The American University in Cairo
MENG 3605
Applied Thermodynamics
Lab report #2
Refrigeration & Heat Pump Performance
Group members:
Enjy Katary
900183501
Mohammed Taha
900183180
Zainab Abdein
900183279
Submitted on 2nd of April
Instructor: Dr. Salah El-Haggar
TA: Ahmed Sayed
1
Abstract
An experiment was conducted on a refrigeration unit to study the cycle and change of states of
the refrigeration fluid throughout the cycle. The coefficient of performance varied with the change in
the flow rate of the inlet cooling water. The cooled air temperature decreased with the increase in
cooling water flow rate and compressor exerted work.
2
Table of Contents
List of figures ................................................................................................................................................. 2
List of tables .................................................................................................................................................. 2
Nomenclature ................................................................................................................................................ 3
Introduction ................................................................................................................................................... 4
Objective ....................................................................................................................................................... 5
Theory ........................................................................................................................................................... 7
Description of Experiment & Apparatus ...................................................................................................... 9
Procedure .................................................................................................................................................... 11
Results ......................................................................................................................................................... 12
Discussion: .................................................................................................................................................. 15
Conclusions & Recommendations .............................................................................................................. 17
References ................................................................................................................................................... 18
List of figures
Figure 1 TS diagram of ideal refrigeration cycle ........................................................................................... 7
Figure 2 T-s Diagram for first cycle ............................................................................................................. 13
Figure 3 T-s Diagram for second cycle ........................................................................................................ 13
Figure 4 T-s Diagram for third cycle ............................................................................................................ 14
Figure 5 Flow rate vs COP ........................................................................................................................... 14
List of tables
Table 1 Experimental Data .......................................................................................................................... 12
Table 2 Experimental Data (2) .................................................................................................................... 12
Table 3 Corresponding Mass flow rate and enthalpy for each point ......................................................... 12
Table 4 Corresponding entropy, power and coefficient of performance ................................................... 12
3
Nomenclature
Symbol
Units
Description
H
kJ
Enthalpy
K
-
Heat capacity ratio
𝒎̇
Kg/s
Mass flow rate
N
rpm
Rotational speed
P
kPa
Pressure
P
W
Power
Q
W
Heat transfer
T
K
Temperature
U
kJ
Internal energy
V
v
Voltage
W
W
Work
η
-
Efficiency
COP (HP)
-
Coefficient of performance of
heat pump
COP (Ref)
-
Coefficient of performance of
refrigerator
4
Introduction
The process of heat transfer occurs in nature when there is a difference in temperature between two
regions, and the direction of this process is naturally from the higher temperature reservoir to the lower
temperature reservoir. In this experiment, the process of heat transfer occurs in the reverse mode from the
lower temperature reservoir in a cycle named the refrigeration cycle which needs a power input to operate
in contrast to the natural heat transfer process. This cycle takes heat from the lower temperature reservoir,
the refrigerator cabinet, to higher temperature reservoir, the room temperature. It is usually found in home
fridge that is used to reserve food by keeping it in a low temperature or air conditioners.
The working fluid in all previously mentioned refrigerator cycles is the refrigerant which has tabulated
properties at different pressures and temperatures. For illustration as shown in figure (1-a), the
refrigeration cycle is supplied by 𝑊𝑛𝑒𝑡 to perform its operation which happens as the following: the cycle
removes the cooling load 𝑄𝐿 at the lower temperature 𝑇𝐿 and rejects the heating load 𝑄𝐻 is it to the
warm environment 𝑇𝐻 . Therefore, it keeps the lower temperature region at a constant low
temperature.
On the other side of the figure, there is a heat pump which has the opposite objective of keeping
the warm places at a constant high temperature𝑇𝐻 which is used in vapor compression cycles.
