
CHAPTER ONE: INTRODUCTION
1.0.1 Background of the study
1.0.2 Refrigerants and Refrigerant blends:
1.0.3 Refrigeration cycles
1.0.4 Classification of refrigerants
1.1
Aim of project
1.2
Objective of the Research
1.3
Definition of Problem
1.4
Research Goals
1.5
Scope /Limitation of research
1.6
Contribution to knowledge
1
1
2
3
4
4
4
5
5
5
5
CHAPTER TWO: LITERATURE REVIEW
2.1
History and development of refrigerants
2.2
History of Natural refrigerants
2.3
Application of Natural Refrigerants
2.4
Types of Refrigerants
2.5
Principle of Operation of refrigeration systems
2.6
Designation of Refrigerants
6
6
7
7
7
8
9
CHAPTER THREE: RESEARCH METHODOLOGY
3.1
Thermodynamic cycle and working fluid
11
11
3.2
12
Mathematical formulation
Reference
14
3
LIST OF FIGURES
Figure
Caption
Page
1.1
Classification of refrigeration methods based on type of the variation of
pressure and flow rate in the cycle during steady-state operation
1
1.2
Schematic of refrigeration cycle showing movement of the refrigerant
2
1.3
Thermodynamic cycle of refrigeration system and heat pump
3
2.1
Overview of U.S. Greenhouse Gas Emissions (%) in 2017
7
2.2
Vapor compression refrigeration circuit
10
2.3
Graphical Representation of Monocholoro-Diffluoro-Methane
10
2.4
Numbering logic for refrigerants
11
4
LIST OF TABLES
Figure
Caption
Page
2.1
Classification of refrigerants
8
3.1
Refrigerant characteristics
14
3.2
Cycle conditions
15
5
CHAPTER ONE
1.0
INTRODUCTION
1.0.1 Background of the study
For many centuries ago, air was used as refrigerants in many refrigeration systems because it
was the safest/cheapest refrigerant (Surendra, 2014). Present day mankind depends very
heavily on refrigeration (which can be defined as artificial production of cold) for daily needs.
These cover a wide range of applications such as food processing, preservation and transport,
comfort cooling, commercial and industrial air conditioning, manufacturing, energy
production, health, recreation, etc. The first known machine to produce continuous cold was
invented by the Frenchman Ferdinand Carre in 1859. This was the earliest version of ‘aqua
ammonia’ absorption system. However, commercially successful compression refrigeration
systems working with ammonia were introduced in 1875. Since then, the refrigeration
technology has grown tremendously, influencing almost all aspects of human life
(Venkatarathnam & Srinivasa, 2012).
Figure 1.1: Classification of refrigeration methods based on type of the variation of pressure
and flow rate in the cycle during steady-state operation(Venkatarathnam & Srinivasa, 2012)
The two global agreements to reduce the ozone layer destruction and global warming are the
Montreal and Kyoto protocols. The Kigali amendment (Montreal Protocol) has already set a
6
deadline for the utilization of HFCs due to their high levels of GWP (Global Warming
Potential). The continuous search for an alternative refrigerant with low GWP is still ongoing.
Several well-known natural refrigerants with low GWP have been used over the years (GTZ,
2008).
The R744 refrigerants have good performance in cold climates and it has been seen as good
option due to its operation in a subcritical cycle, even-though many systems are operating in
trans-critical cycles (Bellos, et al., 2019).
The R744 have notable properties which is not harmful to the ozone layer, it has very low
GWP, it is non-toxic, non-flammable, and non-corrosive. Since it is abundant in the
environment, it is relatively less expensive and it can be discarded to the atmosphere without
causing any environmental anomaly. Other properties of the R744 refrigerant is its high
volumetric efficiency and low pressure drop while in heat exchangers or tubulations, and the
remark to transcritical cycles. Since there is no condensation of the fluid in the gas cooler due
to the refrigerant operates in high pressure, every change in temperature, there is a potential
optimum compressor outlet pressure. This also means that, for each operational condition, there
is an optimum compressor outlet pressure that maximizes the Coefficient of Performance
(COP) (Matheus & Robson, 2021).
