Semester 3 CB303 Ventilation and Air Conditioning Note Flow Control

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Refrigerant Flow Control

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 Hand Expansion Valve
 Capillary tube
 Thermostatic expansion valves – superheat control
 Automatic Expansion Valves – Evaporator Pressure Control
 Low Pressure Float Control
 High Pressure Float Control
Refrigerant Flow Control

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 The refrigerant flow control is one of the four major components in a
vapor compression refrigeration system.
 The function of any refrigerant flow control is twofold[1]：to adjust the
quantity of refrigerant flow into the evaporator according to the
evaporator load; to create a pressure drop from the high side to the low
side of the system in order to permit the refrigerant to vaporize under the
desired low pressure in the evaporator while at the same time condensing
at a high pressure in the condenser.

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 There are various types of refrigerant flow control devices, such as manual
expansion valve, capillary tube, thermostatic expansion valve, float valve
and electronic expansion valve and so on.

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1. Hand Expansion Valves
 Hand expansion valves are also called throttle valves.
 The structure of a hand expansion valve is shown in Fig.1.
 The expansion valve comprises of
main body, valve seat, and hand
wheel which is actuated to change
the opening area around the valve
seat to adjust the frictional
resistance to the refrigerant flow.
Fig.1, Hand expansion valve

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Fig.1, Hand expansion valve
 The rate of the refrigerant flow
through the valve depends on
the pressure differential across
the valve and opening of the
valve.
 Assuming that the pressure drop
across the valve remains the
same, the flow rate through a
hand expansion valve will
remain constant at all times
regardless of the evaporator
pressure and the evaporator
load.

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 The advantage of the hand expansion valve is that it is unresponsive to
changes in the system load and the disadvantage is the valve must be
manually readjusted each time when the load on the system changes in
order to prevent either starving or overfeeding of the evaporator.
 In addition, the valve must be opened and closed manfully each time
when the compressor is cycled on and off.

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2.Capillary tube
 Capillary tubes are widely used as expansion devices in small vapor
compression refrigeration Systems, such as household refrigerators, room
air conditioners, and small package air conditioning units.
 In these system, the capillary tube
is wound into with coils for direct
expansion.
 The tube connects the outlet of
condenser to the inlet of the
evaporator as shown in Fig.2.
Fig.2, Capillary tube

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 Physically the capillary tubes are hollow tubes made with drawn copper,
with internal diameters ranging between 0.51and 2 mm [2].
 Primarily there are two kinds of capillary tubes, namely adiabatic and non-
adiabatic tubes.
 The adiabatic capillary tube expands refrigerant from high pressure to low
pressure adiabatically while in the non-adiabatic situation, the capillary
tube forms a counter-flow heat exchanger with the suction line that joins
the evaporator and the compressor .

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 The refrigerant flow inside the capillary tube is very complex, particularly
in non-adiabatic situations where the capillary tubes are in thermal
contact with the suction lines.
 When the pressure of the sub-cooled liquid refrigerant flowing through
the non-adiabatic capillary tubes drops below the saturation value
(corresponding to its temperature), a part of the refrigerant flashes into
vapor.
 This results in two-phase flow while the refrigerant pressure continues to
drop due to the friction and fluid expansion in the capillary tube.

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3. Thermostatic expansion Valves-
Superheat Control
 At present, thermostatic expansion valve is probably the most widely used
refrigerant flow control device because of its high efficiency and its ready
adaptability to any type of refrigeration applications.
 The thermostatic expansion valve controls the mass flow rate of the
refrigerant into the evaporator according to inspiration vapor degree of
superheat, and at the same time throttles the liquid from condensing
pressure to evaporation pressure.

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I. Internally thermostatic expansion valves
 Fig.3 is an operation diagram of the internal equalizer thermostatic
expansion valve, the main parts including: a needle and seat, a pressure
bellows or diaphragm, a fluid-charged remote bulb, and a spring, the
tension of which is usually adjustable by an adjusting screw.
Fig.3 The principle of internal equalizer thermostatic expansion valve

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 A screen or strainer is usually installed at the liquid inlet for the valve to
prevent the entrance of foreign material which may cause malfunction of
the valve.
 The main important part of the thermostatic expansion valve is the
remote bulb, which responses the superheat of the refrigeration at the
outlet of the evaporator and then move to close or open the valve to
throttle the flow of the liquid to the evaporator.
Fig.3, The principle of internal equalizer thermostatic expansion valve

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 In order to ensure against refrigerant liquid entering the compressor, it is
common practice to have the refrigerant leave the evaporator slightly
superheated.
 Superheat is the difference between the temperature at the bulb and the
evaporating temperature, the former is measured at the point where the remote
bulb is located at the exit of the evaporator coil (seen in Fig.3).
Fig.3, The principle of internal equalizer thermostatic expansion valve
 It is essential that the entire
length of the remote bulb be in
good thermal contact with the
suction line and the outside of
the remote bulb be adiabatic to
ensure the temperature of the
refrigerant in the suction line and
the remote bulb is equal.

