Refrigeration Basics: Optimizing System Performance Using a TXV

The thermostatic expansion valve (TXV) is a precision device designed to regulate the rate at which liquid refrigerant flows into the evaporator. This controlled flow is necessary to prevent the return of liquid refrigerant to the compressor.

The TXV separates the high pressure and low-pressure sides of a refrigeration or air conditioning system. Liquid refrigerant enters the valve under high pressure, but its pressure is reduced when the TXV limits the amount of refrigerant entering the evaporator.

Remember: the TXV controls only one thing: the rate of flow of liquid refrigerant into the evaporator. The TXV is not designed to control air temperature, head pressure, capacity, suction pressure, or humidity. Attempts to use the TXV to control any of these system variables will lead to poor system performance and possible compressor failure.

The TXV responds to the temperature of refrigerant gas as it leaves the evaporator. This temperature is detected by the sensing bulb, which is located near the evaporator outlet. The TXV also responds to the refrigerant pressure within the evaporator, which is transmitted to the TXV by an equalizer line. By responding to these variables, the TXV maintains a predetermined superheat within the evaporator. This is how the TXV keeps the system in balance and operating properly. To understand how this works, we must have a clear understanding of superheat.

Superheat is the difference between two temperatures:

Example of superheat:

When superheat is too low, the point at which all the refrigerant is finally evaporated occurs very close to the evaporator outlet. When this happens, it's possible for liquid refrigerant to be fed back into the compressor, where it will cause serious damage.

When superheat is too high, the liquid refrigerant is fully evaporated long before it reaches the evaporator outlet. As a result, the temperature of the refrigerant vapor continues to rise, raising the superheat of the gas in the suction line from the evaporator to the compressor.

For every one degree rise in the suction gas temperature entering the compressor, there is a corresponding one and a half degree rise in the discharge gas temperature. This can lead to poor system performance and overheating of the compressor. By controlling the rate at which high pressure refrigerant is released into the evaporator, the TXV controls the amount of superheat that will occur.

Basic TXV operation

The TXV controls superheat by controlling the flow of liquid refrigerant. As it does this, it also reduces refrigerant pressure.

As the flow is restricted, several things happen:

Changes in gas temperature at the evaporator outlet are detected by the sensing bulb, which then causes the valve pin to move in or out, regulating the flow of refrigerant through the TXV. In this way, the valve allows just enough refrigerant into the evaporator, to maintain the correct level of superheat in the suction line.

How the TXV Controls Superheat

The TXV controls superheat by varying the size of the orifice through which the refrigerant flows. The pin angle, the size of the stroke (typically 0.015- to 0.035-in.) and the diameter of the orifice itself all affect how much refrigerant can pass through the valve. In addition, all valves have some leakage around the valve pin, although this is normally kept within acceptable limits.

It's important to remember that valve capacity is a function of the orifice diameter, pin angle, and stroke. Adjusting the superheat spring doesn't change valve capacity.

Al Maier is vice president, applications engineering for Emerson Climate Technologies, Flow Controls. Graphics courtesy Emerson Climate Technologies. emersonclimate.com.