In hot climates, with temperatures that are around 30°C, it would be almost impossible to have
R744 subcritical cycles, due to its low critical temperature (around 31°C), without using a
cascade cycle, allowing R744 to condense during the heat rejection process. In transcritical
cycles, the output temperature of the heat rejection process will be higher than the critical
temperature, meaning that there will be no liquid on the high-pressure side of the line for the
classical vapor compression cycle, because of that, those cycles are said to have a gas cooler
instead of a condenser.
1.0.2 Refrigerants and Refrigerant blends
Refrigeration is the process of removing heat from a substance and pumping it to the
surroundings. It can also be described as a process of maintaining and reducing the temperature
of a body below the general temperature its surroundings. The working fluids used for carrying
away heat are called the “Refrigerants”.
7
1.0.3 Refrigeration cycles
There are various methods of refrigeration such as the refrigeration by evaporation, ice,
expansion of air, throttling, dry ice and by vapour.
Figure 1.2: Schematic of refrigeration cycle showing movement of the refrigerant (Rusty,
2010)
There are various types of refrigeration cycles used in refrigerating systems;
(a) Vapour compression cycles
The vapour compression cycle is a common method of transferring heat from a low temperature
to a high temperature.
8
Figure 1.3: Thermodynamic cycle of refrigeration system and heat pump
1.0.4 Classification of refrigerants
There are four main groups of refrigerants and they are determined by their chemical
constituents but are generally classified as Chlorofluorocarbons (CFCs), Hydro
chlorofluorocarbons (HCFCs), Hydro fluorocarbons (HFCs) and Natural Refrigerants.
1.1
Aim
To analyze the Performance of a refrigeration cycle using CO2 (R744) blend to other natural
refrigerants.
1.2
Objectives of the Research
There are some typical refrigeration cycles:
a)
Vapor-Compression – VC;
b)
Absorption Refrigeration – AR;
c)
Evaporative Cooling – EC, that can reduce air temperature by 15°C and are best suited
for dry climates;
d)
Thermoelectric Cooling – TEC (Yu et al., 2020) that uses electric current and a
thermocouple, causing a cooling effect by the Peltier Effect.
As typical transcritical VC cycles for R744 as working fluid are not as good as conventional
phase-change VC cycles, an interesting option is to evaluate if those cycles can have COP
improvements when using R744 and low GWP refrigeration fluids blends. Therefore, the main
objective is to investigate R744 blends to other natural refrigerants and identify combinations
that provide performance improvement. Criteria for best performance considers energy and
exergy efficiency (1st and 2nd Laws of Thermodynamics), mass flow rate, operational pressure
and compressor outlet temperature.
9
1.3
Definition of Problem
There are several natural refrigerants used in the refrigerating systems possessing different
properties that could either be harmful or friendly to our environments. Global warming is
considered as the long-term warming of the planet’s overall temperature. The global warming
is the primary causes of climate changes, sea level rise caused by expansion of warmer seas
and melting ice sheets and glaciers. Flooding and extreme weather conditions can be traced to
the global warming. CO2 does not damage the ozone layer.
For a better future, there is an immediate demand to look for clean refrigerants which are ecofriendly. There is need to look for the refrigerant which do not contribute to the ozone layer
depletion and global warming. The use of natural refrigerants like CO2, water, SO2, NH3 and
hydrocarbons such as R290, R600, R600a and blends of hydrocarbon are possible solution to
this problem and are being used efficiently in many systems. Unfortunately, most of them
proved to be toxic and inflammable. Among the natural refrigerants, CO2 (R744) seems to be
the most promising. CO2 has many excellent advantages in engineering application, such as
no toxicity, inflammability, high volumetric capacity (with a possibility to make the system
compact), lower pressure ratio, superior heat transfer properties, complete compatibility with
normal lubricants. Easy availability, lower price and no recycling issues. In this research work,
analysis will be carried out to compare the CO2 refrigerants with other natural refrigerants.
1.4
Research Goals
The following are the goals that are expected to be achieved at the completion of this research
project.
a) Review several literatures on different refrigerants
b) Understand the working principles of the refrigerants in refrigerating systems.
c) Carry out a parametric analysis of the different natural refrigerants.
d) Based on the analysis, propose a viable solution to the effect of this refrigerants.
1.5
Scope /Limitation of research
a) Four natural refrigerants (CO2, NH3, Isopropane, Isobutane) shall be considered in this
research project.
