Sungkyunkwan University
School of Mechanical Engineering
Smart Air-conditioning System Lab.
☎ 031)-290-7483
Experiments on the Heat Pump System
Laboratory on Thermal/Fluid Engineering
Contents
1. Introduction
1
2. Objective for the Experiment
1
3. Basic Theory and Literature Review
2
3.1 The Performance of the Vapor compression
3.2 The Ideal Single-Stage Compression Cycle
3.3 Comparing the theoretical Single-Stage Compression Cycle with the
Real Single-Stage Cycle
2
4
5
7
3.4 Refrigerants
4. Experimental Apparatus
9
5. Experimental Method
11
6. Experimental Measured Data
12
7. Data Reduction
13
8. Project and Subject
14
9. References
14
1. Introduction
Some literatures define that “Refrigeration” is the transfer of energy from a space, or air supplied to a space, by
virtue of a difference in temperature between the source and the space or air. In the usual cooling process air is
circulated over a surface maintained at a low temperature. The surface may be in the space to be cooled or at
some remote location from it, the air being ducted to and from the space. Usually water or a volatile refrigerant
is the cooling medium. Cooling usually signifies sensible heat transfer, with a decrease in the air temperature.(1)
Most air-conditioning systems installed prior to the 1970s were designed with little attention to energy
conservation, since fuels were abundant and inexpensive. Escalating energy costs since that decade have caused
increased interest in efficiency of operation. During the same period the need for closely controlled
environments in laboratories, hospitals, and industrial facilities continued to grow. A third factor of expanding
awareness was the importance of comfort and indoor air quality for both health and performance. Practitioners
of the arts and sciences of HVAC system design and simulation were challenged as they never had been before.
Developments in electronics, controls and computers have furnished the tools allowing HVAC to become a
high-technology industry. Although tools and methods have changed, and a better understanding of the
parameters that define comfort and indoor air quality have been accomplished, many of the basics of good
system design have not changed. These basic elements of HVAC system design are the emphasis of this test and
furnish a basic for presenting recent developments of importance and procedures for designing functional, wellcontrolled, and energy-efficient systems to maintain human comfort and health as well as industrial productivity.
In this study, therefore, the first interest is to understand the basic theory of the air conditioning system and the
single stage compression cycle by the experiment, second is to calculate the performance of the refrigeration
systems, the Cooling coefficient of performance from the experimental data and the third is to study the
characteristics of general importance for refrigerants. It is because concerns about depletion of atmospheric
ozone and about global warming have focused attention on the release into the atmosphere of certain
refrigerants widely used in the HVAC field.
2. Objective for the Experiment
The basic objectives of this experiment are for the students, who already study the basic theories of the cycle in
the thermodynamics class, to understand the HVAC system and vapor compression cycle system through the
experimental accomplishment.
Therefore, the main objectives of this experiment are as follows:
1) To grasp the difference between the theoretical single-stage compression cycle and the real one.
2) To investigate the performance of the refrigeration systems, the efficiency of the compressors and the cooling
coefficient of performance.
3) To investigate the effect of the fan speed on the performance of the refrigeration systems through the
experiments for the control of the fan speed in the heat exchanger of the condenser and the evaporator.
4) To investigate the things of number 2) and 3) through the P-h diagram
5) To study the characteristics of R-134a which is used in this experiment and general importance for
refrigerants.
3. Basic Theory and Literature Review
3.1 The Performance of the Vapor compression (Reversed Carnot Cycle)
Refrigeration is the transfer of energy from a space, or air supplied to a space, by virtue of a difference in
temperature between the source and the space or air. In the usual cooling process air is circulated over a surface
maintained at a low temperature. So almost every HVAC system that provides a cooling effect depends on a
refrigeration system to provide either a cold liquid such as water or brine or a direct removal of sensible and
latent heat from an air stream. The ideal refrigeration cycle is commonly called the “Reversed Carnot Cycle” (2).
Fig. 1 shows the P-V diagram of the ideal refrigeration cycle.
In studying the performance of refrigeration cycles the concept of the thermodynamically reversible cycle is
useful. Two important characteristics of a reversible cycle are as follows:
1) No refrigeration cycle can have a higher coefficient of performance than that of a reversible cycle
operating between the same source and sink temperatures.
