2. PROBLEM-1
An ideal vapor-compression refrigeration cycle operates at steady state
with Refrigerant 134a as the working fluid. Saturated vapor enters the
compressor at 2 bar, and saturated liquid exits the condenser at 8 bar.
The mass flow rate of refrigerant is 7 kg/min. Determine
a)the compressor power, in kW
b)the refrigerating capacity, in tons
c)the coefficient of performance
4. Ideal Refrigeration Cycle
An ideal cycle has no irreversibilities within the evaporator and
condenser, and there are no frictional pressure drops. Compression
is isentropic. The T-s diagram is shown on the next slide.
Process 1-2s: Isentropic compression of the refrigerant;
Process 2s-3: Heat transfer from refrigerant to outside air, at
constant pressure;
Process 3-4: Throttling process to a two-phase mixture at lower
pressure;
Process 4-1: Heat transfer to the refrigerant as it flows at constant
pressure through the evaporator;
5. Analyzing vapor-Compression
Refrigeration SystemsAs the refrigerant passes through the evaporator, the heat
transfer per unit mass of refrigerant flowing is:
A ton of refrigeration is equal to 200 Btu/min or 211 kJ/min.
Work done by compressor per unit mass flow of refrigerant is
flowmasstrefrigeranismhh
m
Qin
);( 41 −=
);( 12 hh
m
Wc
−=
7. SOLUTION
Let us first get the properties at each state in the cycle.
State 1: p1 = 2 bar, sat vapor. h1 = 241.30 kJ/kg, s1 = 0.9253
kJ/kg.K
State 2: p2 = 8 bar, s2 = s1, h2 = 269.92 kJ/kg
State 3: p3 = 8 bar, sat. liquid, h3 = 93.42 kJ/kg
State 4: Throttling process, h4 = h3 = 93.42 kJ/kg
a)The compressor power is:
CW m h h kg s kJ kg kW( ) ( / )( . . ) / .= − = − =2 1
7
269 92 241 30 3 34
60
8. Solution
b) The refrigerating capacity is
c)The coefficient of performance is
inQ m h h kg kJ kg tons kJ
tons
( ) ( /min)( . . ) / / / /min
.
= − = −
=
1 4 7 241 30 93 42 211
4 91
h h
h h
( ) ( . . )
.
( ) ( . . )
− −
β = = =
− −
1 4
2 1
241 30 93 42
5 17
269 92 241 30
9. • Problem-2
• A refrigerator uses refrigerant-134a as the working fluid and
operates on an ideal vapor-compression refrigeration cycle
between 0.12 and 0.7 MPa. The mass flow rate of the refrigerant
is 0.05 kg/s. Show the cycle on a T-s diagram with respect to
saturation lines. Determine:
• a) the rate of heat removal from the refrigerated space,
• b) the power input to the compressor,
• c) the rate of heat rejection to the environment, and
• d) the coefficient of performance.
11. Problem-3
Consider an ideal refrigeration cycle which uses R-12 as
the working fluid. The temperature of the refrigerant in
the evaporator is –20°C and in the condenser it is 40°C.
The refrigerant is circulated at the rate of 0.03kg/s.
Determine the coefficient of performance and the
capacity of the plant in rate of refrigeration.
For each control volume analyzed, the thermodynamic
model is the R-12 tables. Each process is SSSF with no
change in kinetic or potential energy.
12. Control volume: Compressor.
Inlet state: T1 known, saturated vapor; state fixed.
Exit state: P2 known(saturation pressure at T3).
At T3=40°C
2 1
2 1
cw h h
s s
= −
=
2
1
1 2
2
2
2 1
0.9607
178.61 /
0.7082
50.8
211.38 /
32.77 /
g
o
c
P P MPa
h kJ kg
s s
T C
h kJ kg
w h h kJ kg
= =
=
= =
=
=
= − =
13. Control volume: Expansion valve.
Inlet state: T3 known, saturated liquid; state fixed.
Exit state: T4 known.
Control volume: Evaporator.
Inlet state: State 4 known.
Exit state: State 1 known.
3 4 74.53 /h h kJ kg= =
1 4 104.08 /
3.18
3.12
L
L
c
q h h kJ kg
q
w
Capacity kW
β
= − =
= =
=
14. • Problem-3
Consider a 300 kJ/min refrigeration system that operates on an ideal
vapor-compression refrigeration cycle with refrigerant-134a as the
working fluid. The refrigerant enters the compressor as saturated
vapor at 140 kPa and is compressed to 800 kPa. Show the cycle on a T-s
diagram with respect to saturation lines, and determine the:
a) quality of the refrigerant at evaporator inlet,
b) coefficient of performance, and
c) power input to the compressor.
