LAB 5
Refrigeration Cycle
1.0 OBJECTIVE
1. To study the cycle of refrigeration system.
2. To measure related temperatures and pressure of refrigeration system.
3. To plot P-H diagram for the refrigeration system and to determine coefficient of
performance (COP) of refrigeration system.
2.0 INTRODUCTION AND THEORY
2.1
GENERAL
Heat energy always flows naturally from a higher to a lower temperature level. That
is, hot areas naturally cool off and cold areas naturally warm up. Therefore, moving heat
from a lower to a higher temperature requires the input of work (or heat), usually to create a
pressure differential in the cycle refrigerant.
The refrigerant (acting as a heat transfer fluid) is used to transfer heat energy from a
lower temperature to a higher temperature. The refrigerant is evaporated at a temperature
lower than the desired temperature in the freezer or cooler. The condensing temperature of
the refrigerant is increased by compression so that it can either be rejected to the
environment or recovered as useful heat. The basic refrigeration cycle, with all steps
combined, is shown in figure 1. The main components of refrigeration are:
i.
ii.
iii.
iv.
v.
Compressor
Condenser
Filter and Dryer
Expansion valve or capillary tube
Evaporator
2.2
Refrigeration cycle
A standard refrigeration cycle works as follows:
1 - 2 adiabatic compression of gaseous (i.e. saturated or superheated vapor) refrigerant fluid
from p1, T1 to p2, T2.
2 – 3 isobaric cooling and condensation of the refrigerant fluid. State 3 is saturated liquid or
compressed liquid at p3 = p2. The heat is rejected to the environment.
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3 – 4 adiabatic throttling to p4 = p1. The throttling process is isenthalpic, h3 = h4.
4 – 1 isobaric evaporation (and possibly superheating) of the refrigerant fluid, back to state
1.
The refrigeration cycle can be divided into four phases with all phases is combined in closes
system. These four steps are:
Step 1, Evaporation:
Liquid refrigerant at a sufficiently low pressure is brought into contact with the heat source
(the medium to be cooled). The refrigerant absorbs heat and boils, producing a low-pressure
vapor. The heat exchanger used for this process is called the evaporator.
Step 2, Compression:
The compressor raises the pressure of the refrigerant vapor, normally using an electric motor
drive. This increases the temperature at which the vapors will condense to a temperature
above the temperature of the heat sink. Most common compressors are reciprocating (piston
and cylinder) or screw (looking much like an old meat grinder) compressor designs.
Step 3, Condensing:
The high-pressure refrigerant gas now carrying the heat energy absorbed at the evaporator
plus the work energy from the compressor enters the condenser. Since the refrigerant's
condensing temperature is higher than that of the heat sink, heat transfer will take place,
condensing the refrigerant from a high-pressure vapor to a high-pressure liquid.
Step 4, Expansion:
The condensed liquid's pressure is reduced (called "throttled") to the lower pressure
evaporator using a valve, orifice plate or capillary tube device. In actual practice, the
condenser cools the refrigerant a bit more, sub cooling it below the condensing temperature.
This is an important efficiency improving attribute to the cycle, since it reduces the amount
of refrigerant liquid that has to evaporate (it is called flashing at this stage in the cycle) to a
gas in the expansion valve to reduce the pressure and temperature of the liquid entering the
evaporator. This reduction in flash gas is important to improve system performance.
The refrigeration process flow are describe below :
a)
Compressor starts.
b)
Low pressure vapour refrigerant is compressed and discharged out of
the Compressor.
c)
The refrigerant, at this time is at High Pressure.
d)
The refrigerant flows through the discharge line to the condenser.
e)
The condenser gives up heat absorbed and changes from vapour to liquid
f)
The high pressure liquid then flows through the Filter Drier to the Thermal
Expansion Valve(TXV) or Capillary tube.
g)
The Thermal Expansion Valve (TXV) control the liquid refrigerant into the
Evaporator.
h)
As the refrigerant flows through the expansion device, changes to Low
Pressure liquid.
i)
In the Evaporator, heat is absorbed and the refrigerant begin to change in state
from liquid to vapour.
j)
The low pressure vapour refrigerant moves back to the compressor.
k)
The cycle start over again
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2.2
Pressure-Enthalpy Diagram
The Pressure-Enthalpy (P-H) diagram is another way of looking at the refrigeration cycle. It
has the advantage of graphically showing the process, the cooling effect and the work
required to make it happen.
Figure 2 : P-H Chart
Figure 2 shows the Pressure-Enthalpy (PH) diagram for the refrigeration. The process for
each of the components is indicated.
Point 1 to 2 - Compression process.
The work is the change in enthalpy from point 1 to point 2. Simply, Btu/lb. times the lb./min
equals compressor power. Compressors end up with the work of compression as heat in the
refrigerant. The vertical aspect of the curve shows the rise in refrigerant pressure (and
temperature) from 1 to 2.
Point 2 to 3 - Condensation process.
The process takes place in the condenser. Once the refrigerant is saturated, condensation
occurs and the refrigerant changes from a gas to a liquid. Like the evaporator, the line is
horizontal indicating constant pressure.
