Repairing a refrigeration circuit is a complex technical task that requires not only theoretical knowledge of thermodynamics, but also specific skills in working with non-ferrous and ferrous metals. The question of how to solder refrigerator tubes is often faced by craftsmen who are faced with a refrigerant leak or the need to replace the compressor. The wrong choice of tools or consumables can lead to repeated depressurization of the system after a short time or, even worse, to damage to expensive components of the unit.
Modern refrigeration units are a closed system operating under high pressure, where even a microscopic seam defect is unacceptable. To create a reliable connection, it is necessary to take into account the physical and chemical properties of copper and steel, since these two metals most often have to be combined in household refrigerators. It is important to understand that soldering of refrigeration tubes is fundamentally different from plumbing or electrical soldering in terms of temperature conditions and the alloys used.
In this article we will analyze in detail what equipment is necessary for high-quality work, what nuances exist when working with different types of metals and why The use of unsuitable materials is strictly unacceptable. You will learn the differences between hard and soft solders, as well as the critical role of fluxes in the weld formation process. The correct approach to the selection of tools is a guarantee of longevity of the repair.
Fundamentals of soldering technology in a refrigeration circuit
The process of joining metal tubes in refrigeration technology is based on the principle of capillary spreading of molten metal in the gap between the connected surfaces. To ensure a seal that can withstand refrigerant pressure and compressor vibration, the melting point of the solder must be well above the operating temperature of the system, but below the melting temperature of the tubes themselves. In household refrigerators, it is most often used hard solderingusing silver or copper-phosphorus alloys.
The key point is surface preparation. The oxide film that inevitably forms when copper is heated in air prevents the adhesion of the solder to the base metal. Therefore, the use of the correct fluxes or soldering in a protective gas environment (nitrogen) is a prerequisite. Copper-phosphorus solders, for example, they have the property of self-fluxing when soldering copper to copper, but require the addition of flux when connecting copper to steel.
Temperature regime also plays a decisive role. Overheating the soldering area can cause the copper to anneal, making it soft and brittle, or burn out the solder components, reducing its strength characteristics. Underheating will lead to the formation of a so-called “cold solder” - a porous structure that does not provide tightness. Therefore, control of the burner flame and uniform heating of the connection area are the basic skills of the master.
⚠️ Attention: Never use lead-tin solders (PLS) intended for electronics to repair the refrigeration circuit. Their melting point is too low, and their strength will not withstand the operating pressure and vibrations of the compressor, which will lead to instant depressurization.
Understanding the physics of the process allows you to avoid common mistakes. When you heat a joint, the heat should be applied to the tubes and not directly to the solder rod. The metal must melt upon contact with the hot tube, which ensures proper spreading and filling of the gap. Violation of this rule is one of the most common causes of defective connections among beginners.
Necessary equipment: burners and heat sources
The choice of heat source directly depends on the scale of the work and the conditions in which the repairs are carried out. For professional activities that involve daily soldering of many connections, the optimal solution is to use oxygen-propane oxy-acetylene torches. They provide a high flame temperature, sufficient to work with any refractory solders, and allow you to quickly heat up massive components.
However, for one-time repairs or work in conditions of limited space in an apartment, portable gas burners are often used. The most popular models are those that run on a mixture of propane-butane and suboxygen (for example MAPP gas or its analogues in cans). Such devices are compact, do not require bulky cylinders and are capable of developing a temperature sufficient to melt most silver and copper-phosphorus solders.
When choosing a burner, pay attention to the ability to adjust the flame. A soft, focused flame with a well-defined inner cone zone (where the temperature is highest) is preferable to a roaring torch. It is also important to take into account the material of the nozzle: copper tips give off heat better, but wear out faster, while brass or steel tips are more durable.
The issue of safety when working with open fire deserves special attention. In the confined space of an apartment, the use of open fire requires extreme caution. Make sure that there are no flammable materials within a radius of several meters, and that the refrigerator itself is moved away from walls and furniture. Always have a fire extinguisher on hand.
