1. Sanjivani Rural Education Society’s
Sanjivani College of Engineering, Kopargaon-423 603
(An Autonomous Institute, Affiliated to Savitribai Phule Pune University, Pune)
NAAC ‘A’ Grade Accredited, ISO 9001:2015 Certified
Department of Mechanical Engineering
Subject:- Heating, Ventilation and Air
Conditioning (ME 411)
B.Tech.Mechanical
Purushottam W. Ingle
Assistant Professor
3. • First three expansion devices fall in the category of fixed opening
type.
• And remaining three are falling into variable opening type.
12/27/2023 P.W. Ingle Department Of Mechanical Engineering, Sanjivani COE, Kopargaon
7. • The capillary tube, as shown in Fig. 12.1, is used as an expansion
device in small capacity hermetic sealed refrigeration units such as
in domestic refrigerators, water coolers, room air conditioners and
freezers.
• It is a copper tube of small internal diameter and of varying length
depending upon the application. The inside diameter of the tube
used in refrigeration work is generally about 0.5 mm to 2.25 mm and
the length varies from 0.5 m to 5 m.
• It is installed in the liquid line between the condenser and the
evaporator.
12/27/2023 P.W. Ingle Department Of Mechanical Engineering, Sanjivani COE, Kopargaon
8. • In its operation, the liquid refrigerant from the condenser enters the
capillary tube. Due to the frictional resistance offered by a small
diameter tube, the pressure drops.
• Since the frictional resistance is directly proportional to the length
and inversely proportional to the diameter, therefore longer the
capillary tube and smaller its inside diameter, greater is the pressure
drop created in the refrigerant flow.
• In other words, greater pressure difference between the condenser
and evaporator
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12. • The automatic expansion valve is also known as constant pressure
expansion valve, because it maintains constant evaporator pressure
regardless of the load on the evaporator.
• Its main moving force is the evaporator pressure. It is used with dry
expansion evaporators where the load is relatively constant.
• The automatic expansion valve, as shown in Fig., consists of a needle
valve and a seat (which forms an orifice), a metallic diaphragm or
bellows, spring and an adjusting screw.
12/27/2023 P.W. Ingle Department Of Mechanical Engineering, Sanjivani COE, Kopargaon
13. • The opening and closing of the valve with respect to the seat depends
upon the following two opposing forces acting on the diaphragm :
• 1. The spring pressure and atmospheric pressure acting on the top of
the diaphragm, and
• 2. The evaporator pressure acting below the diaphragm.
• When the compressor is running, the valve maintains an evaporator pressure in
equilibrium with the spring pressure and the atmospheric pressure. The spring
pressure can be varied by adjusting the tension of the spring with the help of
spring adjusting screw.
• Once the spring is adjusted for a desired evaporator pressure, then the valve
operates automatically to maintain constant evaporator pressure by controlling
the flow of refrigerant to the evaporator.
12/27/2023 P.W. Ingle Department Of Mechanical Engineering, Sanjivani COE, Kopargaon
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15. • When the evaporator pressure falls down, the diaphragm moves
downwards to open the valve. This allows more liquid refrigerant to
enter into the evaporator and thus increasing the evaporator pressure
till the desired evaporator pressure is reached.
• On the other hand, when the evaporator pressure rises, the
diaphragm moves upwards to reduce the opening of the valve.
• This decreases the flow of liquid refrigerant to the evaporator which,
in turn, lowers the evaporator pressure till the desired evaporator
pressure is reached.
12/27/2023 P.W. Ingle Department Of Mechanical Engineering, Sanjivani COE, Kopargaon
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18. • The thermostatic expansion valve is the most commonly used
expansion device in commercial and industrial refrigeration systems.
• This is also called constant superheat valve.
• because it maintains constant superheat of the vapor refrigerant at
the end of the evaporator coil, by controlling the flow of liquid
refrigerant through the evaporator.
• The thermostatic expansion valve , as shown in Fig.consists of a
needle valve and a seat, a metallic diaphragm, spring and an
adjusting screw.
• In addition to this, it has a feeler or thermal bulb which is mounted
on the suction line near the outlet of the evaporator coil.
12/27/2023 P.W. Ingle Department Of Mechanical Engineering, Sanjivani COE, Kopargaon
19. • The feeler bulb is partly filled with the same liquid refrigerant as used in
the refrigeration system. The opening and closing of the valve depends
upon the following forces acting on the diaphragm :
• 1. The spring pressure (PS ) acting on the bottom of the diaphragm,
• 2. The evaporator pressure (PE ) acting on the bottom of the diaphragm,
and
• 3. The feeler bulb pressure (PB ) acting on the top of the diaphragm.
• Since the feeler bulb is installed on the suction line, therefore it will be at
the same temperature as the refrigerant at that point. Any change in the
temperature of the refrigerant will cause a change in pressure in the
feeler bulb which will be transmitted to the top of the diaphragm.
12/27/2023 P.W. Ingle Department Of Mechanical Engineering, Sanjivani COE, Kopargaon
20. • Under normal operating conditions, the feeler bulb pressure acting at the
top of the diaphragm is balanced by the spring pressure and the
evaporator pressure acting at the bottom of the diaphragm.
