This laboratory report describes an experiment that studied the performance of a vapor compression refrigeration cycle in an air conditioning unit. Temperature and humidity readings of the air were taken at different points as it was heated, humidified, cooled, dehumidified, and reheated. Readings of the refrigerant temperatures and pressures were also recorded. The air conditions were plotted on a psychrometric chart, and the refrigerant cycle was plotted on a p-h diagram. The cooling loads of the air and refrigerant were calculated, and the coefficient of performance (COP) was determined based on each. The COP value based on the refrigerant was considered more accurate due to inaccuracies in the wet bulb temperature readings for the air.
The aim of this experiment is to measurement linear thermal along z direction conductivity and to investigate and verify Fourier’s Law for linear heat conduction along z direction and we proved that K is inversely proportional with ΔT, and we have many errors in our experiment that made the result not clear.
ME010 801 Design of Transmission Elements
(Common with AU010 801)
Teaching scheme Credits: 4
2 hours lecture, 2 hour tutorial and 1 hour drawing per week
Objectives
To provide basic design skill with regard to various transmission elements like clutches, brakes, bearings and
gears.
Module I (20 Hrs)
Clutches - friction clutches- design considerations-multiple disc clutches-cone clutch- centrifugal clutch -
Brakes- Block brake- band brake- band and block brake-internal expanding shoe brake.
Module II (17 Hrs)
Design of bearings - Types - Selection of a bearing type - bearing life - Rolling contact bearings - static
and dynamic load capacity - axial and radial loads - selection of bearings - dynamic equivalent load -
lubrication and lubricants - viscosity - Journal bearings - hydrodynamic theory - design considerations -
heat balance - bearing characteristic number - hydrostatic bearings.
Module III (19 Hrs)
Gears- classification- Gear nomenclature - Tooth profiles - Materials of gears - design of spur, helical,
bevel gears and worm & worm wheel - Law of gearing - virtual or formative number of teeth- gear tooth
failures- Beam strength - Lewis equation- Buckingham’s equation for dynamic load- wear loadendurance strength of tooth- surface durability- heat dissipation - lubrication of gears - Merits and
demerits of each type of gears.
Module IV (16 Hrs)
Design of Internal Combustion Engine parts- Piston, Cylinder, Connecting rod, Flywheel
Design recommendations for Forgings- castings and welded products- rolled sections- turned parts,
screw machined products- Parts produced on milling machines. Design for manufacturing - preparation
of working drawings - working drawings for manufacture of parts with complete specifications including
manufacturing details.
Note: Any one of the following data book is permitted for reference in the final University examination:
1. Machine Design Data hand book by K. Lingaiah, Suma Publishers, Bangalore/ Tata Mc Graw Hill
2. PSG Design Data, DPV Printers, Coimbatore.
Text Books
1. C.S,Sarma, Kamlesh Purohit, Design of Machine Elements Prentice Hall of India Ltd NewDelhi
2. V.B.Bhandari, Design of Machine Elements McGraw Hill Book Company
3. M. F. Spotts, T. E. Shoup, Design of Machine Elements, Pearson Education.
Reference Books
1. J. E. Shigley, Mechanical Engineering Design, McGraw Hill Book Company.
2. Juvinall R.C & Marshek K.M., Fundamentals of Machine Component Design, John Wiley
3. Doughtie V.L., & Vallance A.V., Design of Machine Elements, McGraw Hill Book Company.
4. Siegel, Maleev & Hartman, Mechanical Design of Machines, International Book Company
The aim of this experiment is to measurement linear thermal along z direction conductivity and to investigate and verify Fourier’s Law for linear heat conduction along z direction and we proved that K is inversely proportional with ΔT, and we have many errors in our experiment that made the result not clear.
ME010 801 Design of Transmission Elements
(Common with AU010 801)
Teaching scheme Credits: 4
2 hours lecture, 2 hour tutorial and 1 hour drawing per week
Objectives
To provide basic design skill with regard to various transmission elements like clutches, brakes, bearings and
gears.
Module I (20 Hrs)
Clutches - friction clutches- design considerations-multiple disc clutches-cone clutch- centrifugal clutch -
Brakes- Block brake- band brake- band and block brake-internal expanding shoe brake.
Module II (17 Hrs)
Design of bearings - Types - Selection of a bearing type - bearing life - Rolling contact bearings - static
and dynamic load capacity - axial and radial loads - selection of bearings - dynamic equivalent load -
lubrication and lubricants - viscosity - Journal bearings - hydrodynamic theory - design considerations -
heat balance - bearing characteristic number - hydrostatic bearings.
