DEPARTMENT OF MECHANICAL AND INDUSTRIAL ENGINEERING
FACULTY OF ENGINEERING AND BUILT ENVIRONMENT
Refrigeration Bench Experiment
SURNAME
: NOPAPAZA
INITIALS
: A.C
STUDENT NO
: 217033248
COURSE
: Refrigeration and Air Conditioning 3B
MODULE
: RACMIB3
DATE
: 2021/11/03
I confirm that this assignment is my work, is not copied from any other person's work, and has
not previously submitted for assessment either at the University of Johannesburg or elsewhere.
Signed: NOPAPAZA AC.
Date: 14 Oct. 20
Table of Contents
Introduction ................................................................................................................................ 2
Comparing Vapour Compression Systems to Air Refrigeration Systems ............................. 3
Advantages of VCRS over Air Refrigeration Systems ................................................. 4
Disadvantages of VCRS over Air Refrigeration Systems ............................................ 4
Classification of Evaporators ................................................................................................. 4
Aim ............................................................................................................................................ 7
Apparatus and Materials ............................................................................................................ 8
Assumptions............................................................................................................................... 8
Observations .............................................................................................................................. 8
Analysis of Results .................................................................................................................... 9
Observation 1 ......................................................................................................................... 9
Properties of R22 at 3.13 bar .......................................................................................... 9
Observation 2 ....................................................................................................................... 11
P-H diagram of the system ................................................................................................... 12
Discussion ................................................................................................................................ 13
Recommendations ................................................................................................................ 14
Conclusion ............................................................................................................................... 14
Bibliography ............................................................................................................................ 15
Appendix .................................................................................................................................. 16
1
Figure 1: Schematic of Vapour Compression Home Refrigeration System [1] ........................ 3
Figure 2: Lab Vapour Refrigeration System .............................................................................. 8
Figure 3: Pressure Enthalpy Diagram for Superheated Vapour at Evaporator Exit. ............... 12
Table 1: Observation table ......................................................................................................... 8
Table 2:Interpolation to find properties at 3.13 bar ................................................................... 9
Table 3:Interpolation to find exact values of point 1 ............................................................... 10
Table 4: Interpolation to find point 2 properties at 5.9bar ....................................................... 10
Table 5: Interpolation to find the exact values at point 2 ........................................................ 10
Table 6: Point 3 values reading ................................................................................................ 11
Table 7:Summary of data points (Observation 1) .................................................................... 11
Table 8: Summary of data points (Observation 2) ................................................................... 12
Figure_AX 1: Observation 1 Calculations ............................................................................... 16
Figure_AX 2:Observation 2 Calculations ................................................................................ 17
2
Introduction
Refrigeration systems are ubiquitous in modern life. They are used in food preservation,
manufacturing and processing. Refrigeration systems are also found in industrial and
commercial applications where they are needed to keep an environment at a constant
temperature. There are different types of refrigeration systems and methods of transferring
heat.
Vapour compression systems use the condensation and vaporisation of substances called
refrigerants that have low boiling points and can be condensed in a wide range of pressures and
temperatures.
Figure 1: Schematic of Vapour Compression Home Refrigeration System [1]
Comparing Vapour Compression Systems to Air Refrigeration Systems
Vapour compression systems are commonly used for all air conditioning and air conditioning
systems. They are said to be an improvement in refrigeration from gas cycle systems. The first
distinction of vapour compression systems from gas cycle refrigeration systems is their
3
working principle. Gas cycle refrigeration systems use sensible heating and cooling to extract
heat and refrigerate the warm region. Vapour compression refrigeration systems use latent heat
to refrigerate the warm space as the refrigerant undergoes phase change during vaporisation.
Some of the other distinctions between the VCRS and air refrigeration systems are discussed
in the form of the advantages and disadvantages VCRS have compared to air refrigeration
systems:
Advantages of VCRS over Air Refrigeration Systems

VCRS have a high refrigeration capacity while requiring small mass flow rates. This
means VCRS can be much smaller than air refrigeration systems for the same output.

