Capillary the tube in the refrigerator system acts as a throttle, creating the necessary resistance for the circulation of the refrigerant. It is this thin copper element that divides the circuit into a high and low pressure zone, which is critical for the phase transition of freon. If the length of this part does not correspond to the factory parameters, the entire thermodynamic chain of operation of the compressor is disrupted.
Many craftsmen mistakenly believe that when replacing they can use the tube “by eye”, as long as it fits into the dimensions of the housing. Hydraulic resistance the system directly depends on the geometry of the channel, and even a small deviation changes the balancing.
In this article we will analyze in detail the physical processes that occur when the length of the capillary changes, and we will explain why accurate compliance with the dimensions is a prerequisite for the durability of the unit.
The principle of operation of a capillary tube in a circuit
The main function of a capillary tube is to throttle the refrigerant flow. Liquid freon under high pressure leaves the condenser and enters a narrow channel with a diameter of less than a millimeter. Passing along this path, the substance loses pressure and partially evaporates, turning into a vapor-liquid mixture before entering the evaporator.
The length of the tube determines the time the refrigerant remains in the high-pressure zone. If the channel is too short, the freon will not have time to transfer heat to the condenser and will lose insufficient energy. As a result, too much liquid phase will enter the evaporator, which can lead to hydraulic hammer or liquid entering the compressor.
On the other hand, an excessively long tube creates excess resistance. It becomes difficult for the compressor to push gas, the load on the motor windings increases, and the amount of circulating freon decreases. The refrigerator begins to work "pull", consuming more electricity with less cooling efficiency.
⚠️ Attention: The capillary tube does not adjust automatically. Its throughput is fixed by the manufacturer for a specific compressor power and evaporator volume.
It is important to understand that modern systems often use soldered filter drierto which a capillary of a given length is already welded. Replacing the entire bundle assembly is the most reliable solution, eliminating errors in soldering and sizing.
Physics of the process: how length affects pressure
The law of resistance in hydraulics states that pressure loss is directly proportional to the length of the pipeline at a constant diameter. In the context of a refrigerator, this means that increasing the length of the capillary leads to an increase in the pressure at the inlet (in the condenser) and a decrease in pressure at the outlet (in the evaporator).
Consider the situation where a tube of greater length than required by the specification is installed:
- 📉 The condensation pressure increases, since the compressor cannot effectively force gas through a long channel.
- 🌡️ The condensation temperature increases, which worsens heat transfer in the radiator.
- ⚡ The compressor current consumption increases due to the increased load on the piston group.
- ❄️ The evaporation pressure drops below the calculated one, which can lead to freezing of the evaporator.
The opposite situation - a tube that is too short - is also dangerous. The resistance drops and freon flows too quickly from the high pressure area to the low pressure area. The pressure in the condenser drops, condensation worsens, and a mixture with a high liquid content enters the evaporator.
The critical parameter here is pressure drop. It is this that ensures that the refrigerant boils at a low temperature. Violation of the channel geometry throws off this setting, turning an efficient refrigeration machine into an inefficient heater with poor cold.
Consequences of installing a tube of the wrong length
Ignoring factory parameters when replacing a capillary tube leads to a cascade of problems. The compressor is the first to suffer. If the tube is longer than normal, it operates in overload mode. The thermal relay may not immediately turn off the motor, but constant operation at the limit of capacity reduces the life of bearings and valves.
If the tube is shorter than necessary, a risk arises hydraulic hammer. Liquid freon, which has not had time to evaporate in the capillary, enters the compression chamber. Since the liquid is incompressible, this can lead to destruction of the valve plate or even jamming of the crankshaft.
⚠️ Attention: An attempt to compensate for the length of the tube by changing the amount of freon charged is a gross mistake. This will not restore the correct pressure ratio in the system.
The heat removal system also suffers. If the capillary length is incorrect, the temperature regime of the capacitor is disrupted. It can either overheat or, conversely, remain cold, which indicates ineffective operation of the circuit.