Figure 1) refrigeration and heat pump cycle
5
Ideal cycle:
Ideally the refrigerant enters the compressor as a saturated vapor where it gets compressed isentropic ally
at constant entropy and therefore it experiences an increase in its pressure and temperature until it leaves
in the superheated region. After leaving the compressor, it goes to the condensation device to leave in a
saturated liquid form at the same pressure it entered the condenser with while it loses this heat to the
environment𝑄𝐿 . Then in order to make a pressure drop to the refrigerant and make it ready to enter the
evaporator, it passes through a throttling valve. In the evaporator it gains heat from the low temperature
space and it evaporates to reenter the compressor again. Here where the cycle repeats itself to assure
constant low temperature in the refrigerating space as shown in figure (2).
Figure 2 ideal cycle
Practical cycle:
In order to reserve the compressor and avoid its damage, in design it is made sure that the refrigerant
enters the compressor in a superheated form and not as a saturated vapor. Considering the losses that
happens within the pipes due to fluid friction; therefore a significant heat transfer in the pipes because of
irreversibility. Since the compressor also operates at a specific efficiency <%100 , it is not expected to
have an ideal isentropic compression as shown in figure (3), note the difference between point 2 and point
2’. Due to irreversibility and shifting from the ideal situation, the required power input for the cycle to
operate increases and consequently the costs.
Figure 3 actual cycle
6
Objective
1. Sketch the cycle on the T-S diagram for performed refrigeration cycle
2. Analyze the performance of that refrigeration cycle by comparing it to the actual cycle
3. Using the measured properties and collected data, calculate the COP, the coefficient of
performance for the refrigerator and the heat pump.
4. Plot the relation between the flow rate, the independent variable, and the coefficient of
performance, the dependent variable.
7
Theory
The refrigeration cycle undergoes four main processes:
1-2 Isentropic compression
2-3 Heat rejection by the condenser
3-4 throttling by the expansion valve
4-1 heat absorption by evaporator
Figure 4 TS diagram of ideal refrigeration cycle
The whole process operates under steady-flow fluid. Therefore, energy equation can be used to study the
mechanism and calculate the COP of the refrigerator and heat pump:
(𝑞𝑖𝑛 − 𝑞𝑜𝑢𝑡 ) − (𝑤𝑖𝑛 − 𝑤𝑜𝑢𝑡 ) = (ℎ𝑒 − ℎ𝑖 )
(1)
The heat absorbed by the evaporator through process 4-1 can be determined using the following equation:
𝑞𝐿 = 𝑚̇(ℎ1 − ℎ4 )
(2)
The compressor work 1-2 can be determined by the following equation:
𝑤̇𝑐 = 𝑚̇(ℎ2 − ℎ1 )
(3)
8
The heat rejected by the condenser:
𝑞𝐻 = 𝑚̇(ℎ2 − ℎ3 )
(4)
The cop of the refrigerator when the desired is 𝑞𝐿 is described as the following:
𝑞
𝐶𝑂𝑃𝑅𝑒𝑓 = 𝑤̇𝐿
𝑐
(5)
However, when the desired is 𝑞𝐻 the COP is calculated for the heat pump as the following:
𝐶𝑂𝑃𝐻𝑃 =
𝑞𝐻
𝑤̇𝑐
(6)
9
Description of Experiment & Apparatus
In this experiment the apparatus shown in figure (6) is replaced by the apparatus shown in figure (5)
because the latter apparatus has recording gauges and thermocouples to be used in collecting the
necessary data; however, the first apparatus shown in figure (6) has a better display of the components for
a better understanding and illustration.
The apparatus setup includes the following:
1. A rotameter and its function is to measure the flow rate.
2. A mechanical arrangement of piston-cylinder device to perform the compressor’s function in the
refrigeration cycle.
3. A condenser: for heat rejection and transforming the refrigerant to saturated liquid
4. Expansion valve: used to make pressure drop in the refrigerant before entering the evaporator
5. Power switch: supply required power to the cycle
6. Pressure gauges to the record the maximum and minimum pressure limits which mainly happens
respectively before and after the compressor.