Air refrigeration cycles require significant modifications to be made to the system to
obtain maximum output. In contrast, VCRS require less equipment for the improvement
of performance comparable to the air refrigeration systems, hence they have less
operational costs [1].

Vapour compression refrigeration systems have a higher coefficient of performance
than air refrigeration systems. This means that they produce a higher refrigeration effect
with less work input required.

Due to the very low boiling points of the refrigerants and wideband of condensation
temperatures, VCRS can be used in a wide range of temperatures, which is increased
versatility [2].
Disadvantages of VCRS over Air Refrigeration Systems

The substances (Ammonia, Carbon Dioxide, Chlorofluorocarbons) used in VCRS can
be harmful to humans, animals and the environment when out in the atmosphere. VCRS
require a significant amount of attention and resources to prevent leakages [1].

In large industrial applications, the equipment needed to complete the vapour
compression cycle (evaporators, compressors, condensers and expansion systems) can
be an expensive initial cost [1].

As mentioned, the substances used in VCRS are dangerous to the environment and may
be subjected to supply bans and limitations affecting ease of use.
Classification of Evaporators
Evaporators are the devices that are responsible for the refrigerating effect or the removal of
heat from the system to the environment. This is done by maintaining the refrigerant at a
4
temperature that is lower than that of the area intended to be cooled. From the second law of
thermodynamics, the heat is removed from the area and carried by the refrigerant, which
evaporates, hence the naming of the equipment as evaporators. There are different kinds of
evaporators and their classification can be a function of their construction, method of feeding
the refrigerant, the type of heat transfer and their operating conditions. The different types of
evaporators are detailed and summarized as follows:
Construction
1. Bare tube coil evaporator – These are also known as prime-surface evaporators. Bare
tube coil evaporators feature a bare tube that is constructed in a way to allow for
vaporisation of the refrigerant. Critical parameters that affect the performance of a bare
time coil evaporator include the length of the tube, which increases the surface area of
heat transfer, the capacity of the expansion valve – the expansion valve needs to be
suited to the length of the evaporator, the diameter of the tube relative to tube length –
this should be sized to allow a sufficient flow velocity for vaporisation [2].
2. Finned tube evaporator – This is the most common type of heat exchanger used in
the interface between air and the refrigerant. Fins are attached to the tubes carrying the
refrigerant, the effectively increase the contact surfaces for heat transfer. Finned tube
evaporators are used in low-temperature applications slightly above (0°C). Finned tube
evaporators are prone to frost which can clog airflow and affect the performance of the
refrigerator. The length of the tubes is to be controlled to prevent high-pressure drop
across the evaporator [2].
3. Plate evaporator – As it has been established that increasing the heat transfer contact
surface increases the performance of evaporators, plate evaporators achieve this by
having a plate welded to one or both sides of the evaporator tubes [2].
4. Shell and tube evaporator – The construction consists of a shell vessel which has the
liquid that is to be chilled and several horizontal tubes that house the refrigerant. As
the liquid refrigerant is moved through the network of horizontal pipes it exchanges
heat with the warmer water or brine solution that has to be chilled. The water or brine
solution goes through inlet and outlet headers with perforated metal tube sheets. Shell
and tube evaporators can be operated as dry expansion evaporators, with the refrigerant
housed in the tube and as flooded evaporators, with the refrigerant circulating in the
shell, the later increases refrigeration capacity [2].
5
