The University of Calgary
Faculty of Engineering
ENME 485 Mechanical Engineering Thermodynamics Laboratory
Experiment 1: Refrigeration and Mechanical Heat Pump Experiment
Objectives:
i) To demonstrate the principles of a refrigeration cycle (Chapter 10 of Cengel and Boles).
ii) To demonstrate the principles of a heat pump (Chapter 10 of Cengel and Boles).
Introduction:
It has been estimated that at least 85% of the refrigeration processes in use today are powered
by vapor compression systems. The applications embrace many varied disciplines including
catering, public health, architecture, food storage, transport and food processing. An improved
understanding of refrigerating techniques is demanded of engineers, particularly in the fields of
building services and environmental control. The purpose of the experiment is to demonstrate
the basic principles of refrigeration, i.e. how heat can be transferred from a cooler object to a
hotter object. The mechanical heat pump unit, shown in Fig. 1 (a), consists of a standard
compressor-condenser unit, a watt meter, and controls and instrumentation including flow meters,
thermocouples and pressure gauges.
Figure 1 (a): Picture of Mechanical Heat Pump
The Mechanical Heat Pump was designed solely for educational purposes and yields data in a
quantitative and qualitative form in a manner easily understood by the student regardless of their
level of interest in the subject. The equipment is compact, bench mounted and instrumented
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Experiment #1: Heat Pump Experiment
sufficiently for students to make measurements and perform calculations. All instruments are
located at the actual point of measurement (Fig. 1(b)) and connecting pipes are open to view to
allow the lab instructor to conveniently describe the sequence of events occurring in the
refrigeration cycle.
P2,
T3
P1,
T2
P2,
T4
P1,T1
P2,
T4
Blue Tank
Red Tank
Figure 1 (b): Schematic of the Mechanical Heat Pump
Cycle of Operation:
The method of operation of the Heat Pump can be explained by reference to the schematic
diagram in Fig. 1 (b). The working fluid or heat transfer medium is the refrigerant
Dichlorodifluoromethane (Freon 12 or R-12) and the heat source and heat sink is cold running
water.
The sealed compressor, rated at 0.5 h.p. and operating on either 220V-50Hz or 110V-60 Hz acts
as an external energy source. The super-heated refrigerant is compressed and pumped through
insulated copper pipes to the nickel plated; copper condensing coil at pressure P2 and
temperature T3. The coil is immersed in cold flowing water in the stainless steel condensing tank.
The refrigerant pressure remains substantially constant at P2 while passing through the coil, but
falls in temperature from T3 to T4, losing its super-heat and all of its latent heat or heat of change
of phase to the water. It reaches the refrigerant flowmeter as a sub-cooled liquid still at pressure
P2 and temperature T4. The function of the silica gel drier is to eliminate water, not liquid
refrigerant. The pipe work here is nickel plated copper with brass fittings.
The refrigerant now expands through the manually variable nickel-plated brass expansion valve
where it expands to a lower pressure P1 and commences to boil. The boiling temperature or wet
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Experiment #1: Heat Pump Experiment
vapor temperature T1 is measured before the refrigerant passes through the nickel-plated copper
evaporator coil. This coil is also immersed in cold flowing water in the stainless steel evaporating
tank. During its passage through the coil the refrigerant absorbs from the water the latent heat of
evaporation or heat of change of phase, and in addition may receive some further heat to superheat the refrigerant to temperature T2. The boiling or evaporation process occurs at constant
pressure P1. The super-heated vapor refrigerant leaves the evaporator to return to the
compressor through insulated copper pipes to begin the cycle again.
The heat source and sink water is taken from the cold mains supply and passed through a small
pressure reducing valve to ensure a standard flow rate by reducing mains pressure fluctuations.
Separate control valves are used to adjust the low of water to the condensing and evaporating
tanks. The water flow rates through the stainless steel tanks are determined by flowmeters. An
annular tank design is employed to reduce thermal inertia. Both water and refrigerant
temperatures are measured by standard thermocouples with each refrigerant thermocouple
resting in sealed pockets in the refrigerant circuit. Pressures are measured by refrigeration quality
pressure gauges. Electrical power input to the Heat Pump compressor is measured by an
integrating watt hour meter where power consumed is a direct function of the speed of the
rotating disc and by timing a revolution of the disc a very accurate power figure can be obtained.
The relationships between compressor shaft speed and torque versus power are given later in
this handout.
Potential Experiments:
Some of the thermodynamic and refrigeration experiments of which the equipment is capable of
performing are listed below. We will perform the experiments that are in bold font.