7. Temperature recording screen: used for data collection to perform the analysis.
8. Temperature knob: To get the reading of temperature at different points in the cycle.
-
Between the compressor and the condenser
-
Between the compressor and the expansion valve
-
Between the expansion valve and the evaporator
-
Between the evaporator and the compressor
-
The inlet temperature of the entering water into condenser
-
The outlet temperature of the water flowing out of the condenser
-
The refrigerant space temperature 𝑇𝐿
-
The room temperature 𝑇𝐻
10
Figure 5) experiment refrigeration apparatus with recording screens
Figure 6 refrigeration original setup
11
Procedure
12345-
Connect the unit to the cooling water supply.
Switch the unit on.
Fix the flow rate of the cooling water.
Wait for the temperature readings to stabilize.
Record the 8 temperature readings, the cooling water flow rate as well as the minimum and
maximum pressures.
6- Repeat the experiment with different cooling water flow rates.
7- Ensure the unit is turned off after use.
12
Results
Table 1 Experimental Data
Volume t1 (°C)
t2 (°C)
t3 (°C)
t4 (°C)
t5 (°C)
t6 (°C)
t7 (°C)
t8 (°C)
P min
flow
(gauge)
rate
(bar)
(lit./hr.)
30
45
33
-6
19
21
35
10
20
1.8
50
44
31
-6
21
22
32
10
22
1.8
80
52
31
-3
20
23
31
10
23
2
P max
(abs.)
(kg/cm^
2)
9.5
9
8.5
Table 2 Experimental Data (2)
Volume
t1 (°C)
t2 (°C)
t3 (°C)
t4 (°C)
t5 (°C)
t6 (°C)
t7 (°C)
t8 (°C)
P min
P max
flow rate
(abs.)
(abs.)
(m^3/s)
(kPa)
(kPa)
0.0005
45
33
-6
19
21
35
10
20
280
931.95
0.000833
44
31
-6
21
22
32
10
22
280
882.9
0.001333
52
31
-3
20
23
31
10
23
300
833.85
Table 3 Corresponding Mass flow rate and enthalpy for each point
Mass
Flow
Rate
(Water)
(kg/s)
0.5
0.833333
1.333333
Mass
Flow
Rate
(R-12)
(kg/s)
0.207683
0.243711
0.298479
h1
(kJ/kg)
h2
(kJ/kg)
h3
(kJ/kg)
h4
(kJ/kg)
373.147 232.0735 194.4205 365.1948
373.1747 230.0575 194.4205 366.4537
379.6303 230.0541 197.2038 365.5389
h5
(kJ/kg)
h6
(kJ/kg)
87.8955 146.4925
92.081 133.936
96.2665 129.7505
h7
(kJ/kg)
2.87
2.87
2.87
h8
(kJ/kg)
5.74
6.314
6.601
Table 4 Corresponding entropy, power and coefficient of performance
s1
s2
s3
s4
Qh (kW)
(kJ/kg.K) (kJ/kg.K) (kJ/kg.K) (kJ/kg.K)
1.5614
1.5646
1.588
1.1092
1.1027
1.1028
0.9794
0.9794
0.9897
Ql (kW)
Win
COP
COP
(refrigerator) (heat
pump)
1.6094 29.2985 35.46684 1.651533
21.4751 17.74019
1.6137 34.87917 41.9263 1.637978
25.59637 21.29403
1.6061 44.64533 50.24447 4.205985
11.94595 10.61472
13
Figure 7 T-s Diagram for first cycle
Figure 8 T-s Diagram for second cycle
14
Figure 9 T-s Diagram for third cycle
Figure 10 Flow rate vs COP
15
Discussion:
The first step in the refrigeration cycle is the compressor. Before and after the compressor, the
temperature was recorded for different mass flow rates. The main function of the compressor is
to increase the pressure of the refrigerant since the refrigerant enters with low-pressure and low
temperature and leaves with higher values. As shown in table 1, adjusting the mass flow rate to
30 l/hr, the pressure increased from 1.8 to 9.5 using the compressor.