5. Shell and coil evaporators - These are similar to shell and tube evaporators in their
construction and use of cooling water but differ in that they use coils to circulate the
refrigerants [2].
6. Tube-in-tube evaporators – These evaporators have the same working principle as
coaxial heat exchangers, as one tube is nested in another. They provide higher heat
transfer but have the drawback of requiring more space for the same refrigerating
capacity as the other types of evaporators. They are used in petrochemical and
beverages industries [2].
Feeding Mechanism
1. Flooded evaporator – In these refrigeration systems, a surge tank is used to maintain
a constant level of liquid refrigerant in the evaporator. The liquid is constantly
vaporised in the evaporator and the level of the fluid drops, the surge tanks makes up
the drop in the liquid. The refrigerant level in the accumulator drops and is sensed by a
float level which is linked to a float valve that opens and lets in liquid refrigerant from
the receiver until the float level returns to the required level. Higher heat transfer is
achieved through the constant contact of the evaporator coil with the liquid refrigerant.
Flooded evaporators are used in the chemical and food processing industries [2].
2. Dry expansion evaporator – In contrast to flooded refrigeration systems, dry
expansion systems have the flow of liquid refrigerant regulated by the expansion valve.
This configuration allows for the vaporisation of liquid refrigerant to occur with less
space and less refrigerant volume. Dry expansion evaporators are well suited for
compact refrigeration systems. The flow of the refrigerant is in one direction and the
best efficiency is achieved when the liquid and vapour in the coil is separated with the
liquid preferably located at the bottom of the coil and the gas located at the top of the
evaporator coil. The flow regulation of the expansion valve and the diameter of the coil
should be configured to reduce the chances of bubbles developing in the system that
reduce the heat transfer of the system [2].
Heat transfer mode
1. Natural convection evaporators – The flow of air in these types of evaporators is
guided by the natural movement of air. They use the fundamental guiding principle that
warm air rises and cold air descends. An example of this is the high vertical placement
6
of evaporators in domestic refrigeration to allow the cold air to descend on to the
refrigeration chamber [2].
2. Forced convection evaporator – The movement of air over the refrigerant is pushed
through a fan driven by an electric motor. This is a more efficient method of heat
transfer as less surface area is required and load on the compressor can be achieved
through increasing evaporator pressures.
Evaporator operation conditions
This refers to the temperature operating conditions that the evaporator is operating in. As
mentioned, VCRS have a wide band of operating temperatures and they can be outlined as
follows [2]:
1. Frosting evaporator – These evaporators operate below the freezing point of water
(0°C). This caused moisture from the atmosphere to freeze on the surface of the
evaporator and cause frost. This frost has to be removed manually or automatically as
the build-up of frost negatively impacts the performance of the refrigerator.
2. Non-frosting evaporator – They operate above but close to (0°C). Therefore, frosting
does not occur. This application is suited for high-temperature applications where
shrinkage and dilution due to frosting are avoided as in bakery refrigeration.
3. Defrosting evaporator – They use the on and off cycles of evaporators to manage to
frost. During the on-cycle of the compressor, frosting occurs and defrosting take place
during the off-cycle of the compressor. This is done through rapid heat transfer through
the type of evaporator construction [2].
Aim
The experiment aims to familiarize the student with vapour compression systems and the use
of pressure-enthalpy diagrams and property tables to determine the system Coefficient of
Performance (COP)
7
Apparatus and Materials
Figure 2: Lab Vapour Refrigeration System
Assumptions
The following assumptions were made for the experiment:

