ABSTRACT. This experiment demonstrated the general operation of vapor compression heat pump. In order to determine the power input and coefficient of performance of the heat pump, the temperature and pressure of water and refrigerant are observed. The process cycle of heat pump is clearly illustrated in pressure enthalpy diagram. The compressor pressure ratio and volumetric efficiency are also calculated. The objectives of this experiment successfully achieved. This experiment is successful achieved although the experiment do not produce a good result since the execution of the procedure is not fully accurate. . INTRODUCTION. A refrigeration unit is a unit composed of several machineries that can transfer heat from a low-temperature region to a high-temperature region. Normally heat can only be transfer from a high temperature region to low, so to reverse this process, a refrigerator is needed. Refrigerator consists of four main components which are; compressor, evaporator, expansion valve and condenser. Each component works together to perform a series of process. The process includes a cycle of refrigerant and a flow of cooling water. In the experiment, the refrigeration unit used is SOLTEQ Mechanical Heap Pump (Model: EH165). The device is design to give understanding to the user on how refrigeration process worked. The SOLTEQ Mechanical Heat Pump (Model: HE165) is a bench top unit with all components and instrumentations mounted on the sturdy base. The heat pump consists of a hermetic compressor, a water-cooled plate heat exchanger, a thermostatic expansion valve and a water heated plate heat exchanger. The arrangements of the components are in a manner similar to many domestic air- water heat pumps where they are visible from the front of the unit. During the operation, slightly superheated refrigerant (R-134a) vapor enters the compressor from the evaporator and its pressure is increased. Thus, the temperature rises and the hot vapor then enters the water cooled condenser. Heat is given up to the cooling water and the refrigerant condenses to liquid before passing to the expansion valve. Upon passing through the expansion valve, the pressure of the liquid refrigerant is reduced. This causes the saturation temperature to fell to below that the atmospheric. Thus, as it flows through the evaporator, there is a temperature difference between the refrigerant and the water being drawn across the coils. The resulting heat transfer causes the refrigerant to boil,
and upon leaving the evaporator it has become slightly superheated vapor, ready to return to the compressor. The temperature at which heat is delivered in the condenser and the evaporator is controlled by the water flow rate and its inlet temperature. Instrumentations are all provided for the measurement of flow rates of both refrigerant and cooling water, power input to the compressor and all relevant temperatures.
AIMS/OBKECTIVES. i.
Experiment 1 The objectives of this experiment is to determine the power input, heat output and coefficient of perfomance.
ii.
Experiment 2 The objectives of this experiment is to determine the production of heat pump perfomance curves over a range of source and delivery temperatures.
iii.
Experiment 3 The experiment is to determine the production of vapor compression cycle on p-h diagram nd energy balance study
iv.
Experiment 4 The experiment is to study determine the production of heat pump perfomance curves over a range of evaporating and condensation temperature.
v.
Experiment 5 The objectives of this experiment is to estimate the effect of compressor pressure ratio on volumetric efficiency.
THEORY. A heat pump is a mechanism that absorbs heat from waste source or surrounding to
produce valuable heat on a higher temperature level than that of the heat source. The fundamental idea of all heat pumps is that heat is absorbed by a medium, which releases the heat at a required temperature which is higher after a physical or chemical transformation.
Heat pump technology has attracted increasing attention as one of the most promising technologies to save energy. Areas of interest include heating of buildings, recovery of industrial waste heat for steam production and heating of process water for e.g. cleaning, sanitation.
Generally, there are three types of heat pump systems: i) ii)
Closed cycle vapor compression heat pumps (electric and engine driven) Heat transformers (a type of absorption heat pump)
iii)
Mechanical vapor recompression heat pumps operating at about at 200°C
Closed Cycle Vapor Compression Heat Pump Most of the heat pumps operate on the principle of the vapor compression cycle. In this cycle, the circulating substance is physically separated from the heat source and heat delivery, and is cycling in a close stream, therefore called ‘closed cycle’. In the heat pump process, the following processes take place:
i)
In the evaporator the heat is extracted from the heat source to boil the circulating
ii)
substance The circulating substance is compressed by the compressor, raising its pressure
iii)
and temperature The heat is delivered to the condenser
iv)
The pressure of the circulating substance (working fluid) is reduced back to the evaporator condition in the throttling valve.
