The internal structure of a modern refrigeration unit is a closed system where each the element performs a strictly defined function, ensuring stable production of cold. The central link in this process is refrigerator capillary tube, which, despite its simple design, plays the role of a throttle that regulates the refrigerant pressure. It is in this unit that there is a sharp drop in the pressure of liquid freon before it enters the evaporator, which starts the process of boiling and active absorption of heat from the chambers.
Understanding how this component works is necessary not only for design engineers, but also for household appliance repairmen, as well as enthusiasts who want to understand the causes of malfunctions of their equipment. Any violation of the patency or tightness of the capillary leads to a complete failure of the system or incorrect operation of the temperature regime. In this article, we will analyze in detail the physical essence of the throttling process, consider typical malfunctions and methods for eliminating them.
Physical essence of the refrigerant throttling process
The principle of operation of a capillary tube is based on the law of conservation of energy and the properties of throttling of gases and liquids. The refrigerant leaving the condenser is under high pressure and at a temperature close to ambient temperature, while remaining in a liquid state. Getting into the narrow channel of the capillary, freon encounters high hydraulic resistance, which is created due to friction against the walls and internal friction of the liquid itself.
As a result of this resistance, the pressure at the outlet of the tube drops sharply. According to thermodynamic laws, with a sharp decrease in pressure, part of the liquid instantly boils, turning into steam, which is accompanied by a significant decrease in the temperature of the remaining liquid phase. This process, called throttling, does not require external energy and occurs solely due to the pressure difference created by the compressor. Throttle effect is a key point in the refrigeration cycle.
The length and internal diameter of the tube are selected by the manufacturer with high accuracy for a specific compressor model and type of refrigerant. If the channel is made too short or wide, the pressure will not have time to drop to the desired level and the efficiency of the evaporator will be low. Conversely, an excessively long or narrow tube will create excessive resistance, the compressor will work with overload, and the cooling capacity will drop due to insufficient freon supply.
Design features and location in the system
Structurally, the capillary tube is a long copper pipeline with a very small internal diameter, usually ranging from 0.6 to 2 millimeters. For compact placement inside the refrigerator body or in the foamed part, the tube is rolled into a spiral or laid in a snake. In modern models, especially those using environmentally friendly refrigerants, the length of the capillary can reach several meters, which allows you to precisely adjust the hydraulic resistance.
The location of the element in the system also has its own characteristics. Often the capillary tube is thermally bonded (soldered or pressed tightly) to the suction piping running from the evaporator to the compressor. This heat exchanger performs two important functions: it prevents liquid freon from entering the compressor (which can cause water hammer) and provides additional subcooling of the liquid in front of the throttle, increasing the efficiency of the cycle.
The manufacturing material is oxygen-free copper, which has high ductility and excellent thermal conductivity. It is important to understand that even microscopic defects in the internal surface or bends at an acute angle can disrupt laminar flow and cause noise or vibration. Hydraulic resistance system directly depends on the condition of the internal walls.
- 🔹 The internal diameter varies depending on the compressor power and type freon.
- 🔹 The length of the tube can be from 1.5 to 4 meters in household refrigerators.
- 🔹 Material - exclusively high-purity copper to prevent corrosion.
- 🔹 Design - spiral or zigzag to save space.
Why copper?
Copper was not chosen by chance. It is easily soldered, which simplifies installation, has high thermal conductivity for an effective heat exchanger and, critically, does not react with most types of oils and refrigerants used in refrigeration equipment.
Symptoms of blockage and mechanical damage
The most common cause of failure of the refrigeration circuit is clogged capillary tube. This may be caused by moisture entering the system, which at low temperatures turns into an ice plug, or by the presence of solid particles (oxidation products, dust, flux residues). Also, the cause may be polymerization of the oil inside the tube when the compressor overheats, which leads to the formation of tarry deposits.
The first sign of a problem is often incorrect operation of the compressor. The unit can turn on, operate for several minutes, and then turn off via a thermal relay, since it cannot push freon through a clogged channel. In other cases, the motor-compressor works almost without stopping, but the cold in the chambers does not accumulate, or the temperature drops only to positive values.
⚠️ Attention: If you hear gurgling sounds in the area of the evaporator or capillary, this may indicate the presence of air or moisture in the system, which requires immediate evacuation and refilling.
Mechanical damage, such as kinks or creases, often occurs due to careless transportation of the refrigerator or improper defrosting (for example, when breaking off the ice with a knife). The crease in the tube completely blocks the refrigerant flow, which makes the refrigerator impossible to operate. The hall can be visually identified only by disassembling the foamed part or in accessible places in the circuit.
