ACHRNEWS

TX valves vs. automatic expansion

September 13, 2000
In refrigeration and air conditioning applications, there are two main types of expansion valves: thermostatic expansion valves (TXVs), and automatic, or constant-pressure, expansion valves (AXVs).

This article will take a closer look at the differences between these valves, and provide information to help select the best type for your particular application.

TXVS

Function: The thermostatic expansion valve is a metering device. It regulates the flow of high-pressure liquid refrigerant to the low-pressure evaporator in response to the heat load on the evaporator.

The valve’s primary function is to maintain maximum evaporator efficiency and protect the compressor by preventing liquid refrigerant from passing through the suction line to the compressor. It does this by controlling the flow of liquid refrigerant into the evaporator, so that it equals the rate at which the liquid is completely vaporized by the heat load on the evaporator. The resulting suction gas is superheated.

The TXV controls flow to maintain a predetermined level of superheat. A typical valve is shown in Figure 1.

Operation: Conventional TXVs respond to four forces (see Figure 2).

Force 1 — Thermal bulb pressure times the diaphragm effective area; this force acts on the top of the diaphragm, which tends to open the valve.

Force 2 — Evaporator pressure times the diaphragm effective area; this force acts on the underside of the diaphragm. It tends to close the valve. This force is transmitted to the diaphragm through the valve body with internal equalized valves, or through the external connection in external equalized valves.

Force 3 — Superheat spring force that assists in closing the valve; and

Force 4 — High- and low-side pressure differential times the port area; this differential pressure force tends to open the valve.

Design: Parker TXVs incorporate the power piston (balanced port). The balanced port power element assembly incorporates heavy-duty diaphragm housings and a high-strength, stainless steel flat diaphragm to withstand severe high pressures.

The assembly also includes a “buffer” ring for additional support and subsequent additional endurance. The flat steel diaphragm provides a smooth stroke without “snap.”

The stainless steel piston assembly uses an o-ring packing compound for refrigerant use. The element is protected from any system contaminants by virtue of the piston seal on the power piston. Additionally, this seal prevents any leakage from the high to low side of the valve.

This balanced-port design offers the ability to compensate for wide variations in high- to low-side pressure; sufficient capacity to allow for intermittent flash gas; the ability to compensate for wide variations in evaporator load; and the ability to compensate for changes in liquid line temperatures.

AXVS

Function: The constant pressure or automatic expansion valve responds to outlet pressure. It meters liquid refrigerant to the evaporator, maintaining a constant evaporator pressure.

Refrigerant flows at a rate that exactly matches compressor capacity. A typical AXV is shown in Figure 3.

Operation: The valve incorporates a diaphragm that separates atmospheric pressure and system pressure. A range spring with an adjustable pressure setting is located above the diaphragm.

Beneath the diaphragm is a stainless steel push rod and ball assembly backed by a closing spring. An o-ring located on the push rod creates a balanced port. This feature balances out the effect of increasing or decreasing inlet pressure, and aids in maintaining a constant outlet pressure. An equalizer passageway is incorporated into the valve body.

Three forces control the operation of the valve (Figure 4).

Force 1 — The adjustable range spring above the diaphragm, which moves the diaphragm down, opening the valve;

Force 2 — The closing spring beneath the diaphragm, which moves the push rod and ball assembly up, closing the valves; and

Force 3 — The outlet pressure acting under the diaphragm; this is the pressure the valve controls when the spring force, F1, is equal to the sum of forces F2 and F3.

Design: The AXV incorporates a diaphragm that separates atmospheric pressure and system pressure. A range spring with an adjustable pressure setting is located above the diaphragm.

Beneath the diaphragm is a stainless steel push rod and ball assembly backed by a closing spring. An o-ring located on the push rod creates a balanced port, and an equalizer passageway is incorporated into the valve body. These design features provide the following user benefits.

  • The balanced-port feature balances out the effect of increasing or decreasing inlet pressure and aids in maintaining a constant outlet pressure. In other words, changes in inlet (condenser) pressure have little effect on outlet pressure control.

  • AXVs are position insensitive. The valve diaphragm is not affected by oil migration even at low temperatures, or if the valve is installed upside-down.

  • Freeze-up protection is provided by keeping the evaporator above the freezing point at low loads.

  • A constant evaporator pressure is maintained under all load conditions.

  • And there is no remote bulb to attach to the suction line.


Applications

Many oem’s apply TXVs in applications where it may be possible to use an AXV. If the application has a fairly constant load, such as a flaked ice machine or water cooler that is installed in a conditioned environment, an AXV can often be used in place of a TXV.

The oem’s use TXVs over AXVs to improve the performance of their units at abnormally high or low operating conditions. The TXV is capable of producing more ice and it is more efficient; however, the reduction in ice capacity at the 70/50 condition is only 3.5% and the reduction in efficiency is only 4.2%.

If these small reductions in capacity and efficiency can be tolerated and the unit is being used in a conditioned environment, there are some advantages to using an AXV.

The AXV has no remote bulb; therefore, the installation is easier and there is no bulb charge that can possibly leak out over time. Additionally, AXVs are less expensive when compared to conventional TXVs.

AXVs are also easier to adjust after the valve is installed in the system. Since AXVs control evaporator pressure, it is not necessary to measure suction temperature when installing the valve. Simply adjust the range spring until the desired evaporator pressure is reached.

If the service technician is unsure of the needed evaporator pressure, then measuring suction temperature is necessary. The service technician needs to adjust the evaporator pressure until a desired superheat is achieved.

Constant-pressure expansion valves can also be used in place of or in parallel with capillary tubes in air conditioning applications.

At low-load conditions, the AXV maintains a higher suction pressure, thus preventing coil freeze-up. Additionally, at higher load conditions, the AXV maintains a lower suction pressure, thus reducing power consumption and protecting the compressor against overload.

Also, since the AXV maintains a constant suction pressure, this results in more efficient compressor operation. It allows the system designer to save on up-front costs through reduced condenser surface area and the use of a lower-capacity compressor.

The benefits of AXVs over capillary tubes can easily offset the additional cost for the automatic expansion device.