Figure 1: The closed loop compression cycle
Vapor Compression Heat Pump System Principles
Figure 2: Vapor compression heat pump cycle The components are; 1) 2) 3) 4)
Condenser Compressor Expansion Valve Evaporator
Four basic processes or changes in the condition of the refrigerant occur in a Vapor
Compression Heat Pump Cycle. These four processes shall be illustrated in the most simplistic way with the aid of above figure. i)
Compression Process (
t1 ¿ t2 )
The refrigerant at the pump suction is in gas at low temperature and low Pressure. In order to be able to use it to achieve the heat pump effect continuously, it must be brought to the liquid form at a high pressure. The first step in this process is to increase the pressure of the refrigerant gas by using a compressor. Compressing the gas also results in increasing its temperature.
ii)
Condensing Process (
t2 ¿ t3 )
The refrigerant leaves the compressor as a gas at high temperature and pressure. In order to change it to a liquid, heat must be removed from it. This is accomplished in a heat exchanger called the condenser. The refrigerant flows through one circuit in the condenser. In the other circuit, a cooling fluid flows (normally air or water), at a temperature lower than the refrigerant. Heat is therefore transferred from the Refrigerant to the Cooling fluid and as a result, the refrigerant condenses to a liquid state (3). This is where the heating takes place. iii)
Expansion Process (
t3 ¿ t 4 )
At Point (3), the refrigerant is in liquid state at a relatively high pressure and temperature. It flows to (4) through a restriction called the flow control device or expansion valve. The refrigerant loses pressure going through the restriction. The Pressure at (4) is so low that a small portion of the refrigerant flashes (vaporizes) into a gaseous. In order to vaporize, it must gain heat (which it takes from that portion of the refrigerant that did not vaporize). iv)
Vaporizing Process (
t 4 ¿t 1 )
The refrigerant flows through a heat exchanger called the evaporator. The heat source is at a slightly higher temperature than the refrigerant, therefore heat is transferred from it to the refrigerant. The refrigerant boils because of the heat it receives in the evaporator. By the time it leaves the evaporator (4) it is completely vaporized.
Understanding the Pressure Enthalpy Diagram
Figure 3: A pictorial representation of a P-H diagram Using the chart of R-134a Refrigerant (Figure 3), we shall attempt to explain the use of it: The Chart is divided into THREE areas. These three areas are separated from each other by the following: a) Saturated liquid b) Saturated vapor line
The area on the chart to the left of the Saturated Liquid line is called the SUBCOOLED region. At any point in the sub-cooled region, the refrigerant is in the LIQUID phase and its Temperature is below Saturation temperature corresponding to its Pressure. The area to the right of the Saturated Vapor line is the SUPERHEATED region, and the refrigerant is in the form of a SUPERHEATED VAPOR.
The area between the SATURATED LIQUID and the SATURATED VAPOR lines is the
mixture region and represents the change in phase of the refrigerant between the liquid and Vapor phases. Thus, at any point between the two saturation lines the refrigerant is in the form of liquid-vapor mixture. The distance between the two lines along any constant pressure line is known as the “latent heat of vaporization” at that pressure. The Saturated Liquid line and Saturated Vapor line are not exactly parallel to each other because the "latent heat of vaporization" varies with the pressure at which the change in phase occurs. This change of phase from liquid to vapor phase takes place progressively from LEFT to RIGHT and the change in phase from vapor to liquid phase occurs from RIGHT to LEFT. At any point on the Saturated Liquid line, the refrigerant is at SATURATED LIQUID and at any point along the SATURATED VAPOR LINE, the Refrigerant is a SATURATED VAPOR.
The HORIZONTAL LINES, extending across the chart are CONSTANT PRESSURE Lines. The VERTICAL Lines are Lines of CONSTANT ENTHALPY. The Lines of Constant Temperature varies, depending on the phase stage. It is almost VERTICAL in the SUBCOOLED region and is PAPALLEL to lines of CONSTANT ENTHALPY. It however changes at the CENTRE section, since the refrigerant changes state at a CONSTANT TEMPERATURE AND PRESSURE, the lines of Constant Temperature are now, parallel to constant Pressure line. At the Saturated Vapor line, the lines of Constant Temperature changes direction again and upon entering the SUPERHEATED VAPOR REGION, it falls off sharply towards the bottom of the chart. The ENHALPY Values are found on the Horizontal scale at the bottom of the chart.