The system had no leaks.

The evaporator and condenser pressures stayed constant throughout.

There was no undercooling at the condenser exit, i.e. saturated liquid refrigerant.
Observations
The following readings were observed for the vapour compression refrigeration system:
Table 1: Observation table
Description
Evaporator Pressure
Condenser Pressure
Evaporator
Outlet
Temperature
Symbol
Pe
Pc
t1
Units
kPa
kPa
°C
Observation 1
313
590
-3
Observation 2
335
590
-4
8
Analysis of Results
The observation results from the laboratory experiment were used to find the enthalpies,
temperatures and the performance characteristics of the laboratory refrigeration system. The
detailed calculations with detailed explanations of the methodology are attached in the
Appendix. The following was established about the system:

The refrigerant at the evaporator exit is superheated.

The refrigerant at the condenser exit is not supercooled.
The values that were obtained from the laboratory were not present as exact values in the R22
property table. Interpolation was employed to find the properties of the refrigerant at the
various points in the system. A summary of the analysis results is presented. To aid in concise
reading, the detailed steps followed for the second set of results are attached in the Appendix.
Observation 1
Properties of R22 at 3.13 bar
Table 2:Interpolation to find properties at 3.13 bar
Tsat [°C]
Pressure
h[kJ/kg]
s[kJ/kg]
h[kJ/kg]
s[kJ/kg]
[bar]
[ΔTsup = 10K]
[ΔTsup = 10K]
[ΔTsup = 20K]
[ΔTsup = 20K]
-15
2.9570
406.24
1.8009
412.97
1.8255
-13.5
3.13
406.88
1.7985
413.65
1.8201
-10
3.5430
408.41
1.7927
415.26
1.8174
From the temperature at evaporator exit, we see that the vapour is superheated as the
temperature at exit (-3°C) is higher than the saturation temperature (-13.5°C). We then find the
degree of superheat:
9
Δ𝑇 = 𝑡𝑠𝑢𝑝 − 𝑡𝑠𝑎𝑡
= −3 − (−13.5)
= 10.5𝐾
It was observed that the exact values for point 1 lie between 10K and 20K degree of superheat,
therefore, linear interpolation is to be done in between these values:
Table 3:Interpolation to find exact values of point 1
T [°C]
P[bar]
h[kJ/kg]
s[kJ/kgK]
-13.5+10=-3.5
3.13
406.88
1.7985
-13.5+10.5= -3
407.22
1.7996
-13.5+20 = 6.5
413.65
1.8201
𝑘𝐽
𝑘𝐽
Therefore, at evaporator exit, the properties are ℎ1 = 407.22 𝑘𝑔 , 𝑠1 = 1.7996 𝑘𝑔 , 𝑡1 = −3°𝐶.
𝑘𝐽
As we assumed isentropic compression ∴ 𝑠1 = 𝑠2 = 1.7996 𝑘𝑔, we then use this entropy value
to find the temperature of the gas and other properties at compressor pressure (590kPa) through
linear interpolation:
Table 4: Interpolation to find point 2 properties at 5.9bar
T [°C]
P[bar]
s[kJ/kgK]
[Δ𝑇 = 20𝐾]
s[kJ/kgK]
[Δ𝑇 = 30𝐾]
h[kJ/kg]
h[kJ/kg]
[Δ𝑇 = 20𝐾]
[Δ𝑇 = 30𝐾]
5
5.8378
1.7956
1.8200
421.02
429.23
5.32
5.9
1.7952
1.8196
422.02
429.37
10
6.8078
1.7894
1.8137
423.97
431.47
It is seen that point 2 (compressor exit) lies between the 20 and 30K superheat. We again use
interpolation to get the temperature of the refrigerant after compression and enthalpy.
Table 5: Interpolation to find the exact values at point 2
T [°C]
P[bar]
s[kJ/kgK]
h[kJ/kg]
5.32+20=25.32
5.9
1.7952
422.02
27.12
1.7996
423.335
5.32+30 = 35.32
1.8196
429.37
10
Finding point 3, after the condenser where the refrigerant is in a liquid phase. Enthalpies are
found through the interpolation table.
Table 6: Point 3 values reading
T [°C]
5
5.32
10
P[bar]
5.8378
5.9
6.8078
hf [kJ/kg]
205.9
206.28
211.88
The bolded characters in the table are the properties of the saturated liquid at compressor exit.
Since the fluid goes through an expansion valve, the enthalpy remains constant, h3 = h4 = 206.28
kJ/kg.
The table of properties at the various points in the refrigeration system is presented as follows:
Table 7:Summary of data points (Observation 1)
Point
1
2
3
4
Temperature
-3
27.12
5.32
H[kJ/kg]
407.22
423.35
206.28
206.28
S[kJ/kgK]
1.7996
1.7996
The coefficient of performance is then given as:
𝐶𝑂𝑃 =
=
ℎ1 − ℎ4
ℎ2 − ℎ1
407.2 − 206.8
423.35 − 407.22
= 12.43
Observation 2
As previously mentioned, the detailed analysis of the second set of results is outlined in the
Appendix section. A few salient points about this process were summarized below and the table
of results is provided:

At the exit of the evaporator, the degree of superheat was found to be = -4°C – (-11.70
°C) = 7.7K.