Module III (19 Hrs)
Gears- classification- Gear nomenclature - Tooth profiles - Materials of gears - design of spur, helical,
bevel gears and worm & worm wheel - Law of gearing - virtual or formative number of teeth- gear tooth
failures- Beam strength - Lewis equation- Buckingham’s equation for dynamic load- wear loadendurance strength of tooth- surface durability- heat dissipation - lubrication of gears - Merits and
demerits of each type of gears.
Module IV (16 Hrs)
Design of Internal Combustion Engine parts- Piston, Cylinder, Connecting rod, Flywheel
Design recommendations for Forgings- castings and welded products- rolled sections- turned parts,
screw machined products- Parts produced on milling machines. Design for manufacturing - preparation
of working drawings - working drawings for manufacture of parts with complete specifications including
manufacturing details.
Note: Any one of the following data book is permitted for reference in the final University examination:
1. Machine Design Data hand book by K. Lingaiah, Suma Publishers, Bangalore/ Tata Mc Graw Hill
2. PSG Design Data, DPV Printers, Coimbatore.
Text Books
1. C.S,Sarma, Kamlesh Purohit, Design of Machine Elements Prentice Hall of India Ltd NewDelhi
2. V.B.Bhandari, Design of Machine Elements McGraw Hill Book Company
3. M. F. Spotts, T. E. Shoup, Design of Machine Elements, Pearson Education.
Reference Books
1. J. E. Shigley, Mechanical Engineering Design, McGraw Hill Book Company.
2. Juvinall R.C & Marshek K.M., Fundamentals of Machine Component Design, John Wiley
3. Doughtie V.L., & Vallance A.V., Design of Machine Elements, McGraw Hill Book Company.
4. Siegel, Maleev & Hartman, Mechanical Design of Machines, International Book Company
This file contains slides on One-dimensional, steady state heat conduction without heat generation. The slides were prepared while teaching Heat Transfer course to the M.Tech. students.
Topics covered: Plane slab - composite slabs – contact resistance – cylindrical Systems – composite cylinders - spherical systems – composite spheres - critical thickness of insulation – optimum thickness – systems with variable thermal conductivity
Thermal Radiation-II- View factors and Radiation energy exchange between blac...tmuliya
This file contains slides on THERMAL RADIATION-II: View factors and Radiation energy exchange between black bodies.
The slides were prepared while teaching Heat Transfer course to the M.Tech. students in Mechanical Engineering Dept. of St. Joseph Engineering College, Vamanjoor, Mangalore, India, during Sept. – Dec. 2010.
Contents: View factor – general relations – radiation energy exchange between black bodies – properties of view factor and view factor algebra – view factor formulas and graphs – Problems
Recognize numerous types of heat exchangers, and classify them.
Develop an awareness of fouling on surfaces, and determine the overall heat transfer coefficient for a heat exchanger.
Perform a general energy analysis on heat exchangers.
Obtain a relation for the logarithmic mean temperature difference for use in the LMTD method, and modify it for different types of heat exchangers using the correction factor.
Develop relations for effectiveness, and analyze heat exchangers when outlet temperatures are not known using the effectiveness-NTU method.
Know the primary considerations in the selection of heat exchangers.
In the material testing laboratory, a Charpy impact test was performed on three different types (hot,cold,and steel alloy)of steels testing each variety at four different temperatures (32°C(RT), 100°C,0°C and -22°C ). From results (shown below), we determined that the a transition is from ductile failures to brittle failures
Heat transfer from extended surfaces (or fins)tmuliya
This file contains slides on Heat Transfer from Extended Surfaces (FINS). The slides were prepared while teaching Heat Transfer course to the M.Tech. students in Mechanical Engineering Dept. of St. Joseph Engineering College, Vamanjoor, Mangalore, India.
Contents: Governing differential eqn – different boundary conditions – temp. distribution and heat transfer rate for: infinitely long fin, fin with insulated end, fin losing heat from its end, and fin with specified temperatures at its ends – performance of fins - ‘fin efficiency’ and ‘fin effectiveness’ – fins of non-uniform cross-section- thermal resistance and total surface efficiency of fins – estimation of error in temperature measurement - Problems
This file contains slides on One-dimensional, steady state heat conduction without heat generation. The slides were prepared while teaching Heat Transfer course to the M.Tech. students.
Topics covered: Plane slab - composite slabs – contact resistance – cylindrical Systems – composite cylinders - spherical systems – composite spheres - critical thickness of insulation – optimum thickness – systems with variable thermal conductivity
Thermal Radiation-II- View factors and Radiation energy exchange between blac...tmuliya
This file contains slides on THERMAL RADIATION-II: View factors and Radiation energy exchange between black bodies.