15. PSYCHROMETRY & AIR PROCESSES
Psychrometry is the science dealing with the physical laws of air
– water mixtures.
When designing an air conditioning system, the temperature and
moisture content of the air to be conditioned, and the same
properties of the air needed to produce the desired air
conditioning effect, must be known.
Once these properties are known, the air conditioning task can
be determined.
16. COMPOSITION OF AIR
ATMOSPHERIC AIR
Atmospheric air is not completely dry but a mixture of dry air
and water vapor.
In atmospheric air, the content water vapor varies from 0 to
3% by mass.
The processes of air-conditioning and food refrigeration often
involve removing water from the air (dehumidifying), and
adding water to the air (humidifying).
17. THERMAL PROPERTIES OF MOIST AIR
(1) Dry bulb temperature t:
Dry bulb temperature is the temperature of the air, as measured by an
ordinary thermometer.
The temperature of water vapor is the same as that of the dry air in
moist air.
Such a thermometer is called a dry-bulb thermometer in psychrometry,
because its bulb is dry.
(2) Wet bulb temperature tWB:
Wet bulb temperature is thermodynamic adiabatic temperature in an
adiabatic saturation process, and measured by a wet bulb
thermometer.
The wet bulb temperature will be discussed in the next paragraph.
19. (3) Dew point temperature tDP:
When the unsaturated moist air is
cooled at constant vapor pressure or
at constant humidity ratio, to a
temperature, the moist air becomes
saturated and the condensation of
moisture starts, this temperature is
called dew point temperature of the
moist air.
Condensation occurs at the Dew
Point Temperature
20. (4) Relative humidity Ф:
Relative humidity is defined as the ratio of the mole fraction of the water
vapor in a given moist air to the mole fraction of water vapor in a saturated
moist air at the same temperature and the same atmospheric pressure.
Relative humidity is usually expressed in percentage (%).
From the ideal gas relations, relative humidity can be expressed as
---- the mole fraction of the water vapor in moist air;
---- mole fraction of water vapor in a saturated moist air
at the same temperature;
satw
w
satw
w
P
P
x
x
,,
==φ
wx
satwx ,
Relative
Humidity
(%)
Amount of moisture that a given amount
of air is holding
=
Amount of moisture that a given
amount of air can hold
21. (5) Degree of Saturation μ:
Degree of saturation is defined as the ratio of the
humidity ratio of moist air w to the humidity ratio of
saturated moist air wsat at the same temperature and
atmosheric pressure.
(6) Humidity ratio (Moisture Content) w:
The humidity ratio is the mass kg of water vapor
interspersed in each kg of dry air.
It should be noted that the mass of water refers only to
the moisture in actual vapor state, and not to any
moisture in the liquid state, such as dew, frost, fog or
rain.
The humididy ratio, like other several properties to be
studied- enthalpy and specific volume-is based on 1kg of
dry air.
w
satw
sat PB
PB
w
w
−
−
×== ,
φµ
22. (7) Specific Volume/Moist Volume v:
Specific volume of moist air v , m3
/kgdry is defined as the total volume of
the moist air (dry air and water vapor mixture) per kg of dry air.
(8) Specific Enthalpy:
Specific enthalpy of moist air h (kJ/kgdry) is defined as the total
enthalpy of the dry air and water vapor mixture per kg of dry air.
Enthalpy values are always based on some datum plane.
Usually the zero value of the dry air is chosen as air at 0℃, and the zero
value of the water vapor is the saturated liquid water at 0℃.
23. ADIABATIC SATURATION
PROCESS
An unsturated air, which has dry bulb
temperature t1, humidity ratio w1 and
enthalpy h1, flows through a spray of
water, as shown in Figure.
The spray can provide enough surface
area so that the air leaves the spray
chamber in equilibrium with the water,
with respect to both the temperature
and the vapor pressue.
In order to perpetuate the process, it is
necessary to provide makeup water
(w2-w1) to compensate for the amount of
water evaporated into the air.
The temperature of the makeup water
is the same as that in the sump.
If the device is adiabatic, then the
process is called adiabatic saturation
process.
24. THERMODYNAMIC WET BULB
TEMPERATURE
In an adiabatic saturation process, the temperature of the water
in the sump is called the thermodynamic wet bulb temperature
tWB of the air , or simply the wet bulb temperature of the air.
