Comparing the Coefficient of Performance
for Varying Flowrates of Refrigerant
through the Refrigeration System
The process of refrigeration is an essential process in the industry. This process is
accomplished by the use of a working fluid and mechanical energy to facilitate the transfer of heat
from cold region to a hotter region. The process is affected by multiple parameters but in this paper
of interest is the effect of the compressor setting and the refrigerant flow on the coefficient of
performance (COP) of the refrigeration cycle. For this experiment the relationship between compressor
speed and coefficient performance of the Armfield RA-1 MKII was determined. The performance of a
refrigeration equipment can be determined using the coefficient of performance that relates the amount
of heat transferred per unit of work added to the system. From the data gathered, maximum COP was
achieved at 1% compressor setting with a value of 8.2 ± 0.32 while the lowest at the 100% setting
with a value of 4.8 ± 0.38. It could be concluded that process is more efficient at low compressor
settings with low refrigerant flow thus resulting in low effluent pressure. This is indicative of the
overdesigned refrigeration cycle for the process simulated thus it could be used for a more
intensive cooling process. . It was determined that with a 95% confidence level the sample means were
not equal. Therefore, increasing the compressor speed had a significant effect on the coefficient of
performance of the RA-1 MKII refrigeration unit.
Refrigeration is the process of lowering the temperature of an enclosed space by forcefully
removing the heat and transferring it elsewhere by means of a working fluid. Traditionally the
process of refrigeration was made to occur naturally by means of temperature gradient where heat
moved freely from an area of high temperature to an area of low temperature as applied in ice
boxes . Currently, refrigeration is accomplished by means of mechanical work which aids the
transfer of heat through a cycle. The purpose of refrigeration is to create a low temperature zone
in the space or object heat is absorbed . The principle is applied in air conditioning equipment
to provide the required cool environment in rooms for human comfort. The most popular use of
refrigeration however is in preservation of perishables such as in homes, supermarkets and hotels.
The goods are kept at low temperatures to discourage the growth of bacteria. In medicine,
refrigeration is employed to provide very low temperatures for preservation of drugs and body
tissue samples. Most industrial cooling processes such as condensation of gases before storage
and transportation and the cold treatment of materials also use the concept of refrigeration. The
Objective of the experiment is to establish how refrigeration loop coefficient of performance is
affected by varying the compressor speed, and to determine whether there is an optimum operation
condition that would lead to the highest coefficient of performance.
Refrigerators are the heat transfer from a low temperature sink to a high temperature
source. It uses refrigerant in cyclic devices to remove heat from the cold medium. The refrigeration
cycle obeys the first law of thermodynamics. Such that energy can neither be created nor destroyed
but it can only be transferred from one medium to another.
𝑑𝑑𝑑𝑑 = 𝛿𝛿𝛿𝛿 + 𝛿𝛿𝛿𝛿
The maximum efficiency of a cycle and with less work used is the ideal case of the Carnot
refrigeration cycle. The Carnot cycle
consists of four processes: isentropic compression in a
compressor, isobaric heat rejection in a condenser, throttling in an expansion device, and isobaric
heat absorption in an evaporator. Since this process will not be reversible, an actual vaporcompression refrigeration cycle would be more likely to look at. The fluid friction and heat
transfers to or from the surroundings are the biggest contributors to the process being irreversible.
Both of these have effects on the entropy of the system.
To find the efficiency of the refrigerator, the coefficient of performance (COP). The COP
is the compares the desired output with the required input. It is always a positive number and can
be greater than one. The purpose of the refrigerator is to remove heat from a cold space and release
it into the surrounding . COP can then be found by:
With 𝑄𝑄𝐿𝐿 as the amount of heat absorbed from the water and 𝑊𝑊𝑖𝑖𝑖𝑖 is the power used by the
compressor. The 𝑄𝑄𝐿𝐿 can be found by:
𝑄𝑄𝐿𝐿 = 𝑚𝑚̇(ℎ1 − ℎ4 )
with the 𝑚𝑚̇ is the mass flow rate of the liquid and ℎ1 and ℎ4 are the enthalpies corresponding to
points 1 and 4, Figure 1.