After that, the condenser removes heat from the refrigerant to condense the water to be in the
saturated liquid state. As shown in table 4, the heat rejected increases as the mass flow rate
increases. The value increased from 29.2 to 44.6. This can be illustrated using equation (4) since
the amount of heat rejected depends mainly on the mass flow rate and temperature change.
However, there is an error due to the difference between the temperature at point 3 and the
saturation temperate of the measured pressure, which was not equal to zero. This means there is a
deviation between the actual temperature and the theoretical one, resulting in inaccurate plotting
of the T-S diagram for the three mass flow rates.
Then comes the throttling valve, which creates a pressure drop in the refrigerant after it leaves
the condenser to help the evaporator perform its function. As shown in table 1, the pressure
decreased from 9.5 kPag to 1.8 kPag for the first mass flow rate.
The final step is the evaporator which exchanges heat between the cycle and the ambient. It
absorbs heat from the ambient following equation 2. As shown in table 4, the value of heat
absorbed increased from 35.56 to 50.2 as the flow rate increased. This increase in the values is
16
illustrated using equation (2) which shows a direct relationship between the mass flow rate and
the amount of heat absorbed into the system.
The COP for the refrigerant and the heat pump was calculated based on three flow rates, as
shown in figure (5). COP of the heat pump depends on the amount of heat rejected and the
amount of work supplied by the compressor, based on equation (6). Therefore, when the mass
flow rate results in a higher difference between enthalpy 2 and 3 than the difference between
enthalpy 2 and 1, the COP of the heat pump increases, and vice versa. In this case, it increased
from 17.7 to 21.29. The same applies to the COP of refrigeration, and it grew from 21.4 to 25.9,
as shown in the plot. This can be described theoretically in terms of temperature and heat
transfer. When the flow rate increases, the temperature becomes unstable to adjust to the new
change, and then the COP and evaporation temperature decrease. If the heat exchange is assumed
to be constant, the amount of work needed to compress the refrigerate is higher. Therefore, the
COP decreases. However, the plot didn’t follow this trend due to errors.
Sources of error:
1- Inaccurate measurements
2- The device is not calibrated before using
3- The compressor is assumed to be isentropic without heat loses
4- The T taken at the sections was not following the theoretical ones according to the
saturation temperatures.
17
Conclusions & Recommendations
A direct relation between the mass flow rate flowing in the condenser and the coefficient of
performance, The COP, of the cycle was deduced from the findings of this experiment as for the
refrigerator and the heat pump cycle. The results showed how the mass flow rate affect the amount of heat
rejection at the condenser in case of a heat pump or the amount of heat absorbed by the evaporator. The
increase of the entering mass flow rate affects the COP negatively since it increases the amount of power
needed for the cycle to operate. Because of human error and irreversibility in the process, the resulted
cycle wasn’t ideal and it included some discrepancies, and the value of actual entropy deviated from the
ideal expected value. Any point that was plotted on the T-S outside its expected region was neglected and
replaced with its theoretical values to perform later calculation and analysis. A possible recommendation
to reduce the error in this case is to insulate the compressor to avoid unaccounted and unnecessary heat
rejection.
18
References
Çengel Yunus A. and M. A. Boles, Thermodynamics: An engineering approach. 8th edition,
Singapore: McGraw-Hill Education, 2020.
“Thermodynamics & Transport Properties Calculation-refrigerant: Enthalpy: Water&steam:
Air,” Thermodynamics & Transport Properties Calculation-Refrigerant | enthalpy |
Water&Steam| air. [Online]. Available: http://www.ethermo.us/. [Accessed: 02-Apr-2022].