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What is a capillary tube and why do we need it

What is a capillary tube and for what purpose it is used

Capillary tube can be considered as an expansion device, this device is also known as a metering device it is a throttling device and it is used in air conditioning and refrigeration systems. This device is a fixed restriction type device. It is a narrow tube that connects the conductor directly to the evaporator. So instead of an orifice, a length of small diameter tube gives the restrictive effect, which is the same as the orifice. Capillary tubes are small diameter tubes the inside diameter of the capillary used in refrigeration is generally about 0.5-2.28mm which means they are very small. It can create a greater pressure drop in the refrigerant flow if the capillary tube is long and has a smaller inside diameter.

How capillary tubes are constructed and how big is a capillary tube

The capillary tube is a smaller tube with a small inside diameter, mostly it has diameter of about .5 to 2.2mm and it is used in the domestic refrigerant, deep freezers, water coolers, and air conditioners. Copper is used for the tube construction the pressure drop in the capillary tube is developed due to friction and acceleration. The construction of the capillary tube is really simple. They have no moving parts and directly connects condenser and evaporator. The length and diameter of the tube is depended on the required mass flow rate and pressure drop. The capillary tubes are coiled to many turns so that it would only use less space. Capillary tubes are not capable to adjust to load change. They are mostly used where the load is relatively constant with no large fluctuations. Small-bore size tube can get blocked with metal particles or even with high viscosity oils. So mostly a strainer is used at the inlet to the device. The tube of this device is selected with proper length and bore so that it can maintain the required pressure differential between the condenser and the evaporator.

What is the difference between the thermostatic expansion valve and the capillary tube

The thermostatic expansion valve is more efficient than the capillary tubes because thermostatic valves can vary the refrigerant flow according to the load. The thermostatic expansion valve has a better cooling feature in ambient temperature than the capillary tube. Thermostatic expansion valves have larger refrigerant charge range than the capillary. The capillary would need the right refrigerant charge in the system but the TEV has a greater variation in charge without any bad effect in the performance.

Why do we need a capillary tube and what is its function

The capillary tube will control the refrigerant flow by the pressure drop across it in a refrigeration system. The diameter and length of the tube would determine the flow at a given pressure. It won’t maintain evaporator pressure or superheat. It doesn’t have any moving parts so there is no wear out. They are used when the load is relatively constant.

How do capillary tubes work and how does it work in a refrigerator? Why capillary tube is narrow

The pressure of the refrigerant would drop down when the refrigerant enters the capillary tube from the condenser, the pressure is decreased because of the low diameter of the tube. If the diameter is small and has a long length then the pressure drop is achieved. The capillary tube is a non-adjustable device so that the refrigerant flow cannot be controlled, so the flow of refrigerant would change according to the variation in the surrounding. So the capillary tubes are designed for certain ambient conditions. So if we select a capillary tube properly then it can work well for a wide range of conditions.

How to install a capillary tube and where is the capillary tube located

While installing the capillary tube the personnel should be careful when joining the capillary to the condenser and the evaporator coil. If the brazing is done deep inside the capillary could get blocked because of the smaller diameter of the tube. So the person who is installing the capillary must be an experienced person. If the refrigerant is charged into the refrigerators or deep freezers then the capillary must be changed. It is because if the machine is stopped some of the oil particles may clog the capillary as the refrigerant leaks to the atmosphere. Sometimes accumulators are used in refrigeration system with the capillary tube because the refrigerant that leaves the evaporator carries liquid particles, and these particles are separated by the help of the accumulator. This liquid refrigerant would vaporize and then would be sucked by the compressor. Liquid refrigerant flooding can also be prevented by the accumulator. Mostly the accumulator is connected between the evaporator and the compressor of the refrigeration system.

The receiver must not be installed in the refrigeration system with the capillary because, when the refrigeration plant stops the pressure on both sides of the capillary equalizes. During the stopping of the refrigeration plant, the system must not be overcharged with the refrigerant, if it is overcharged then the discharge pressure from the compressor will be high and the compressor would be overloaded. So the refrigerant charge in the air condition and refrigeration system with the capillary tube is a little critical.

What are the advantages of the capillary tube

  • It is not expensive
  • It doesn’t need maintenance
  • It would reduce the starting torque needed for the motor because of the same pressure on two sides of the compressor
  • It can be used for hermetic compressor-based systems, that is critically charged and factory assembled

What are the disadvantages of the capillary tubes

  • These valves are not capable to adjust itself to the change of flow as per changing ambient temperature and load.
  • Clogging could happen because of the narrow bore of the tube
  • The condenser and evaporator are connected and because of this the evaporator could be flooded and the liquid refrigerant could flow to the compressor.


post-graduate in Electronics & communication.

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