1. Construction of elementary energy balances.
2. Determination of compressor efficiencies.
3. Calculation of performance coefficients.
4. Calculation of refrigerating efficiency
5. Comprehensive use of refrigerant tables.
6. Analysis of refrigerating cycles using the temperature-entropy diagram.
7. Analysis of refrigerating cycles using the pressure-enthalpy diagram.
8. Comparing the practical reversible cycle with the reversible ideal Carnot cycle.
9. Comparing theoretical results with actual practical results and understanding the
limitations of a practical refrigerator.
10. Investigating the performance of a vapor compression refrigerator acting as a Heat
Pump.
11. Investigating the improvement in performance as a result of superheating the refrigerant
during evaporation.
12. Investigating the improvement in performance as a result of supercooling the
refrigerant during condensation.
Safety Features:
Refrigerant 12 used in the heat pump is incombustible and non-toxic so that leakages of
refrigerant arising from an accident or from student abuse will not cause any harm (except
perhaps to the ozone layer).
To eliminate the possibility of an excessive refrigerant pressure from occurring in the condensing
coil a differential pressure switch is fitted in the refrigerant circuit. The switch will isolate the
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Experiment #1: Heat Pump Experiment
compressor drive motor. The maximum condenser pressure is marked on the dial of the pressure
gauge.
The compressor motor is protected by thermal overload relays and a thermal switch. Should the
compressor be switched on under conditions of high load e.g. a high compression ratio across
the compressor, the motor starting current will also be high and there is a risk of the compressor
and motor being stalled. Under these conditions the relay will go open circuit and isolate the
motor.
Whenever the compressor is in continuous use for several hours under high load it will generate
heavy thermal losses due to mechanical friction and motor winding losses. These must be
dissipated through the casing of the compressor to the surroundings. This is the function of the air
cooling fan. To protect the resin insulation of the motor windings the internal temperature of the
compressor casing must not exceed the fusion temperature of the resin. The purpose of the
thermal switch attached to the inside of the casing is to isolate the motor under conditions of
excessive casing temperature.
In both situations described above, allow the compressor to cool before attempting to re-start. As
an added precaution a fuse link is located in a housing on the front of the instrument panel.
Set Up and Operating Instructions:
The apparatus is delivered complete, charged with refrigerant, ready to operate with the minimum
of preparatory work. A single phase electricity supply, a water supply and water drain are the only
external facilities required. The following order of instructions should be followed:
1. Stand the unit on a firm even surface higher than the water drain which is to be used.
2. Provide two 15mm (1/2”) bore plastic or rubber pipes to lead from the drain to main drainage.
These pipes are a push-fit and do not need further securing.
3. Remove the red blanking cap from the water inlet to the pressure reducing valve.
4. Provide 8mm (5/16”) bore copper pipe to lead from tie water supply to the pressure reducing
valve. Fit pipe and secure to both the valve inlet and the supply tap by wire or clips. Do not turn
the water on at this stage.
5. Position thermocouples in the pockets provided in accordance with the range.
6. Plug the lead from the unit into the laboratory socket. Do not switch on at this stage.
7. Turn on the mains water supply and by a process of trial and error set the pressure reducing
valve and the two adjustable tank supply valves to give a suitable steady flow of water.
8 Set to refrigerant expansion valve to approximately the middle of its range.
9. Switch on the mains electricity supply and switch on the compressor.
10. As the unit reaches equilibrium, there wilt be some ice formation evident in the right- hand
evaporating tank. This is normal but the water flow should be adjusted so that this ice formation
does not become excessive.
The unit will take about 30 minutes to obtain equilibrium from its initial start-up. The water tank
supply valves (not the pressure reducing valve), and the refrigerant expansion valve can now be
adjusted to obtain the required conditions. Once the apparatus has been running for some time a
substantial change of conditions will require approximately 15-20 minutes to obtain equilibrium.
When the experiments have been completed, the electricity should be first switched off
and then the water supply, not the reverse order.
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Experiment #1: Heat Pump Experiment
Performance Analysis of the Mechanical Heat Pump:
Assumptions
Practical limitations are imposed on the analysis of all vapor compression refrigeration systems
such that care must be taken to ensure that all assumptions are relevant to the particular system
under analysis. For example, a refrigerant condenser having numerous sharp bends and where
the bore of the pipe is small enough to cause high velocity flow will lead to excessive refrigerant
pressure drop from viscous friction. Significant errors could arise in calculations if the condensing
process is assumed to occur at constant pressure. Such limitations must be carefully considered
in the design of educational equipment to produce meaningful results.