2) All reversible refrigeration cycles operates between the same source and sink temperatures have identical
coefficients of performance.
P
Q1
1
T1=C
2
PVk=C
PVk=C
3-2 : Reversible adiabatic pumping process
2-1 : Isothermal compression
1-4 : Reversible adiabatic expansion
4
4-3 : Isothermal expansion
3
T2=C
Q2
V
Fig. 1 P-V diagram of the ideal refrigeration cycle
The most convenient reversible cycle to use as an ideal refrigerating cycle is the Carnot refrigeration cycle,
consisting of two isothermal processes and two adiabatic processes. Such a cycle is shown in Fig. 1 on P-V
coordinates. Because of characteristic 2 listed above, no particular working medium needs to be specified.
Notice that in the Carnot refrigeration cycle all of the heat is absorbed at the lower (evaporator) temperature T2
in process 4-3 at constant temperature, and all of the heat is rejected at a constant higher (condenser)
temperature T1 in process 2-1.
The instantaneous performance of any refrigerating system when used for cooling is expressed in terms of the
cooling coefficient of performance (COP)
COP =
useful refrigerating effect
= Q2 / WC = Q2 /  Q1  Q2  = T2 / T1  T2 
net energy input
COP is a dimensionless quantity, expressible as a pure number.
In an ideal refrigerator system, T1 is equivalent to the temperature of the concentrated refrigerant leaving the
condenser, T2 is equivalent to the temperature of the evaporative refrigerant leaving the evaporator. Therefore
the COP will increase with the higher T1 and the lower T2 (7).
However the theoritical refrigerator system is different from the ideal one. Fig. 2 shows the P-h diagrams of an
ideal cycle and a theoretical single-stage cycle. 1-2-3-4-5 is the theoretical single-stage cycle, and 1-a-3-b-5-1 is
an ideal cycle. The isothermal changes in an ideal refrigerator system become the constant-pressure changes in a
theoretical one. And the reversible adiabatic expansion becomes the isenthalpic expansion.
Fig. 2 P-h diagram showing the relationship between the ideal refrigeration cycle and the theoretical
refrigeration cycle
3.2 The real refrigeration cycle equipment components
Fig. 3 illustrates a real single-stage cycle schematically. The real aspects of the compressor are shown as state
points A and B. Other factors cause the complete cycle to deviate from the theoretical. These are the pressure
losses in all connecting turbine that increase the power requirement for the cycle, and the heat transfer to and
from the various components. Heat transfer will generally be away from the system on the high-pressure side of
the cycle and will improve cycle performance. Exposed surfaces on the low-pressure side of the cycle will
generally be at a lower temperature than the environment, and any heat transfer will generally degrade cycle
performance.
A : Inlet of the compressor
B : Outlet of the compressor
A-B : Pumping process
B-C-D’-D: Heat is transferred from the working fluid
in process
D-E : Throttling process in the expansion valve
E-A : Evaporating process in the evaporator
Fig. 3 The real refrigeration cycle
Fig. 4 P-h, T-s diagrams of the real refrigeration cycle
3.3 Comparing the theoretical Single-Stage Compression Cycle with the Real Single-Stage Cycle
Fig. 5 shows the P-h diagram of the real compression cycle. The P-h diagram can be used easily for the
calculations of the amount of the compression work (kJ/kg), the refrigerating rate, the COP and so on.
Fig. 5 P-h diagram of the real compression cycle
The compression work (kg/kg) is determined as h5-h1 from the first law of thermodynamics with zero kinetic
and potential energy, that it may be written
h5+q=h1+w
The heat transfer is “0” (adiabatic process) so that the amount of work is h5-h1.
Heat rejection in condenser (kj/kg) is h2-h1 from the refrigerant. The refrigerating rate (kg/kg) is h5-h3. So the
COP is defined as
COP =
h5  h3
h1  h5
As shown in Fig. 5, q4-5 increases compressor load. It should be mentioned that the condenser and evaporator
heat transfer surfaces have pressure losses that also contribute to the overall compressor power requirement.
