What Is a Capillary Tube?
Capillary tubes are thin tubes with an inner diameter of 0.6 to 2 mm.
Capillary tubes for refrigeration cycles used in air conditioners and refrigerators are thin tubes made of copper or steel that control the flow of refrigerant. As the refrigerant passes through the narrow passage, the pressure drops due to flow resistance, a function known as throttling expansion. This drop in refrigerant pressure causes the refrigerant to evaporate and absorb heat from its surroundings.
A similar function is performed by an expansion valve. Expansion valves are more controllable, but they are more expensive. High-precision glass capillary tubes, called capillary tubes, are used for X-ray diffraction and research.
Uses of Capillary Tubes
Capillary tubes are mainly used as cooling devices in small refrigerators, room air conditioners, and electric refrigerators. They are also useful in gas appliances, oil fan heaters, fire alarms, and terminal equipment. They are mainly used in small refrigeration cycles with relatively stable operating conditions.
In the same air conditioner, expansion valves are used in car air conditioners for automobiles because of their wide range of operating conditions. This is because it is difficult for capillary tubes to perform under a wide range of conditions, with outside air temperatures ranging from low to high, and engine-driven compressors from low to high speed.
Principle of Capillary Tubes
Refrigeration cycles for air conditioners and refrigerators consist of a compressor, condenser, evaporator, receiver, and expansion mechanism. Expansion valves and capillary tubes are used in the expansion mechanism.
When refrigerant gas is compressed in the compressor and cooled in the condenser, it becomes a high-pressure liquid refrigerant. The condenser is located in the outdoor unit in air conditioners and in the cabinet in refrigerators. The high-pressure liquid refrigerant is then depressurized by the expansion valve and capillary tubes and enters the evaporator. The evaporator is located in the indoor unit in air conditioners and in the cabinet cooler in refrigerators.
Meanwhile, as the compressor draws refrigerant from the evaporator, the pressure in the evaporator is reduced to a low pressure, and the liquid refrigerant evaporates to become gas. The evaporator is a heat exchanger between the refrigerant and the air, and the heat of evaporation of the refrigerant cools the air. Expansion valves and capillary tubes control the pressure and refrigerant flow rate in the evaporator through the throttling effect, thereby adjusting the cooling capacity.
Types of Capillary Tubes
Capillary tubes are available in many types of fineness and length. Capillary tubes may be connected using copper flare tubes or by brazing. When using copper flared capillary tubing, select the type with nuts on both ends, which already have nuts on both ends of the capillary tubes.
1. Straight Type
Straight capillary tubing is used in simple refrigeration systems where the inner diameter is constant and the refrigerant flow is steady.
2. Coil Type
Coiled capillary tubes are spiral wound and are used when space is tight.
3. Multi-Port Type
Multi-port capillary tubes have multiple ports along their length and are used in systems with multiple evaporators or compressors.
4. Low-Profile Type
Low-profile capillary tubes are very thin and are used when space is limited.
5. Insulated Type
Capillary tubes have an insulating layer on the outside to help maintain the temperature of the refrigerant as it flows through the tube.
How to Select Capillary Tubes
When selecting capillary tubes, there are several considerations for optimal performance and efficiency
1. Refrigerant Type
Capillary tubes should be sized for the refrigerant to be used.
2. Refrigerant Flow Rate
Capillary tubes should be sized to handle the required refrigerant flow rate of the refrigeration system.
3. Working Pressure
Capillary tubes should be sized to handle the working pressure of the refrigeration system.
4. Superheat
Capillary tubes should be sized so that the superheat value of the refrigerant at the evaporator outlet is appropriate. Superheat is the difference between the saturation temperature, which corresponds to the refrigerant pressure, and the actual refrigerant gas temperature.
If the superheat at the evaporator outlet is small, there is sufficient evaporation in the evaporator, but if some of the liquid refrigerant is sucked into the compressor, it can cause compressor failure. If the superheat is too large, the evaporation function in the evaporator is inadequate, resulting in insufficient performance.