Replacing a compressor or a freon leak often leads craftsmen to the need to replace the capillary tube, but simply installing the part “by eye” is the path to unstable operation of the unit. Capillary tube is a throttling device that creates the necessary resistance to separate the high and low pressure circuits. An error in calculating the diameter or length will either overload the motor or cause insufficient cooling of the evaporator.
Many craftsmen rely only on correspondence tables, forgetting that each refrigeration circuit is unique and requires an individual approach. In this article, we will examine the physical principles of throttle operation, methods for accurately calculating length based on pressure and hydraulic resistance, and also consider modern approaches to balancing the system. You will learn why a length of 4 meters for R134a can be a fatal mistake if the internal diameter is not taken into account.
Before you start soldering, you need to understand that The calculated hydraulic resistance of the tube must exactly correspond to the performance of a particular compressor for a given refrigerant. Ignoring this rule turns repairs into a lottery, where winning is an accident and not the result of professionalism. Let's look at the technical details that will help you avoid repeated visits to the client.
Physical principles of operation of a capillary throttle
The main function of the capillary tube is to create a pressure difference between condenser and evaporator. Liquid refrigerant under high pressure enters a narrow tube, where, due to frictional forces and local resistance, a sharp drop in pressure occurs. As a result, part of the liquid instantly evaporates, forming a vapor-liquid mixture, which enters the evaporator to extract heat.
The key parameter here is hydraulic resistance, which directly depends on the length of the tube, its internal diameter and the roughness of the internal walls. If the resistance is too low, the condensing pressure will drop, but the suction pressure will increase, which can lead to liquid fraction entering the compressor - a phenomenon known as “water hammer”.
⚠️ Caution: Installing a tube with a smaller diameter or longer than required will dramatically increase the discharge pressure. This leads to overheating of the compressor, activation of the thermal relay and eventual burnout of the motor windings.
It is important to consider that the throughput of the tube depends nonlinearly on its diameter. Reducing the internal diameter even by hundredths of a millimeter requires a significant increase in length to maintain the same mass flow of refrigerant. That is why when selecting analogues for original tubes it is necessary to recalculate, and not just copy the length.
Methods for selecting lengths using a correspondence table
The most common method of initial selection is the use of empirical tables compiled by compressor manufacturers and capillary tubes. This data is based on thousands of hours of testing and allows you to quickly determine the starting length for subsequent fine tuning. However, it is worth remembering that the tabular data is for reference only.
Below is a table of the approximate lengths of capillary tubes for various refrigerants and compressor capacities. Please note the difference in lengths for R134a and R600aas these gases have different viscosities and densities.
| Refrigerant | Compressor power (hp) | Inner diameter (mm) | Approximate length (m) |
|---|---|---|---|
| R134a | 1/6 | 0.66 | 3.0 - 3.5 |
| R134a | 1/4 | 0.71 | 3.5 - 4.2 |
| R600a | 1/6 | 0.66 | 3.5 - 4.0 |
| R600a | 1/4 | 0.71 | 4.0 - 4.8 |
| R12 (old models) | 1/6 | 0.79 | 2.5 - 3.0 |
The use of tables is effective when you change the handset to exactly the same model or its direct analogue from a trusted supplier. However, if you are using a universal coil tube or rebuilding a system with a modified evaporator volume, the table values are just a starting point. The exact length is selected experimentally during the filling process.
Calculation using discharge pressure and suction
The most accurate method that allows you to adapt the system to specific conditions is calculation and selection based on operating pressures. The essence of the method is to create conditions as close as possible to the standard ones and measure the operating parameters of the system. To do this, you will need a pressure gauge station and a cylinder with the appropriate type of gas.
The setup process begins with installing a tube of a obviously longer length than indicated in the tables. The system is sealed, evacuated and charged with the calculated amount of refrigerant. Then the compressor is turned on, and the technician observes the readings of the low and high pressure gauges, as well as the current consumption of the electric motor.
If the suction pressure is too low and the motor current is low, this indicates excessive tube resistance. In this case, the tube must be shortened in small sections (10-20 cm), each time repeating the vacuum and filling cycle. The goal is to achieve the nominal values indicated in the compressor data sheet or on the refrigerator nameplate.