Methodology for diagnostics and troubleshooting
Diagnostics of the state of the capillary system requires a specialized tool: a pressure gauge station, vacuum pump and nitrogen source. The initial inspection begins with a visual inspection of accessible sections of the pipeline for dents, oxides or traces of oil that may indicate microcracks. However, internal blockages are not visually detected.
For accurate diagnosis, the technician connects a pressure gauge to the service fitting and starts the compressor. If a deep vacuum is created on the suction side, and the pressure on the discharge side does not increase or increases very slowly, this is a sure sign ( - clogged/blocked) of the capillary. You can also use the purge method: by disconnecting the tube from the filter drier, compressed air or nitrogen is supplied. Resistance to air flow should be noticeable, but uniform.
It is important to distinguish between an ice plug and a technical blockage. Ice dams are often temporary: after unplugging the refrigerator and defrosting for 24 hours, the ice will melt and the system will start running again until the moisture freezes again. Technical blockage caused by dirt or oil will not disappear on its own and requires mechanical or chemical cleaning.
| Symptom | Probable cause | Confirmation method |
|---|---|---|
| Compressor hot, cold evaporator | Complete capillary blockage | No flow during purging |
| Cyclic operation (on/off) | Partial blockage or moisture | Pressure monitoring pressure gauge |
| Noisy bubbling in the tubes | Excess refrigerant or air | Analysis of pressure readings |
| Low discharge pressure | Leaks or leaks | Nitrogen leak test |
Technology for cleaning and replacing the capillary tube
The process of restoring capillary patency is labor-intensive and requires high qualifications. If the blockage is caused by moisture, the system is evacuated for a long time (up to several hours) with the compressor running or an external pump, sometimes using heating of the unit housing to evaporate the moisture. However, in cases of oil-resin plugs or foreign objects, it is necessary replacement of the capillary tube.
To replace it, it is necessary to unsolder the old capillary, having first discharged the refrigerant in accordance with environmental standards. The new capillary is selected strictly according to the internal diameter and length, similar to the factory one. Installing a new tube requires care: copper is very soft and can be easily deformed. After installation, the connections are soldered in a nitrogen environment or using high-quality fluxes that do not leave corrosive traces.
⚠️ Attention: It is strictly forbidden to try to clean the capillary tube with water or aggressive solvents that do not evaporate completely. Remaining liquid will lead to re-freezing and failure of the compressor.
After physically replacing the tube and filter-drier (which always changes when the circuit is depressurized), the vacuum stage follows. This is a critical step to remove air and water vapor from the system. Only after reaching a deep vacuum (-1 Bar) is charging with the exact amount of refrigerant indicated on the refrigerator nameplate.
☑️ Algorithm for replacing the capillary
The influence of the type of refrigerant on the operation of the capillary
Modern refrigeration technology is switching to the use of refrigerants R600a (isobutane) i R134a, which replaced freon R12. These gases have different physical properties, viscosity and operating pressure, which directly affects the design of the capillary system. For example, isobutane requires smaller diameter and longer capillaries due to its high efficiency and flammability, which dictates special requirements for tightness.
Using the wrong type of refrigerant or installing a capillary that is not designed for a specific gas will lead to an imbalance in mass and temperatures. Throttle effect will be either insufficient or excessive. For example, when filling R134a into a system designed for R12, without replacing the capillary, the discharge pressure will be too high, which will lead to overheating of the compressor.
When repairing, it is important to consider that the oils used with different freons are also different (mineral for R12 and R600a, polyester for R134a). Mixing oils or using the wrong lubricant can result in sediment that will quickly clog the fine channel of the capillary tube.
⚠️ Attention: Specifications and component requirements may vary depending on the manufacturer and model year. Always check the service documentation (service manual) for a specific refrigerator model before starting work.
Is it possible to shorten the capillary tube during repairs?
No, shortening the capillary tube is strictly not recommended without special calculations. Reducing the length will reduce hydraulic resistance, which will lead to a drop in condensation pressure and disruption of refrigerant circulation. The compressor may begin to “throw in” liquid freon, which will cause its breakdown. The length must correspond to the factory parameters.
How often do you need to change the filter drier?
The filter drier is a disposable element and is changed every time the refrigeration circuit is depressurized, even if the system has been open for only a few minutes. The sorbent inside the filter is quickly saturated with moisture from the air, and repeated use will lead to the formation of ice plugs.
Why does the capillary sometimes freeze?
Freezing of the capillary tube or filter-drier usually indicates the presence of moisture in the system (ice plug) or that there is not enough freon in the system, and the pressure drops too early. This may also be a sign of blockage when throttling occurs earlier than expected.
Does tilting the refrigerator affect the operation of the capillary?
Strong tilting or laying the refrigerator on its side can lead to oil flowing from the compressor into the circuit, including the capillary tube. Solidified oil will create a plug. That is why, after transportation, it is recommended to let the refrigerator sit for several hours before turning it on.