The Magnitude' of the Pressure in bar/MPa is read on the vertical scale at the left side of the chart. Temperature values in degrees Celsius are found adjacent to constant temperature lines in sub-cooled and superheated regions of the chart on both Saturated Liquid and
Saturated Lines. It is worthwhile to note that the p-h diagram is based on a limb mass of the refrigerant, the volume given is the specific volume, the Enthalpy is in kJ per kg, and the entropy is in kJ per kg per degree of absolute temperature. Obtain the Enthalpy Values from P-H Diagram To obtain the following values, we first refresh our memory from the previous chapter on: a) Flow diagram of a Simple Saturated Cycle b) Enthalpy or p-h diagram of R-134A, Simple Saturated Cycle, as shown below;
Figure 4: Flow diagram of a simple saturated cycle
Figure 5: Comparison of two simple saturated cycles operating at different vaporizing temperatures (figure distorted) (Refrigerant-134a)
Figure 6a: Skeleton P-H chart illustrating the three regions of the chart and the direction of phase changing
Figure 6b: Skeleton P-H chart showing oaths of constant pressure, constant temperature constant volume, constant enthalpy, and constant entropy (Refrigerant-134a)
Figure 6c: Pressure-enthalpy diagram of a simple saturated cycle operating at a vaporization temperature of 200F and a condensing temperature of 1000◦F (Refrigerant – 134a)
In recalling, and referring back to Figure 5 and 6, of a Simple Saturated Cycle, we now thus obtained the following values:
h1
= The Enthalpy at Point 1, which is the point where “Compression Process" begins (This is also where we obtained the Temperature Reading, TT1 for the process)
h2
= The Enthalpy at Point 2, which is the point where "Compression Process" ends.
h3
= The Enthalpy at Point 3, which is the point where "Condensation" is complete. (This is also where we obtained the Temperature Reading, TT3 for the process)
Thus, h2 – h3 = Refrigerating Effect (See figure 5) While, h2 – h1 = Heat of Compression (See figure 5)
Figure 7: Pressure-enthalpy diagram of a simple saturated cycle operating at a vaporizing temperature of 20oF and a condensing of 100◦F (Refrigerant- 134a)
Coefficient of Performance The Coefficient of Performance, (COPH) of a heat pump cycle is an expression of the cycle efficiency and is stated as the ratio of the heat removed in the heated space to the heat energy equivalent of the energy supplied to the Compressor. COPH = Heat removed from heated space / Heat energy equivalent of the Energy supplied to the compressor. Thus, for the Theoretical Simple Cycle, this may be written as:
COPH =
=
Heating Effect Heat of Compression
(h 2−h3) (h 2−h1)
APPARATUS.
Figure 8: Unit construction for Mechanical Heat Pump (Model: HE165)
1. 2. 3. 4. 5. 6. 7. 8. 9.
Pressure switch Receiver tank Compressor Condenser Pressure transmitter Control panel Evaporator Refrigerant flow meter Water flow meter
PROCEDURES. i.
General Operating Procedures A. General Start-Up Procedures 1. The unit and all instruments are checked to ensure they are in proper conditions. 2. The water source and drain are checked to ensure they are connected. The water supply is open and the cooling water flowrate is set as 1.0 LPM 3. The drain hose at the condensate collector is checked to ensure it is connected. 4. Power supply is connected. The main switch is on at the main power and main switch on control panel. 5. Refrigerant compressor is switch on.when the temperature and pressure is constant, the unit is ready for experiment. B. General Shut-Down Procedures 1. The compressor is switched off followed by the main switch and power supply. 2. The water supply is closed.
1. 2. 3. 4.
1. 2. 3. 4. 5. 6.