⚠️ Attention: When working with gas burners indoors, be sure to provide a flow of fresh air. Burning gas consumes oxygen and can release carbon monoxide, which is hazardous to health in an unventilated room.
Selection of solder: hard versus soft alloys
The heart of any solder joint is the solder itself. In refrigeration technology, only hard solders with a melting point above 450°C are used. The most common group is copper-phosphorus alloys (for example, PMF-7, PMF-9). They have excellent fluidity, high strength and, due to the phosphorus content, reduce the melting point of copper and prevent oxidation.
The second popular group is silver solders (PSr-15, PSr-25, PSr-45 and their foreign analogues). Silver significantly increases the ductility of the weld, which makes the joint more resistant to vibration - a critical parameter for the discharge section of the compressor. The higher the silver content, the higher the ductility and lower the melting point, but also the higher the cost of the material.
Table of melting temperatures of popular solders
PMF-7 (copper-phosphorus): 710-750°C|PSr-15 (silver): 640-680°C|POS-61 (tin-lead): 183-190°C (NOT APPLICABLE)|L-CuP6 (analogous to PMF): 700-720°C
There is a common misconception that the refractory the solder, the stronger the seam. In fact, for refrigeration tubes, the elasticity of the connection is more important. A tough, brittle weld can crack due to vibration of the compressor or thermal expansion of the tubes. Therefore, for areas subject to vibration (compressor inlet and outlet), it is recommended to use solders with a high silver content.
When choosing a material, it is also worth considering the type of metals being joined. For the copper-copper pair, copper-phosphorus solders without flux are excellent. For a copper-steel pair (for example, a condenser tube and a compressor fitting), phosphorus solders cannot be used, since brittle iron phosphides are formed. Here you need silver solder in combination with active flux.
The role of fluxes and protection against oxidation
Flux is a chemical substance designed to remove the oxide film from the surface of metals and prevent its re-formation during the heating process. Without a high-quality flux, solder will not be able to wet the surface of copper or steel, gathering into a ball, like mercury on glass. For hard soldering, special high-temperature fluxes are used, often called boron-containing or flux pastes.
Modern fluxes for refrigeration soldering are usually a viscous paste, convenient for application with a brush. They are active at high temperatures and are easily removed after cooling with water or special solvents. It is important to apply flux evenly to both surfaces to be joined before assembling the assembly. Excess flux is just as harmful as its lack: excess can coke inside the tube and clog the capillary.
An alternative to chemical fluxes is soldering in an inert gas environment, most often nitrogen. Purging the tubes with nitrogen during soldering displaces oxygen, preventing the formation of oxides within the system. This is a professional method that allows you to get perfectly clean connections from the inside, which is especially important for modern refrigerators with narrow capillary tubes and sensitive expansion valves.
The mistake of many beginners is to use rosin intended for tinning wires. Rosin burns out at hard soldering temperatures without having time to perform its function, and leaves behind carbon deposits that interfere with the normal spreading of solder. Use only specialized fluxes marked “for hard soldering” or “for copper pipes.”
Step-by-step instructions for soldering pipes
The soldering process requires sequential operations. First you need to prepare the ends of the tubes: chamfer, clean the surfaces to a metallic shine using an abrasive sponge or sandpaper. Then one tube is inserted into another (or into a fitting) with a gap of about 0.1-0.2 mm. A thin layer of flux is applied to the prepared area.
☑️ Checklist for preparation for soldering
Then follows uniform heating of the joint. The burner flame must constantly move, covering the entire circumference of the tubes to avoid local overheating. When the metal heats up to the desired temperature (copper will turn dark cherry), a solder rod is brought to the joint area. If the temperature is sufficient, the solder melts and is drawn into the gap by capillary forces.
