Expansion Devices
Expansion devices play very important role in the vapour compression refrigeration system.
Functions :
● It must reduce the pressure of the refrigerant coming from the consent temperature als per the requirement the system
● It must regulate the flow of the refrigerant as per the load on the evaporator.
These are classified as follows
(1) Capillary tube,
(2) Thermostatic expansion valve,
(3) Automatic expansion or constant pressure valve.
(4) Low and high pressure
Capillary Tube:
● It is extremely small diameter tube ranging from 0.5 mm to 25 mm of about 0.5 m to 5 m long
● In its operation, liquid refrigerant enters the capillary tube and during the flow, there is a pressure drop due to friction. Some of the liquid flashes into vapour as refrigerant flow through the tube.
● Once the sine and length of the is selected, no modification is possible to adjust itself to variation in discharge pressure, suction pressure and load care must be taken to prevent the plugging of the tube by ice or any other decomposed material.
● The capillary tube is substitute for the conventional liquid line from the condenser and soldered to a length of the suction line to form a single heat exchanger.
Advantages of Capillary Tube :
(1) The cost of the capillary tube is less than other expansion devices.
(2) Since refrigerant charge in the capillary tube system is critical, there is no receiver is necessary.
3) When the compressor stops, the refrigerant continue to flow into the evaporator and equalizes the pressure between the high side and low side of the system. This considerably decreases the starting load on the compressor. Thus low starting torque motor can be used to drive the compressor, which is great advantages.
Disadvantages of Capillary Tube:
(1) t cannot be used for high fluctuating load plant.
(2) f the refrigerant contains some moisture and dirt, it will choke the tube and stop the flow of refrigerant.
Thermostatic Expansion Valve
● This is most popular and very efficient type of expansion device.
● The operation of this valve is based on the principle of constant degree of superheat for the evaporator exiLE by. controlling the flow of liquid refrigerant through the evaporator
● The thermostatic expansion valve consists of a needle valve and a seat, a metallic diaphragm spring and adjusting screw.
● In addition to this it has a feeler or thermal bulb which is firmly clamped on the suction line of compressor near the outlet of the evaporator coil.
● The filler bulb is partly filled with the same liquid refrigerant as used in refrigeration system
● The opening or closing of valve is depend upon the forces acting on the diaphragm .
Operation :
● The remote bulb is charged with fluid which is open on one side of the diaphragm through capillary tube is clamped firmly to the evaporator outlet.
● The pressure of the fluid in the bulb tends to open the valve. This pressure is balanced by pressure due to spring and in the evaporator.
● If the evaporator temperature is high or the load on the evaporator increase, more fluid from feeler bulb will be vaporized and bulb pressure will rises which exert this force on diaphragm.
● This will widen the valve opening and the refrigerant flow will increase to meet load demand and if load on evaporator decrease reverse action takes place. Thus, it automatically maintain the degree of superheat of refrigerant vapour after coming out of evaporator.
Automatic Expansion Valve or Constant Pressure Valve:
● This valve simply works on the principle of maintaining a set pressure in the evaporator by adjusting the spring.
● The constant pressure in the evaporator is maintained by two opposing forces namely evaporator pressure and spring pressure.
● The evaporator pressure acting on the diaphragm tries to close the valve and the spring pressure tries to open the valve.
● Suppose evaporator pressure falls below the set value for which spring is set.
● The valve will open more and increase the liquid (refrigerant) flow to the evaporator resulting in flooding more evaporator surface.
● When more evaporator surface effective, the refrigerant vaporize more result in rise in evaporator pressure till it again balance the spring pressure.
● if the evaporator pressure Rises above the set value, the valve tends to closed, less refrigerant flow into the evaporator. this lead to reduced in pressure in operator till again it is equal to Spring pressure.
● When the plant in the off, the evaporator pressure will built up and keep the valve also firmly shut.
● On starting the plant the suction from the evaporator start resulting in the fall in pressure into the evaporator below the spring pressure to Resume the flow of liquid through the valve.
● It is imperative that rate of vaporization should keep constant to maintain evaporator pressure unaltered.
● It is used with dry expansion evaporator where load is relatively constant
Comparisons :
◆ Comparison between Thermostatic expansion valve and capillary Tube :
Thermostatic expansion valve → Capillary tube
1. It is used in large refrigeration capacity system → It is used in small refrigeration system
2. Pressure drop is high → Pressure drop is low.
3. Efficiency is high → Efficiency is low.
4. Complicate in construction → Simple in construction
5. Receiver is required. → Receiver is not required
6. Cost is high → Cost is low.
7. Starting torque of motor driven compressor is high. → Starting torque of motor driven compressor is low.
Comparison between thermostatic expansion valve and automatic
Thermostatic expansion valve → Automatic expansion valve.
1. Work on constant degree of superheat → Work on constant pressure of refrigerant.
2. Pressure drop is large → Pressure drop is small
3. External equalization is required. → Equalization not required.
4. Used in flooded type evaporator. → Used in dry expansion evaporator
5. Frictional losses are more. → Frictional losses are less
6. Flow patterns can affect due to change in pressure. → Efficiency and degree of superheat can affect due to change in load.
7. Valve functioning is controlled by spring, evaporator pressure and feeler bulb pressure. → Valve functioning is controlled by spring pressure and evaporator pressure only.
