Principles of the Refrigeration Cycle
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Transcript of Principles of the Refrigeration Cycle
MCAST BTEC NATIONAL DIPLOMA IN BUILDING SERVICES ENGINEERING
Refrigeration Technology in Building Services Engineering
Principles of the Refrigeration Cycle
Joseph Gatt
Principles of the Refrigeration Cycle 19/11/2009
Joseph Gatt Page 2 of 22
Contents
Section A - P.37.1 ...............................................................................................................................................4
1. Temperature Conversions ..........................................................................................................................4
2. Laws of Thermodynamics as Regards to Refrigeration and A/C ................................................................4
First Law of Thermodynamics .....................................................................................................................4
Second Law of Thermodynamics ................................................................................................................4
3. Heat Transfer ..............................................................................................................................................4
Conduction ..................................................................................................................................................4
Convection ...................................................................................................................................................5
Radiation .....................................................................................................................................................5
4. Pressure Conversions .................................................................................................................................5
5. Boiling Point of a Liquid Refrigerant ............................................................................................................5
6. Calculations .................................................................................................................................................6
7. Dew Point, Wet Bulb and Dry Bulb Temperatures ......................................................................................7
Dew Point Temperature ..............................................................................................................................7
Dry Bulb Temperature .................................................................................................................................7
Wet Bulb Temperature ................................................................................................................................7
Section B – P.37.2 ..............................................................................................................................................9
8. Boiling Points, Specific Heat Capacities, and Latent Heat Values for Various Refrigerants ......................9
9. Refrigerants and Their Impact on the Environment ....................................................................................9
CFC – ChloroFloroCarbons .........................................................................................................................9
HCFC – HydroChloroFloroCarbons ............................................................................................................9
HFC – HydroFloroCarbons ....................................................................................................................... 10
HC – HydroCarbons ................................................................................................................................. 10
Section C – P.37.3 ........................................................................................................................................... 11
10. Refrigeration Vapour Compression Cycle.............................................................................................. 11
Compressor .............................................................................................................................................. 11
Condenser ................................................................................................................................................ 11
Liquid Receiver ......................................................................................................................................... 12
Filter Dryer ................................................................................................................................................ 12
Expansion Valve ....................................................................................................................................... 12
Evaporator ................................................................................................................................................ 12
Principles of the Refrigeration Cycle 19/11/2009
Joseph Gatt Page 3 of 22
11. Compressors and Metering Devices ...................................................................................................... 12
Compressors ............................................................................................................................................ 12
Metering Devices ...................................................................................................................................... 14
Section C – M.37.1 .......................................................................................................................................... 17
About Hilton A660 Air Conditioning Laboratory Unit .................................................................................... 17
Procedure ................................................................................................................................................. 18
Pressure/Enthalpy Chart and Thermodynamic States of R134a ............................................................. 18
References ...................................................................................................................................................... 22
Principles of the Refrigeration Cycle 19/11/2009
Joseph Gatt Page 4 of 22
Section A - P.37.1
1. Temperature Conversions
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2. Laws of Thermodynamics as Regards to Refrigeration and A/C
First Law of Thermodynamics
Energy cannot be created nor destroyed. Energy can only be converted into other forms. The work done
in a system is equal to the heat supplied to the system. By means of mechanical energy and by the
process of heating or cooling, heat is transferred to and from its surroundings. Whether it becomes cool
or warm, the refrigerant returns to its original state, hence it will have the same amount of energy.
Second Law of Thermodynamics
Heat only travels from a warmer body to a cooler body. The process cannot be reversed. In a freezing
compartment, the heat energy stored in the food travels to the cooler refrigerant in order to emit the heat
away, until the food reach the same temperature of the compartment.
3. Heat Transfer
Heat transfer may be described as the transfer of thermal energy or heat exchange. Heat Transfer occurs
due to temperature difference between two bodies. Heat transfer process ends once the two bodies reach
thermal equilibrium. The greater the temperature difference, the greater the rate of heat transfer required.1
Heat transfer travels from a hotter body to a cooler body by means of three basic processes.
Conduction
Conduction is the transfer of heat energy through a material without the molecules of the material
changing their basic position.2 Conductive heat occurs in solids, liquids, and gases. Heat transfer by
conduction occurs in such way that two bodies are touching directly with each other. A practical example
is the heat conduction through copper pipes found in refrigerators. Metals are the best conductors of heat
and are widely used in building services engineering, such as heat pumps.