An adiabatic saturation process is a constant wet bulb
temperature process, the wet bulb temperatures of air at the
inlet and the outlet are same, and they are equal to tWB.
That is
21 ⋅⋅ = WBWB tt
25. MEASUREMENT OF WET BULB
TEMPERATURE
A simple wet bulb thermometer used to measure the wet bulb temperature
is based on the principle of the adiabatic saturation process.
It is an ordinary thermometer being wrapt with a cloth sleeve of wool or
flannel, around its bulb.
The cloth sleeve should be clean and free of oil and thoroughly wet with
clean fresh water.
The water in the cloth sleeve evaporates as the air flows at a high velocity
(≥2.5m/sec).
The evaporation, which takes the heat from the thermometer bulb, lowers
the temperature of the bulb.
The thermometer indicates approximately the wet bulb temperature tWB.
The difference between the dry-bulb and wet-bulb temperatures is called
the wet-bulb depression.
If the air is saturated, evaporation cannot take place, and the wet-bulb
temperature is the same as the dry-bulb temperature; and the wet-bulb
depression equals zero.
29. PSYCHROMETRIC CHART
A psychrometric chart graphically
represents the thermodynamic properties
of moist air and it is very useful in
presenting the air conditioning processes.
The psychrometric chart is bounded by two
perpendicular axes and a curved line:
• 1) The horizontal ordinate axis
represents the dry bulb temperature
line t, in℃
• 2) The vertical ordinate axis represents
the humidity ratio line w, in kgw/kgdry.air
• 3) The curved line shows the saturated
air, it is corresponding to the relative
humidity Ф=100% .
30. PROPERTIES IN PSYCHROMETRIC CHART
The psychrometric chart incorporates seven parameters and properties.
They are dry bulb temperature t , relative humidity Ф , wet bulb
temperature tWB, dew point temperature tDP , specific volume v, humidity ratio
w and enthalpy h.
a. Dry-bulb temperature t is shown along the bottom axis of
the psychrometric chart.
The vertical lines extending upward from this axis are
constant- temperature lines.
b. Relative humidity lines Ф are shown on the chart as curved lines
that move upward to the left in 10%
increments.
The line representing saturated air ( Ф= 100% ) is the
uppermost curved line on the chart. And the line of Ф = 0% is a
horizontal ordinate axis itself.
c. Wet-bulb temperature tWB : On the chart, the constant wet-bulb lines
slope a little upward to the left, and the wet bulb temperature is read
following a constant wet-bulb line from the state-point to the
saturation line.
31. PROPERTIES IN PSYCHROMETRIC CHART
d. Dew point temperature tDP : This temperature is read by following a
horizontal line from the state-point to the saturation line.
e. Specific volume v: It is shown from the constant-volume lines
slanting upward to the left.
f. Humidity ratio w: it is indicated along the right-hand axis of the
chart.
g. Enthalpy h: It is read from where the constant enthalpy line crosses
the diagonal scale above the saturation curve. The constant enthalpy
lines, being slanted lines, are almost coincidental as the constant
wet-bulb temperature lines.
Only two properties are needed to characterize the moist air because
the point of intersection of any two properties lines defines the state-
point of air on a psychrometric chart.
Once this point is located on the chart, the other air properties can be
read directly.
32. AIR CONDITIONS ON THE
PSYCHROMETRIC CHART
wet bulb
dew point
30
Dry-Bulb Temperature (°F)Dry-Bulb Temperature (°F)
HumidityRatio(grains/lbofdryair)HumidityRatio(grains/lbofdryair)
25 35 40 45 50 55 60 65 70 75 80 85 90 95 100 105 110
20
40
60
80
100
120
140
160
180
200
220
humidity ratio
dry bulb
relative humidity
33. AIR CONDITIONS ON THE
PSYCHROMETRIC CHART
• There is one more property of air that is displayed on the psychrometric chart
—specific volume. It is defined as the volume of one Kg of dry air at a specific
temperature and pressure. As one pound of air is heated it occupies more
space—the specific volume increases.