Figure 1: A pressure vs enthalpy diagram for a refrigeration cycle .
The unit used in the experiment was the RA1-MKII, which used a computer based interface
to control the cycle. Below is a schematic of the unit and the locations of the sensors which were
presented in the software. Along with Figure 2, Figure A1 illustrates the unit used for better
Figure 2: Schematic Diagram of the RA1-MKII Showing the places of the sensors with respect
to the cycle .
The coefficients of performance were calculated using a refrigeration system described in
the Armfield Refrigeration unit instruction manual for compressor speeds of 1, 50, and 100%. The
experiment was repeated three times to compare the results statistically. The Armfield RA1-MKII
software generated a table showing the COP and P2 as functions of time at intervals of 10 seconds
for two minute. The COP values were then compared using statistical analysis at each speed
explained in the methods of analysis. Two plots were generated: COP versus compressor speed
and COP versus P2 to determine the effect that compressor speed and pressure have on the COP.
Results and Discussion
The obtained results are graphically presented in the succeeding figures in order to establish
a trend for the relationship of the different variables such as the COP, compressor setting,
refrigerant flow, and P2 (pressure).
From the data gathered in table 1 to table 6, it can been seen that the coefficient of performance is
highly dependent on the refrigerant flow and subsequently the pressure of the effluent from the
From Figure 3, an inverse relationship could be seen between the COP and the compressor setting
thus a decrease in the COP is observed with an increase in the compressor setting. The declining
trend is an indication that the compressor may be overdesigned for the process thus requires
minimal setting of the compressor.
A plot of COP against Compressor Setting
Figure 3: A graph of the coefficient of performance (COP) vs. compressor setting
A similar trend is observed with the relationship of the COP with the refrigerant flow. The COP
decreases with an increase in the refrigerant flow thus indicating that the process simulated actually
requires minimal refrigerant for the set temperature. This similar trend is understandable given that
the relationship between the two parameters which is shown in Figure 5. A direct relationship is
established between the compressor setting and the refrigerant flow which is expected between the
two parameters since the compressor drives the refrigerant (working fluid) into the system.
Refrigerant Flow (L/hr)
Figure 4: A graph of the coefficient of performance (COP) vs. refrigerant flow
REfrigerant Flow (L/hr)
Figure 5: A graph presenting the relationship between the refrigerant flow and the compressor
Figure 6: A graph of the relationship between the coefficient of performance (COP) and the
pressure from the compressor to the condenser (P2)
The relationship between the effluent pressure and the COP follows a similar decreasing trend sa
with the refrigerant flow and the compressor setting. This is evident of the overdesigned
refrigeration cycle for the process.
T-tests were conducted among the trials and it can be concluded that the samples from the different
compressor settings present different values and are not of the same sample population thus gives
variance on the experimental set-up. This is backed up by the p-values from table 10 which are
0.000837 between the COPs at 1% and 50% and 0.012889 for the 50% and 100% trials. The trends
observed between the different settings thus could be considered valid and varied enough to be
The objective of the experiment was achieved since the relationship between the COP and
coefficient performance of the Armfield RA-1 MKII was determined. The COP value achieved at
1% compressor setting had a value of 8.2 ± 0.32 while the lowest achieved value was at 100%
setting that had a value of 4.8 ± 0.39. It could be concluded that process is more efficient at low
compressor settings with low refrigerant flow thus resulting in low effluent pressure. This is
indicative of the overdesigned refrigeration cycle for the process simulated. For better results to
be obtained, more time should be given to the compressor to allow it to settle before data is taken
after each interval of measurements done. Besides, the data could be taken over longer periods of
 Cengel, Yunus A., John M. Cimbala, and Robert H. Turner. Fundamentals of Thermalfluidsciences. New York: McGraw-Hill Higher Education, 2012. Print.
