Heat pump, air conditioning, refrigeration - brazing of heat exchangers, pipes, etc.

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Posted on: 2024-05-09
 

Soldering in refrigeration devices and installations - basics

Soldering is one of the essential skills for any professional involved in the installation or service of air conditioning or heat pumps - generally speaking, refrigeration units that use refrigerants.

✅ The specific nature of refrigerants or similar media leaves no choice; the joining of components by soldering is necessary. However, what does this process really mean, why is soldering indispensable, and what types of solders are available? Further, what materials should be chosen and what should be paid special attention to during their selection? In this post, we will try to briefly answer these and other questions, discussing the essence of soldering and related (albeit important) issues. This is an explanatory article for laypeople or enthusiasts who want, for example, to convert an air conditioner into a heat pump or perform similar modifications themselves.

Soldering in refrigeration and heating

Individual components of systems must always be properly connected - in accordance with the art appropriate for the media used in a particular system. The way elements such as pipes in water-to-water systems are connected differs from those in systems with other media (even down to the selection of materials - fittings used to connect pipes, threads, etc.). It is evident that yet another method of connecting pipes, for example, with components of a heat pump (like a condenser, etc.), is required for gaseous media - i.e., refrigerants.

✅ Here - in refrigeration - we use soldering - it is a method of connecting individual parts of air conditioning systems, heat pumps, and similar devices - such as refrigeration units.

Difference between welding and soldering

From a layperson's perspective, welding and soldering may seem similar, at least in effect - they permanently join two elements. However, from a physics standpoint, there are significant differences - in welding, the original material is partially melted, to which another element is joined. This leads to a permanent connection, with the original material itself acting as the binding agent. In the case of soldering - the solder is the binding agent, and only it is melted, while the structure of the original material remains unchanged. Therefore, in air conditioning units, heat pumps, and heat exchangers for refrigerants - there is no question of welding, only soldering.

✅ Welding and soldering are not the same; they are different physical processes, although from a layperson's perspective they lead to the same outcomes.

Soldering AC lines heat exchangers

Hard soldering and soft soldering - and what is brazing?

Of course, there are different types of soldering, brazing etc - we most commonly encounter hard soldering and soft soldering. The difference lies in the melting temperature of the solder, as naturally, solder melting is achieved by applying high heat.

✅ Hard soldering is what interests us in the context of our article - because it is usually this type of soldering that is used in air conditioning systems, refrigeration units and heat pumps. It is a process that requires a temperature higher than 450°C (840°F). On the other hand, soft soldering - is a process that occurs at temperatures lower than the aforementioned 450 degrees Celsius. It is less interesting to us, as it is less commonly used in refrigeration.

Additionally, we can also encounter the concept of high-temperature soldering - some people confuse it with hard soldering. However, it is not quite the same, as it is a process of applying solder at even higher temperatures - above 1000°C.

☑️ Hard soldering and high-temperature soldering are not the same - hard soldering occurs at temperatures > 450°C, while high-temperature soldering occurs at much higher, > 1000°C.

Sometimes we meet the term brazing... is it the same?

From a layperson's perspective, brazing and soldering might seem the same, but technically, they are not the same process. In English, the term "brazing" is commonly used to refer to what is known as hard soldering.
The sole distinction so - between soldering and brazing - lies in the temperature levels used for each process. Soldering is conducted at temperatures below 840°F  or 450°C, while brazing is carried out at temperatures above 450°C (or 840°F).

Of course, this leads to further differences; the joint created by brazing is stronger than that resulting from soldering, hence it can be used in more demanding applications. However, we will not expand on this topic further.

☑️ Brazing is in fact the hard soldering, so requires at least 450°C to be done.

Capillarity - Capillary attraction - the basic property of liquid solder that determines its usefulness

Capillarity or Capillary attraction is a word characteristic of the soldering process.
✅ Capillary attraction is simply the ability of liquid solder to penetrate into the soldering gap. This occurs even against gravity. By the soldering gap, we mean the gap between elements at the soldering temperature. The smaller this gap, the easier it will be to fill the gaps with solder. (To every type of soldering there may be different types of recommandations).

☑️ In the context of the soldering gap - specifically its size and the accompanying capillary attraction - it is very important to ensure that the soldering gap, or actually the space between the different soldered elements, is suitably small. Then, capillarity will do its job and help us effectively bind the elements together. It is important to remember this during the planning stages, to ensure that individual elements such as pipes are relatively compatible in size.

Wettability of solder - what is it and what does it mean in practice?

The most important aspect here is the practical ability to apply solder, although using technical terms might seem daunting to a layperson. However, these are the absolute basics that will help you better understand the phenomena you will encounter in practice.

Solder has a certain ability to spread evenly over the surface of the soldered material, forming a thin, regular layer without lumps and what might be called "clots".
✅ The ability for such even spreading significantly facilitates the work - this is called the wettability of the solder. To achieve it at an appropriate level, it is necessary to properly select the materials used for the solder, possibly improve them by adding so-called flux, but also to take care of the basic preparation of the soldered material (it must be clean).