A practical refrigerant cycle for the Hilton Mechanical Heat Pump is outlined in Fig. 2 on a
Pressure-Enthalpy diagram. The changes in properties between state points are exaggerated for
clarity.
Figure 2: Realistic P-h Cycle Diagram
Process 1-2 is vaporization of the refrigerant in the evaporator followed by some useful
superheating, 2-3, of the refrigerant while still in the evaporator. The pressure drop shown
between states 1-3 will be negligible and has been measured to be less than 7 kN/m2 (1 psi). The
average operating increase in pressure across the compressor is approximately 800 kN/m2 and
the pressure drop through the evaporator is less than 1% of this. It may be safely assumed, with
little error that vaporization occurs at constant pressure.
Process 3-4 takes place in the compressor suction line, i.e. between the outlet from the
evaporator and inlet to the compressor casing. There are two sources of error here:
1. The refrigerant leaving the evaporator will, in most operating conditions, be below ambient
temperature resulting in a heat gain from the surroundings to increase superheat. The suction line
is adequately insulated to allow the assumption that this does not occur.
2. Viscous friction in the suction line producing a pressure drop. Apart from the fact that the
refrigerant is in the vapor state giving low frictional losses, the suction line is very short in length
and a pressure drop will be quite small.
It is reasonable to assume, therefore, that no ambient super-heating occurs and the pressure
drop may be ignored.
Process 4-5. The compressor is of the fully hermetic type where both the compressor and its
drive motor are immersed in the refrigerant suction vapor. A pressure drop may occur as the
refrigerant expands into the compressor casing from the suction line, but the refrigerant also
simultaneously absorbs the drive motor winding losses (Process 5-6) and mechanical friction
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Experiment #1: Heat Pump Experiment
losses from the compressor. It is therefore assumed unlikely that a drop in pressure on entry into
the casing will be significant.
Process 6-7 results from frictional losses in the refrigerant in flowing around the compressor
suction flap valve and passages into the cylinder. Some super-heating occurs by mechanical
friction heat transfer from the cylinder wall.
These errors are difficult to quantify but it can be assumed frictional losses are small in
comparison with the actual work done on the refrigerant during compression. Pressure energy
losses arising from viscous friction when the refrigerant is in the vapor state are quite small in
comparison to the overall changes in energy being measured.
Process 7-8. Compression.
Process 8-9. Fall in pressure caused by the opening of the compressor discharge flap valve
pressure and by flow losses around the valve and passages.
The design of the Mechanical Heat Pump does not attempt to extract and isolate the errors
described between state points 4 and 9 for reasons already mentioned. The purpose of the
exercise is to make the student aware of the effect practical limitations can have upon a
theoretical study.
Process 9-10 occurs in the compressor discharge line, i.e. between the outlet from the
compressor and inlet to the condenser. Similar comments used in describing process 3-4 may be
applied here.
Process 10-11 is condensation of the refrigerant followed by some liquid sub-cooling (process 1112). Generally, similar assumptions may be made for condensing as for vaporization with the
exception that liquid viscous friction causes a greater pressure drop.
Process 12-1, throttling, is usually assumed to take place at constant enthalpy.
Throttling is discussed in detail in most text books dealing with Applied Thermodynamics. See the
list of references given in the end of this handout.
Fig. 3 shows the cycle diagram in its simplified form. Compression is shown as a dotted line
indicating an estimated process. The state points representing the start and finish of compression
are those given by the temperature and pressure measured at the outlet from the evaporator and
inlet to the condenser.
Figure 3: Idealized P-h Cycle Diagram
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Experiment #1: Heat Pump Experiment
Procedure:
DAY ONE:
Experiment #1: Refrigeration Performance Analysis
1. Turn on the main water supply and by a process of trial and error set the evaporator flow rate
to 70 kg/hr, the condenser flow rate to 30 kg/hr, and the water supply temperature to 12°C.
2. Set the refrigerant expansion valve to approximately half the middle of its range.
3. Switch on the compressor.
4. Set the refrigerant flow rate to 10 kg/hr.
6. Allow the system to stabilize by monitoring the temperatures in the LabVIEW program. This
will probably take around 30 minutes.
7. Collect data as shown in Table 1 (should be similar to Column 1 data).
NOTE: As the unit reaches equilibrium, there will be some ice formation evident in the right-hand
(blue) evaporating tank. This is normal but the water flow should be adjusted so that this ice
formation does not become excessive. Also be careful not to overflow either of the two
tanks.
