A thermostatic expansion valve (TXV) is built up around a thermostatic element separated from the valve body by a diaphragm. Its purpose is to regulate the rate at which refrigerant flows into the evaporator.

TXVs can contain one of three different types of charge:

• universal charge
• Maximum operating pressure (MOP) charge.
• MOP charge with ballast (standard for Danfoss expansion valves with MOP).

Expansion valves with Universal charge are used in most refrigeration systems where there is no pressure limitation requirement and where the bulb can be located armer than the element or at high evaporating temperature/evaporating pressure.

Expansion valves with MOP charge are typically used on factory-made units where suction pressure limitation on starting is required such as in the transport sector and in air conditioning systems. All expansion valves with MOP have a very small charge in the bulb. This means that the valve or the element must be located warmer than the bulb. If it's not, charge can migrate from the bulb to the element and prevent the expansion valve from functioning.

Expansion valves with MOP ballast charges are used mainly in refrigeration systems with "high dynamic" evaporators, such as in air conditioning systems and plate heat exchangers with high heat transfer.

Installation Procedures
The expansion valve must be installed in the liquid line, ahead of the evaporator, with bulb fastened to the suction line as close to the evaporator as possible. If there's external pressure equalization, the equalizing line must be connected to the suction line immediately after the bulb. (Figure 1)

The bulb is best mounted on a horizontal suction line tube and in a position corresponding to between 1 o'clock and 4 o'clock. The location depends on the outside diameter of the tube. (Figure 2) Note:The bulb must never be located at the bottom of the suction line due to the possibility of oil laying in the bottom of the pipe causing false signals.

The bulb must be able to sense the temperature of the superheated suction vapour and must therefore not be located in a position that will expose it to extraneous heat/cold.

The bulb must not be installed after a heat exchanger. In this position it will give false signals to the expansion valve (Figure 4). Additionally, the bulb must not be installed close to components of large mass, as this also will give rise to false signals to the expansion valve.

As previously mentioned, the bulb must be installed to the horizontal part of the suction line immediately after the evaporator. It must not be installed to a collection tube or a riser after an oil pocket. The expansion valve bulb must always be installed ahead of any liquid lock.

— Information courtesy Danfoss

How the TXV Controls Superheat
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:

• the saturation temperature of the refrigerant (the temperature at which the refrigerant changes from a liquid state to vapor. This is the same as its boiling point. For water at sea level, the saturation temperature is 212F. The saturation temperature of a liquid increases as pressure increases.
• the actual temperature of the refrigerant (the temperature of refrigerant vapor by the time it reaches the evaporator outlet).

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

Click here to download a convenient TXV Troubleshooting PDFfrom Danfoss