The slides were prepared while teaching Heat Transfer course to the M.Tech. students in Mechanical Engineering Dept. of St. Joseph Engineering College, Vamanjoor, Mangalore, India, during Sept. – Dec. 2010.
Contents: View factor – general relations – radiation energy exchange between black bodies – properties of view factor and view factor algebra – view factor formulas and graphs – Problems
Recognize numerous types of heat exchangers, and classify them.
Develop an awareness of fouling on surfaces, and determine the overall heat transfer coefficient for a heat exchanger.
Perform a general energy analysis on heat exchangers.
Obtain a relation for the logarithmic mean temperature difference for use in the LMTD method, and modify it for different types of heat exchangers using the correction factor.
Develop relations for effectiveness, and analyze heat exchangers when outlet temperatures are not known using the effectiveness-NTU method.
Know the primary considerations in the selection of heat exchangers.
In the material testing laboratory, a Charpy impact test was performed on three different types (hot,cold,and steel alloy)of steels testing each variety at four different temperatures (32°C(RT), 100°C,0°C and -22°C ). From results (shown below), we determined that the a transition is from ductile failures to brittle failures
Heat transfer from extended surfaces (or fins)tmuliya
This file contains slides on Heat Transfer from Extended Surfaces (FINS). The slides were prepared while teaching Heat Transfer course to the M.Tech. students in Mechanical Engineering Dept. of St. Joseph Engineering College, Vamanjoor, Mangalore, India.
Contents: Governing differential eqn – different boundary conditions – temp. distribution and heat transfer rate for: infinitely long fin, fin with insulated end, fin losing heat from its end, and fin with specified temperatures at its ends – performance of fins - ‘fin efficiency’ and ‘fin effectiveness’ – fins of non-uniform cross-section- thermal resistance and total surface efficiency of fins – estimation of error in temperature measurement - Problems
To assess the performance of the vapor compression cycle as a refrigerator and as a heat pump and its dependence on various parameters. To learn how to use the equipment to measure temperatures at various test points and the flow rates for liquids and gases.
The COP of the refrigeration increasing the performance and to get high efficiency of the refrigeration system. By using nano coating over the evaporator of the refrigeration component the objective can be achieved. The improper heat dissipation occurred in the heat exchanger components causes effect in performance. The vapour compression refrigeration system consuming the high power. Though the energy taken for the refrigeration process has increased and leads to more power consumption. In order to increase the performance, Nano coating Copper Oxide has been applied over the evaporator. By applying the Nano coating Copper Oxide over the evaporator the COP increased. In result the energy required for the refrigeration process and global warming problems has been reduced. By addition of nanoparticles to the refrigeration results in improvements in the COP of the refrigeration, thereby improving the performance of the refrigeration system. In this experiment the effect of using CuO-R134a in the vapour compression system expected COP will be increased by 5% with nano coating.
Applied thermodynamics and engineering fifth edition by t.d eastop and a. mc ...Asi Asim
Introduction to Thermodynamics
Some useful constants in thermodynamics:
1 eV = 9.6522E4 J/mol
k, Boltzmann's constant = 1.38E-23 J/K
volume: 1 cm3 = 0.1 kJ/kbar = 0.1 J/bar
mole: 1 mole of a substance contains Avogadro's number (N = 6.02E23) of molecules. Abbreviated as 'mol'.
atomic weights are based around the definition that 12C is exactly 12 g/mol
R gas constant = Nk = 8.314 J mol-1 K-1
Units of Temperature: Degrees Celsius and Kelvin
The Celsius scale is based on defining 0 °C as the freezing point of water and 100°C as the boiling point.
The Kelvin scale is based on defining 0 K, "absolute zero," as the temperature at zero pressure where the volumes of all gases is zero--this turns out to be -273.15 °C. This definition means that the freezing temperature of water is 273.15 K. All thermodynamic calculations are done in Kelvin!
kilo and kelvin: write k for 1000's and K for kelvin. Never write °K.
Units of Energy: Joules and Calories
Joules and calories and kilocalories: A calorie is defined as the amount of energy required to raise the temperature of 1 g of water from 14.5 to 15.5°C at 1 atm.