10
b) Open-source software DWSIM shall be used in order to estimate the mixtures
properties of the refrigerants
c) The results obtained shall be used to make recommendations to the refrigerants
industries.
1.6
Contribution
(a) This research project when completed may be used as referendum researchers.
(b) This research project when completed may also be used as teaching/learning aid.
(b) This research carryout to promote greener environment.
11
CHAPTER TWO
2.0
LITERATURE REVIEW
2.1
History and development of refrigerants
The dawn of mechanical refrigeration preceded the possibility of using chemical synthesis to
obtain compounds foreign to nature, and therefore surely the first refrigerants have been natural
fluids. Some of them have completely disappeared, others are still widely applied today, and
some others, although abandoned in the past, are now reconsidered. This happens under the
emergency of the environmental issues created by the use of synthesized refrigerants, that is
the depletion of the stratospheric ozone and the display of the anthropogenic greenhouse effect.
John Hague built the first compression machine that utilizes the caoutchoucine as working
fluid. This refrigerant has disappeared owing it adverse effect on the environment. The
dangerous sulphuric dioxide is a natural fluid also used in the past as a refrigerant, and then
abandoned without any change of reconsideration (Alberto & Claudio, 2020).
Gustave
Lorentzen and his team developed the initial modern CO2 refrigeration technology, then in the
late 80’s and 30 years ago it started entering to the refrigeration sectors (Armin & Angel, 2019).
The US Department of Energy is increasing minimum energy efficiency requirements for air
conditioning and refrigeration system (Department Greenhouse Gas Emissions, 2017).
Figure 2.1: Overview of U.S. Greenhouse Gas Emissions(%) in 2017 (Department
Greenhouse Gas Emissions, 2017)
12
2.2
History of Natural refrigerants
Lorentzen, proposed CO2 as a possible natural refrigerant, a number of studies have been
performed for different types of HP, A/C and refrigeration systems. In 1973, Prof. James
Lovelock discovered Freon to possess high ozone layer depletion potential (ODP) (Calm,
2018). The natural refrigerants like water, methyl chloride, sulphur dioxide, carbon dioxide
and ammonia were used in the beginning for the invention of mechanical refrigeration (Madhu
& Bijan, 2018). Madhu et al, (2018) further suggested that the natural refrigerants like CO2,
NH 3 and hydrocarbons have zero ODP and GWP and are considered to be the long term
replacements to CFCs and HFCs. The natural refrigerants, ammonia, carbon dioxide and
hydrocarbons have a broader range of application (Madhu & Bijan, 2018).
2.3
Application of Natural Refrigerants
Domestic refrigerators and freezers are used throughout the world for food storage in homes
and commercial areas such as offices. Isobutane (HC-600a) and HFC-134a are the dominant
alternative refrigerants for replacing CFC-12 in new domestic refrigerants. The HC-600a uses
historically familiar mineral oil lubricants. HFC-134a uses moisture-sensitive polysolester oils.
The use of the hydrocarbon blends propane (HC-290)/isobutane (HC-600a) allows CFC-12
volumetric capacity. These blends introduce manufacturing complexities and require the use
of charging techniques suitable for refrigerants bends which have components with different
boiling points (Sukumar & Stephan, 2002).
2.4
Types of Refrigerants
The different types of refrigerants can be grouped as given below.
Table 2.1: Classification of refrigerants (ASHRAE, 2008)
Methan Ethan Propan Zeotrop Azeotro
Organic
Inorganic
e
e
e
e
pe
compounds Compoun
Group Grou Group mixture mixtures
ds
p
s
10
100
200
400
500
600 series
700 + mw
series
series
series
series
series
series
As per Numbering Logic
Numbering Convention does not work
13
Series
with
isolated
carbon
>1100
series
As per
numberin
g logic
R11
R123
R404a
R12
R123a
R407c
R22
R507c
R410a
Etc..
Etc..
Etc..
Etc..
Etc..
600
Hydrocarbo
ns
610 oxygen
compounds
620 sulphur
compounds
630
Nitrogen
compounds
R717
ammonia
NH3
R718
water
R744 –
CO2
R729 - Air
R1100s
R1200s
R1234ze
R1234yf
R1270
etc..
