2. 5.1 Introduction
The mechanism used for lowering or
producing low temp. in a body or a space,
whose temp. is already below the temp. of
its surrounding, is called the refrigeration
system.
Here the heat is being generally
pumped from low level to the higher one
& is rejected at high temp.
3. Objectives
•
•
•
•
Basic operation of refrigeration and AC
systems
Principle components of refrigeration and
AC systems
Thermodynamic principles of refrigeration
cycle
Safety considerations
5. Refrigeration
The term refrigeration may be defined as
the process of removing heat from a
substance under controlled conditions.
It also includes the process of reducing
heat & maintaining the temp. of a body
below the general temp. of its
surroundings.
6. Contd….
In other words the refrigeration means a
continued extraction of heat from a body
whose temp is already below the temp. of
its surroundings.
7. Coefficient of performance(COP)
C.O.P is a measure of efficiency of a refrigeration cycle/
system.
It is the ratio of refrigerating effect to the energy spend.
Refrigerating effect is the amount of heat removal/
absorbed from the substance to be cooled.
The energy spend may e in the form of work in VCR or
heat in VAR.
8. REFRIGERATORS AND HEAT PUMPS
The transfer of heat from a low-temperature
region to a high-temperature one requires
special devices called refrigerators.
Another device that transfers heat from a
low-temperature medium to a hightemperature one is the heat pump.
Refrigerators and heat pumps are essentially
the same devices; they differ in their
objectives only.
The objective of a refrigerator is to remove heat
(QL) from the cold medium; the objective of a heat
pump is to supply heat (QH) to a warm medium.
for fixed values of
QL and QH
9. Refrigerator & Refrigerant
A refrigerator is a reversed heat engine or
a heat pump which takes out heat from a
cold body & delivers it to a hot body.
The refrigerant is a heat carrying medium
which during their cycle in a refrigeration
system absorbs heat from a low temp.
system & delivers it to a higher temp.
system.
10. Unit of refrigeration
The capacity of refrigeration unit is generally expressed
in “Tons” of refrigeration.
The rate of removal of heat in cooling operation was
expressed in terms of kilograms or tons of ice required
per unit time usually or a day.
11. One ton of refrigeration
The quantity of heat required to remove from one ton ice
within 24 hours when initial condition of water is zero
degree centigrade, because the same cooling effect will
be given by melting the same ice.
12. Uses of Systems
•
•
and cargo
Cooling of food stores
Cooling of electronic spaces and
equipment
•
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CIC (computers and consoles)
Radio (communications gear)
Radars
Sonar
Air conditioning for crew comfort
14. Refrigeration Cycle
In refrigeration system the heat is being
generally pumped from low level to
higher one & rejected at that temp.
This rejection of heat from low level to
higher level of temp. can only be
performed with the help of external work
according to second law of
thermodynamics.
15. Contd….
The total amount of heat being rejected to
the outside body consist of two parts:- the heat extracted from the body to be
cooled .
- the heat equivalent to the mechanical
work required for extracting it.
19. Vapour compression cycle
The superheated vapour enters the
compressor where its pressure is
raised
3
Condenser
High
Pressure
Side
4
Expansion
Device
Compressor
2
1
Evaporator
Low
Pressure
Side
20. Vapour compression cycle
Low pressure liquid refrigerant in
evaporator absorbs heat and
changes to a gas
3
Condenser
High
Pressure
Side
4
Expansion
Device
Compressor
2
1
Evaporator
Low
Pressure
Side
20
21. Vapour compression cycle
The high pressure superheated gas
is cooled in several stages in the
condenser
3
Condenser
High
Pressure
Side
4
Expansion
Device
Compressor
2
1
Evaporator
Low
Pressure
Side
24. Evaporator/Chiller
•
•
•
Located in space to be refrigerated
Cooling coil acts as an indirect heat
exchanger
Absorbs heat from surroundings and
vaporizes
•
•
Latent Heat of Vaporization
Sensible Heat of surroundings
26. Compressor
•
Superheated Vapour:
•
•
•
•
•
Enters as low press, low temp vapour
Exits as high press, high temp vapour
Temp: creates differential (DT)
promotes heat transfer
Press: Tsat
allows for condensation
at warmer temps
Increase in energy provides the driving
force to circulate refrigerant through the
system
27. Condenser
•
•
•
Refrigerant rejects latent heat to cooling
medium
Latent heat of condensation (LHC)
Indirect heat exchanger: seawater absorbs
the heat and discharges it overboard
28. Receiver
•
•
Temporary storage space & surge volume
for the sub-cooled refrigerant
Serves as a vapor seal to prevent vapor
from entering the expansion valve
29. Expansion Device
•
•
•
Thermostatic Expansion Valve (TXV)
Liquid Freon enters the expansion valve
at high pressure and leaves as a low
pressure wet vapor (vapor forms as
refrigerant enters saturation region)
Controls:
•
•
Pressure reduction
Amount of refrigerant entering evaporator
controls capacity
33. Vapour absorption refrigeration system
In the absorption refrigeration system, refrigeration effect is
produced mainly by the use of energy as heat. In such a
system, the refrigerant is usually dissolved in a liquid. A
concentrated solution of ammonia is boiled in a
vapour generator producing ammonia vapour at high
pressure. The high pressure ammonia vapour is fed to a
condenser where it is condensed to liquid ammonia by
rejecting energy as heat to the surroundings. Then, the liquid
ammonia is throttled through a valve to a low pressure.