Experiment 1 Geeral start up procedure is performed. The cooling wate r flow rate is adjusted to 40% The system is run for 15minutes. The reading is recorded. Experiment 2 The The general start-up was run. The cooling water flow rate was adjusted to 60%.. The system was allowed to run for 15 minutes. All the necessary reading was recorded in the table. The procedure 1 to 4 was repeated with different cooling water flow rate. (40% and 20%) All the necessary reading was recorded in the table .
Experiment 3 1. 2. 3. 4.
The general start-up was run. The cooling water flow rate was adjusted to 40%. The system was allowed to run for 15 minutes. All the necessary reading was recorded in the table.
Experiment 4 1. The general start-up was run. 2. The cooling water flow rate was adjusted to 60%. 3. The system was allowed to run for 15 minutes.
4. All the necessary reading was recorded in the table. 5. The procedure 1 to 4 was repeated with different cooling water flow rate. (40% and 20%) 6. All the necessary reading was recorded in the table.
Experiment 5 1. 2. 3. 4.
The general start-up was run. The cooling water flow rate was adjusted to 40%. The system was allowed to run for 15 minutes. All the necessary reading was recorded in the table.
RESULTS AND CALCULATIONS EXPERIMENT 1 Cooling Water Flow Rate, FT1 Cooling Water Flow Rate, FT1 Cooling Water Inlet Temperature, TT5 Cooling Water Outlet Temperature , TT6 Compressor Power Input
% LPM °C °C W
Cooling water flow rate (LPM)
Refrigerant flow rate (LPM)
=
=
Cooling water flow rate () 100
Refrigerant flow rate( ) 100
x 1.26 LPM
Heat output 2000
mL min
x
1g 1 mL
x
1 kg 1000 g
T ( K )=T (℃)+273.15
C p @ 25 ℃−50℃ =4.18
kJ kg . K
Q=m C p dT
(
¿ 0.033 kg 4180
¿ 165.53W
J (31.5−30.3) K kg . K
)
x
1 min 60 s
x 5 LPM
= 0.033 kg/s
40 2 30.3 31.5 165
Coefficient of performance
W net ,∈¿ CO PR =
¿
Desired output Q L = Required input ¿
165.53 165.0
¿ 1.0032
EXPERIMENT 2
Cooling Water Flow Rate, FT1 Cooling Water Flow Rate, FT1 Cooling Water Inlet Temperature, TT5 Cooling Water Outlet Temperature , TT6 Compressor Power Input Heat Output
CO PR
*The calculations are similar to that in Experiment 1.
% LPM °C °C W W
1 60 3 30.2 31.3 164 151.73 0.92
2 40 2 30.3 31.3 165 206.91 1.25
3 20 1 30.2 33.0 167 386.23 2.31
Power input vs temperature
Power Input (W)
167.5 167 167 166.5 166 165.5 165 165 164.5 164 164 163.5 163 162.5 31.2 31.4 31.6 31.8 32 32.2 32.4 32.6 32.8 33 33.2 Temperature (℃)
Heat output vs temperature 450
386.23
400 350 300 250 Heat output (W)
206.91
200 151.73 150 100 50 0 31.2 31.4 31.6 31.8
32
32.2 32.4 32.6 32.8
Temperature (℃)
33
33.2
COP vs temperature 2.31
2.5 2 1.5 COP
1
1.25 0.92
0.5 0 31.2
31.4
31.6
31.8
32
32.2
32.4
32.6
32.8
33
33.2
Temperature (℃)
EXPERIMENT 3 Refrigerant Flow Rate, FT2 Refrigerant Flow Rate, FT2 Refrigerant Pressure (Low), P1
% LPM Bar
61.3 0.77 2.0
Refrigerant Pressure (High), P2
(abs) Bar
7.1
Refrigerant Temperature, TT1 Refrigerant Temperature, TT2 Refrigerant Temperature, TT3 Refrigerant Temperature, TT4 Cooling Water Flow Rate, FT1 Cooling Water Flow Rate, FT1 Cooling Water Inlet Temperature, TT5 Cooling Water Outlet Temperature , TT6 Compressor Power Input
(abs) °C °C °C °C % LPM °C °C W
28.7 82.0 81.1 24.7 40.1 2.01 30.3 31.9 165
Point Pressure (bar)
1 2.0
2 7.1
3 7.1
4 2.0
Temperature ( ℃ ¿ Enthalpy (kJ/kg)
28.7
82.0
31.1
24.7
277.76
320.15
269.56
274.27
*All of the enthalpy value was obtained from the property table of R-134a (interpolation)
P 2−P 1 P 3−P 1
=
T 2−T 1 T 3−T 1
Pressure 8
7.1
7
7.1
6 5 Pressure (bar)
4 3 2
2
2
1 0 277.76
320.14999999999998
269.56
Enthalpy (kJ/kg)
*The cycle is out of the curve (all of the state is in superheated vapor)
274.27
Figure 9: Ideal vapor compression cycle Compressor energy balance
Q W m(h pe ke)
Q 0, ke 0, pe 0
W m h
Condenser energy balance
Q W m(h pe ke)
W 0, ke 0, pe 0
Q m h
Q mh mh in
out
EXPERIMENT 4