Water is the best known refrigerants due to its availability and ease of use. It is safe and less
expensive. The refrigerating capacity of water is 20 times higher than that of R-12 but a
compressor with very large capacity needs to be employed. Carbon dioxide is a non-flammable,
non-toxic, odourless, inexpensive workign fluid with zeoro ODP and zero effective GWP.
Ammonia has good tolerance to normal mineral oils, low sensivity to small amoung of water
in the system but it is unsuitable for domestic, automative and small commercial refrgeration
and heat pump system due to pungent smell and flammability. Hydrocargons (propylene and
blends with propane, butane and isobutane. It is not suitable for the environment but can reduce
energy consumption. (Madhu & Bijan, 2018). The peformance of hydrocarbon refrigerant,
R600a was observed as an alternative for R-12 household auto-defrost refrigerator/freezer
(Kim, Lim, & Chu, 1998).
2.5
Principle of Operation of refrigeration systems
The vapour compression refrigeration cycle has four components: evaporator, compressor,
condenser and expansion valve. The refrigerant enters the compressor as a saturated vapour
and is cooled to the saturated liquid state in the condenser. It is the throttled to the evaporator
pressure and vaporizes as it absorbs heat from the refrigerated space.
comparison for refrigeration cycles is the reversed Carnot cycle.
14
The standard of
Figure 2.2: Vapor compression refrigeration circuit (Selvaraji & Aseem, 2013)
The four process can be summarized as follows:
1-2
Isentropic compression
2-3
Constant pressure heat rejection in the condenser
3-4
Throttle in an expansion valve
4-1
Constant pressure heat addition in the evaporator
2.6
Designation of Refrigerants
The American Society of Refrigerating Engineers (ASRE) developed certain conventions for
use in naming different types of refrigerants. These naming conventions differ according to the
type of refrigerant. Each refrigerant type is denoted by a different series. Thus, we have separate
series for halogenated refrigerants and other types. The naming conventions are simple and
easy to follow. These conventions are now accepted worldwide and help to name the large
variety of refrigerants available commercially nowadays. For the halocarbon Compounds;
according to the above mentioned convention,
No. of C atoms in R22: C – 1 = 0 => C = 1
No. of H atoms in R22: H + 1 = 2 => H = 1
No. of F atoms in R22: F = 2
15
Figure 2:3: Graphical Representation of Monocholoro-Diffluoro-Methane
Numbering logic for a refrigerant can be explained as follows:
R () 1 3 4 a
Isomer
No. fluorine atom per molecules
No. of hydrogen atoms + 1 per molecule
No of carbon atoms -1 per mole (left off when 0)
No. of unsaturated carbon bonds (left off when 0)
Figure 2.4: Numbering logic for refrigerants (Selvaraji & Aseem, 2013)
16
CHAPTER THREE
3.0
METHODOLOGY
3.1
Thermodynamic cycle and working fluid
There are several types of thermodynamic cycles and working fluid (refrigerants) that are
utilized in refrigeration systems. The thermodynamic cycles include; Vapor-Compression
(VC), Absorption Refrigeration (AR), Carnot cycle (CC), Thermoelectric Cooling (TEC),
Cascading cycle, etc. For the purpose of this research, the Vapour Compression Cycle will be
utilized. The working fluid (refrigerant) to be considered in this research are the natural
refrigerants which includes CO2 (R744), NH3 (R717), Isobutane(R600a) and Propane(R290).
The performance of these natural refrigerants in terms of their energy and exergy will be
analyzed.
Properties of carbon dioxide (R774)
R744 refrigerant has its properties different from other refrigerants. Before now, the most
widely used refrigerant was the R22. Table 2.1 shows an overview of the different properties
of the refrigerant under consideration.