14.0
13.0
13.5
14.5
specific volume lines
(cubic feet / pound of dry air)
30
Dry-Bulb Temperature (°F)Dry-Bulb Temperature (°F)
HumidityRatio(grains/lbofdryair)HumidityRatio(grains/lbofdryair)
25 35 40 45 50 55 60 65 70 75 80 85 90 95 100 105 110
20
40
60
80
100
120
140
160
180
200
220
34. COMFORT ZONE
The specifications for human comfort
Relative humidity:
Air flow rate:
dry-bulb temperature
humidityratio
wet-bulb temperature
80°F
[26.7°C
]
70°F
[21.2°C
]
60 %
RH
40 % RHA
CtC 00
2722 ≤≤
%60%40 ≤≤ φ
sec/25.0 m≈
36. SENSIBLE COOLING• The sensible cooling happens when the air is cooled without altering the specific
humidity. During this process, the relative humidity of the air will increase.
• The sensible cooling can only take place under the condition when the temperature
of the cooling coil is not below the dew point temperature of the air being
processed.
37. SENSIBLE HEATING• The sensible heating is similar to sensible cooling, but with the dry bulb temperature
increasing.
• It should be noted that there should be no water within the heating system because the
evaporation of the water will increase the specific humidity of the air.
38. HUMIDIFICATION
• In humidification process the moisture is added to air without changing
the dry-bulb temperature, the air condition moves upward along a dry-
bulb temperature line.
39. DEHUMIDIFICATION
• In dehumidification process the moisture is removed from the air
without changing its dry-bulb temperature, the air condition moves
downward along a dry-bulb temperature line.
42. HEAT AND MASS TRANSFER BETWEEN
MOIST AIR AND SOLID SURFACE
There are a lot of heat exchangers in refrigeration and air conditioning
systems.
Moist air makes heat transfer and/or mass transfer with the solid
surface of the heat exchangers.
If the solid surface is dry during the process, there is heat transfer only.
However, if the solid surface is wetted, there are both heat transfer and
mass transfer.
1. Sensible heat transfer between moist air and solid surface
The sensible heat transfer rate dqsensible, KW, from the wetted surface to
the moist air is )( assensible ttdAdq −⋅⋅= α
43. HEAT AND MASS TRANSFER BETWEEN
MOIST AIR AND SOLID SURFACE
2. Mass transfer between moist air and solid surface
The mass transfer rate of water vapor dm, kgw/s, from the wetted
surface to the moist air is
3. Latent heat transfer between moist air and solid surface
The latent heat transfer rate dqlatent , KW, from the wetted surface to the
moist air is
)( as wwdADdm −⋅⋅=
fgaslatent hwwdADdq )( −⋅⋅=
54. TYPES OF AIR CONDITIONERS
1. Room air conditioners
Window type air conditioner
Split type air conditioner
1. Central air conditioning systems
2. Heat pumps
3. Evaporative coolers
55. TYPES OF AIR CONDITIONERS
Room air conditioners use the standard compressor cycle and are sized to cool
just one room. To cool an entire house, several room units are necessary.
Central air conditioning systems also operate on the compressor cycle
principle and are designed to cool the entire house. The cooled air is distributed
throughout the house using air ducts, which may be the same ducts that are used
by the heating system.
Heat pumps, described in the heating section, use the compressor cycle, but it is
reversible. In the summer, the heat pump transfers heat from indoors to
outdoors. In the winter, the heat pump transfers heat from outdoors to indoors.
Heat pumps may be powered by electricity or natural gas.
Evaporative coolers, also called "swamp coolers", do not use the compressor
cycle. Instead, they cool air by blowing it over a wet surface. You have
experienced this phenomenon when you get out of a swimming pool while a
breeze is blowing. As water evaporates, it absorbs heat from the air. Evaporative
cooling systems depend on the ability of air to absorb moisture, and so they only
work in dry climates such as the Southwest U.S.
58. WINDOW TYPE AIR
CONDITIONERWindow type air conditioners cool rooms rather than the entire home.
Less expensive to operate than central units
Their efficiency is generally lower than that of central air conditioners.
Can be plugged into any 15- or 20-amp, 115-volt household circuit that
is not shared with any other major appliances
It is a packaged AC contains all components in single unit.
59. • Split type air conditioners is like
window type AC but it contains
two units such as indoor and
outdoor units .
• An outdoor metal cabinet contains
the condenser and compressor,
and an indoor cabinet contains the
evaporator and expansion device.
60. CENTRAL AIR CONDITIONING
Circulate cool air through a system of supply and
return ducts. Supply ducts and registers (i.e.,
openings in the walls, floors, or ceilings covered by
grills) carry cooled air from the air conditioner to
the home.