3.4 Refrigerants
The fluid used for energy exchanges in a refrigerating or heat pump system is called the refrigerant. The
refrigerant usually absorbs heat while undergoing a phase change (in the evaporator) and then is compressed to a
higher pressure and a higher temperature, allowing it to transfer that energy (in the condenser) directly or
indirectly to the atmosphere or to a medium being purposefully heated. A refrigerant’s suitability for a given
application depends on many factors including its thermodynamic, physical, and chemical properties, and its
safety. The significance or relative importance of each characteristic varies from one application to the next, and
there is no such thing as an ideal refrigerant for all applications. Some of the characteristics of general
importance are as follows:
1) Thermodynamic Characteristics
- High latent enthalpy of vaporization: This means a large refrigerating effect per unit mass of the refrigerant
circulated. In small-capacity systems, however, the resulting low flow rate may actually lead to problems.
- Low freezing temperature: The refrigerant must not solidify during normal operating conditions.
- Relatively high critical temperatures: Large amount of power would otherwise be required for compression.
- Positive evaporating pressure: Pressure in the evaporator should be above atmospheric to prevent air from
leaking into the system.
- Relatively low condensing pressure: Otherwise expensive piping and equipment will be required.
2) Physical and Chemical Characteristics
- High dielectric strength of vapor: This permits use in hermetically sealed compressors where vapor may come
in contact with motor windings.
- Good heat-transfer characteristics: Thermo-physical properties (density, specific heat, thermal conductivity,
and viscosity) should be such that high heat-transfer coefficients can be obtained.
- Satisfactory oil solubility: Oil can dissolve in some refrigerants or some refrigerants in the oil. This can affect
lubrication and heat-transfer characteristics and lead to oil logging in the evaporator. A system must be designed
with the oil solubility characteristics in mind.
- Low water solubility: Water in a refrigerant can lead either to freeze-up in the expansion devices or corrosion.
- Inertness and stability: The refrigerant must not react with materials that will contact it, and its own chemical
makeup must not change with time.
3) Safety
- Non-flammability: The refrigerant should not burn or support combustion when mixed with air.
- Non-toxicity: The refrigerant should not be harmful to humans, either directly through food stuffs.
- Non-irritability: The refrigerant should not irritate humans (eyes, nose, lungs, or skin)
4) Effect on the environment
- Ozone depletion potential: The refrigerant’s potential to deplete the ozone in the upper atmosphere should be
low.
- Global warming potential: The refrigerant’s potential to persist in the upper atmosphere and to trap the
radiation emitted by the earth (the green house effect) should be low.
4. Experimental Apparatus
The experimental apparatus for this experiment is the refrigerant cycle and heat pump for fundamental
experiments and made by Daeil-Bio Tech. It mainly consists of compressor, condenser, evaporator and so on.
The equipment was described briefly at the below.
(1) Compressor
A compressor is mechanically the most complex and is often the most expensive single item in the system. A
compressor will be at partial load much of the time. With small systems, the compressor can be started and
stopped by thermostat action when the demand for cooling is satisfied. However, it is not desirable to start and
stop larger compressors at frequent intervals. In this case a technique is employed to reduce the compressor
capacity, referred to as compressor unloading. This may be done by reducing the compressor speed, holding the
intake valves open, or increasing the clearance volume. Fig. 6 shows the 5/4 HP compressor for this experiment.
Fig. 6 Compressor for this experiment
(2) Condenser and Evaporators
Aside from the compressors, the heat exchangers (condensers and evaporators) are major cost items in a
refrigerating system and very often take up the most space. Their proper configuration and capacity are critical
to proper performance.
A condenser is a heat exchanger that usually rejects all the heat from the refrigeration system. This includes not
only the heat absorbed by the evaporator but also the energy input to the compressor. The condenser accepts hot,
high-pressure refrigerant, usually a superheated gas, from the compressor and rejects heat from the gas to some
cooler substance, usually air or water. As energy is removed from the gas, it condenses, and the condensate is
drained so that it may continue its path back through the expansion valve or capillary to the evaporator.