Table 2.1: Characteristics of some natural refrigerant for vapour compression cycle
(Tauseef, et al., 2022)
Properties
R744
R717
Ozone depletion potential
0
0
0
Global warming potential
1
0
3
Atmospheric life in years
N/A
N/A
N/A
Density ratio - liquid to gas - at 00C
9.5
N/A
N/A
Critical pressure [kPa]
7377
1142
4250
Critical temperature [◦C]
30.98
113
96.7
Volumetric refrigerant capacity [0C]
22545
4382
3907
Maximum hot water temperature[0C]
80-100
Phase out date
N/A
17
R600a
R290
Even though the R744 is a greenhouse gas, it has the lowest Global Warming Potential (GWP)
of all the refrigerants that are currently in use. The use of R744 according to Lorentzen, (1995)
has the following benefits:

Availability

Generally affordable

No special lubricants or material required

Low global warming potential

No ozone depletion potential

Reduced pressure ration leading to smaller pressure differences over the compressors
Irrespective of the numerous advantages of R744, one notable disadvantage is its higher
operating pressure. The operational pressure of R744 could rise up to 10 times higher than the
other refrigerants currently in use (Kim, et al., 2004).
Vapour Compression Cycle
The conventional refrigeration cycle used in domestic refrigeration system, food processing,
freezers, industrial refrigeration system, transport refrigeration and electronic cooling, etc
(Tamil, et al., 2019). The cycle relies on the condensation temperature with increase in
pressure. A vapour from the evaporator when compressed will condensed at a higher
temperature, corresponding to the new higher pressure. Changes in pressure exist between the
evaporation and the condenser (Hundy, et al., 2016). Understanding the performance of the
system is of essence to improve the refrigerating effect or reduce the power consumption for
the same refrigerating effect.
Heat flows naturally from hot to colder body but it is opposite in refrigeration systems. The
refrigerant used absorbs heat and hence evaporates at a low pressure to form a gas. This gas is
then compressed to a higher pressure, such that it transfers the heat it has gained to ambient air
or water and condenses into a liquid. Thus, heat is absorbed, or removed, from a low
temperature source and transferred to a higher temperature source. The refrigerating cycle can
be broken down into the following steps as illustrated in Figure 3.3, 3.4 and 3.5
18
Figure 3.3: Thermodynamic cycle of a vapour compression refrigeration system
(Tauseef, et al., 2022)
From Figure 3.3, the saturated vapour enters the compressors where it pressure is raised. A big
increase in temperature will be experienced because a proportion of the energy input into the
compression process is transferred to the refrigerant 2-3, the high pressure superheated vapour
passes from the compressor into the condenser. There will be decrease in temperature due to
condensation process. The cooling for this process is usually achieved by using air. After
condensation, refrigerant enters the expansion valve 3’-3, shell and coil heat exchanger is
installed between the host refrigeration system compression and condenser. Water is circulated
through one side of heat exchanger and hot refrigerant gas from the compressor is routed
through the other side. Heat is transferred from the hot refrigerant gas to the water thus
refrigerating effect increases and power consumption or work input decreases. Thus,
performance of cycle is improved. Along with this waste heat also recovered. 3-4, the highpressure liquid refrigerant passes through the expansion device, which both reduces its pressure
and controls the flow into the evaporator. 4-1, Low pressure liquid refrigerant in the evaporator
absorbs heat from its surroundings. During this process it changes its state from a liquid to a
gas, and at the evaporator exit is slightly superheated.
19
Figure 3.4: P-V diagram of Vapour compression refrigeration system (Tauseef, et al., 2022)
Figure 3.5: T-S diagram of vapour compression refrigeration system (Tauseef, et al., 2022)
20
In summary, the cycle shown in the Figure 3.3, the processes occur respectively as follows.
1-2 Adiabatic compression (in compressor)
2-3 Condensation at constant pressure and temperature (in condenser)
3-4 Pressure decrease in constant enthalpy (in expansion valve)
4-1 Evaporation at constant temperature and pressure (in evaporator).
The cycle (Figure 3.3) for all the chosen refrigerants were investigated for several operational
conditions. First, the evaporator inlet temperature (T3) will be evaluated from -30°C up to 0°C,
with a 10°C step. The gas cooler outlet temperature (T2) will be fixed on 35°C. For each one
of T3 values, the four (4) refrigeration fluids were considered. The Coefficient of Performance
(COP) for each of the gases were also investigated.
Other assumptions regarding the simplifications of the thermodynamic cycle:
a)
Steady state operation;
b)
No pressure drops in the gas cooler, evaporator or heat exchanger;
c)
Isentropic efficiency of the compressor: 85%;
d)
Heat transfer efficiency on the internal heat exchanger: 75%. This value is used to find
the actual heat transfer, multiplying it by the estimate of the maximum heat transfer,
calculated by checking the inlet streams properties (Wagner, 2021). In the case of the
heat transfer effectiveness, a temperature difference ratio would be the calculation
method;
e)
Ideal throttling valve, inlet enthalpy is equal to outlet enthalpy;
f)
In order to determine the mass flow rate, the refrigeration capacity was set as 1 kW to
all simulations.
