This article describes capillary tube, the commonly used throttling device in the domestic refrigeration and air conditioning systems.
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What is Capillary Tube in Refrigerators?
Capillary tube is one of the most commonly used throttling devices in the refrigeration and the air conditioning systems. The capillary tube is a copper tube of very small internal diameter. It is of very long length and it is coiled to several turns so that it would occupy less space. The internal diameter of the capillary tube used for the refrigeration and air conditioning applications varies from 0.5 to 2.28 mm (0.020 to 0.09 inches). Capillary tube used as the throttling device in the domestic refrigerators, deep freezers, water coolers and air conditioners.
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Capillary Tube Refrigeration
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How Capillary Tube Works?
When the refrigerant leaves the condenser and enters the capillary tube its pressure drops down suddenly due to very small diameter of the capillary. In capillary the fall in pressure of the refrigerant takes place not due to the orifice but due to the small opening of the capillary.
The decrease in pressure of the refrigerant through the capillary depends on the diameter of the capillary and the length of the capillary. Smaller is the diameter and more is the length of the capillary more is the drop in pressure of the refrigerant as it passes through it.
In the normal working conditions of the refrigeration plant there is drop in pressure of the refrigerant across the capillary but when the plant stops the refrigerant pressure across the two sides of the capillary equalize. Due to this reason when the compressor restarts there won’t be much load on it. Also, due to this reason one cannot over-charge the refrigeration system with the refrigerant and no receiver is used.
The capillary tube is non-adjustable device that means one cannot control the flow of the refrigerant through it as one can do in the automatic throttling valve. Due to this the flow of the refrigerant through the capillary changes as the surrounding conditions changes. For instance as the condenser pressure increases due to high atmospheric pressure and the evaporator pressure reduces due to lesser refrigeration load the flow of the refrigerant through the capillary changes. Thus the capillary tube is designed for certain ambient conditions. However, if it is selected properly, it can work reasonably well over a wide range of conditions.
The length of the capillary of particular diameter required for the refrigeration applications cannot be found by fixed formula rather it is calculated by the empirical calculations. Some approximate length required for certain application is found out and it is then corrected by the experiments.
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When the refrigerant leaves the condenser and enters the capillary tube its pressure drops down suddenly due to very small diameter of the capillary. In capillary the fall in pressure of the refrigerant takes place not due to the orifice but due to the small opening of the capillary. The decrease in pressure of the refrigerant through the capillary depends on the diameter of the capillary and the length of the capillary. Smaller is the diameter and more is the length of the capillary more is the drop in pressure of the refrigerant as it passes through it.
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Advantages of the Capillary Tube
Here are some of the advantages of using capillary tube as the throttling device in the refrigeration and the air conditioning systems:
1) The capillary tube is a very simple device that can be manufactured easily and it is not very costly.
2) The capillary tube limits the maximum amount of the refrigerant that can be charged in the refrigeration system due to which the receiver is not required in these systems.
3) When the refrigeration plant stops the pressure across the capillary tube becomes same and also along the whole refrigeration cycle the pressure is constant. This means that when the plant is stopped the pressure at the suction and discharge side of the compressor are same. Thus when the compressor is restarted there is not much load on it since it does not have to overcome very high pressures. Due to this the compressor motor of smaller torque can be selected for driving the compressor, thus reducing the cost of the compressor. This along with the above two advantages helps reducing the overall cost of the refrigeration and the air conditioning systems.
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Important Points to Remember when Installing Refrigeration Plants with Capillary Tube
Since the pressure on the two sides of the capillary equalizes when the refrigeration plant is stopped, one should not overcharge the system with the refrigerant. Due to this reason the receiver is also not installed in the refrigeration system with the capillary. If the system is overcharged the discharge pressure from the compressor will be high and the compressor would get overloaded. Thus the refrigerant charge in the refrigeration and air conditioning systems with the capillary tube is critical.
The technician should be very careful while brazing the capillary to the condenser and the evaporator coil. Since the diameter of the capillary is very small the capillary may get blocked if the brazing is done deep inside. Brazing the capillary requires lots of expertise and patience.
In most of the cases, when the fresh refrigerant is charged into the refrigerator or the deep freezers, the capillary of the system should also be changed. This is because when the machine is stopped some oil particles may clog the capillary as the refrigerant leaks to the atmosphere.
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Accumulator Used in the Refrigeration Systems with Capillary Tube
Accumulator is a small hollow cylindrical shape vessel made of copper. It is fitted between the evaporator and the compressor of the refrigeration system towards the suction side of the compressor. Sometimes the refrigerant leaving the evaporator carries liquid particles. These particles get separated in the accumulator. The liquid refrigerant collected in the accumulator slowly gets vaporized and is then sucked by the compressor. The accumulator also prevents the flooding of the liquid refrigerant to the compressor when the load on the evaporator drops down drastically.
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Refrigeration System with Capillary Tube and Accumulator
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1) Book: Basic Refrigeration and Air Conditioning by P. N. Ananthanarayanan, Second Edition, Tata Mc-Graw-Hill Publishing Company Limited
Throttling Devices, Expansion Valves in Refrigeration & Air Conditioning Systems
This is the series of articles that describes what the throttling valves are for the refrigeration and air conditioning systems. It also describes the functions of the throttling valves and also its types like capillary tube, automatic expansion valve, thermostatic expansion valve etc.