During throttling, ammonia is partially vapourized and
its temperature decreases.
34. Vapour absorption refrigeration system
is fed to an evaporator
This low temperature ammonia
where it is vaporized removing energy from the
evaporator. Then this low-pressure ammonia Vapour is
absorbed in the weak solution of ammonia. The
resulting strong ammonia solution is pumped back to
the Vapour generator and the cycle is completed. The
COP of the absorption system can be evaluated by
considering it as a combination of a heat pump and
a heat engine
37. ICE plant
The simple VCR cycle is used in primary circuit using
ammonia as a refrigerant and brine is a secondary
circuit.
This is indirect method of cooling used for ice
production.
The ice can contain water which reject heat to the brine
which is circulated in secondary circuit.
In evaporator the heat of brine is transfer to the
refrigerant in primary circuit and brine is cooled.
38. ICE plant
The vapour refrigerant form in evaporator is suck by
compressor then it compressed to a high pressure and
this is condensed in a condenser with the help of cooling
water.
The high pressure liquid ammonia is collected in the
receiver and then it is passed through expansion valve.
The throttle liquid ammonia at low pressure and
temperature enter in an evaporator, so brine in cooled
and ammonia absorb heat and form vapour.
39. 5.3 Psychrometry and
Air Processes
1. 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).
40. 2. The thermal parameters of moist
air
t
(1) Dry bulb temperature
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.
41. (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.
(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
xw
Pw
xw, sat
Pw, sat
42. (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.
43. (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℃.
45. Psychrometric chart
A psychrometric chart graphically represents the
thermodynamic properties of moist air.
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% .
46.
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.
①Dry-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.
②Relative 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.
47. 2. Main air handing processes and their
variations in properties
(1) Sensible cooling along a cooling coil, or sensible
heating along a heating coil
tDp
rel
ati
ve
2a
1a
1b
2b
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.
tW
3
dew-point temp
B
dry-bulb temp
wet
-bu
lb t
em
p
w humidity ratio
During this process, the
relative humidity of the air
will increase.
φ
hu
mi
di
ty
The sensible cooling happens
when the air is cooled without
altering the specific humidity.
φ
=1
00
%
48. Cont.….
tDp
re
la
tiv
e
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.
φ
2a
1a
1b
2b
tW
3
dew-point temp
B
dry-bulb temp
wet
-bu
lb t
emp
w humidity ratio
The sensible heating is
similar to sensible cooling,
but with the dry bulb
temperature increasing.
φ
=1
00
%
hu
mi
di
ty
49. (2) Adiabatic humidification and dehumidification using
a humidifier or chemical dehumidifier
tDp
re
lat
iv
e
2a
1a
1b
2b
tW
3
During the adiabatic humidification
process along the constant wet bulb
temperature line, the specific humidity
of air will increase.
Reduction in dry bulb temperature will
happen as the evaporated water will
absorb heat .
dew-point temp
B
dry-bulb temp
wet
-bu
lb t
em
p
w humidity ratio
A humidifier performs this function by
supplying the water vapor.
φ
hu
mi
di
ty
The adiabatic humidification occurs
when water vapor, of which
temperature is near the wet bulb
temperature of the moist air, is added
to the air .
φ
=1
00
%
50. 5.4 Air Conditioning
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•
•
Purpose: maintain the atmosphere of an
enclosed space at a required temp, humidity and
purity
Refrigeration system is at heart of AC system
Types Used:
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•
Self-contained
Refrigerant circulating
Chill water circulating
51. Air conditioning
Air conditioning is the science which deals with the
supply and maintaining desirable internal atmospheric
condition irrespective of external condition.
Air conditioning may be defined as simultaneous
control of temperature, humidity, motion of air and
purity of air within the enclosed space.
52. Classification of air conditioning
a) According to purpose.
i) comfort air conditioning system
ii) Industrial air conditioning
b) According season of year.
i) Winter air conditioning
ii) Summer air conditioning
iii) Year round (All weather air conditioning)
53. Classification of air conditioning
c) According to equipment arrangement
i) central air conditioning
ii) unitary air conditioning
d) According to working substance used.
i) All air system
ii) chilled water system
iii) Air water system