Experiment #2: Operation of the Equipment as a Heat Pump
1. Turn on the main water supply and by a process of trial and error set the evaporator flow rate
to 120 kg/hr, the condenser flow rate to 120 kg/hr, and the water supply temperature to 15°C. Be
careful not to overflow either tank.
2. Set the refrigerant expansion valve to fully open.
3. Switch on the compressor.
4. Allow the system to stabilize by monitoring the temperatures in the LabVIEW program. This
will probably take around 30 minutes.
7. Collect data as shown in Table 1 (should be similar to Column 2 data).
8. Reduce the condenser (red tank) water flow rate in small increments (5 kg/hr), allow the
equipment to stabilize (by watching the temperatures), and collect data.
9. Repeat step 8 until the condenser water flow rate gets to 5 kg/hr.
DAY TWO:
Experiment #3: Refrigeration Performance by Supercooling during Condensation
1. Turn on the main water supply and by a process of trial and error set the evaporator (blue
tank) flow rate to 120 kg/hr, the condenser (red tank) flow rate to 20 kg/hr, and the water supply
temperature to 15°C. Be careful not to overflow either tank.
2. Set the refrigerant expansion valve to approximately half the middle of its range.
3. Switch on the compressor.
4. Allow the system to stabilize by monitoring the temperatures in the LabVIEW program. This
will probably take around 30 minutes.
7. Collect data as shown in Table 1 (should be similar to Column 2 data).
8. Reduce the condenser (red tank) water flow rate in small increments (5 kg/hr), allow the
equipment to stabilize (by watching the temperatures), and collect data.
9. Repeat step 8 until the condenser water flow rate has been turned off.
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Experiment #1: Heat Pump Experiment
Calculations:
Experiment #1: Refrigeration Performance Analysis
Students are often introduced to the topic of vapor compression refrigeration via a discussion of
the Carnot Cycle and the Temperature-Entropy properties diagram. A T-s diagram has been
supplied and analysis is conducted using the T-s diagram. This exercise will demonstrate the use
of thermodynamic charts and their respective advantages and disadvantages.
Temperature-Entropy Diagram
Figure 4: T-s Cycle Diagram, Idealized Straight-Line Compression
In the cycle diagram shown in Fig. 4 it is assumed that the compression process approximates to
a straight line. Using the T-s diagram for R-12, the Refrigerating Effect can be computed using:
Refrigerating Effect = Area dbhj = cbhk + dckj = R.E. = T1 (sk − sh ) + (hd − hc )
The work done by the compressor can be computed using:
W&comp = he − hd
The work done by the compressor can also be computed graphically using:
W&comp = Area adef
This can be estimated by integrating the area within the curve, and noting that for the linear
scales on the T-s diagram 2.7X2.7 cm2 corresponds to 2.5 kJ/kg. Compute the theoretical
Coefficient of Performance using:
COPR ,theoretical =
R.E.
W&
comp
Comparison to Ideal Reversible Carnot Cycle:
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Experiment #1: Heat Pump Experiment
By superimposing the ideal reversible Carnot cycle on the practical T-s cycle, as shown in Fig. 5,
one can examine deviations from the ideal and calculate the refrigeration efficiency. Refrigeration
efficiency expresses the approach of the practical cycle to the ideal Carnot cycle.
Figure 5: T-s Cycle Diagram, Practical Cycle versus Carnot Cycle
Refrigerating Efficiency =
COPR
COPR ,Carnot
Ideal refrigerating effect:
= T1 (s1 − s2 )
Ideal compressor work:
= (T5 − T1 )(s1 − s2 )
Ideal Carnot Coefficient of Performance:
COPR ,Carnot =
R.E.
COPR
With this and COPR ,theoretical , the theoretical refrigerating efficiency can be computed.
From Fig. 5 it is seen that the practical refrigerating effect is made larger than the ideal by
subcooling the liquid refrigerant in the condenser before expansion through the throttle and also
superheating the refrigerant during evaporation. These gains appear to give a very efficient cycle
but this is only true on a theoretical basis. No account is taken of the compressor losses and
other thermal losses in the system.
Consider the actual energy consumed by the compressor as measured by the Watt-hour meter.
In this particular test the instrument measuring disc is calibrated at 333.33 revs/h = 1 kW. The
elapsed time for one revolution of the disc is measured using a stopwatch. Say this time is
t seconds, then the power going to the compressor can be computed as:
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Experiment #1: Heat Pump Experiment
1000 * 3600
W&comp =
333.33 * t
The total refrigerating effect can be computed knowing the mass flow rate of refrigerant and the
refrigerating effect:
QL = Area(dbhj ) * m& r
The overall practical Coefficient of Performance can be computed using:
COPR , practical =
Q& L
W&comp
With this, the practical refrigerating efficiency can be computed.