After filling the seam, it is necessary to allow the joint to cool naturally, without blowing on it or cooling it with water. Rapid cooling can lead to the appearance of microcracks in the structure of the weld metal. Only after complete cooling can the remaining flux be removed and a visual inspection be carried out.
The quality of the seam is assessed visually: it should be smooth, shiny (for silver solders) or matte golden (for copper-phosphorus), without cavities, sagging and pores. The solder should evenly encircle the entire circumference of the tube. Any doubts about the tightness should be resolved by re-soldering or re-welding the unit.
Comparative characteristics of materials for soldering
To systematize knowledge about the materials used in the repair of refrigerators, it is convenient to use a comparative table. It will help you quickly navigate the choice of solder and flux depending on the specific task and type of metals being connected.
| Type of connection | Recommended solder | Necessity of flux | Temperature soldering |
|---|---|---|---|
| Copper - Copper | Copper-phosphorus (PMF-7, PMF-9) | Not required (self-fluxing) | 700-750°C |
| Copper - Brass/Bronze | Copper-phosphorus or Silver (PSr-15) | Desirable | 650-750°C |
| Copper - Steel | Silver (PSr-25, PSr-45) | Required (special flux) | 650-700°C |
| Steel - Steel | Copper or Silver | Required (active flux) | 800-900°C |
The table shows that a universal solution for the home craftsman, which can collide with different metals is the presence of silver solder (for example, with a silver content of 15-25%) and high-quality flux. This will allow you to qualitatively connect the copper tube of the evaporator with the steel pipe of the compressor or condenser.
The cost of silver solders is higher than copper-phosphorus ones, but the reliability of connections based on them is worth it. Especially when you consider that the cost of calling a technician again or losing products due to defrosting will significantly exceed the savings on a couple of centimeters of a solder bar.
Common mistakes and safety measures
One of the most serious mistakes is soldering under pressure or without preliminary vacuumization of the system. Residual refrigerant or oil in the pipes may ignite on contact with an open flame or create excess pressure leading to an explosion. Before starting work, make sure that the system is completely depressurized, the refrigerant is removed, and the oil is drained.
Another common mistake is overheating of the soldering area. Prolonged exposure to high temperatures causes scale to form inside the tube, which can subsequently fall off and clog the filter drier or capillary tube. This will lead to the refrigerator not working immediately after refueling.
⚠️ Attention: Never point the burner flame at the refrigerator’s plastic elements, wires or insulation. Use metal screens (reflectors) made of tin or foil to protect adjacent units from thermal radiation.
It is also worth remembering the toxicity of flux vapors and combustion products. Work in a well-ventilated area, use a respirator if necessary. Vapors of zinc (if brass is used) or cadmium (in some old solders) can be hazardous to the respiratory tract.
FAQ: Frequently asked questions
Is it possible to solder refrigerator tubes with a regular electric soldering iron?
No, a regular soldering iron for electronics develops temperature of about 300-350°C, which is not enough to melt hard solders (requiring 600-700°C). The use of soft solders (tin-lead) in the refrigeration circuit is prohibited due to the low burst pressure and vibration instability of such connections.
How to replace nitrogen purge at home?
Under ideal conditions, nitrogen is required. At home, craftsmen sometimes use blowing with the refrigerant itself (if you don’t mind it) before the final sealing, or try to make the connection as quickly as possible with minimal air access, using excess flux, although this is less reliable.
How to determine that the solder has spread correctly?
If it has spread correctly, you will see a uniform ring of solder around the entire perimeter connections. If the solder lays down only on one side or forms drops, it means that the heating was uneven, the surface was dirty or there was not enough flux. Such a connection needs to be redone.
Is it necessary to strip the tubes inside before soldering?
It is mechanically impossible and unnecessary to strip the inside. However, it is important not to allow chips, dirt or excess flux to get in there. When cutting the tube with a pipe cutter, always hold it with the hole down so that the chips do not get inside the system.