At the compressor exit, the exact point 2 lies in between the 10K and 20K degree of
superheat values.
11
Table 8: Summary of data points (Observation 2)
Point
Temperature[°C]
H[kJ/kg]
S[kJ/kgK]
1
-4
406.126
1.7996
2
22.94
420.25
1.7996
3
5.32
206.28
4
206.28
The coefficient of performance is given as:
𝐶𝑂𝑃 =
=
ℎ1 − ℎ4
ℎ2 − ℎ1
406.126 − 206.28
420.25 − 406.126
= 14.15
P-H diagram of the system
Figure 3: Pressure Enthalpy Diagram for Superheated Vapour at Evaporator
Exit.
The pressure-enthalpy of a refrigeration system with superheated vapor at evaporator exit in
Figure 3
12
Discussion
The vapour compression refrigeration system was observed in practice and the parameters of
the system were noted from the different gauges in the apparatus. The following salient points
were noted about the experiment:

The temperatures at evaporator exit revealed some information about the modification
to the basic vapour compression cycle.

The refrigerant was superheated at the exit of the evaporator. This was established
through the calculation of the saturation temperature of the refrigerant at the given
pressure.

For the first set of observations, the degree of superheat from the saturation temperature
(-13.5 °C) was Δ𝑇 = 10.5𝐾 and from the second set of readings, the degree of
superheat from the saturation temperature (-11.70 °C) was Δ𝑇 = 7.7𝐾.

From the table of observations, it was noted that the evaporator pressure was raised.
The effect of this modification is discussed later.

The laboratory technician outlined another parameter that was changed and that is the
rate of cooling. For the first set of readings, the rate of cooling was reported to be 40
and for the second set of readings, the rate of cooling was reported to be 60.

From the analysis of results, it could be seen that the second set of values yielded a
higher coefficient of performance 14.15 compared to 12.43.

The increase in the coefficient of performance could be due to the increase in evaporator
pressure. This agrees with the theory from the literature that the lowering of the
evaporator pressure decreases the performance of the refrigerator.

The effect of the evaporator pressure can be observed in the COP formula. The
compression process 1-2 is the denominator in the COP formula, therefore it is
inversely proportional to COP, i.e. an increase in the work done by the compressor
decreases the performance of the refrigeration system.

The second set of readings has a lower refrigeration capacity but has a lower work input,
hence improving the performance.

Superheating at both the evaporator and compressor exit improves the performance of
the refrigerator, this is however limited by how low the evaporator pressure and
temperature is.
13

The rate of cooling is a function of different parameters namely, volumetric fluid flow,
surface area, thermal conductivity and the temperature difference between the
refrigerant and the environment [3]. The mechanism of increasing the cooling rate in
the refrigerator was not outlined to the group of students.
Recommendations
Only a single recommendation was humbly made by the student upon reflection on the lab:

A walk-around, by the technician, for the different knobs and buttons could be helpful.
Conclusion
In conclusion, the operation of a vapour refrigeration system was appreciated practically and
the parameters that were used in theory calculations were read from the physical machine.
Thus, part of the aim of the experiment was met. Pressure enthalpy (p-h) graphs were used to
gain an understanding of the cycle in the graphs and the changes that come about modifying
different parameters. Property tables and interpolation was used to find the values that were
not present in the standard R22 refrigerant table. The concept of the rate of cooling was
introduced as a performance characteristic and was found to be affected by parameters such as
flow rate and surface area. It was observed that the increase in the evaporator pressure increases
the coefficient of performance (COP) as it decreased the amount of work required from the
compressor. Finally, with all the observations, learnings and discussion about the operating of
a vapour compression system, its comparative nature against air refrigeration systems and the
different types of evaporators, the aim of the experiment was successfully met.
14
Bibliography
[1] J. Tomczyk, E. Silberstein, B. Whitman and B. Johnson, “Refrigeration,” in Refrigeration
and Air Conditioning Technology, Boston, Cengage Learning, 2016, pp. 498-520.
[2] R. Khurmi and J. Gupta, A Textbook of Refrigeration and Air Conditioning (SI Units),
New Dehli: Eurasia Publishing House (P) Ltd. , 2009.
[3] Posthavest Management of Vegetables, “Cooling Rates,” Posthavest Management of
Vegetables, Sydney.
15
Appendix
Figure_AX 1: Observation 1 Calculations
16
Figure_AX 2:Observation 2 Calculations
17