 Armfield Ltd. Vapor-Compression Refrigeration Unit: Instruction Manual. Issue 2. August
2013. Print. RA1:MkII.
Verify the USB connection exists between the RA1-MK11 unit and the PC.
Check the RCD/combined circuit breaker is on - in the up position
Switch the unit on by flipping the power switch on the console.
Check to see what the temperature, pressure, and other values are; should be about ambient.
Set Pump 1 to desired speed and Pump 2 to desired speed. The condenser water pump
(Pump 1) should be set to 40% and the evaporator water pump (Pump 2) should be 60%.
Verify water is flowing through the compressor and evaporator. Typically 1.5L/min and
5.5L/min respectively on the mimic diagram
Set the compressor motor speed to 10%, about 2000RPM, then click compressor on, on the
screen. Allow system 30 seconds to reach 2000 RPM set point. Check the flowmeter F3 on RA1MK11 to ensure working fluid flows through the system
Set the sample options as ‘Automatic / 10 second intervals’ and click GO. The readings
from the sensors will now be recorded.
While looking at the graphs of T1, T3, and T7 on the Y-axis and P1 & P2 on the secondary
Y-axis, allow the system until the values are reasonably steady i.e. the changes within 5%.
10. Record the refrigerant flow rate on the variable area flowmeter F3 and enter the value on the
11. Increase the compressor speed to 20% around 2460 RPM, and repeat steps 9 and 10.
12. Repeat this process for 40% (2945 RPM), 60% (3430 RPM), 80% (3915 RPM) and 100%
13. Set the compressor speed back to 50% and let the system reach dynamic equilibrium
14. Click the ‘Stop button’ to discontinue recording data and next click ‘Compressor on’ to stop
Method of Analysis
The null hypothesis for the experiment was that the Coefficient of Performance for 1, 50 and
100% compressor speeds respectively could be explained by random chance.
Once all the data were obtained from the refrigeration unit, the optimum operation condition was
be determined. The compressor’s speed which lead to the highest COP was determined to be the
optimum setting for the unit. Moreover, the set of COPs from the three trials at the same speed
were compared statistically to another set at a different speed. This is to analyze the effect of
compressor speed on the COP. Out of all the data, two plots were generated, COP versus
compressor speed and COP versus the outlet pressure from the compressor (P2).
Safety precautionary measures and regulations must be observed to guarantee the safety of
the users and the condition of the equipment.
An immediate risk in this experiment setting is the presence of hot surfaces which can
cause serious burns. It is recommended that users have to allow time for the equipment to cool
before handling it. The ‘hot surfaces’ labeled surfaces should never be touched or brought close to
Since the equipment operates on an electrical supply, electrical safety measures have to be
adhered to. The RA1-MKII should be operated with the prescribed frequency and voltage and it
should be made certain that all the panels are in place before connection. The refrigeration unit is
fitted with a Residual Current Device (RCD) which switches off electrical supply in case the
system becomes electrically flawed. This device should be checked regularly and maintained in
good working condition.
The equipment contains rotating parts which demands special safety attention. It should be
ensured that the protective guards against these parts are in place before operation. The users are
required to clear off from the rotating parts when in operation and refrain from inserting any object
past the protective guards. The parts must be left to come to rest after switching off the equipment
before attending to them. By fact that the equipment is heavy and hot, it should be maintained in
a strong frame and handled carefully. The equipment should not be left to run unattended.
Electrical safety - the Armfields RAI-MKII refrigeration unit should be operated on the
specified voltage and frequency. The Residual Current Device (RCD) should be checked regularly
to ensure that it is functioning well. The experiment involves the use of water and care should be
taken for possible spillage. All electrical connections and adjacent devices should be secured to
avoid water from reaching them.
Raw data sheet.
Figure A1: T-s phase diagram for the cycle .