The model shall be developed using figure 3.3. The cycle configuration shall be analysed on
the classic four (4) stage refrigeration cycle as shown in figure 3.3. The criteria for
thermodynamic 2nd law or exergy efficiency will be applied taking into consideration the
diagram in figure 3.6, considering the three exergy transportation forms: mass flow rate, work
(mechanical power), and heat (thermal power), by convention they’re positive on the direction
displayed below.
21
Figure 3.6: Exergy analysis of vapour compression cycle (Matheus & Robson, 2021)
3.2
Mathematical formulation
All the equations for the energy and exergy analysis are presented below. The combination of
first law and second law of thermodynamics is used in the exergy analysis of the Vapour
Compression Refrigeration System (VCRS) and this analysis is important for the design,
optimization and performance assessment of the systems. The exergy of the VCRS is the
maximum work obtained if the system is permitted to reach equilibrium with the environment.
The exergy analysis of the VCRS predicts inefficiencies in the system and the amount of exergy
destroyed within each component of the VCRS. The second law of thermodynamics is
employed to carry out the exergy analysis of the different components in the system. This
analysis defines all loses in the components of the system and also the overall system and
determines the maximum performance of the system.
To estimate the performance characteristics of the natural refrigerants considered in this
research project, seven parameters was evaluated which included Coefficient of Performance
(COP), Cooling capacity, discharge temperature, exergy destruction rates of all the
components, relative exergy destruction of all the components, second-law efficiency of each
component and efficiency defect of each component.
22
Evaporator Unit:
The evaporator is made up of coils of pipe which transports the refrigerants at low temperature
and pressure. The refrigerants at the stage absorbs the latent heat of evaporation required from
the medium to be cooled.
Figure 3.7: Schematics diagram of an evaporator coil
The amount of cooling produced in the evaporator is called cooling capacity and is given by
equation 3.5.
𝑄𝑒𝑣𝑎𝑝 = 𝑚̇𝑅
𝑄𝑒𝑣𝑎𝑝 = 𝑚(ℎ1 − ℎ5 )
3.5
Where:
𝑄𝑒𝑣𝑎𝑝 is the cooling capacity of the refrigerants
𝑅 is the refrigeration effect, which is equal to the heat transferred at the evaporator per kilogram
of refrigerant.
ℎ1 is the specific enthalpy at the exit of the evaporator (kJ/kg)
ℎ5 is the specific enthalpy at the inlet of the evaporator (kJ/kg)
𝑚 is the mass flow rate of refrigerant (kg/s)
23
Compressor Unit:
It is a vital component in a refrigeration system which is responsible for maintaining the
evaporator pressure corresponding to the requirement of low temperature. It conveys the
refrigerants from the evaporator to the condenser. The power input of the compressor or the
work done by the compressor in compressing the refrigerants from low to high pressure is given
by equation 3.6
𝑊̇𝑐 = 𝑚̇𝑊
𝑊̇𝑐 = 𝑚̇(ℎ2 − ℎ1 )
3.6
Where:
𝑊̇𝑐 is the work done by the compressor on the refrigerants
ℎ2 is the specific enthalpy at the outlet of the compressor (kJ/kg)
Condenser Unit:
The condenser unit consists of a condenser which heat rejects heat absorbed from the
compressor to the surrounding, resulting in the condensation of the refrigerants. The heat
transfer rate at the condenser 𝑄𝑐𝑜𝑛𝑑 , is given by equation 3.7.