• The force tending to close the valve is dependent upon the spring
pressure and the evaporator pressure which, in turn, depends upon the
saturation temperature of the refrigerant in the evaporator coil.
• The force tending to open the valve depends upon the feeler bulb
pressure which, in turn, depends upon the temperature of refrigerant in
the bulb.
• Thus the operation of valve is controlled by the difference between the
two temperatures (i.e. saturation temperature and feeler bulb
temperature) which is the superheat.
12/27/2023 P.W. Ingle Department Of Mechanical Engineering, Sanjivani COE, Kopargaon
21. • If the load on the evaporator increases, it causes the liquid refrigerant
to boil faster in the evaporator coil.
• The temperature of the feeler bulb increases due to early
vaporization of the liquid refrigerant. Thus the feeler bulb pressure
increases and this pressure is transmitted through the capillary tube
to the diaphragm.
• The diaphragm moves downwards and opens the valve to admit more
quantity of liquid refrigerant to the evaporator.
• This continues till the pressure equilibrium on the diaphragm is
reached.
12/27/2023 P.W. Ingle Department Of Mechanical Engineering, Sanjivani COE, Kopargaon
22. • On the other hand, when the load on the evaporator decreases, less
liquid refrigerant evaporates in the evaporator coil. The excess liquid
refrigerant flows towards the evaporator outlet which cools the feeler
bulb with the result the feeler bulb pressure decreases due to
decrease in its temperature.
• The low feeler bulb pressure is transmitted through the capillary
tube to the diaphragm and moves it upward. This reduces the
opening of the valve and thus the flow of liquid refrigerant to the
evaporator. The evaporator pressure decreases due to reduced
quantity of liquid refrigerant flowing to the evaporator.
• This continues till the evaporator pressure and the spring an pressure
maintain equilibrium with the feeler bulb pressure.
12/27/2023 P.W. Ingle Department Of Mechanical Engineering, Sanjivani COE, Kopargaon
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24. Cooling tower
• Water from the condenser when it absorbs heat from the refrigerant,
is made to flow through the cooling tower.
• Then this water is made to flow the packaging unit, so here comes in
contact with the air and air is flowing in counter flow manner, as
shown.
• So air takes away the heat from water, during this some water get
evaporates, and remaining water is filled in the sump.
• The cold water from sump again recirculated through condenser, with
the help of condenser pump.
• It is applicable for water cooled condenser only.
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26. • Here water flows from top to bottom, so continuously water
temperature decreases, at the same time air goes from bottom to
top, so air temperature continuously increases.
• The fall in the temperature of water in cooling tower is called as
RANGE, and the difference between lowest temperature of water and
lowest temperature of air is known as APPROACH.
• Lower approach and higher range is the desirable characteristics of
any type of cooling tower.
•
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28. Natural Draught cooling tower
12/27/2023 P.W. Ingle Department Of Mechanical Engineering, Sanjivani COE, Kopargaon
29. • The atmospheric natural draft (spray type) cooling tower, as shown in
Fig. 10.11. consists of a box-shaped structure with louvers.
• The louvers allow the atmospheric air to pass through the tower, but
slant down towards the inside of the tower to retain water in it. The
framework and louvers are usually made of steel.
• The size of the cooling lower depends upon the capacity of the unit.
The atmospheric natural draft (spray type) cooling towers should be
located in the open space or on the roof of a building where the air
can blow freely through them.
12/27/2023 P.W. Ingle Department Of Mechanical Engineering, Sanjivani COE, Kopargaon
30. • In this type of cooling tower, warm water from the condenser is pumped to a
spray header provided at the top of a tower. It is sprayed down into the tower
through the nozzles.
• Since the heat transfer from water to air is dependent upon the surface of water
exposed lo the air stream, therefore a spray nozzle having finer spray pattern is
essential for good performance of the cooling tower.
• It may be noted that the finer spray exposes more water surface to air. However,
if the spray is too fine, too much water is blown away. The water spray blown
away by the air is called drift.
• The drift increases water loss in the tower, but does not affect the
cooling action.
12/27/2023 P.W. Ingle Department Of Mechanical Engineering, Sanjivani COE, Kopargaon
31. Mechanical Draft Cooling Towers
12/27/2023 P.W. Ingle Department Of Mechanical Engineering, Sanjivani COE, Kopargaon
32. • In the forced draft cooling tower, as shown in Fig. 10.13, a fan forces
the air through the tower. In its operation, the warm water from the
condenser is sprayed at the top of the tower through the spray
nozzles. The air is forced upward through the tower by the propeller
fan provided on the side near the bottom of the tower as shown in
the figure.
• The condenser warm water is cooled by means of evaporation as
discussed earlier. The effectiveness of the cooling tower may be
improved by increasing the height of the tower, area of water surface
exposed to air or the velocity of air.
• The air velocities from 75 to 120 m/min is recommended with a flow
of 90 to 120 per tons of refrigeration capacity.
12/27/2023 P.W. Ingle Department Of Mechanical Engineering, Sanjivani COE, Kopargaon
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34. • In the induced draft cooling tower, as shown in Fig. 10.14, the fan
sucks the air through the tower.
• The induced draft cooling towers are similar to forced draft cooling
towers except that the fans are located at the top instead of at the
bottom and draw the air upward through the tower
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