How are Thermostatic Expansion valves and Electronic Expansion valves

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  • It is a variable restriction type expansion valve.
  • It maintains a constant degree of suction superheats at the evaporator outlet, therefore, it is also called a constant superheat expansion valve.
  • The thermostatic expansion valve is probably the most widely used refrigerant control device at the present time, due to its high efficiency and ready adaptability.
  • It is particularly suitable for systems, which are subjected to wide and frequent variations in loading.

Construction:

  • The conventional liquid-charged thermostatic expansion valve.
  • The principal parts of the valve, are:
    • Needle and seat
    • Metallic diaphragm
    • Spring with an adjusting screw
    • Feeler bulb
    • Strainer
  • The opening and closing of the valve depend upon the following:
    • Spring pressure – acting on the bottom of the diaphragm
    • Feeler bulb pressure – acting on the top of the diaphragm
    • evaporator pressure – acting on the bottom of the diaphragm
  • The feeler bulb is partly filled with the same liquid refrigerant as used in the refrigerating system.
  • The feeler bulb is connected to the end of an evaporator coil and any change in the temperature of the refrigerant will cause a change in pressure in the feeler bulb. It will be transmitted to the top of the diaphragm through a capillary tube.

Operation:

  1. under normal conditions, the spring pressure and evaporator pressure acting at the bottom of the diaphragm balance the feeler bulb pressure acting at the top of the diaphragm.
  2. The force tending to close the valve depends on spring pressure and evaporator pressure. The evaporator pressure depends upon the saturation temperature of the refrigerant in the evaporator coil.
  3. The force tending to open the valve depends upon the feeler bulb pressure. It depends upon the temperature of the refrigerant in the bulb.
  4. When the valve is set for a certain superheat (difference between two temperatures), it maintains the setting under all load conditions on the evaporator.

When the load on the evaporator increases:

  • It causes, faster evaporation in the evaporator, which increases the feeler bulb temperature, in turn, increases the feeler bulb pressure.
  • This increase in pressure is transmitted to the diaphragm. The diaphragm moves downward, which admits more quantity of liquid refrigerant to the evaporator this continues till the pressure equilibrium is reached.

When the load on the evaporator decreases:

  • It causes, less liquid refrigerant evaporates in the evaporator, which cools the feeler bulb. Thus the feeler bulb pressure decreases due to decreases in its temperature.
  • The feeler bulb pressure is transmitted to the diaphragm, which moves it upwards. This reduces the opening of the valve. The flow of liquid refrigerant to the evaporator is reduced, This continues till the pressure equilibrium is reached.

Applications:

  1. Most suitable for applications in air conditioning and refrigeration plants.
  2. Used in both commercial and industrial refrigerant systems.
Thermostatic Expansion valves

Electronic Expansion valve

  • It is a variable restriction type expansion valve.
  • It controls the flow of refrigerant entering a direct expansion evaporator.
  • The control of this valve is independent of refrigerant and refrigerant pressure hence it works in a reverse flow direction also.
  • It is convenient to use in year-round – air – conditioning systems, which serve as heat pumps in winter with reverse flow.

Construction:

  • The schematic diagram of an electronic expansion valve.
  • The principal parts of the valve, are:
    • orifice and needle
    • The heating element or stepper motor with controller
    • Liquid sensing thermistor
  • The opening and closing of the valve depend on the following:
    • Heater current – which depends upon
    • Thermistor resistance – that depends upon
    • Refrigerant condition.
  • Since the area available for refrigerant flow in the expansion device is normally very small. There is a danger of valve blockage due to some impurities present in the system. Hence, it is essential to use a filter before the expansion device.
  • In another version of EEV, the heater is replaced by a stepper motor with the controller, which opens and closes the valve with great precision giving a proportional control in response to the temperature sensed by an element.

Operation:

  1. The needle moves up and down in response to the magnitude of the current in the heating element. A small resistance allows more current to flow through the heater of the expansion valve, as a result, the valve opens wider.
  2. A small negative coefficient thermistor is used if superheat control is desired. The thermistor is placed in series with the heater of the expansion valve.
  3. Exposure of thermistor to superheated vapor permits thermistor to self-heat lowering its resistance and increasing the heater current. This opens the valve wider and increases the mass flow vapor becomes saturated and some liquid refrigerant droplets appear.
  4. A thermistor is nothing but a resistor that changes its resistance as its temperature changes other sensors are located at the evaporator inlet and outlet to sense the evaporator superheat. This protects the compressor from any liquid flooding back under low superheat conditions.
  5. The liquid refrigerant will cool the thermistor and increases its resistance hence in presence of liquid droplets its thermistor offers a large resistance, which allows a small current to flow through the heater making the valve opening narrower.
  6. If operations are not proper certain practical problems are encountered with this expansion device an oversized expansion device will overfeed the refrigerant or hunt ( too frequent closing and opening ) and not achieve the balance point

Applications:

  1. Useful in many diverse applications of air conditioning and refrigerating plants
  2. Used in both commercial and industrial refrigerant systems.
Electronic Expansion valve

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