4.184 J = 1 cal; all food 'calories' are really kcal.
Many times it is easiest to solve equations or problems by conducting "dimensional analysis," which just means using the same units throughout an equation, seeing that both sides of an equation contain balanced units, and that the answer is cast in terms of units that you want. As an example, consider the difference between temperature (units of K) and heat (units of J). Two bodies may have the same temperature, but contain different amounts of heat; likewise, two bodies may contain the same heat, but be at different temperatures. The quantity that links these two variables must have units of J/K or K/J. In fact, the heat capacity C describes the amount of heat dQ involved in changing one mole of a substance by a given temperature increment dT:
dQ = CdT
The heat capacity C is then
C = dQ/dT
and must have units of J K-1 mol-1. (The specific heat is essentially the same number, but is expressed per gram rather than per mole.)
Don't forget significant digits. 1*2=2; 1.1*2=2; 1.1*2.0=2.2; 1.0*2.0=2.0
Why Thermodynamics?
Think about some everyday experiences you have with chemical reactions.
Your ability to melt and refreeze ice shows you that H2O has two phases and that the reaction transforming one to the other is reversible--apparently the crystallization of ice requires removing some heat.
Frying an egg is an example of an irreversible reaction.
If you dissolve halite in water you can tell that the NaCl is still present in some form by tasting the water. Why does the NaCl dissolve? Does it give off heat? Does it require energy?
How is it that diamond, a high-pressure form of C, can coexist with the low pressure form, graphite, at Earth's surface? Do diamond and graphite both have the same energy? If you burn graphite and diamond, which gives you more e
A full-blown audit of the city-owned Philadelphia Gas Works, commissioned by the PA Public Utilities Commission. The biggest way PGW can save money, according to the audit, is to buy more of its natural gas supplies from the nearby PA Marcellus--rather than continue to buy gas from the Gulf Coast as it does now.
3. Page 3 of 13
2 - Introduction
Introduction
Heat pumps are very important in today’s society, being the basis for refrigerators and
freezers as well as some heating methods. Heat pumps are machines which move heat from a lower
temperature reservoir (called the source) to a higher temperature reservoir (called the heat sink)
driven by a work input. Depending on the aim, they can be heaters by rejecting heat into the heat
sink, or refrigerators by absorbing heat from the source. Their performance is measured by the ratio
of cooling obtained to work required, called the coefficient of performance (COP) shown in equation
1 below.
!"#$ =
'(
)
, +,. 1
Where:
'( = cooling load (heat removed from source) (W)
) = Work input (W)
COP values will generally be above 1 as more heat will be removed from the source than work
is put in due to the nature of the cycle (discussed in 3 – Theory of Vapour Compression Refrigeration
Cycles).
This experiment aims to find the COP of a vapour compression refrigeration cycle as it is used
to cool and dehumidify air. Irreversibilities of the process and other points of interest will be discussed.
3 - Aims of Experiment
An air conditioning unit operating a vapour compression refrigeration cycle will be used to heat
air to a warm and humid state, then dehumidify and cool air. Temperatures of the air and the
refrigerant working fluid will be recorded from the air conditioning unit. Other working parameters
including differential pressures, mass flow rates and voltage/current values will be recorded. These
set of results will allow:
• The path of the air conditioning process to be plotted on a psychometric chart
• The state of the air at each point to be found and comments to be made on the final quality
of the air
• The cooling load from the air during the air conditioning cycle to be calculated
• The ideal and real cycle for the refrigerant on a p-h diagram to be plotted
• The cooling load for the refrigerant during the cycle to be calculated
• The coefficient of performance of the refrigerator (COP) to be calculated
4. Page 4 of 13
4 - Theory of Vapour Compression Refrigeration Cycles
Carnot Cycle
This experiment uses a vapour compression refrigeration cycle which cools air down. Though
the air is heated and cooled in the air conditioning unit it is the refrigeration cycle (using R12
refrigerant as the working fluid) which is the main focus of the experiment. The refrigeration cycle
removes heat from the air and rejects it to the environment, acting as a reverse heat engine –
commonly called a heat pump. Heat pumps are based around reverse Carnot cycles, which are exactly
the same as Carnot cycle but the process directions and hence work and heat inputs and outputs are
reversed. Figure 1 and 2 below are the pressure-volume (p-v) and temperature-entropy (T-s) diagrams
for a reverse Carnot cycle.
Figure 1: Pressure Volume diagram showing Figure 2: Temperature-Entropy graph with
Carnot cycle. ssssssssssss sssssssssssddfffffffffffhsslkcsd a saturation curve showing Carnot cycle.
