Thermostatic expansion valve pressure calculations - How to calculate expansion valve?

Thermostatic Expansion Valve

Capacity expansion valve (TRV) depends on the following factors:
  • The size of the hole.
  • Pressure difference between the high side and the low side.
  • Temperature and state of the refrigerant in the liquid line.
The capacity of most thermostatic valves size selected needle hole and Assembly. The same body can be used for many opportunities. More than a hole, the more liquid refrigerant can be served in the evaporator in a given time.

Valves are calculated in tonnes of refrigeration. However, the same hole, as a rule, has three different tonnage of capacities. This power range depends on the pressure of the high side and the low side. Increasing this pressure drop will increase the level of refrigerant.

For example, if the valve is used on the R-12 refrigerant system, the following occurs: the valve, which has a 1/2 ton (.455 metric ton) rating at the level of 13 psi (193 kPa) pressure on the low side will have a 3/4 ton 1 ton (.72 to .98 metric ton) capacity of 5" (12.7 cm) Hg vacuum at the frozen products unit.

The same valve has only 1/3 tonne (.3 metric ton) capacity at low side pressure of 40 psi (380 kPa) for air conditioning.

In the first case, a 130 - 13=117 psi (807 kPa) pressure difference, assuming 130 pounds per square inch (1 000 kPa) pressure head. In the second case, it is a question of 130 plus 2 1/2 psi (5-vacuum=2 1/2 psi=84 kPa) -132 1/2 psi (914 kPa) pressure difference. In the latter case it is 130 - 40=90 psi (621 kPa) pressure difference.

It is important to use the valve required carrying capacity. With undersized valve outlet of the evaporator will be starved, regardless of overheating. The full capacity of the evaporator can be achieved.

If the outlet is oversized, the valve will hunting, or burst. When the valve is opened too much refrigerant will be held in the evaporator. Suction line will sweat frost and heat element can close the valve. Increase overheating fix this condition results in the evaporator starve a lot of time.


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