REFRIGERATION BASICS: Optimizing System Performance Using a TXV

Basic TXV operation

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

• Liquid refrigerant enters the TXV under high pressure.

• As the flow of liquid refrigerant is reduced, its pressure drops.

• The refrigerant leaving the TXV is now a combination of low-pressure liquid and vapor.

As the flow is restricted, several things happen:

• The pressure on the liquid refrigerant drops

• A small amount of the liquid refrigerant is converted to gas, in response to the drop in pressure

• This “flash gas” represents a high degree of energy transfer, as the sensible heat of the refrigerant is converted to latent heat

• The low-pressure liquid and vapor combination moves into the evaporator, where the rest of the liquid refrigerant “boils off” into its gaseous state as it absorbs heat from its surroundings.

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.