The processes relating to the points in figure 1 and 2 are detailed below:
1-2: Isentropic compression – work is done on the working fluid at a constant entropy, resulting in
an increase in temperature
2-3: Isothermal heat rejection – heat is rejected into the high temperature reservoir
3-4: Isentropic expansion – work is done by the working fluid at a constant entropy, resulting in a
decrease in temperature
4-1: Isothermal heat absorption – heat is absorbed ('() from a low temperature reservoir
Note that figure 2 shows the saturation curve for the cycle. Enclosed in the saturation curve
the refrigerant is a mixed state of vapour and liquid. To the left of the saturated liquid line the
refrigerant is a sub-cooled liquid, to the right of the saturated vapour line the refrigerant is
superheated vapour. The importance of this is discussed below.
To base a refrigeration cycle around the reverse Carnot cycle, a working fluid must be found
that is capable of isothermal heat rejection and absorption at the temperatures of the high and low
temperature reservoirs (TH and TL). This can be obtained by using fluids that condense and evaporate
around these temperatures as phase changes are processes in which heat is input or output with no
temperature change. This can be understood more by looking at the Carnot cycle encased in a
saturation curve (figure 2). By choosing a refrigerant which has saturation temperatures (TH and TL)
at the required pressures, the phase changes of the refrigerant can be used to facilitate the isothermal
processes 2-3 and 4-1. The refrigerant will have a low boiling point allowing it to change phase (and
absorb heat) from an already cool temperature.
P
V
11
3
4
2
T
s
1
3
4
2TH
TL
6. Page 6 of 13
5 - Set-up and Procedure
Equipment
The apparatus used is an air conditioning unit. The unit is divided into stations where various
readings can be taken. At station A air is taken in from the room and enters the unit. Station B is for
mixing re-circulating air that has already passed through the unit in with the fresh air from the room.
Air will not be re-circulated during the experiment so any readings from station A and B should be
identical. Between station B and C electric pre-heaters and steam injection occurs to bring the air to
the conditions of a warm humid climate. Between station C and D the air passes though an
evaporator where it is cooled and excess moisture is condensed out. The evaporator uses
dichlorodifluoromethane (R12) refrigerant – the cycle of the refrigerant is of key importance to the
experiment and will be discussed in more detail in Section 7 – Discussion and Analysis of Results.
Between station D and E there is re-heating of the air to increase its temperature and reduce its
humidity. Each station has two thermocoupled thermometers – a wet and a dry bulb. By taking these
two readings we can determine the state of the air (its relative humidity). Other readings about the
unit such as mass flow rates, current in compressors, relative pressures and temperatures in the
refrigerant cycle will be recorded.
Set-up and Procedure
Hot and humid air must be produced for the air conditioning cycle. This air will then be
cooled and dehumidified and then reheated to a required temperature of 20°C.
The unit is turned on and air flow is set to a minimum of 0.07kgs-1
(any lower and the reliability
of the wet bulb thermocouples will be poor). The boilers and refrigerator units are turned on and
allowed to stabilise for about 5 minutes. Once steam is being produced by the boilers, two of the three
boilers are turned off and the re-heater between station D and E is turned on, adjusting it so that the
air before the cooling is heated to 25°C and a relative humidity of 90% (a warm humid climate). The
unit is then left again to stabilise for 10 minutes, after which the re-heater is adjusted to give a
temperature of about 20°C and relative humidity of 50-60% after the cooling as it exits the unit. After
allowing the unit to stabilise for 10 minutes once more, thermocouple readings and other parameters
at each station can be taken. The results can be seen in the section below.
6 - Observations and Results
The temperature readings from the thermocouples (wet and dry bulbs) were recorded and
shown in table 1. The corrected values for the wet bulb temperature are also shown – the correction
value was found using figure 6 which shows the relationship between ‘Screen’ and ‘Sling’ wet bulb
temperatures when the dry bulb temperature is known. Figure 6 and a brief explanation of how
correction values are obtained is shown in the Appendix.
Station Dry Bulb Temperature
(°C)
Wet Bulb
Temperature
(observed) (°C)
Wet Bulb
Temperature
(corrected) (°C)
A – Intake 20.0 16.0 16.4
B – After Mixing 21.0 17.0 17.4
C – After Pre-heating
and Steam Injection
27.5 27.5 27.5
D – After Cooling and
Dehumidification
18.3 18.3 18.3
E – After Re-heating 24.0 22.0 22.2
Table 1: Dry, Wet bulb and corrected wet bulb temperature readings from each station.
7. Page 7 of 13
Note that when the readings for station C and D were taken the wet bulb temperatures were
higher than the dry bulb temperatures. The wet bulb is meant to be less than or equal to the dry bulb
so the wet bulbs were given the same temperature as the dry bulb. No correction values were
obtained for these temperatures. This is further discussed in the section 8 - Sources of Error.