A. Raw Data
Table 1: Raw data for Trail 1 at 1% compressor speed
Table 2: Raw data for Trial 1 at 50% compressor setting
Table 3: Raw data for Trial 1 at 100% compressor setting
Table 4: Raw data for Trial 2 at 1% compressor setting
Table 5: Raw data for Trial 2 at 50% compression setting
Table 6: Raw data for Trial 2 at 100% compression setting
Table 7: Raw data for Trial 3 at 1% compressor setting
Table 8: Raw data for Trial 3 at 50% compressor setting
Table 9: Raw data for Trial 3 at 100% compressor setting
Table 10: t-test comparing 1% and 50%compressor settings
t-Test: Two-Sample Assuming Unequal Variances
t Critical one-tail
t Critical two-tail
Table 11: t-test comparing 50% and 100$ compressor settings
t-Test: Two-Sample Assuming Unequal Variances
t Critical one-tail
t Critical two-tail
µ is the sample mean, x is each data point, and N is the sample size.
Standard Deviation σ
σ = � �(𝑥𝑥𝑖𝑖 − 𝜇𝜇)2
µ is the sample mean, x is each data point, and N is the sample size.
σ = �(xi − µ)2
Pooled Standard Variation
(𝑛𝑛1 − 1)σ𝑋𝑋1 2 + (𝑛𝑛2 − 1)σ𝑋𝑋2 2
σ𝑝𝑝 = �
𝑛𝑛1 + 𝑛𝑛2 − 2
σ2𝑋𝑋1 is the variance of one of the samples.
𝑋𝑋1 − 𝑋𝑋
σ𝑝𝑝 � + 𝑛𝑛
𝑋𝑋1 is the mean of population 1, ���
𝑋𝑋2 is the mean of population 2
T-test: If the value is smaller than the critical t value the null hypothesis that the two sample
means are equal is rejected.
Troubleshooting an Armfield RA1-MKII Vapor Compression Refrigeration Unit
Abstract: (Final Report)
The vapor-compression refrigeration system (VCRS) is a system commonly used in buildings
and cars which has the purpose of providing air conditioning. However, it is not limited to air
conditioning, it can also be used in preservation or chilling of produce and goods in domestic or
industrial use refrigerators. The VCRS is a system composed of four components, a compressor,
a condenser, an expansion valve, and an evaporator. The system operates using a refrigerant
which undergoes a repeatable cycle of phase changes, displacing hot air, and producing an air
conditioning or cooling affect. The industries that most commonly utilize this method of
refrigeration are oil refineries or natural gas processing plants, food production plants, and
chemical processing plants. In oil refineries and natural gas processing plants, these types of
refrigeration systems are used to condense product gases into a liquid form in order to transport
the product more effectively. In food production plants refrigeration is used to provide the chiller
with a place to displace heat in order to provide cold storage for products to extend the lifetime
by inhibiting bacterial growth. Finally, refrigeration is used in chemical processing plants to
control the humidity of processes as well as condense gases into liquids as needed (King).
These types of refrigeration systems are often effective and reliable, however, with all systems, it
is important to understand the unit operation in order to troubleshoot issues that occur with the
equipment. By understanding the system, it becomes possible to recognize where maintenance is
required. In industry, troubleshooting of VCRSs often begins with identifying the issue of the
system. This often consists of examining the four main components of the system and running
tests to identify which element of the system is producing the issue. Common issues with the
system, include but are not limited to, the compressor not running, the fixed temperature being
too high, having either a high or low discharge pressure, and having either a high or low suction
pressure (King). Each of these issues can be addressed in different ways but correspond to the
main components of the system which help to identify the problem.
The objective of the experiment is to troubleshoot an Armfield Vapor-Compression
Refrigeration Unit by collecting data by running the damaged refrigeration system and
comparing it with previously obtained data of the same refrigeration system operating at normal
operating conditions. Once the data is obtained, the system will be evaluated and repaired to
operate at normal op ...
Purchase answer to see full attachment