The table below compares symptoms with different lengths of the installed tube:
| Parameter | The tube is longer than normal | The tube is shorter norms | Norm |
|---|---|---|---|
| Discharge pressure | Increased | Reduced | Calculated |
| Compressor current | Above nominal | Normal or below | Nominal |
| Evaporator temperature | Too low (freezing) | High (undercooling) | Uniform |
| Operation noise | Humming, overload | Gurgling, bubbling | Even hum |
Methods for selecting and calculating length
Professional repairs do not allow the use of the “poke” method. The length of the capillary tube is calculated by engineers at the model design stage and depends on many factors: compressor power, type of refrigerant (R134a, R600a, R12), evaporator volume and even climate class.
There are several approaches to determining the required size in the absence of service documentation:
- 📏 Measurement of the original: When replacing a burnt one capillary, it is necessary to restore the length and internal diameter of the old tube as accurately as possible.
- 📊 Tabular data: Use of compressor manufacturers' reference books, which indicate the recommended length for a specific model.
- 🧪 Experimental method: Selection of length based on outlet pressure when operating compressor (requires high qualifications).
Often craftsmen use the “comparative flow” method. They take a piece of tube of a reference length, supply air at the inlet at a certain pressure and measure the time when a certain volume comes out. Then a new tube is selected, achieving identical time indicators.
Diameter accuracy
The internal diameter of the capillary tube varies from 0.6 mm to 1.4 mm. A difference of one tenth of a millimeter dramatically changes the flow capacity, so it is important to use a tube with an identical internal cross-section, not just external geometry.
It is important to note that different refrigerants require different lengths. For example, isobutane (R600a), which is often used in modern refrigerators, is characterized by different pressure parameters than R134a freon. Replacing a tube without taking into account the type of gas is unacceptable.
Replacement and soldering technology
The process of replacing a capillary tube requires not only accurate selection of length, but also compliance with installation technology. The copper of the capillary is very soft and the walls are thin, so inaccurate soldering can easily block the flow area or create microcracks.
When soldering, it is necessary to use solder with a melting point corresponding to the material of the tubes, and be sure to use a flux that does not require rinsing or nitrogen protection to avoid oxidation inside the channel.
☑️ Replacement quality control
Particular attention should be paid to the entrance to the capillary tube. A mesh filter is often installed there, which traps mechanical impurities. If scale or solder gets inside during soldering, the system will be clogged instantly.
After installing a new tube and sealing the system, a vacuum must be carried out. This removes moisture and air. The evacuation time must be sufficient to evaporate moisture that could have entered the system during installation.
Diagnostics of problems after replacement
How to understand that the length is correct? After filling the refrigerator with freon and entering the mode, it is necessary to carry out comprehensive diagnostics. The initial inspection includes checking the temperature of the condenser - it should warm up evenly over 2/3 of its length.
Next, the operation of the evaporator is assessed. In refrigerators with a weeping evaporator (“weeping”), freezing should be uniform over the entire surface. If only a cold part or, conversely, frost extends beyond the evaporator onto the return tube, this is a sign of an incorrect selection of the capillary.
⚠️ Attention: If the return (suction) tube is covered with frost all the way to the compressor, this is a sure sign that the capillary is too short or there is an excess of freon in the system.
You should also listen to the sound of the compressor. A steady hum indicates normal load. Intermittent operation, relay clicks or an excessively hot motor housing indicate a problem with the pressure balance.
FAQ: Frequently Asked Questions
Is it possible to twist the extra length of the capillary tube into a ring?
Yes, this is standard practice. The capillary tube is often wound in a spiral around the return tube (heat exchanger) or placed in rings in a specially designated area. The main thing is to avoid creases that will block the cross-section, and not to narrow the internal diameter when bending.
Does the cleanliness of the capillary tube affect its effective length?
Absolutely. The presence of internal contaminants, oxides or moisture narrows the effective flow area, which is hydraulically equivalent to increasing the length of the tube. That is why the system must be dry and clean.
What happens if you install a universal long tube and just bite off the excess?
This method (“by eye”) is extremely risky. By biting off a centimeter at a time, you change the resistance of the system. Without monitoring the pressure at the gauge station, you will not understand the moment when the length becomes optimal. There is a high risk of “overshooting” and getting a tube that is too short, which will lead to compressor breakdown.
Is it necessary to change the filter drier when replacing the capillary tube?
Yes, definitely. The filter drier is a disposable element. When the circuit is opened, it becomes saturated with moisture from the air and loses its properties. Installing an old filter will negate all repair efforts, since moisture will quickly cause an ice plug in the new capillary.