The air at A and B is expected to be identical as there is no change in the air between these
two points. However at station B there is an increase in temperature of 1°C for both the wet and the
dry bulb. This could be due to the air conditioning unit acting as an insulation to the outside
environment so air further in the unit (station B) will be warmer whereas station A is not well enclosed.
It could also be due to a too short stabilisation period.
The temperature readings from the refrigerant cycle are shown in table 2. These will be used
in Section 7 – Discussion and Analysis of Results to plot a more realistic cycle on a p-h diagram.
R12 Temperature: Temperature (°C)
Before Expansion valve 33
After Expansion valve 0
After Evaporator 21.5
After Compressor 92
Table 2: Temperature of R12 refrigerant at various stages
through cycle.
The voltage supply, and currents through the relevant equipment are recorded, shown in table 3.
Reading
Voltage (V) 233
Pre-heater current (A) 2
Boiler current (A) 8
Compressor and cooling fan current (A) 9.2
Re-heater current (A) 2
Circulating fan current (A) 0.8
Table 3: Voltage and current readings from the unit.
Using the value of voltage and compressor and cooling fan current we can calculate the
amount of power the compressor and cooling fan require. This is calculated using equation 2 below
and we obtain a power of 2.144kW.
#/0+1 = 23 = 9.2×233 = 2143.6), +,. 2
This value of power is the work input to the refrigerant cycle. It will be used later to find a
value for the COPR of the refrigerant cycle.
Other variables of the unit which were recorded are shown in table 4. They will be used in
later calculations finding the COP and mass flow rates of the air and refrigerant.
Reading Units
Orifice Differential (intake) mmH20 0.7
Orifice Differential (outlet) mmH20 0.7
Evaporator Pressure (gauge) kPa 200
Condenser Pressure (gauge) kPa 750
Refrigerant mass flow rate kgs-1
0.024
Table 4: Orifice differential intake and outlet, evaporator and condenser pressure and refrigerant mass
flow rate as recorded from the unit.
9. Page 9 of 13
added back during the re-heating process which may have decreased the % saturation more, giving a
closer value of both % saturation and temperature to the aim.
Other differences between the aimed and actual values are commented on in the section 8 -
Sources of Error.
Cooling Load
The cooling load on the air is the amount of heat removed from it as it passes through the
heat exchanger where the cooled refrigerant flows. It can be calculated using equation 3 below.
'( = :(ℎ= − ℎ?), +,. 3
Where:
: = mass flow rate (kgs-1
) (air)
ℎ= = specific enthalpy of air before cooling (station C) (kJ/kg)
ℎ? = specific enthalpy of air after cooling (station D) (kJ/kg)
The values of ℎ= and ℎ? from table 5 are 87 kJ/kg and 50 kJ/kg respectively. The mass flow
rate of air can be calculated using equation 4 shown below.
: = 0.0757
∆E
FG
, +,. 4
Where:
∆E = intake orifice differential pressure (mmH20)
FG = specific volume at intake (station A) (m3
/kg)
The value for ∆E was recorded to be 0.7 mmH20 (table 4) and the value of FG was found to be
0.843 m3
/kg (graph 1 and table 5). These two values can be substituted into equation 4 and give a
value of 0.0690kgs-1
for the air mass flow rate. Substituting the mass flow rate and both specific
enthalpies for station D and C (table 5) into equation 3 a cooling load on the air of 2.55kW is obtained.
Refrigerant Calculations
Refrigeration Cycle on pressure-enthalpy Diagram
The readings that were taken during the experiment can be used to plot a p-h diagram for the
refrigerant cycle. A pressure enthalpy (p-h) diagram shown in figure 5 [2] can then be used to find the
enthalpy of the refrigerant at each stage and hence the cooling load can be calculated. This can then
be used to calculate the COPR of the refrigeration cycle.
A pressure enthalpy diagram for dichlorodifluoromethane (R12) was used, by plotting the
points the changes in phase of the refrigerant can clearly be seen. Note that on this graph there are
multiple axes. In order to plot the points the absolute pressures at which the condenser and
evaporator are running on must be calculated. The gauge running pressures of the evaporator and
condenser were recorded to be 200kPa and 750kPa respectively (table 4). To convert the values to
absolute pressure the atmospheric pressure (taken as 100 kPa) is added to each value. From this the
evaporator pressure is found to be 300kPa and the condenser pressure is found to be 850kPa.