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 Superheat is used as a signal to regulate liquid injection through the
expansion valve.
 Resulting from the pressure exerted by the saturated liquid-vapor mixture
in the remote bulb, stem and valve head, the bulb pressure pb acts on the
top of the diaphragm to open the valve Fig.13-5.
 Besides the bulb pressure, there are two other pressures under the
diaphragm to move the valve toward an open or closed position: the
spring pressure ps acts on the pin and the evaporator pressure pe acts on
the bottom of the diaphragm to close the valve.
 When the opening and closing pressures balance each other, the valve pin
is in a stable fixed position.

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 When the bulb pressure is greater than the sum of the spring pressure
and the evaporator pressure, the valve will move to a position more
open than it was and so allow more refrigerant to flow in the evaporator.
 On the other hand, the valve will move toward a less open position if the
closing pressures are greater than the opening pressure.

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II. Externally thermostatic expansion valves
 When there is significant pressure drop between the valve outlet and the
evaporator exit, the thermostatic expansion valve with an internal
equalizer construction will not function properly.
 That is because the saturation temperature of the refrigerant is always
lower at the evaporator outlet than at the evaporator inlet because of
friction as it flows through the evaporator.
 Then it necessitates a higher degree of suction superheat in order to bring
the valve into equilibrium.

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 The problem can be solved by using an external equalizer thermostatic
expansion valve.
 The evaporator pressure pe is replaced by the refrigerant pressure pw of
the evaporator outlet to balance the diaphragm.
 This is accomplished by completely isolating the valve diaphragm from the
evaporator inlet pressure, while at the same time permitting the
evaporator outlet pressure to be exerted on the diaphragm through a
small diameter tube which is connected to the evaporator outlet or the
suction line 150 to 200 mm beyond the remote bulb location on the
compressor side.

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4.Automatic Expansion Valves -
Evaporator pressure Control
 Fig.4 is a schematic diagram of
an automatic expansion valve
showing the principal part of
the valve, which includes a
needle and seat, a pressure
bellows or diaphragm and a
spring, the tension of which is
variable by means of an
adjusting screw.
Bellows or diaphragm
Needle and seat
Strainer
Inlet from receiver
Spring
Out
Spring pressure
Evaporator pressure
Adjusting screw
Fig.4, Automatic Expansion Valve

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Bellows or diaphragm
Needle and seat
Strainer
Inlet from receiver
Spring
Out
Spring pressure
Evaporator pressure
Adjusting screw
Fig.4, Automatic Expansion Valve
 As in the case of the
thermostatic expansion valve
and all other refrigerant
controls, a screen or strainer
is usually installed at the
liquid inlet of the valve in
order to prevent the entrance
of foreign materials which
may cause stoppage of the
valve.

21. in the evaporator in which it is
controlling
the liquid level or it may be
installed external to these units in
a separate float chamber.
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5. Low-Pressure Float Control
 The low pressure float control (low
side float) acts to maintain a constant
level of liquid in the evaporator by
regulating the flow of liquid
refrigerant into that unit in
accordance with the rate at which the
supply of liquid is being depleted by
vaporization . It is responsive only to
the level of liquid in the evaporator
and will maintain the evaporator filled
with liquid refrigerant to the desired
level under all conditions of loading
without regard for the evaporator
temperature and pressure. The low

22. Low-Pressure Float Control
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6. High-Pressure Float Control
• The high pressure float valve is a
liquid level actuated refrigerant flow
control that regulates the flow of
liquid to the evaporator in accordance
with the rate at which the liquid is
being vaporized. These device is
located on the high pressure side of
the system and controls the amount
of liquid in the evaporator indirectly
by maintaining a constant liquid level
in the high side pressure float control

24. High-Pressure Float Control
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25. High-Pressure Float Control
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REFERENCES
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1978
2. Capillary tube of copper and copper alloys, GB/T 1531
3. Appliances Components Companies，
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nformaciontecnica#TablasGraficos
5. Stoecker W.F., Jones J.W., Refrigeration and air conditioning, 2nd Edition, McGraw-Hill
Book Company, New York, USA.,1982
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Valve，World Shipping，2004, 27(5)44-45

27. 27
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465
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