Expansion Valves and Devices

You have to know by now, that expansion valves and devices in air conditioning systems have the function of,

  • allowing pressure build up from the compressor’s outlet, and hence separating the high pressure and low pressure sides of an air conditioner, and

  • regulating the flow of liquid refrigerant, into expansion zone. Too much liquid into expansion zone is not good. Too little is just as bad

It is a form of flow limitation device to enable pressure drop across the inlet and outlet lines, as explained here.

So, what is too much and too little of flow?

It depends on the size of the evaporator and refrigerant type used.

The evaporator and refrigerant are matched to give desired heat removal capacity of an air conditioning unit. Therefore, the expansion device has to make sure that the amount of liquid fed into the expansion zone, will be completely boiled at the end of the evaporator.

Anything less than that, is too little. Anything beyond it, is too much.

The common types of expansion valves or devices:


  • capillary tube, and

  • the thermostatic expansion valve

are the most commonly used in air conditioning systems. The former is normally used in small sized units, and the latter is used in larger sized units.

The capillary tube expansion device

This device is not a valve, but a plain tube with similar material to the main air conditioner tubing.

There is no device to control the flow, except for the tube’s internal diameter, length and its spiral shape.

These factors will create necessary restriction, and allowing necessary amount of refrigerant into expansion zone.

Thermostatic expansion valves

Now, this is a valve with moving parts involved.

The valve will have a,

  • sensing bulb connected to a tube,

  • diaphragm,

  • spring loaded valve seat,

  • liquid inlet line,

  • liquid outlet line,

  • filter at the inlet line

  • adjusting knob

These expansion valves will have the sensing bulb placed at the exit line of the evaporator.

Pressure within the connected tube will rise if the exit temperature is too high, i.e. when there is insufficient boiling within the evaporator.

Pressure in the connected tube will drop if the exit temperature is too low, i.e. there is too much fluid going into the evaporator.

Illistration of a thermostatic expansion valve.

The rise and drop in pressure will push, or pull the diaphragm. This diaphragm is directly connected to the spring loaded valve seat. The seat is arranged such that it will increase flow under high pressure, and vice versa.

Therefore, these expansion valves may operate under varying load conditions.

Filter is used to trap unnecessary solids from equipment wear and tear, into the valve seat of the expansion valves.

Adjusting knob or dial, is used to tune the valves so that there will be approximately extra 5 oC (41 oF) of refrigerant superheat at the evaporator’s exit. Liquid flooding into the compressor will be avoided by such setting.

Comparison between thermostatic expansion valves and capillary tubes:

Capillary tubeTEV
AdvantagesCheap, simple design, maintenance freeCan operate under varying load requirements
DisadvantagesMay operate under tight load variations only. Consider tube replacement from compressor’s exit to evaporator’s inlet, if defectiveMore expensive than capillary tubes. Requires tuning during first assembly

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