𝑄𝑐𝑜𝑛𝑑 = 𝑚(ℎ3 − ℎ4 )
3.7
ℎ3 is the specific enthalpy at the inlet of the condenser (kJ/kg)
ℎ4 is the specific enthalpy at the outlet of the condenser (kJ/kg)
Expansion Device
The expansion device or throttling device regulates the flow of the refrigerants back into the
evaporator at a reduced pressure and temperature. This is also an isenthalpic expansion process
𝑚̇ℎ4 = 𝑚̇ℎ5
3.8
ℎ5 is the specific enthalpy at the outlet of the expansion device (kJ/kg)
The ratio of the cooling capacity of the evaporator to the input power of the compressor is
known as the coefficient of performance (COP) is given in equation 3.7
24
𝐶𝑂𝑃 =
𝑄𝑒𝑣𝑎𝑝
𝑊̇𝑐
=
𝑚(ℎ1 − ℎ4 )
𝑚(ℎ2 − ℎ1 )
=
(ℎ1 − ℎ4 )
3.7
(ℎ2 − ℎ1 )
Where:
𝐶𝑂𝑃 is the Coefficient of performance
Exergy Analysis
The concept of exergy analysis is considered an effect tool for the conversation of mass and
energy principles also with the second law for the design and analysis of vapour compression
refrigeration system. It is a powerful tool for making system and process more efficient and a
key tool for determining the locations, types and true magnitude of wastes and loss (Rahul
ukey, 2012).
General exergy balance with respect to time is expressed as given in equation
The exergy destruction in the compressor, condenser, expansion valve, and evaporator and the
total exergy destruction are expressed by equations
COMPRESSOR:
𝐸𝑥,𝑖𝑛 − 𝐸𝑥,𝑜𝑢𝑡 − 𝐸𝑥,𝑑𝑒𝑠𝑡,1−2 = 0
𝐸𝑥,𝑑𝑒𝑠𝑡,1−2 = 𝐸𝑥,𝑖𝑛 − 𝐸𝑥,𝑜𝑢𝑡
𝐸𝑥,𝑑𝑒𝑠𝑡,1−2 = 𝑊 + 𝐸𝑥,1 − 𝐸𝑥,2
𝐸𝑥,𝑑𝑒𝑠𝑡,1−2 = 𝑊 − ∆𝐸𝑥,12
= 𝑊 − [ℎ2 − ℎ1 − 𝑇0 (𝑆2 − 𝑆1 )]
= 𝑊 − 𝑊𝑟𝑒𝑣
𝐸𝑥,𝑑𝑒𝑠𝑡,1−2 = 𝑇0 𝑆𝑔𝑒𝑛,1−2 = 𝑚𝑇0 (𝑆2 − 𝑆1 )
𝜂𝑒𝑥,𝑐𝑜𝑚𝑝 =
𝑊𝑟𝑒𝑣
𝐸𝑥,𝑑𝑒𝑠𝑡,1−2
=1−
𝑊
𝑊
CONDENSER:
𝐸𝑥,𝑑𝑒𝑠𝑡,3−4 = 𝐸𝑥,𝑖𝑛 − 𝐸𝑥,𝑜𝑢𝑡
𝐸𝑥,𝑑𝑒𝑠𝑡,3−4 = (𝐸𝑥3 − 𝐸𝑥,4 ) − 𝐸𝑥,𝑄𝐻
= 𝑚[ℎ3 − ℎ4 − 𝑇0 (𝑆3 − 𝑆4 ) − 𝑄𝐻 (1 −
