How thermostatic expansion valves work - Danfoss

There are three different forces at work in a TXV: bulb pressure, spring pressure, and evaporator pressure (see Figure 4). Bulb pressure comes from the bulb that is mounted at the outlet of the evaporator; the bulb senses the suction temperature and drives the diaphragm down if there is an increase. Spring pressure is constant and pushes up against the diaphragm, counter to the bulb pressure. The spring pressure is calibrated when the valve is set by the equipment manufacturer or the installer. Evaporator pressure pushes the diaphragm up when the suction pressure increases and comes from the evaporator load on the system, which varies according to different operating conditions, such as room temperature changes. Based on the balance between these three pressures, the valve will either open or close.

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Figure 6 shows two types of port designs. In conventional or single port designs, the diaphragm can be influenced by pressure changes in the condenser. The general rule of thumb is that a conventional port design works best in systems with less than five tons of refrigerant, while larger systems work best with a balanced port design (though it is not uncommon to use a balanced port valve in smaller systems). A balanced port design isolates the condenser pressure from affecting the opening of the valve, necessitating the use of O-rings. However, the more O-rings used, the more friction will be created, requiring design measures to negate frictional loss in the TXV.

While there are several types of bulb charges, two common charges used in air conditioning systems are universal charge and anti-hunt charge (see Figure 7).

With a universal charge, the bulb filled with a liquid cross charge. Whenever the bulb senses an increase in suction line temperature, the liquid expands, increasing the pressure in the fixed volume, and pushes the diaphragm down, thereby opening the valve and allowing more liquid refrigerant into the evaporator. Unfortunately, vaporization is a dynamic process, which can produce sporadic superheat at the evaporator outlet. Think of liquid refrigerant changing to vapor like a pot of boiling water: the liquid does not instantaneously become a gas once the boiling point is reached, but changes into steam erratically. Similarly, the bulb might sense vapor one instant and liquid the next. In this scenario, a bulb with a universal charge will rapidly open and close the valve, a process called hunting. Hunting reduces the system’s efficiency, shortens the valves lifespan, and increases the risk of liquid refrigerant making its way to the compressor, which will damage it.

To avoid hunting, some TXVs add a ballast to the bulb (usually a clay brick), creating what is known as an anti-hunt charge. The ballast dampens the rate of expansion within the bulb, stabilizing the bulb pressure against the diaphragm by dampening the rate of temperature change to the bulb charge compared to the rate of temperature change of the suction line. This stabilization ensures that the TXV operates more efficiently and better protects the compressor.

There are two common approaches to what makes up bulb charge fluid. The first approach is to use the same refrigerant that is used in the system, i.e., using R-410A in the bulb for an R-410A system. The other common approach—and the one that Danfoss recommends—is called a cross charge. Cross charged bulbs mix a combination of different refrigerants with gases to flatten the pressure-temperature (P-T) curve (see Figure 8). Cross charges enable the TXV to perform similarly regarding the change in opening degree for a given change in superheat across a range of evaporator temperatures.

Whether it's air conditioning systems, refrigeration applications in supermarkets, or large cooling or freezing plants, commercial refrigeration valves play a huge role in ensuring the heating, cooling, and freezing processes run smoothly.

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Besides handling large temperature variations, these valves also need to deal with large pressure differences, and corrosive, hazardous, and often dangerous refrigerant gases including propane, ammonia, hydrofluorocarbons, and in some cases the highly ozone-depleting, and now banned CFC's and HCFCs.

What are commercial refrigeration systems?

Commercial refrigeration is the use of cold storage equipment in commercial spaces. These include the reach-in freezers and refrigerators that can be found in supermarkets, food stores, grocery markets, and convenience stores. A walk-in refrigerator and freezer in a restaurant or cafeteria are also considered commercial refrigeration equipment.

In industrial settings, refrigeration units can be used during the production of plastic or metal, or to help with the fermentation process of beverages.

Environmental testing of parts and components can also be performed using industrial refrigeration. In some cases, refrigeration systems can create extremely cold environments to test whether or not parts will fail or have shortened life cycles. To make all this work efficiently a wide range of valves is implemented throughout the industry.

What are commercial refrigeration valves?

Within refrigeration systems, there is a huge variety of valves used for all parts of the system. These valves all fulfill different duties within refrigeration systems. Some supply refrigerant to the system, some play a part in cooling of the system, some work to avoid contamination. These include: 

Solenoid Valves: Solenoid valves are widely used in refrigeration to control the amount of flow of media in the systems. The liquid line in almost every dry-expansion refrigeration system over a certain capacity is controlled by solenoid valves. They use an electrical current to activate a solenoid coil which in turn activates a valve to open or close, opening or closing flow.

For refrigeration systems, the choice for a solenoid valve depends on system capacity, refrigerant, type of fluid, temperature, and pressures where normally the choice would be made based on Cv values and line size. Solenoid valves can be a complex subject matter, read this article to understand solenoid valves better.  

Thermal expansion valves: The volume of refrigerant released into an evaporator is controlled by a thermal expansion valve. Its purpose is to maintain a constant temperature of the refrigerant that flows out of the evaporator. Even though an expansion valve is often described as a thermostatic valve, it cannot precisely control the evaporator's temperature. The temperature of the evaporator will only vary with evaporating pressure.

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