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Refrigerants

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03/01/07

So you want to know about a T.X.V

The standard TXV is a dynamic valve that has three main functions:

1) Provide a constant amount of evaporator superheat under varying evaporator heat load conditions, provided the range and capacity of the valve have not been exceeded.

2) Keep the entire evaporator full of liquid/vapor refrigerant under all evap. heat loading conditions for an efficient system.

3) Prevent liquid floodback of refrigerant to the compressor under varying evap. heat loading conditions.

What standard TXVs do not do:

---Control evaporator pressure
---Cycle the compressor
---Control running time
---Control box temperature

The three main working forces which act on the TXV are:

----Remote bulb or sensing bulb pressure (opening force)
----Spring pressure (closing force)
----Evaporator pressure (closing force)

Note: A fourth liquid (opening) force is often negligible on smaller tonnage valves because of the smaller surface areas it acts on. This force will be mentioned in a later article on balanced port TXVs.

In explaining the principal behind the internal and external equalized TXV, the closing force of the evaporator pressure must be explored in some detail.

Internally Equalized TXV
The evaporator pressure acts on the underside of the flexible bellows or diaphragm. This will have a closing effect on the valve as the evap. pressure increases. Also notice that the evap. pressure that acts on the underside of the bellows in the TXV is pressure from the evap. inlet or the evaporator coil's entrance. This evap. inlet or entrance pressure acts through a drilled passageway called the internal equalizer port on it's way to the underside of the TXV's flexible bellows. This is how the internal equalizer TXV gets it's name. Anytime the evap. entrance pressure acts on the underside of the TXV's bellows, the TXV is referred to as an internally equalized TXV. Internally equalized TXVs are used on systems with smaller evaporators because of their low pressure drop characteristics. As refrigerant travels from the beginning to the end of the evaporator coil, the pressure remains relatively constant and a consistent evaporator superheat can be accomplished by the TXV.

Externally Equalized TXV
In larger evaporators, pressure drop from fluid friction as the refrigerant travels the length and turns of the evaporator can effect TXV performance. When this pressure drop is appreciable, an externally equalized TXV must be used. If not, high evap. superheat reading and inefficient system performance will be experienced. Externally equalized TXVs sense evaporator pressure at the evap. outlet instead of at the entrance. Evaporator outlet pressure is sensed through a small diameter tube that is connected to the evaporator outlet beyond the remote sensing bulb. This is accomplished by completely isolating the valve diaphragm from the evaporator inlet pressure. Mechanical seals allow the main shaft to move while sealing evaporator inlet pressure from reaching the TXV's diaphragm. The external equalizer port should never be closed off because evaporator pressure will never be sensed, and the valve will be inoperable.

The TXVs are made either internal or external equalized and cannot be changed in the field. External equalizer connections allow evaporator outlet pressure to be sensed and compensate for any evaporator pressure drop. This type of connection does not get rid of the pressure drop, it only compensates for the pressure drop and permits full and effective use of the evap. surface.
Remember, evaporator pressure is a closing force on a TXV, and because evaporator outlet pressure and not evaporator entrance pressure is sensed, there will be less of a closing force upon the underside of the TXV's diaphragm when an external equalizer is used. This smaller closing force will cause the valve to remain more open and fill out more of the evaporator with refrigerant for better system efficiency.

When to Use Externally Equalized TXVs

----Evaporators with large pressure drops--A basic rule of thumb is when the pressure drop causes a:
--3 degree temp. change in a/c ranges (high temp. applications)
--2 degree temp. change in the medium temp. commercial refrigeration ranges
--1 degree temperature change in the low temp. application range
----When a refrigerant distributor is used in conjunction with the TXV because of it's
inherently large pressure drop characteristics.

Questions ?

- Mark

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