This cooled air becomes warmer as it circulates
through the home; then it flows back to the central
air conditioner through return ducts and registers
60
61. CENTRAL AIR CONDITIONING
SYSTEMS
Outside air is drawn in, filtered
and heated before it passes
through the main air
conditioning devices. The
colored lines in the lower part
of the diagram show the
changes of temperature and of
water vapor concentration (not
RH) as the air flows through
the system.
62. TOTAL AIR CONDITIONING
• Variable fresh air mixer and dust and pollutant filtration.
• Supplementary heating with radiators in the outer rooms and individual
mini heater and
• Humidifier in the air stream to each room.
Notas del editor
That is all there is to the part of the system in the room, which is sketched on the left in figure 2. The bit that is more difficult to understand, or at least unfamiliar to most people, is how the cooling fluid is produced and controlled. That is the part on the right of the diagram.
The cooling fluid used to be a chlorofluorocarbon compound, and often still is, though they all more or less ravage the earth's ozone layer. The essential characteristics of these fluids is that they have quite a low boiling point at atmospheric pressure and that they can stay in the pipes for a long time without decomposing either themselves or the pipes. Finally they need to have some lubricating ability, or the ability to carry a lubricant, because the fluid has to be compressed and pumped round the system. This rare set of necessary properties has proved difficult to combine with friendliness to the earth's atmosphere.
The liquid is let into the cooling unit through a valve marked B on the diagram. It evaporates while it passes through the pipe, taking heat from the air just as water evaporating from a towel laid on your fevered brow cools you when on holiday in the Mediterranean. The temperature in the cooling coil depends partly on the amount of fluid let in by the valve, which is controlled by the thermostat or the humidistat. But now comes a crucial difference from your Mediterranean experience: the minimum temperature at the cold surface can be fixed by controlling the pressure in the cooling coil, with the valve marked A on the diagram. The boiling point of any liquid depends on the pressure. One could use water in the cooling coil, if the pressure is kept low enough. At 1000 Pa pressure, which seems a lot but is just 1% of atmospheric pressure, water boils at 7 degrees. It isn't used in cooling coils of this evaporative type because it has practical disadvantages.
The reason for wanting to limit the minimum temperature is to stop ice clogging the air passage. There are clever systems which notice when ice has formed and hold a melting pause, but that adds to the cost. The pressure controller is therefore set to make the cooling fluid boil at the lowest temperature that is likely to be needed to control the humidity, but always over zero degrees. The temperature needed for cooling is nearly always higher than that needed for dehumidification so it is the RH setting that is decisive.
This brings me to the first point that conservators need to understand: it is expensive to produce air at a dew point below about 4 degrees in this type of equipment. This dewpoint corresponds to 50% RH at 15°C. This sort of air conditioning is entirely suitable for keeping people comfortable but it is not good for specialised stores, for films or for furs, for example, where one needs a temperature below 15 degrees. Such equipment is, however, often used for such places. A better solution is to use an absorption dehumidifier, which will be described in a later article.
Now back to the main story: The vapour that emerges through the pressure controller is gathered up by a compressor. The compression also heats the gas, as will be understood by anyone who has pumped up a cycle tyre. The hot gas is then led away from the room, to be cooled down. This is often done on the roof or in a small enclosure which vibrates to the roar of the fan blowing air over the fins of a condenser. The cooled, now liquid coolant is piped back to the reservoir, ready for its next tour through the room air conditioner.
The entire process described above is inefficient and uses electricity, which is itself produced by inefficient conversion of heat energy. Such systems are therefore confined to small places where the inefficiency is compensated by the generally high reliability and freedom from maintenance.
The principle of operation is the same as that of the small system described above except that the cooling fluid is usually water, which has itself been cooled by the refrigeration system described above. The air is circulated through ducts, with a portion of fresh air added. There is therefore a pre-heater, because the outside air may be below zero and will therefore freeze the water in the cooling coil. A humidifier and various filters have also been added in figure
Some refinements to the basic system compensate for the different heat requirements of different rooms in the building. Figure 4 shows a complete system, with two details that have not been mentioned yet: the outer zone of the building, which loses more heat in winter, has radiators to supplement the heat supply through the air conditioning. The inner zone has, in this example, an archive room that is not much used and so is cooler, and drier, than the rooms with people, computers and coffee machines. To keep the climate uniform throughout the building there is a little local heater and humidifier placed just before the air reaches the room. The main air supply is kept a little too cold and a little too dry. Any one of these local humidifiers can give trouble, with rapid over-humidification of the room. Again, here is a dangerous detail that is provided by the engineer to protect himself against complaints that the equipment does not achieve the standard required.