Condensers may be water-cooled, air-cooled, or evaporative (air- and water-cooled.) Important design criteria
for condensers are given in the ASHRAE Handbook, HVAC Systems and Equipment Volume 6. Standards
pertaining to the design, testing and rating of HVAC condensers and condensing units are listed in each of the
ASHRAE handbooks. As pressure vessels, most condensers must meet the requirements of national, state, and
local codes, which are often patterned from the ASME Boiler and Pressure Vessel Code, Unfired Pressure
Vessels.
Energy is picked up in the evaporator by heat transfer from a medium at some slightly higher temperature,
causing the refrigerant to evaporate. Where an expansion valve or capillary tube is used, the refrigerant is
usually received in the evaporator in a two-phase, low-temperature condition, having partially evaporated in the
throttling process. Most evaporators are designed and controlled to bring the refrigerant to a small degree of
superheat as it leaves the evaporator so as to protect the compressor downstream from the damaging effects of
liquid. The medium transferring heat to the evaporator may be the air stream to be cooled or may be water or
brine, as in the case of chillers. Where a small temperature difference is required between the refrigerant and the
medium being cooled, a flooded evaporator is sometimes used. In this case the evaporator coil is supplied
refrigerant liquid, which circulates from the coil to a surge tank, where refrigerant vapor is drawn into the
compressor suction line. Circulation between the surge tank and the evaporator coil may be controlled by a
thermosyphon effect or by forced pump circulation.
Fig. 7 Condenser and Evaporators
(3) Expansion valve
An expansion valve is a slide valve used in a steam engine to control the cut-off. It rides on the back of an
adapted main D slide valve and is driven by an additional eccentric that has more advance than the main
eccentric. The cut-off is adjusted in one of two ways. The stroke of the expansion valve may be altered by
adjusting the throw of the eccentric or by an expansion link and radius rod arrangement, usually under the
control of a centrifugal governor. Alternatively, the effective length of the expansion vale can be altered. The
Meyer expansion valve has two heads mounted on opposite-handed threads on a rotatable valve rod, so that
rotating the rod moves the heads either together or apart, according to the direction of rotation. In this
arrangement the cut-off is normally controlled manually. The engines at Coleham Pumping Station have Meyer
expansion valves on the high-pressure cylinders.
Fig. 8 Expansion valve
(5) Flow-meter
Both gas and liquid flow can be measured in volumetric or mass flow rates (such as litres per second or kg/s).
These measurements can be converted between one another if the material's density is known. The density for a
liquid is almost independent of the liquid conditions; however, this is not the case for a gas, the density of which
highly depends upon pressure, temperature and to a lesser extent, the gas composition. When gases or liquids
are transferred for their energy content (such as the sale of natural gas) the flow rate may also be expressed in
terms of energy flow, such as GJ/hour or BTU/day. The energy flow rate is the volume flow rate multiplied by
the energy content per unit volume or mass flow rate multiplied by the energy content per unit mass. Where
accurate energy flow rate is desired, most flow meters will be used to calculate the volume or mass flow rate
which is then adjusted to the energy flow rate by the use of a flow computer. In engineering contexts, the
volumetric flow rate is usually given the symbol Q and the mass flow rate the symbol .
(6) Extra equipments
- Dryer: The dryer can be provided to remove the moisture, the acid radical and sewage or dust.
- Liquid receiver: The liquid receiver acts as a stock of liquid refrigerant for the evaporators. However, the
receiver should be sized to hold the full system charge during service work.
- Oil separator: Oil separator can be provided to retrofit existing concrete vaults and tanks to improve their
separation performance and increase flow rates.
5. Experimental Method
① For the test, at first, the atmospheric pressure, the dry and wet bulb temperatures are measured.
② Enter up the fan power of the Low, Medium and High while only main power is turned on. This is to
calculate the power of the compressor.
③ Turn off the fan and to close all valves without V-1, V-4, V-5, V-7, V-10 and V-11.
④ Start the apparatus with turning on the compressor power.
⑤ Drive the apparatus for 5 minutes and to maintain the fan on the low level.
⑥ After 5 minutes, measure the flow meter ( m ).
⑦ Repeat the ⑤ and ⑥ with the fan on the medium.