Refrigerant Flow Rate, FT2 Refrigerant Flow Rate, FT2 Refrigerant Pressure (Low), P1 Refrigerant Pressure (High), P2 Refrigerant Temperature, TT1 Refrigerant Temperature, TT2 Refrigerant Temperature, TT3 Refrigerant Temperature, TT4 Entalphy 1 (P1, TT1)
% LPM Bar (abs) Bar (abs) °C °C °C °C kJ/kg
1 61.2 0.77 2.0 7.1 28.8 82.2 30.8 24.5 277.84
2 61.2 0.77 2.0 7.1 28.6 82.0 31.1 24.4 277.67
3 61.3 0.77 2.0 7.1 28.3 81.0 32.0 24.5 277.41
Entalphy 2 (P2, TT2) Entalphy 3 (P2, TT3) Evaporating Temperature (TT4) Condensing Temperature (TT3) Compressor Power Input Heat Delivered in Condenser (Refrigerant) COPH
kJ/kg kJ/kg °C °C W W
320.35 269.18 24.5 30.8 163 0.665 1.204
320.15 269.46 24.4 31.1 165 0.659 1.193
319.15 270.37 24.5 32.30 164 0.634 1.169
*All of the enthalpy value was obtained from the property table of R-134a (interpolation)
L min
0.77
X
1000 mL L
Trial 1 Q
= m∆h = m(h2-h3) = 0.013(320.35-269.18) = 0.665 kW
COPH =
Heating Effect Heat of Compression
=
(h 2−h3) (h 2−h1)
=
321.55−268.7 321.55−276.74
= 1.179
X
g mL
X
kg 1000 g
X
min 60 s
= 0.013 kg/s
*The same calculations are used for Trial 2 and 3.
EXPERIMENT 5 Refrigerant Flow Rate, FT2 Refrigerant Flow Rate, FT2 Refrigerant Pressure (Low), P1
% LPM Bar
61.1 0.77 2.0
Refrigerant Pressure (High), P2
(abs) Bar
7.1
Refrigerant Temperature, TT1
(abs) °C
30.2
Compressor pressure ratio
Suction pressure of refrigerant
= Discharge pressure of refrigerant
=
2 7.1
= 0.282
Volumetric efficiency
=
Actual mass flow rate Theoretical mass flow rate
=
P1 P2
DISCUSSIONS. Boyle’s law stated that the pressure of gas inversely proportional to the volume of a container. From the results recorded, some calculation have been made in order to know the difference value between before and after of the experiment one. These values are very small and close with the theoretical value, therefore the Boyles’s Law is verified. According to the data tabulated, it can been said that the pressure and volume inversely proportional. When the pressure increase, the volume start to decrease. This is happen because if the gas of the same pressure with constant temperature injected into small and big container which means have different volume. The gas molecule in small container have less spacious room and will collide to the wall and with each other more often which exert more pressure. Gay-Lussac’s Law stated that pressure is directly proportional to the temperature which means if the pressure increase, the temperature also increase with constant volume. Experiment two has been conducted in order to know the relationship between pressure and temperature. Therefore, from the data tabulated and graph plotted, it can be said that the Gay-Lussac’s Law is verified. The same concept applied here, if the temperature of a gas in a container increase, the heat energy of the system transfer its energy into the molecule of gas which actually increase the frequency of collision in that container which exert more pressure. Isentropic expansion process occur when the system are reversible and adiabatic where no heat will be transferred in or out and no energy transformation occurs. From the data recorded, a constant k are now known which is equal to 1.443. It was obtained that both temperature and pressure of the gas before expansion were higher compared to after the expansion. The process is said to be isentropic since there was no change in the entropy throughout the process. Stepwise
depressurizationis
a
strategy
to
adopt
an
equal
time-stepwise
