Internally Equalized Thermostatic Expansion Valves VS. External Thermostatic Expansion Valve

What is Thermostatic Expansion Valve?

The thermal expansion valve is an important part of the refrigeration requirement. It is one of the most important parts in the refrigeration system. Small in size, but huge effect. The thermostatic expansion valve controls the opening of the expansion valve through the superheat of the gaseous refrigerant at the outlet of the evaporator.

What’s The Function of Thermostatic Expansion Valve in Refrigeration System?

The Thermostatic Expansion Valve in Refrigeration System has functions:

(1) Throttle and Pressure Reduction.

The high-temperature and high-pressure refrigerant is condensed by the condenser, then it is throttled and reduced pressure in the expansion valve. The high-temperature and high-pressure refrigerant becomes a low-temperature and low-pressure vapor-liquid mixture. Then it enters the evaporator to evaporate, and absorbs external heat.

(2) Adjusting The Flow Rate.

According to the temperature signal obtained by the temperature sensing bulb, the expansion valve can automatically adjust the refrigerant flow rate, the refrigerant entering the evaporator to meet the changing needs of the cooling load.

(3) Maintain a Certain Degree of Superheat, Prevent Liquid Shock and Abnormal Overheating.

The expansion valve can adjust the flow rate to make the evaporator have a certain degree of superheat, ensure the effective use of the total volume of the evaporator, and avoid liquid refrigerant entering the compressor to cause liquid shock; at the same time, it can Control the superheat within a certain range to prevent abnormal overheating.

What Are The Two Types Of Thermal Expansion Valve?

There are two main types of thermostatic expansion valve in refrigeration system: internally equalized thermostatic expansion valves and externally equalized TXV.

The external expansion valve can more accurately control the outlet superheat of the evaporator, make full use of the heat exchange area of the evaporator, and is mainly used for evaporators with large pressure drop.

The internal expansion valve measures the expansion valve outlet pressure, but because there is no need to connect a external equalizer line, it is easy to install and reduce equipment failure points. The price is also cheaper than that of the externally equalized TXV. It mainly used for small evaporator.

What Are Internally Equalized Thermostatic Expansion Valves?

The Internally Equalized Thermostatic Expansion Valve is also called internally equalized TXV. It is composed of temperature sensing package, capillary, valve seat, diaphragm, ejector rod, valve needle and regulating mechanism.

The internally equalized TXV is connected to the liquid inlet pipe of the evaporator, and the temperature sensing bulb is laid on the outlet pipe of the evaporator. Usually, the working fluid filled in the temperature sensing bulb is the same as the refrigerant in the system. The working principle of the thermal expansion valve is based on the balance of forces. When working, the upper part of the elastic metal diaphragm is affected by the pressure P3 of the working medium in the temperature sensor, and the lower part is affected by the refrigerant pressure P1 and the spring force P2. The diaphragm bulges up or down under the action of three forces, so that the valve hole is closed or opened to adjust the liquid supply of the evaporator.

When the amount of liquid entering the evaporator is less than the heat load requirement of the evaporator, the superheat of the steam at the outlet of the evaporator increases, and the pressure above the diaphragm is greater than the pressure below, which forces the diaphragm to bulge downward. The ejector rod compresses the spring, and pushes the valve needle back, so that the valve hole is opened larger, and the liquid supply volume increases.

On the contrary, when the liquid supply volume is greater than the heat load requirement of the evaporator, the superheat degree of the steam at the outlet decreases. The pressure in the temperature sensing system decreases, and when the force on the top of the diaphragm is smaller than the force on the bottom, the diaphragm will bulge upward, the spring will stretch, the push rod will move up and the valve hole will be reduced, and the liquid supply to the evaporator will be reduced. The quantity also decreases.

In the operating condition, the three forces will reach a balance, that is, P1+P2=P3. At this time, the diaphragm does not move, the valve core does not move, and the valve opening is constant.

What Is The External Thermostatic Expansion Valve?

The external thermostatic expansion valve is also called externally equalized TXV. It is composed of three parts, valve body, temperature sensing bulb and external equalizer line.

The principle of the externally balanced expansion valve is basically the same as that of the internally balanced expansion valve. The main difference is that the externally equalized TXV has a external equalizer line. The function of the external equalizer line is to measure the pressure of the evaporator outlet (at bulb location) and transmit it to the TXV diaphragm. Because large evaporator experience a pressure drop, while the effects of severe pressure drop can be offset by using an externally equalized thermostatic expansion valve.

What Are The Main Difference Between Internally Thermostatic Expansion Valves and Externally Thermostatic Expansion Valves?

In an internally equalized TXV, the closing force that equalizes the bulb’s opening pressure is taken at the evaporator inlet. The measurement is internal to the valve at the evaporator inlet.

While in an externally equalized TXV, the closing force that equalizes the bulb’s opens opeing pressure is taken at the evaporator outlet (at the bulb location). An external equalized line connects the underside of the diaphragm to the outlet of the evaporator instead, thus it can measure the superheat correctly, ensuring the evaporator coil area is fully utilized, maintaining high efficiency.

When Should an Externally Equalized TXV Valve Be Used?

An externally equalized TXV mainly used in large evaporator or the evaporator with a multiple refrigerant circuits and a distributor. Because they experience a pressure drop, while the effects of severe pressure drop can be offset by using an externally equalized thermostatic expansion valve.

When is a Internally Equalized TXV Valve Needed?

An Internally Equalized TXV Valve mainly used in an small refrigeration system or refrigeration system with relatively low pressure drop.

Glen Refrigeration-Leading Condensing Unit and Refrigeration Unit Manufacturer and Supplier in China

Glen Refrigeration is a professional Condensing Unit and Refrigeration Unit Manufacturer in China. Our products includes refrigerator condensing unit, horizontal condensing unit, indoor condensing unit, outdoor condensing unit, air cooled condensing unit, monoblock refrigeration unit, evaporator air cooler and other products. We have been leading the way of energy efficiency and ECO friendly products for many years, including R290 condensing unit, water-cooled condensing unit, inverter condensing unit, condensing unit with hot gas defrost and other products.

Except the standard condensing unit, Glen Refrigeration also supply custom condensing unit and custom refrigeration system according to customers’ special requirement.

If you don’t know how to choose the right TXV for your commercial refrigeration system, feel free to contact us!

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