1 What is the Transfer of Heat? – C.Cassar & D. Privitera – Pg. 2
2 Environmental Science in Buildings – 4
th Edition - Randall McMullan – Pg. 17
Principles of the Refrigeration Cycle 19/11/2009
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Convection
Convection is the transfer of heat energy through a material by the bodily movement of particles.3
Convectional heat occurs in fluids (liquids or gases). Heat by convection is divided into two.
Natural Convection
A practical example of natural convection is a refrigerator. The food in the compartment cools by natural
convectional.
Forced Convection
Forced convectional current occurs by means of a mechanical source, such as pump or fan. The air
conditioner cools a room by means of a blower or fan, hence forced convection.
Radiation
Radiation is the transfer of heat energy by electromagnetic waves.4 Radiant heat occurs only from a heat
source (warm element). All materials radiate heat in relation with their temperature. A practical example of
radiant heat is the black surface of a compressor. Actually, bodies with black surfaces emit heat very
quickly.
4. Pressure Conversions
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c. 10 ,-. �� ;<- 10 � 100 ���� =>9
d. 1 ?<- �� /01 �
*.**@A �+" 234
e. 1 B�.. �� C1D.�E 1 � 1000 ���� F4G:HI
5. Boiling Point of a Liquid Refrigerant
The temperature (boiling point) of a liquid refrigerant varies in relation to the pressure applied. The higher
the pressure, the higher is the boiling point. The following table illustrates the boiling point for R22.
Temperature (T) °C Pressure (p) Bar
-41 1
-25 2
3 Environmental Science in Buildings – 4
th Edition - Randall McMullan – Pg. 17
4 Environmental Science in Buildings – 4
th Edition - Randall McMullan – Pg. 18
Principles of the Refrigeration Cycle 19/11/2009
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6. Calculations
Calculate the heat required to convert 15kg of ice at -10°C into steam at 100°C. The specific heat
capacities of water and ice are, respectively, 4,200J/kgK and 2,100J/kgK; the specific latent heat of fusion
of ice is 340,000J/kg, and the specific latent heat of vaporization of water is 2,300,000J/kg.
Data: m = 15 kg
T1 = -15 °C
T2 = 100 °C
Cwater = 4,200 J/kgK
Cice = 2,100 J/kgK
Cfusion = 340,000 J/kg
Cvaporisation = 2,300,000 J/kg
JKLM CD∆B
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Principles of the Refrigeration Cycle 19/11/2009
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7. Dew Point, Wet Bulb and Dry Bulb Temperatures
http://userpages.umbc.edu/~martins/PHYS650/Class8_PsychrometricChart-SeaLevel-SI.jpg
Dew Point Temperature
Dew point can also be referred as the saturated condition. It can be read from the saturation line in the
psychrometric chart. The higher the moisture content of air, the higher is the dew point. Dew point refers
to the water vapour when condenses out of the air, or saturated. At this point in time, the moisture content
remains in the air. The relative humidity is high if the dew point temperature is close to the air
temperature. The relative humidity is low if the dew point temperature is below the air temperature. Dry
bulb, wet bulb, and dew point temperatures are the same when the air is saturated.
Dry Bulb Temperature
Dry bulb measures the temperature of air. It is an indicator of heat content in air. Dry bulb can be
measured either with a normal thermometer or a sling thermometer. It can be read from the vertical lines
in the psychrometric chart.
Wet Bulb Temperature
Wet bulb temperature can also be described as the relative humidity. It is the temperature of adiabatic
saturation. Wet bulb measures the moisture content in air by means of cotton attached to the
thermometer. The cotton absorbs the moisture content in air and gives up some heat, hence reducing the
temperature. The higher the moisture content in air, the higher is the wet bulb temperature. The wet bulb
temperature is always lower than the dry bulb temperature, although both can be equalised when 100%
relative humidity is reached.
Principles of the Refrigeration Cycle 19/11/2009
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Dew point, wet bulb, and dry bulb temperatures do have a relationship between them as follows;
• The dew point temperature is lower than the dry bulb temperature, and the wet bulb temperature
is in between when the air is not saturated but contains some moisture.
• The difference between temperatures becomes less as the amount of moisture in the air
increases and the amount of evaporation decreases.
• The relative humidity becomes 100% when all the three temperatures are the same, hence the
air becomes saturated.