The influence of internal diameter on performance
The internal diameter of the capillary tube is a critical parameter that determines the throughput of the system. Standard sizes range from 0.5mm to 1.2mm, and choosing the wrong size can completely throw off the balance of the circuit. Manufacturers of tubes often mark them with color or use color coding on the packaging.
When replacing the original tube with an analogue with a different diameter, it is necessary to recalculate the length. Increasing the diameter requires a proportional increase in length to maintain resistance, and vice versa. The approximate conversion formula looks complicated, but in practice, craftsmen use the rule: a change in diameter by 0.05 mm requires a change in length by 15-20%.
It is worth noting that the use of tubes with a smaller diameter is dangerous not only due to an increase in pressure, but also the risk of blockage. Micro (microblocking) can occur due to breakdown products of oil or moisture if the system was poorly vacuumed. Therefore, the choice of diameter should be justified not only by calculations, but also by the purity of the circuit.
What to do if there is no tube of the required diameter?
In emergency cases, you can use two tubes of different diameters connected in series, but this is a last resort. The length of each section is calculated in proportion to its resistance. However, this method is not recommended for long-term operation due to the risk of flow stratification and instability.
Practical instructions for installation and balancing
The process of replacing a capillary tube requires a strict sequence of actions, violation of which can nullify all calculations. First you need to remove the old throttle, being careful not to damage the evaporator. Then a new tube is installed with a length reserve of approximately 20-30%.
After soldering the connections (inlet to the condenser and outlet to the evaporator), the system must be evacuated for at least 30-40 minutes. Only after reaching a deep vacuum is the first filling with gas for testing. During operation of the compressor, the free end of the tube is gradually trimmed until the required parameters are achieved.
☑️ Checklist for replacing the capillary tube
The final stage is sealing the end of the tube. After reaching the optimal length, the end is soldered or clamped with a special clamp, if the design allows it. It is important not to overheat the soldering area, so as not to change the properties of the copper and not create internal stress, which can lead to a crack.
⚠️ Attention: When working with an open flame near the insulation of the refrigerator, use screens made of tin or asbestos. Overheating of the internal plastic lining of the camera can lead to the release of toxic substances and deformation of the housing.
Typical errors and troubleshooting
One of the most common mistakes is ignoring the cleanliness of the contour. Even microscopic dust or moisture can block the narrow cross-section of the capillary, especially in the area of \u200b\u200bthe exit to the evaporator, where a sharp expansion of gas and a decrease in temperature occurs. This phenomenon is called “ice jam” or “oil jam.”
Another common problem is the incorrect depth of the tube in the filter drier. If the tube does not reach the filter mesh, “dead zones” form in the system where oil can accumulate. If the tube rests against the bottom of the filter, the flow of refrigerant will be completely blocked and the refrigerator will stop cooling.
The third aspect is the temperature regime. Calculations are valid for standard operating conditions. If the refrigerator is operated in an unheated room at low temperatures, the condensation pressure drops and the standard capillary tube may become a bottleneck that does not allow sufficient freon to pass through. In such cases, installation of a winter kit or pressure adjustment is required.
Frequently asked questions (FAQ)
You can Is it possible to use a capillary tube from another type of refrigerator?
You can use a tube from another type of refrigerator only after careful calculation. If the compressor powers and types of refrigerants are the same, and the diameters of the tubes are identical, then you can try by choosing the length experimentally. However, for systems with R600a (isobutane) and R134a (freon), the approaches to length and diameter differ significantly.
How to determine that the capillary tube is clogged?
The main signs of clogging: the compressor works constantly or with short cycles, the evaporator does not cool completely (cold only in inlet), the suction pressure is below normal and the discharge pressure is above normal. Overheating of the compressor may also be observed.
Is it necessary to change the filter drier when replacing the capillary tube?
Yes, replacing the filter drier is required whenever the circuit is opened and the capillary tube is replaced. The old filter could be saturated with moisture and oil breakdown products, which will quickly damage the new tube and compressor.
What tool is needed to cut the capillary tube?
For cutting, use a special pipe cutter for small-diameter copper pipes. The use of wire cutters or hacksaws is prohibited as they will flatten the pipe or leave chips inside, which will cause a blockage. After trimming, the edges must be calibrated.