depressurization approach in this study yield a more reliable result for an example in the production sector in industries. The molecule in the container affected when the number of them decreasing slowly as they do not have to collide between them more often. The depressurization shown that pressure decrease with time and also affecting the temperature. As the pressure decrease, the temperature also decrease in the system. Brief depressurization shown in the graph plotted in result section which is decrease more linear compared to stepwise. The expansion occur when the pressure of gas increase. Expansion of gas decrease as the gas is free to flow out time by time. Ratio volume can be determined by manipulating the equation of Boyle’s law. Boyle’s law proposed an equation P1V1=P2V2 and after manipulate the equation ratio volume can be determine by V2/V1=P1P2. This experiment test in three different condition where first condition the gas is flow from tank 1 to tank 2, while gas flow from tank 2 to tank 1 in second condition and both were filled with gas in third condition. The theoretical value is 0.495 in this experiment where the error or percentage difference was between 10 and -10. There must be environmental factors that affect the stability of pressure and temperature or random mistake during experiment. The percentage error is high due to some error during conducting the experiment. Some of air probably left from chamber due to not properly close the valve or before the experiment, the gas did not left out completely from the chamber.
Determination of ratio of heat capacity using the expression of the heat capacity ratio and it gives the 1.1584. The theoretical value of this experiment is 1.4. The deviation which now is equal to 21%. The deviation is due to measurement error. The actual intermediate pressure supposed to be lowered that the measured one. Unfortunately the error occur due to heat loss and sensitivity of pressure sensors. Supposed, the intermediate pressure taken as the lowest pressure at the moment the valve is closed. Since the percentage difference is more than 10%, the experiment can be declared as failed.
COCLUSIONS. In conclusion, the experiment was aimed at determining the properties of measurement /PVT according to the Boyle’s law, Gay-Lussac’s Law, isentropic expansion, and heat capacity equation. In fact, in this experiment, we have proven the Boyle’s law and Gay-Lussac’s law. Although our experiment failed, but we have the reason behind the failure. For experiment 7, the failure was due to the fact that an intermediate pressure was not taken after the valve closed. However, the experiment was successfully done in final, and the objective of the experiment was accomplishedly achieved.
RECOMMENDATIONS. There are several improvements that can be performed so as to obtain a more satisfying result in future. Before starting this experiment, we are supposed to do a start-up and shutdown step in order to make sure there is no gas left in the chamber. Most importantly, during recording data, keep an eye on the sensor while monitoring the board because the parameter can increase and decrease really fast and read the procedure carefully. In addition, obtain an average reading by repeating the experiment for three times in order to reduce the range of deviation. Handle the valve carefully and try not to make mistake by choosing the valve because it will affect the data. The place where the experiment is conducted also must be at stable and no vibration. All the equipment must be handled carefully in order to avoid explosion because over-pressure in the tank would cause an explosion.
REFERENCES. Engineering Sciences 182: PVT Measurement And Properties Of a Simple Compressible Substances. (n.d). Retrieved from http://sites.fas.harvard.edu/~es181/handouts/lab01_PVT_f05_v9.pdf Charles's Law. (2010). Retrieved from Sparknotes: http://www.sparknotes.com/testprep/books/sat2/chemistry/chapter5section8.rhtml Charles's Law. (n.d.). Retrieved from how stuff works: http://science.howstuffworks.com/dictionary/physics-terms/charles-law-info.htm Calculating PVT Properties. (n.d). Retrieved from http://petrowiki.org/Calculating_PVT_properties
APPENDIX.
1. 2. 3. 4. 5. 6. 7. 8. 9.
Pressure switch Receiver tank Compressor Condenser Pressure transmitter Control panel Evaporator Refrigerant flow meter Water flow meter