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Section B – P.37.2
8. Boiling Points, Specific Heat Capacities, and Latent Heat Values for Various Refrigerants
9. Refrigerants and Their Impact on the Environment
The major worry at the time being is Global Warming. There is so much action in developing alternative
refrigerants due to their impact on the environment. The ideal refrigerant would be non-toxic, non-
flammable, and eco-friendly, but it does not exist. However, refrigerants are being classified in relation to
their impact on the Ozone Depletion Potential – ODP – and Global Warming Potential – GWP.
ODP is a single molecule potential of the refrigerant to destroy the Ozone Layer. R11 is being used as a
datum reference to all refrigerants. R11 has an ODP of 1.0. The less the value, the better is the refrigerant.
GWP is a measurement of effectiveness that a refrigerant has on Global Warming in relation to Carbon
Dioxide; where CO2 has a GWP of 1. The lower the value, the better is the refrigerant.
CFC – ChloroFloroCarbons
All refrigerant under this title contain chlorine. Due to their negative impact on the environment, these
refrigerants are obsolete. Such refrigerants are R11 (ODP 1, GWP 4000), and R12 (ODP 1, GWP 2400).
HCFC – HydroChloroFloroCarbons
R22 (ODP 0.1, GWP 1700) is an example under this classification. HCFC are less harmful than CFC
since they contain hydrogen. Additionally, HCFC are less stable in the atmosphere, hence dissipate more
rapidly.
Principles of the Refrigeration Cycle 19/11/2009
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HFC – HydroFloroCarbons
This classification indicates that the refrigerant is a mixture of Hydrogen, Fluorine, and Carbon. Such
refrigerant is R134a with 0 ODP and 1300 GWP.
HC – HydroCarbons
These contain 0 ODP as well as O GWP. In turn, these alone are highly flammable and fairly toxic. Such
refrigerants are R170, R290, R600a, R610a, R601.
The following table shows the environmental properties of various refrigerants
http://www.engineeringtoolbox.com/Refrigerants-Environment-Properties-d_1220.html
Principles of the Refrigeration Cycle 19/11/2009
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Section C – P.37.3
10. Refrigeration Vapour Compression Cycle
Compressor
The compressor is the heart of the refrigeration cycle, inserted between the condenser and the
evaporator. The refrigerant compressor serves as a heat pump; circulates the flow of refrigerant. The
compressor sucks the vapour from the evaporator. It compresses low pressure, low temperature vapour
into high pressure, high temperature vapour. The compressor is perfectly designed to suck the vapour
from the evaporator at the same rate it forms in the evaporator, which is called a condition of equilibrium.
Condenser
The condenser changes the refrigerant’s state from vapour to liquid by releases the heat away (outwards)
to a lower temperature (air or water). The heat emitted from the condenser is the same heat absorbed by
the evaporator and the heat created into the compressor.
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Liquid Receiver
The receiver is installed after the condenser, before the expansion valve. The liquid from the condenser is
collected in the receiver.
Filter Dryer
As the name implies, it filters any contaminates out of the refrigerant stream, and absorbs any moisture
content out of the liquid refrigerant. The filter dryer ensures that both contaminate and moisture will be
eliminated from the cycle, hence the cycle will be healthy.
Expansion Valve
The expansion has to be inserted in order to lower the pressure to the same level in the evaporator
pressure. It regulates the flow of the refrigerant. After passing through the expansion valve, the liquid
refrigerant begins to boil and evaporate (saturated).
Evaporator
The liquid refrigerant flow through the evaporator, where it absorbs the heat from the surroundings,
increase its boiling point, hence evaporates. Then the vapour will be sucked by the compressor where it
starts another cycle.
The compressor squeezes the vapour to pressurised vapour. The vapour flows through the condenser
where it changes its state to sub-cooled, pressurized liquid. This occurs because the vapour releases its
excessive heat due to temperature difference between the vapour and the medium. The liquid flows
through the receiver, the filter dryer which absorbs any contaminates and moisture content in the liquid
refrigerant, and to the expansion valve where it changes its state to low-pressure, saturated liquid. This
occurs because the liquid refrigerant starts to boil and evaporate after emerging in the expansion valve.
As a result, a sudden drop in pressure occurs. The liquid flows through the evaporator where it changes
its state to low-pressure vapour. This occurs because the liquid absorbs the heat due to temperature
difference between the liquid and medium. The vapour flows back to the compressor as low-pressure
vapour. The thermodynamic cycle is repeated again.