Experiment #2: Operation of the Equipment as a Heat Pump
While operating as a Heat Pump, heat is removed from a cold source at or below ambient
temperature (the blue water cooled evaporator) and transferred to a sink (the red water cooled
condenser) above ambient temperature. As shown in Fig. 6, this heat could be used for space
heating.
Figure 6: Operation of System as a Heat Pump – Space Heating Example
The performance of the Heat Pump can be quantified by the Coefficient of Performance and is
found to vary as the temperature difference ( ∆T ) between the condenser water outlet
temperature ( Th ) and the ambient air temperature ( Ta ). The temperature gradient is computed
from:
∆T = Th − Ta
From this the heat in the condenser water that is transferred to the ambient air can be computed
as:
Q& H = m& h C p (Th − Ta )
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Experiment #1: Heat Pump Experiment
In the context of this experiment the Coefficient of Performance can be computed by the energy
output from the condenser water divided by the energy input to the compressor.
COPHP =
Q& H
W&
comp
The power into the heat pump can be estimated using the Watt-hour meter reading. Using the
& and W& as a function of the temperature
above equations, compute and plot both Q
H
comp
difference ( ∆T ) between the condenser water and the ambient air temperature. Also compute
and plot the Coefficient of Performance of the heat pump as a function of the same temperature
difference. Comment on the temperature difference over which the heat pump operates most
efficiently (most heat produced for the given amount of compressor work input).
Experiment #3: Refrigeration Performance by Supercooling during Condensation
Supercooling is often described in textbooks as occurring at constant condenser pressure and
constant refrigerant mass flow. In practice, it is not possible to supercool the refrigerant without
changing either or both the mass flow and condenser pressure.
Clearly, changes in condenser pressure are accompanied by a change in compression ratio and
compressor work. Similarly a change in mass flow will affect an energy balance and overall
coefficient of performance. Supercooling in a practical cycle may not therefore result in an
automatic increase in performance.
In this experiment the effect of supercooling is examined while maintaining the electrical power
input to the compressor constant. The analysis should be based upon changes in the overall
coefficient of performance with refrigerating effect, refrigerant mass flow rate, compression ratio,
and liquid refrigerant temperature.
Figure 7: P-h and T-s Diagrams Illustrating Superheating and Supercooling
Compute the Coefficient of Performance of the refrigeration cycle as a function of increased
supercooled refrigerant. Plot the Coefficient of Performance as a function of the Refrigerating
Effect, the Refrigerant Mass Flow Rate, the Compression Ratio, and the Liquid Refrigerant
Temperature. Is supercooling useful and why?
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Experiment #1: Heat Pump Experiment
TABLE 1 – Examples of Data to be Collected
Observed Experimental Data
Symbol
Units
Test 1
Test 2
Water supply temperature
Ts
°C
11.0
15.0
Evaporator water outlet temperature
Tc
°C
5.3
9.0
Condenser water outlet temperature
Th
°C
27.5
36.0
Evaporator water flow rate
m& c
kg/h
70.0
120.0
Condenser water flow rate
m& h
kg/h
33.0
40.0
Refrigerant flow rate
m& r
kg/h
9.4
19.0
Watt-hour meter. Time per revolution.
s
s/rev
61.4
45.0
Ambient temperature
Ta
°C
18.5
18.5
Refrigerant absolute pressure
Pe
kN/m2
135
240
Refrigerant inlet (wet vapor) temperature
T1
°C
-22.0
-6.0
Refrigerant outet (superheat) temperature
T2
°C
-4.0
+8.5
Refrigerant absolute pressure
Pc
kN/m2
870
1050
Refrigerant inlet (superheat) temperature
T3
°C
59.0
73.5
Refrigerant outlet (liquid) temperature
T4
°C
13.0
20.0
EVAPORATOR
CONDENSER
References:
1. Hewett, G. A., “Mechanical Heat Pump (Vapor Compression Refrigerator) – Operating
Instructions and Performance Notes,” P.A. Hilton Ltd., England, 1978.
2. Haywood, R.W., “Thermodynamic Tables in SI (metric) Units,” 2nd Edition, Cambridge
University Press, 1968.
3. Cengel, Y.A., and Boles, M., Thermodynamics: An Engineering Approach, 4th Edition,
McGraw Hill, 2002.
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Experiment #1: Heat Pump Experiment