Explanation:
Expansion valve:
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Expansion valves or throttling valves are used to reduce the refrigerant condensing pressure (high pressure) to the evaporating pressure (low pressure) by a throttling operation and regulate the liquid-refrigerant flow to the evaporator to match the equipment and load characteristics.
These devices are designed to proportion the rate at which the refrigerant enters the cooling coil to the rate of evaporation of the liquid refrigerant in the coil; the amount depends, of course, on the amount of heat being removed from the refrigerated space.
The four main types of throttling device are:
- Capillary tube
- Automatic throttling valve
- Thermostatic expansion valve
- Float valve
Thermostatic expansion valve:
- It is not a temperature control device and it cannot be used to vary evaporator temperature.
- Actually, the thermostatic expansion valve is a throttling device that works automatically, maintaining proper and correct liquid flows as per the dictates of the load on the evaporator.
- Because of its adaptability to any type of dry expansion application, automatic operation, high efficiency, and ability to prevent liquid flood backs, this valve is extensively used.
- Thermostatic expansion valves are generally used with large refrigeration and air conditioning machinery.
- These are also used with low-temperature applications and in package-type air conditioners.
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Functions:
- to keep the evaporator fully active (completely filled)
- to reduce the pressure of the liquid from the condenser pressure to the evaporator pressure
- to continue the flow of liquid to the evaporator according to the load requirements of the evaporator
Note:
Moisture circulating with the refrigerant tends to freeze on the thermostatic expansion valve, causing a build-up of pressure on the condenser side and drop in pressure on the evaporator side due to the blockage.
Capillary Tube:
The constant-restriction type device is the capillary tube which is merely a long tube with a narrow diameter bore.
Automatic Expansion Valve:
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The automatic expansion valve maintains a constant pressure in the evaporator.
- The constant pressure characteristics of the valve result from the interaction of two opposing forces, the evaporator pressure, and the spring pressure.
- The evaporator pressure exerted on one side of the bellows or diaphragm acts to move the valve in a closing direction, whereas the spring pressure, acting on the opposite side of the bellows or diaphragm acts to move the valve in an opening direction. When the compressor is running, the valve functions to maintain the evaporator pressure in equilibrium with the spring pressure.
Float valve:
Float valve maintains the constant level of liquid in the evaporator and condenser, Float valve is used as the throttling device in the large industrial refrigeration plants that fulfill the cooling requirements of the entire process. There are two types of float valves
(1) Low side float valve
(2) High side float valve
Note:
- low side float valve the float chamber is placed in the evaporator on the low-pressure side, in the high-pressure side float valve, and the float chamber is placed on the high-pressure side between the condenser and the evaporator.
- In the low side float valve, the valve opens as the level of the refrigerant drops in the evaporator, but in the high side float valve, the valve opens when the level of the refrigerant increases in the chamber.
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