11. Compressors and Metering Devices
Compressors
http://en.wikipedia.org/wiki/Gas_compressor
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Reciprocating Compressor
http://www.fscc-online.com/"Passing%20Gas"-article/passing_gas.html
Reciprocating compressors can be found as single or multi-stage, and mostly driven by electric motors.
They are widely used in most residential cooling/heating systems which employ the vapour
compression cycle. This type of compressor has an off-centre shaft which drives the crank shaft. The
crank shaft is connected to the rod which forces the cylinder to move back and forth. The cylinder is
completely tight in the outer cylinder by sealing rings. By its motion, the piston sucks and compresses
the refrigerant.
Suction is created in the cylinder as the piston moves downwards, because the pressure in the suction
line is greater than the pressure in the cylinder. The intake valve opens, allowing the refrigerant to
enter. The intake valve closes as the piston starts to move upwards, because the pressure in the
cylinder is now greater than the pressure in the suction line. The refrigerant is now being compressed,
increasing in pressure as the piston moves upwards. The exhaust valve opens, allowing the refrigerant
to flow out of the cylinder, because the pressure in the cylinder is greater than the pressure in the
discharge line. The cycle is then repeated as the piston reaches its upmost position and starts to move
downwards.
The reciprocating compressor is simply constructed, also maintaining a simple working principle.
Financially speaking, it is the cheapest compressor found on the market. Like everything else,
reciprocating compressors have drawbacks. Those have many moving parts that might result in
efficiency and heat losses. Additionally, all the moving parts tend to vibrate, making a lot of noise. The
valves are brittle in such way that those will be damaged if droplets of liquid enter the cylinder, because
the liquid is not compressible.
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Centrifugal Compressor
http://en.wikipedia.org/wiki/Gas_compressor
Centrifugal compressors usually are used widely in various industries where to handling large volume of
low pressure gases or air.
The centrifugal compressor consists of rotating impellers arranged in series, which are closed in
housing. These compressors use centrifugal force in order to compress the refrigerant. The gas flows
from the larger impeller to the smallest one, increasing in velocity and in pressure. The more stages of
impellers, the more is the velocity and pressure of the gas. The centrifugal force depends a lot on the
pitch of the impeller. Moreover, in order to achieve high output pressures, these rotate at very high
revolutions per minutes, in the range of 3000-18000 r.p.m.5
Centrifugal compressors have few moving parts since the impellors rotate on a common shaft, hence
very efficient. Damage might not occur even if liquid refrigerant enters the impellors. If the shaft is a bit
off-centred during its rotation, all the impellers will be damaged and have to be scrapped, and these
cost a lot of money.
Metering Devices
Capillary Tube
http://www.shineyear.com.tw/products/p6/900m.png
The capillary tube is commonly used in refrigeration equipment, mostly in the domestic fridges and air
conditioners. It is a normal coiled tube with a small internal diameter, normally made of copper.
5 Compressors – D. Privitera & C. Cassar
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Capillary tubes control the flow of the liquid refrigerant, with no change in temperature along the tube. It
causes the high pressure liquid refrigerant to back up at its inlet, in order to drop it to a lower pressure
liquid at its outlet. It is situated between the condenser and the evaporator. The capacity of the tube is
affected by its length, internal diameter, and pressure.
Capillary tubes are very cheap and simply designed. They have no moving parts and requires low
starting torque compressor.
Thermostatic Expansion Valve
http://www.e-refrigeration.com/index.php?page=metering-device
The thermostatic expansion valve – TXV - is a very common type of metering device. It is widely used
for both commercial and industrial applications due to its high-efficiency. The TXV is practically
adaptable to any type of refrigeration application. TXVs also drop the pressure in the system. It controls
the flow of liquid refrigerant and is the only metering device that stops the liquid refrigerant from entering
in the compressor. This might happen because the liquid refrigerant tends not to boil all off in the
evaporator. The valve operates on three pressures; P1 = bulb pressure, P2 = evaporator pressure, and
P3 = spring pressure. The spring is used to set the superheat.
http://www.hvacmechanic.com/txv.htm
The valve is attached with a sensing bulb by means of a capillary tube filled with refrigerant. The bulb is
mounted on the suction line at the exit of the evaporator. It adjusts how much refrigerant enters in the
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evaporator because it senses the temperature difference between the inlet and outlet of the evaporator.