Point 3 to 4 - Expansion process.
The process is happen in expansion device such expansion valve or capillary tube. This
appears as a vertical line from point 3 to point 4, indicating the pressure (and temperature)
drop that occurs as the refrigerant passes through the capillary tube.
Point 4 to 1 - Evaporation process.
The evaporator process is from point 4 to point 1. As the refrigerant changes from a liquid
to gas, the pressure (and temperature) stays constant. The heat is being absorbed as a phase
change (latent energy). The refrigeration effect is the change in enthalpy from 4 to 5, simply
expressed as Btu/lb..
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The efficiency of the refrigeration (COP) cycle can be found through the ratio between the
segment 4 -5 and 1-2 :
COP =
h5 - h4
h2 - h1
http://laurens.apogee.net/ces/library/crbcc.asp
http://www.kfupm.edu.sa/me/ 431 refrigeration
www.freeze-co.com/Documents/RefrigerationCycle.pdf
Application Guide AG 31-007, Mc Quay Airconditioning
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3.0
EQUIPMENT / APPARATUS
Figure 3: Refrigeration Unit
1.
2.
3.
4.
5.
6.
7.
8.
9.
10.
Schematic diagram
High and low pressure gauges
Flow meter
Digital Thermometer
Sight Glass
Drier
Evaporator Cabinet
Condenser
Compressor
Control Board.
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4.0 Experiment Procedure
No.
1.
2.
3.
4.
5.
6.
7.
8.
Procedure
Fully open valve V1 , V3, V 4 , V6, V7 and V8
Fully Close valve V2 and V5
Switch ‘ON’ Power supply.
Wait about 5 minutes until the system is stabile.
Record the power of the compressor.
Record the pressure:
i. High pressure (HP) - Red gauge
ii. Low pressure (LP) - Blue gauge
The pressure need to add 1 bar for atm pressure consideration.
Take the temperature probe.
At Compressor :
i. Measure temperature at compressor inlet ( T1)
ii. Measure temperature at compressor outlet ( T2)
9.
At Evaporator
i. Measure temperature at before entering capillary tube ( T3)
ii. Measure temperature at Evaporator outlet ( T evaporator )
10.
11.
12.
13.
14.
Record the Refrigerant flow rate at flow meter.
Fill at the data in Table 1.
Using P-H diagram, Plot your data referring to figure 2.
From the P-H diagram, determine the enthalpy (h) at the all points measured.
End of Experiment
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LAB 5
REFRIGERATION CYCLE
Lab Result
GROUP NUMBER
:___________________________
DATE OF EXPERIMENT :___________________________
GROUP MEMBERS NAME
:
(Reminder: Do not accept your group member to sign if his/her contribution is not satisfy)
1)_______________________________signature:__________
2)_______________________________signature:___________
3)_______________________________signature:__________
4)_______________________________signature:___________
5)_______________________________signature:___________
6)_______________________________signature:___________
Mark :
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5.0 Data & Result
No.
1.
2.
No.
3.
4.
7.
8.
Description
Pressure
(bar)
High Pressure ( HP) + 1 bar (atm)
Low Pressure (LP) + 1 bar (atm)
Description
Temp.
(ºC)
Temperature at compressor inlet ( T1)
Temperature at compressor outlet ( T2)
Temperature at before capillary tube ( T3)
Temperature at Evaporator ( Tevaporator)
Table 1
Note :
1.
2.
3.
4.
5.
6.
Using given P-H diagram, Plot your data referring to figure 2.
Step 1 : Draw the horizontal line of High pressure (HP)
Step 2 : Draw the horizontal line of Low pressure (LP)
Step 3 : At Low Pressure (LP) line plot T1.
Step 4 : At High pressure (HP) line plot T2 and T3
Step 5 : t point T3 draw vertical line that will cut to LP line. This intersection
is point T4.
7. Step 6 : Draw a line that will connecting T1, T2, T3 , T4 and T5 .
8. Step 7 : To find the enthalpy ( x-axis), draw the vertical line to the x- axis.
The intersection is the enthalpy for that point.
9. Record the enthalpy in table 2 :
Point
1
2
3
4
5
Temperature , ºC
Enthalpy (h) , kJ/kg
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6.0
QUESTIONS
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Answer all questions below:
6.1 Calculated the efficiency of the refrigeration (COP) .
6.2 Using your own word, explain the function of Compressor in refrigeration
system?.
6.3 Using your own word, explain the function of Condenser in refrigeration
system?.
6.4 Using your own word, explain the function of evaporator in refrigeration
system?.
6.5 Using your own word, describe the whole refrigeration process
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7.0 DISCUSSION
Include a discussion on the result noting trends in measured data, and comparing measurements with theoretical predictions
when possible. Include the physical interpretation of the results and graphs, the reasons on deviations of your findings from
expected results, your recommendations on further experimentation for verifying your results, and your findings.
8.0
CONCLUSION
Based on data and discussion, make your overall conclusion by refer
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