𝑇0
)
𝑇𝐻
25
𝐸𝑥,𝑑𝑒𝑠𝑡,3−4 = 𝑇0 𝑆𝑔𝑒𝑛,3−4 = 𝑚𝑇0 (𝑆2 − 𝑆1 +
𝜂𝑒𝑥,𝑐𝑜𝑛𝑑
𝑄𝐻
)
𝑇𝐻
𝑇
𝑄𝐻 (1 − 𝑇0 )
𝐸𝑥,𝑄𝐻
𝐻
=
=
𝐸𝑥,3 − 𝐸𝑥,4
𝑚[ℎ3 − ℎ4 − 𝑇0 (𝑆3 − 𝑆4 )]
𝜂𝑒𝑥,𝑐𝑜𝑛𝑑 = 1 −
𝐸𝑥,𝑑𝑒𝑠𝑡,1−2
𝐸𝑥,3 − 𝐸𝑥,4
EXPANSION VALVE
𝐸𝑥,𝑑𝑒𝑠𝑡,4−5 = 𝐸𝑥,𝑖𝑛 − 𝐸𝑥,𝑜𝑢𝑡
𝐸𝑥,𝑑𝑒𝑠𝑡,1−2 = 𝐸𝑥,4 − 𝐸𝑥,5
= 𝑚[ℎ4 − ℎ5 − 𝑇0 (𝑆4 − 𝑆5 )]
𝐸𝑥,𝑑𝑒𝑠𝑡,4−5 = 𝑇0 𝑆𝑔𝑒𝑛,4−5 = 𝑚𝑇0 (𝑆4 − 𝑆5 )
𝜂𝑒𝑥,𝑒𝑥𝑝𝑉𝑎𝑙𝑣𝑒 = 1 −
𝐸𝑥,𝑑𝑒𝑠𝑡,4−5
𝐸𝑥,4 − 𝐸𝑥,5
= 1−
𝐸𝑥,4 − 𝐸𝑥,5
𝐸𝑥,4 − 𝐸𝑥,5
EVAPORATOR:
𝐸𝑥,𝑑𝑒𝑠𝑡,5−1 = 𝐸𝑥,𝑖𝑛 − 𝐸𝑥,𝑜𝑢𝑡
𝐸𝑥,𝑑𝑒𝑠𝑡,5−1 = − 𝐸𝑥,𝑄𝐿 + 𝐸𝑥,5 − 𝐸𝑥,1
𝐸𝑥,𝑑𝑒𝑠𝑡,5−1 = (𝐸𝑥,5 − 𝐸𝑥,1 ) + 𝐸𝑥,𝑄𝐿
= 𝑚[ℎ5 − ℎ1 − 𝑇0 (𝑆5 − 𝑆1 ) − [− 𝑄𝐿 (1 −
𝑇0
)]
𝑇𝐿
𝐸𝑥,𝑑𝑒𝑠𝑡,5−1 = 𝑇0 𝑆𝑔𝑒𝑛,5−1 = 𝑚𝑇0 (𝑆1 − 𝑆5 +
𝑄𝐿
)
𝑇𝐿
𝜂𝑒𝑥,𝑐𝑜𝑛𝑑
𝑇
−𝑄𝐿 (1 − 𝑇0 )
𝐸𝑥,𝑄𝐿
𝐿
=
=
𝐸𝑥,1 − 𝐸𝑥,5
𝑚[ℎ1 − ℎ5 − 𝑇0 (𝑆1 − 𝑆5 )]
𝜂𝑒𝑥,𝑐𝑜𝑛𝑑 = 1 −
𝐸𝑥,𝑑𝑒𝑠𝑡,5−1
𝐸𝑥,1 − 𝐸𝑥,5
The total exergy destruction in the cycle can be determined by adding exergy destruction in
each component:
26
𝐸𝑑𝑒𝑠𝑡,𝑡𝑜𝑡𝑎𝑙 = 𝐸𝑑𝑒𝑠𝑡,1−2 + 𝐸𝑑𝑒𝑠𝑡,3−4 + 𝐸𝑑𝑒𝑠𝑡,4−5 + 𝐸𝑥,𝑑𝑒𝑠𝑡,5−1
The total exergy destruction in the cycle can also be expressed as the difference between the
exergy supplied (power input) and the exergy recovered (the exergy of the heat transferred
from the low temperature medium).
𝐸𝑑𝑒𝑠𝑡,𝑡𝑜𝑡𝑎𝑙 = 𝑊 − 𝐸𝑥,𝑄𝐿
Where the exergy of the heat transferred from the low temperature medium is given by equation
3.9
𝐸𝑥,𝑄𝐿 = −𝑄𝐿 (1 −
𝑇0
)
𝑇𝐿
The minus sign is needed to make the result positive. Note that the exergy of the heat transferred
from the low temperature medium is in fact the minimum power input to accomplish the
required refrigeration load, 𝑄𝐿 :
𝑊𝑚𝑖𝑛 = 𝐸𝑥,𝑄𝐿
The second law efficiency (or exergy efficiency) of the cycle is defined as;
𝐸𝑥,𝑄𝐿
𝑊
𝐸𝑑𝑒𝑠𝑡,𝑡𝑜𝑡𝑎𝑙
𝜂𝐼𝐼 = 1 −
𝑊
𝜂𝐼𝐼 =
27
CHAPTER FOUR
RESULTS AND DISCUSSION
28
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