The flow rate in the evaporator must be equal to the rate at which the refrigerant is being boiled off in
the evaporator. Technically speaking, the bulb causes the valve to open or close against the spring
pressure as the temperature on the bulb increases or decreases.
Thermostatic expansion valves are often more efficient than other valves as they control the liquid
content in the refrigerant. In turn, TXVs have many moving parts that they might not give the utmost
performance.
Principles of the Refrigeration Cycle 19/11/2009
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Section C – M.37.1
About Hilton A660 Air Conditioning Laboratory Unit
http://www.p-a-hilton.co.uk/English/Products/Air_Conditioning/air_conditioning.html
A fully modular and upgradeable system enabling the study of air conditioning from the basic principles of
psychrometry, through recirculation and the use of psychrometric and pressure-enthalpy charts using
manual instrumentation, to PID control and computer linking. In addition an environmental chamber is
available for research and student project work. The following upgrade modules can be factory fitted or
purchased individually and installed at a later date by the end user to enhance a teaching programme:
• Digital temperature upgrade;
• Re-circulating duct upgrade;
• PID control upgrade;
• Computer linked upgrade; and
• Environmental chamber.6
6 http://www.p-a-hilton.co.uk/English/Products/Air_Conditioning/air_conditioning.html
Principles of the Refrigeration Cycle 19/11/2009
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The unit consists of; (please refer to the attached block diagram at the end)
• Positive displacement, reciprocating, hermetic compressor equipped with pressure gauge and
thermometer at its outlet;
• Condensing units which includes liquid receiver, filter dryer, pressure gauge and thermometer at its
outlet;
• Glass flow-meter;
• Evaporating unit which includes TXV metering device, pressure gauge and thermometer at its outlet;
• Air chamber which is equipped with two thermometers and variable blower at its inlet, pre-heaters,
two thermometers, evaporator, two thermometers, re-heaters, and two thermometers;
• Steam cylinder; and
• Charged with R134a.
Procedure
We set the equipment on cooling mode, powered the unit and left it to stabilise for about 15 minutes. In
order to determine the superheat, we had to take a reading (H1) from the pressure gauge located at the
evaporator’s outlet. The pressure was 1 Bar (gauge pressure), hence + 1 (atmospheric pressure) = 2 Bar
(absolute pressure). We then checked the boiling point of the R134a at 2 Bar pressure from the PH chart,
which is -10°C. Another reading (H1’) was taken from the thermometer at the evaporator’s outlet, which
was -4°C. Hence the superheat into the compressor was 10 – 4 = +6°C.
The sub-cooling had to be calculated as well in order to plot the refrigerant’s process cycle onto the PH
chart. We took a reading (H3) from the pressure gauge at the condenser’s outlet, which was 9 Bar
(absoulute pressure). From the PH chart, the boiling temperature of R134a at 9 Bar is 36°C. (H3’) was
then taken from the thermometer at the condenser’s outlet, which was 30°C. Hence, the sub-cooling was
-6°C.
Pressure/Enthalpy Chart and Thermodynamic States of R134a
(Please refer to the attached PH chart at the end)
The process cycle of the Refrigerant 134a starts at point H1, at the evaporator’s outlet and in the suction
line. It is also pin-pointed on the saturation line where it starts the superheat. At this point in time, the
refrigerant is at low-pressure saturated vapour state, having a saturated temperature and pressure.
Moreover, the refrigerant has a boiling point temperature (T) of -9.792 °C, pressure (p) of 2.012 Bar,
enthalpy (h) of 391.49 kJ/kg, specific volume (v) of 0.09884 m3/kg, and an entropy (s) of 1.729 kJ/kgK.
The refrigerant flows to point H1’, at the compressor’s inlet but still in the suction line. At this point in time
the gas has a T of -3.898°C, p of 2.012 Bar, h of 396.63 kJ/kg, v of 0.10179 m3/kg, and an s of 1.748
kJ/kgK. Note that there is no change in pressure. Here, the refrigerant is superheated by 6 °C (9.792 –
3.898), which means that the gas is heated to a higher temperature than the saturated temperature. The
Principles of the Refrigeration Cycle 19/11/2009
Joseph Gatt Page 19 of 22
gas is now in the vapour region at low-pressure state. The energy in the suction line is 5.22 kJ/kg (396.63
– 391.49).
