Learn refrigeration

The metering device is the section of the refrigerator that separates the high and low pressures.

Its job is to hold one side of the refrigerator at a high pressure, while allowing just enough refrigerant to pass through to the low pressure side to do its job in the evaporator.

 

 

There are two popular types of Metering Device used on modern refrigeration

Thermostatic Expansion Valve

Capillary Tubes.

 

Thermostatic Expansion Valve  

 

The thermostatic expansion valve (TXV) is a meter device which controls the flow of liquid refrigerant entering the evaporator.

The valve provides the most widely used refrigerant control for both commercial and industrial facilities, because of its high efficiency and adaptability to any type of refrigeration application. It is the only valve that ensures that the liquid cannot return to the compressor.

When too much liquid enters the evaporator it may not all boil off which could result in liquid entering the compressor. The bulb helps prevent this happening by sensing the low temperatures at the evaporator outlet and then controlling how much liquid enters it.

 

 

The TXV valve is positioned before the entrance to the evaporator. The valve incorporates a bulb sensor that is mounted to the suction line at the exit of the evaporator.

The bulb connects to the TXV via a capillary tube. If the temperature is higher or lower it alters how much refrigerant enters into the evaporator. This is of great advantage to a system as it can adjust the flow of the refrigerant that enters the evaporator.

 

 

The TXV can be adjusted by altering the the tension of the spring in the valve, sometimes referred to as the superheat adjustment. A lot of superheat is usually not required as the effective heat transfer area of the evaporator is reduced. However if the superheat is set too low the valve will lose control of the refrigerant flow and the evaporator will alternatively starve and overfeed, creating very different operating conditions.

Capillary Tubes

 

The Capillary tube is a fixed metering device which acts as a restricting device and cannot be altered to effect performance from the evaporator. It is used in both residential and commercial systems and commonly used on domestic refrigeration.

Made from thin copper tubing its length and internal dimension cause liquid to back up creating a high pressure side, while dropping to a low pressure side at its outlet.

To compare in very simple terms a capillary act similar to a drinking straw, if you pinch the straw while trying to blow in it, it gets harder and builds up a lot of pressure.

The diagram above shows a typical small refrigeration system using a capillary tube with the diagonal line seperating the high side and low side ot the fridge.

Capillary tubes and are sized to offer optimum performance at a single operating condition. They do lend themselves to systems where operating temperatures and requirements vary because they are very dependent on a limited range of conditions.