Each point is plotted on the diagram (figure 5), note that the same nomenclature for the points
is used as in the theory section (4) above. Point one represents the refrigerant as a superheated
vapour as it exits the evaporator. It is plotted at 0.3 bar (evaporator pressure) and on the saturated
vapour curve. Process 1-2 is a constant entropy compression of the refrigerant to the condenser
pressure (0.85 bar), so point 2 is plotted at 0.85 bar (condenser pressure) and at entropy as point 1
(0.7 kJ/kg/K). Process 2-3 is a condensing process where the refrigerant is cooled and phase changes
into a liquid. Point 3 can be plotted at 0.85bar (condenser pressure) and on the saturated liquid curve.
Process 3-4 is a constant enthalpy expansion process where the refrigerant is bought to the
evaporator pressure (0.3 bar) so point 4 is plotted at the same enthalpy as point 3 and on the
evaporator pressure line.
10. Page 10 of 13
Point 1 has been plotted assuming that the evaporator only heats the refrigerant to the
saturated vapour point. The actual point 1 (1a) can be plotted on the graph using the temperature of
the refrigerant as it exits the evaporator which from table 2 is found to be 21.5°C. Note that this is
much higher than the ideal temperature value of 0°C. It is seen that point 1a is in the superheated
region in the p-h diagram, meaning the evaporator has heated the refrigerant more than is expected.
The importance of this will be discussed when calculating the cooling load on the refrigerant (later in
section).
Point 2 has been plotted assuming the process 1-2 is isentropic (constant entropy). In reality
it will not have been an isentropic process, there will have been a small amount of heat transfer into
the refrigerant resulting in a higher final temperature. A more realistic point 2 can be plotted by using
the temperature of the refrigerant at the end of the compression process. From table 2 the value is
found to be 92°C and can be plotted (as point 2a) on the condenser pressure line (0.85 bar) using a
constant temperature line. It is seen that the actual point (2a) is further to the right than the ideal
isentropic point (2), as the ideal point is at a temperature of 40°C. This means that more cooling is
needed to cool the superheated refrigerant to a saturated vapour, and also represents an
irreversibility in the cycle.
Point 1a and 2a have been connected and drawn on figure 5, to allow comparison between the
actual and ideal cycle.
Figure 5: Pressure-enthalpy diagram showing the refrigerant cycle. Note that point 1
and 1a and the isentropic (2) and actual (2a) points of the compression are shown.
Dashed lines represent irreversible processes. [2]
Irreversibilities in the vapour compression refrigeration cycle have been shown on figure 5 by
dashed lines. Process 1a to 2a is irreversible as it does is not an isentropic process (due to there being
small amounts of heat transfer) hence it cannot be reversed. Process 3 to 4 is irreversible due to the
nature of the air conditioning unit, this was discussed in Section 4 – Theory of Vapour Compression
Refrigeration Cycles.
Specic Enthalpy (kJkg)
AbsolutePressure(MPa)
Constant Specic
Entropy (kJkgK)
4
3 2 2
11 a
a
11. Page 11 of 13
Cooling Load
The cooling load on the air can be calculated using equation 3 from above, note that ℎ= and ℎ?
correspond to the specific enthalpies of the refrigerant at points 1 and 4 on figure 5 – 186 kJ/kg and
70 kJkg respectively. These values and the mass flow rate of the refrigerant from table 4 of
0.024 kgs-1
can be substituted into equation 3, giving a cooling load of 2.78kW. Note that this is larger
than the cooling load calculated for the air (2.55kW), showing that the refrigerant gained more energy
than the air in the unit lost. This difference is further discussed in the section 8 - Sources of Error.
Coefficient of Performance
The COPR can be calculated for the refrigeration unit using equation 1 shown below.
!#$ =
'(
)
, +,. 1
Where:
'( = cooling load (W)
) = Work input (power of the compressor) (W)
The work input into this cycle is the compressor, its power was calculated in section 6 -
Observations and Results and was found to be 2.144kW. This and the value for cooling load from
above (2.78kW) can be substituted into equation 1 yielding a COPR of 1.30. This COPR value means that
for every 1kW of work input, 1.3kW of heat will be removed from the air.
The COPR can also be calculated using the cooling load based on the air’s calculations. The
compressor power and the cooling load based on the air (2.55kW) can be substituted into equation 4
to give a COPR of 1.19. This is smaller than the COPR based around the refrigerant, and is due to the
cooling load of the air being lower than the cooling load for the refrigerant. This COPR value means
that for every 1kW of work input, 1.19kW of heat will be removed from the air. The significance of this
is discussed in section 8 – Sources and Discussion of Error.