⑧ Repeat the ⑤ and ⑥ with the fan on the high.
⑨ Perform ⑤,⑥,⑦ and ⑧ every twice and record all data.
<Schematic diagram of refrigerating system>
6. Experimental Measured Data
Number of times
P1 (kPa)
Outlet of the
compressor
T1 (℃)
h1 (kJ/kg)
P2
Outlet of the
condenser
T2
h2
P3
Inlet of the
evaporator
T3
h3
P4
Outlet of the
evaporator
T4
h4
P5
Inlet of the
compressor
T5
h5
m (LPM : l /min)
1
2
7. DATA Reduction
7.1 Process to calculate the COP (Cooling Coefficient of Performance)
(1) Work of compression
wc   h1  h5   q
(2) Power of the real cycle compression
wc,a  m  h1  h5  (kJ/s)
(3) Refrigerating effect
qL  h4  h3 (kJ/kg)
(4) Mass flow rate of the refrigerant ( m )
(5) Compression efficiency (adiabatic)
c 
h1, s  h5
h1  h5
(6) Refrigerating capacity
Qm  m  qL   V  qL (kJ/s)
(7) COP
COP=
(8) Drawing the P-h diagram
Qm h4  h3

wc,i h1  h5
8. Project and Subject
8.1 Data reduction and results
(1) Draw the P-h diagram, #7.1 (8)
(2) Calculate the performance of the refrigeration systems, the efficiency of the compressors and the cooling
coefficient of performance (COP), #7.1 (1)-(7)
9. References
(1) 노승탁, “공업열역학”, 1993, 문운당
(2) 서정윤, “공업열역학”, 1989, 성안당
(3) E. C. Guyer, “Handbook of applied thermal design”, 1989, McGraw-Hill.
(4) W. F. Stoecker and J. W. Jones. “Refrigeration & Air-conditioning”, 1989, McGraw-Hill.
(5) 노상순 외 1 명(공저), “신고 공기조화”, 1991, 동명사
(6) G. S. Van Wylen & R. e. Sonntag(원저), 최인규 외 2 명(공역), “공업열역학", 1992, 보성문화사
(7) 오후규, ”완성냉동공학“, 1992, 한미
(8) Mcquiston, F. C., Parker, J., and Spitler, J., “Heating, Ventilating, and Air conditioning 5 th edition”, 2000,
Wiley
# Annexed paper: R-134a Enthalpy diagram and properties
히트펌프를 이용한 냉동실험 Quiz (손으로 작성)
1. 이상적인 냉동 사이클의 P-v 및 P-h 선도를 그리고, 각 과정이 무슨 과정인지를 간단하게
쓰시오.(Draw P-v, P-h diagram about ideal refrigeration cycle and explain each process
simply.)
2. 이상 냉동 cycle 에서 응축온도 52℃, 증발온도 -7℃이면 성적계수(COP)는 얼마인가?
(if ideal refrigeration cycle, condensing temperature 52℃, evaporating temperature -7℃,
calculate COP.)
3. 다음은 어떤 냉동시스템에서의 냉매의 상태값이다. 항목별로 답하시오.(풀이과정 필수)
(This is the refrigerant system. Solve the problem 1), 2), 3).)
상태
압력(MPa, pressure) 온도(℃, temperature) 엔탈피(kJ/kg, enthalpy)
압축기 입구
0.35
10
405
(Inlet of compressor)
압축기 출구
1.15
60
438
(Outlet of compressor)
증발기 입구
0.35
(Inlet of evaporator)
5
256
단, 증발기 입구와 압축기 입구의 압력강하는 없고, 질량 유동률은 일정하다.
1) 압축기의 압축일량을 구하시오.(kW)(Calculate the compressor work.)
2) 증발기의 냉동능력을 구하시오.(kW) (Calculate the evaporator cooling capacity.)
3) 위 시스템의 성적계수(COP)를 구하시오.(Calculate the COP of this system.)
4. 일반적으로 요구되는 냉매 요구 조건(냉매의 특성)을 3 가지 이상 쓰시오
(Write more than three answers, characteristics of the refrigerant typically required.)