ACHRNEWS

Ice Breaker: How Automatic Expansion Valves Work

February 2, 2009

The automatic expansion valve (AXV) is similar in construction and appearance to the commonly used thermostatic expansion valve (TXV). However, the metering devices operate much differently.

The AXV is a constant pressure regulator. It is designed and constructed to maintain a constant evaporating pressure. The flow of the refrigerant into the evaporator is based on the pressure of the refrigerant in the evaporator. As the pressure in the evaporator decreases, the valve opens and allows more refrigerant to flow through it. As the pressure increases, the valve closes and allows less refrigerant to flow through it.



HOW IT WORKS

An AXV consists of a diaphragm, an adjustable spring, and a needle valve. The diaphragm controls the operation of the needle valve. The adjustable spring is placed on top of the diaphragm and acts to open the needle valve.

The opposing pressure that acts to close the needle valve is the pressure of the refrigerant in the evaporator. As the pressure in the evaporator drops, the spring pressure overcomes the pressure on the underside of the diaphragm and moves the needle valve to a more open position, allowing more refrigerant into the evaporator.

As the refrigerant pressure increases in the evaporator, it imposes a higher pressure on the underside of the diaphragm, which overcomes the spring tension and moves the needle valve to a more closed position, reducing the flow of refrigerant into the evaporator. The pressure applied by the spring can be adjusted to allow for different maximum evaporating pressures. Normally, an adjustment screw on top of the valve body is connected to the spring inside the dome of the valve.



CONSTANT LOADS

This type of metering device is applied similarly to the capillary tube. It should only be used on systems with a fairly constant heat load. Because it does not control the amount of superheat at the outlet of the evaporator, it does not work well on systems with dramatic changes in heat loads. In fact, dramatic changes in heat loads cause problems with the system.

If a high heat load is placed on the evaporator, the valve reduces the refrigerant flow into the evaporator instead of feeding more refrigerant. The evaporator pressure will rise - and because the closing force of the needle valve is the pressure of the refrigerant in the evaporator, it will move to a more closed position, restricting the flow of refrigerant into the evaporator.

During low heat load conditions, the valve will open more, allowing more refrigerant into the evaporator, when it should be restricting flow into the evaporator. On low heat load conditions, the pressure in the evaporator will drop. The spring pressure will keep the valve in a full-open position to feed more refrigerant into the evaporator.

One final note: An AXV looks similar to a TXV but operates much differently. It’s important not to confuse the two while servicing a refrigeration system. An easy way to tell the difference is to look at the top of the valve. If it has an adjustable screw on top of the diaphragm, it’s an AXV. If it has a capillary tube and sensing bulb connected to the top of the diaphragm, it’s a TXV.

Publication date: 02/02/2009