8 - Sources and Discussion of Error
As this experiment was mainly descriptive, it is difficult to quantify any errors. No fits or
expected trends can be compared to the results, instead the systematic and experimental errors and
improvements for the experiment and the air conditioning unit are discussed.
The air conditioning unit was used in a large room with other large pieces of equipment running
nearby. Intermittent construction work was going on in the room adjacent, meaning that the quality
of the air may have fluctuated during the experiment. It is also possible that the air had a higher dust
and particulate content than normal. The experiment was conducted within a 2 hour session, this time
constraint limited the time that could be used for stabilisation of the unit. If the experiment were to
be done again, an isolated, air conditioned and filtered room should be used and longer times should
be allowed for the stabilisation of the unit.
All readings were taken from needle scales which were fluctuating, increasing the error in the
results. The unit was not calibrated before usage and hence the results obtained may have systematic
error. A digital scale may have increased the precision of the results obtained though this would
require adjustment on the unit.
In section 6 the results obtained showed that for two of the recorded temperatures the wet
bulb was higher than the dry bulb temperature - this may have happened due to the unit not having
enough time to fully stabilise.
In section 7 it was shown that the aimed temperature and saturations at stations C and E were
different to the values aimed for. This was suggested to be due to the refrigeration cycle not removing
as much heat from the air as expected. This is supported by the fact that the cooling load on the air
was found to be less than the cooling load gained by the refrigerant (2.55kW to 2.78kW, section 7),
showing the refrigerant was removing more heat than was lost from the air. The extra heat gained by
the refrigerant could be due to it also absorbing heat from the surrounding environment as the unit
12. Page 12 of 13
may not be well insulated on the section where the heat exchange occurs. The extra heat absorption
is also supported by the actual cycle plotted on the p-h diagram where the evaporator increases the
temperature of the refrigerant as well as changes its phase (pushing it into the superheated region).
Two COPR values were calculated, one using the cooling load to the refrigerant (1.3) and one
from the cooling load from the air (1.19). The COPR based on the air is more conservative as it uses
the cooling load that was directly measured from the air and is the desired outcome of the air-
conditioning process. However the refrigerant based COPR is more accurate as it does not have the
inaccuracies in measurement of the wet bulb readings as the air COPR does.
The COPR values obtained for the unit could be improved in a number of ways. From equation
1 it is clear that the COP value can be increased by making the cooling load larger, or the work input
smaller. The cooling load could be made larger by insulating the heat exchange section to ensure that
all heat absorbed by the refrigerant is from the air. It could also be increased by using a contra-current
flow of the refrigerant and air when the exchange takes place as this maintains a higher temperature
gradient increasing the amount of heat exchange. Work input could be reduced by using a more
efficient compressor.
9 – Conclusion
The results obtained allowed us to plot the state of the air on a psychometric chart. It was shown
that the % saturations of the air were different to the values that were aimed for, this was attributed
to the refrigerant removing less heat from the air than expected hence removing less humidity. The
temperatures of the air were relatively close to the aimed values, though the final temperature of the
air was 2.2°C above the aimed value, again suggesting that the refrigeration cycle did not reduce the
air’s temperature enough.
The refrigerant cycle was plotted on a p-h diagram, showing the ideal and actual cycle along
with irreversibilities. The actual cycle moved into the super-heated vapour region of the graph much
more than the actual cycle, meaning that more heat was rejected to the environment than expected.
The cooling loads on the air and to the refrigerant were calculated, giving 2.55kW and 2.78kW
respectively. This higher refrigerant cooling load was said to be due to the air conditioning unit not
being well insulated so heat was removed from the surrounding environment as well as the air.
COPR values were calculated based on the cooling loads for the air and the refrigerant, yielding
values of 1.3 and 1.19 respectively. The COPR based on the air’s cooling load was said to be more
conservative as it used a smaller cooling load and the desired outcome of the refrigeration, however
it is more inaccurate as it includes the measurements of the wet bulb temperatures which are
inherently inaccurate. Improvements for the air conditioning unit were discussed including using a
more efficient compressor and a counter-current flow in the heat exchanger.
10 – References
[1] - Heikal Morgan R., Miller A. J. (2011). AIR CONDITIONING. Available:
http://www.thermopedia.com/content/550. Last accessed 24th Dec 2015. Edited.
[2] - University of Birmingham (2015). Laboratory Experiment MP2.2. Appendix 5 – Enthalpy diagram
for R12. p10. Edited.
[3] - University of Birmingham (2015). Laboratory Experiment MP2.2. Appendix 6: Relationship
between “Screen” and “Sling” wet bulb temperatures. p11. Edited.