R134a is now at point H2 after passing through the compressor. It is at the compressor’s outlet in the
discharge line. The working fluid is still in the vapour region; only this time with a higher pressure and
temperature. Actually, T=47.274°C, p=9.174Bar, h=428.93kJ/kg, v=0.002377, and s=1.748kJ/kgK. Note
that the temperature difference is 47.274 - -3.898 = 51.172°C, and the pressure difference is now 9.174 –
2.012 = 7.162Bar. The work done by the compressor is 428.93 – 396.63 = 32.3kJ/kg.
Having a T of 36.225°C, p of 9.174Bar, h of 416.45kJ/kg, v of 0.02211m3/kg, and an s of 1.708kJ/kgK, the
refrigerant is now at point H2’. It is pin-pointed on the saturation line, at the condenser’s inlet, and still in
the discharge line. Here the refrigerant has saturated temperature and pressure, with no change in
pressure. It is noted that at this point in time the refrigerant is de-superheated due to heat losses from the
discharge line. Actually, the heat lost is 47.274 – 36.225 = 11.049°C. The energy in the discharge line is
416.45 – 428.93 = 12.48kJ/kg.
H3 is the next point, where the refrigerant passed through the condenser, hence the mixture zone. It is
also pin-pointed on the saturation line, having saturation temperature and pressure, and starting the sub-
cooling. Note that the refrigerant had neither a change in temperature or in pressure, but had a change in
state (latent heat). It is now condensed liquid. T=36.225°C, p=9.174Bar, h=250.51kJ/kg, v=0.00395m3/kg,
and s=1.172kJ/kgK. The energy by the condenser is 416.45 – 250.51 = 165.94kJ/kg.
The refrigerant is now at point H3, before entering the metering device. It is in the liquid region, having a
T of 29.640°C, p of 9.174Bar, h of 240.96kJ/kg, and an s of 1.141kJ/kgK. There is no change in pressure
at this point in time, but having a lower temperature. This means that the refrigerant is sub-cooled,
therefore cooled to a lower temperature than the saturated temperature. The energy in the line is 250.51
– 240.96 = 9.55kJ/kg.
H4 is the last point of the cycle, where the refrigerant is in the mixture zone, hence saturated liquid.
Where T= -9.714°C, p=2.030Bar, h=240.96kJ/kg, and s=1.157kJ/kgK. It is now low-pressure liquid due to
a sudden drop in pressure after passing through the expansion valve. A sudden drop in temperature also
occurred. Note that the energy in the metering device is 0, thus constant enthalpy. It shall also be noted
that the pressure drop is 9.174 – 2.030 = 7.144Bar. Therefore, the resultant pressure output from the
compressor is 7Bar continuously.
In order to determine the energy in the evaporator to change the sate from liquid to gas, the enthalpy of
point H4 has to be subtracted by the enthalpy of point H1, which gives 150.53kJ/kg (391.49 – 240.96).
After point H4, the refrigerant flows to point H1 where it starts the process cycle again.
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References
• http://www.engineeringtoolbox.com/dry-wet-bulb-dew-point-air-d_682.html
• http://www.brighthub.com/engineering/mechanical/articles/39619.aspx
• http://www.tpub.com/content/construction/14279/css/14279_241.htm
• http://en.wikipedia.org/wiki/Laws_of_thermodynamics
• http://www.comfort.uk.com/refrigerants.htm
• http://www.memagazine.org/backissues/membersonly/october98/features/refrig/global.html
• Refrigerants – C. Cassar & D. Privitera
• Introduction to HVAC and Refigeration – C. Cassar & D. Privitera
• http://www.engineeringtoolbox.com/Refrigerants-Environment-Properties-d_1220.html
• The refrigeration Circuit – C. Cassar/ D. Privitera
• http://www.answers.com/topic/piston
• http://en.wikipedia.org/wiki/Gas_compressor
• Compressors – D. Privitera & C. Cassar
• http://www.answers.com/topic/centrifugal-compressor
• http://www.central-air-conditioner-and-refrigeration.com/thermostatic_expansion_valve.html
• http://www.longviewweb.com/expansio.htm
• http://www.crtech.com/txvResponse.html
• http://en.wikipedia.org/wiki/Thermal_expansion_valve
• http://www.hvacmechanic.com/txv.htm
• http://www.e-refrigeration.com/index.php?page=metering-device
• http://www.rparts.com/Catalog/Major_Components/filterdryers/filter_dryers.asp
• http://www.p-a-hilton.co.uk/English/Products/Air_Conditioning/air_conditioning.html