What type of solder for air conditioning or heat pumps?

In air conditioning systems or heat pumps, there may be a need to join different materials - or the same materials, such as copper to copper.
☑️ The solder should be chosen based on the materials being joined, in order to follow established patterns and simplify the task. A different solder will be selected for joining copper to copper - and another for joining steel to copper, etc.

✅ What solder to use for connecting copper to copper?

For connecting copper to copper, one of the more readily available solders - copper-phosphorus solder - is sufficient. It is adequate for connecting copper pipes in air conditioning systems, etc. Usually, it is relatively easy to apply due to its good wettability and does not require special preparation, such as adding flux (a substance that increases the wettability of the solder).

Copper-phosphorus solder of course has some drawbacks, e.g., it is not suitable for soldering iron to nickel - but these are not disadvantages in the context of our topic.

It's worth noting - there is a possibility to improve the quality of phosphorus-copper solder by adding a minimal amount of silver solder. This will increase the strength and wettability - helping in our work.

What solder to use for connecting copper and brass?

To make a connection between a copper and a brass component, we must use a solder with a certain silver content, which should be around 40% (the higher the content, the easier it will be to apply, and the solder joint will be more durable). Due to the brass - a flux will need to be used here.

What solder to use for connecting copper to aluminum, or aluminum to aluminum?

For connections between copper and aluminum, or aluminum components with each other, a zinc-aluminum or aluminum-zinc solder should be used - obviously with a flux.

✅ What solder to use for a refrigerant heat exchanger to copper?

The issue of soldering stainless steel (from which plate heat exchangers for air conditioning and heat pumps are made) and copper raises some ambiguities, as various comments by practitioners who have done this themselves and used different methods can be found online. The most important thing is that they achieve the goal.

If you are installing a heat exchanger as a condenser and want to solder steel tubes to copper - we recommend using copper solders with a high silver content, even up to 45%, with the use of flux.

Correctly connecting a heat exchanger, which is to work with refrigerant, is of course crucial for the whole system, so it's not worth skimping on the silver content in the soldering.

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Basic equipment for soldering air conditioning connections

For sporadic use, unless we are professional installers who buy equipment intended for solid exploitation, simple and compact electric soldering irons are generally used. Installers also use them - who simply buy slightly more expensive, more durable equipment. This is a compact and convenient solution, and also relatively safe - and that's what we would recommend.

___________

✔️ Glossary of Terms Used in the Article

AC - commonly used as Air Conditioning or Air Conditioner

Brazing - solder process with temperatures over 450°C, check: hard soldering

Capillarity of solder / Capillary attraction - the ability of solder to spontaneously penetrate into the soldering gap, one might say to "glue itself" into it in an even and trouble-free manner. If a given solder has good capillarity, it will be easy to apply and will fill the soldering gap evenly, even against gravity.

Copper-phosphorus solder - used for connecting copper components, also used in electrical applications. Relatively undemanding, easy to apply, often does not even require the addition of flux.

Flux - a chemical substance aimed at improving the wettability of the solder and protecting metal surfaces from oxidation during the soldering process. Generally, it makes life easier, often without its addition, we would not be able to effectively connect materials with solder.

Hard soldering - a process of applying solder where melting temperatures above 450 °C are used. The solder formed in this way naturally has different properties than in soft soldering. Another term used for this is brazing

Heat exchanger - a device for transferring heat from one medium to another, used in refrigeration, air conditioning, and heat pumps to transfer heat (or cold) to water or air. Additionally, this device comes in many forms, widely used in the industry. Heat exchangers are generally commonly used in heating solutions.

High-temperature soldering - a process of applying solder that requires temperatures above 1000°C.

HP - abbreviation for Heat Pump

HVAC - commonly used for a branch: Heating, Cooling, Air-Conditioning

Refrigeration unit - a device used to generate cool air and maintain low temperatures in the coolant/refrigerant circuit (usually has a closed circuit). The device bears some similarities to air conditioning units.

Refrigerant - gas used in heat pumps, air conditioners, fridges etc.

Refrigerant plate heat exchanger - a type of heat exchanger often used as a condenser or evaporator in heat pumps, air conditioners, etc. Frequently used in converting air conditioners to heat pumps. Connecting it obviously requires soldering its connections.

Soft soldering - a process of applying solder that occurs at relatively low solder melting temperatures (i.e., below 450 °C).

Solder gap - a gap, empty space between the materials we plan to solder. The smaller it is, the easier the whole soldering process will be - however it's worth to check the recommendations in every single case.

Soldering (sometim - a process that leads to the joining of 2 materials by melting solder, in this case - without melting the material itself (unlike welding).

Solder wettability - its ability to spread and adhere to the surface of the soldered material. If it is low, we will have problems with efficient soldering. We can influence it by cleaning the surface or adding so-called flux. It is also important to properly select the type of solder for the materials being soldered.

Welding - a physical process that involves joining materials by melting the bonding filler. It also partially leads to the melting of the original material (unlike soldering or brazing).
Zinc-aluminum solder / Aluminium-Zinc soler - used for connecting aluminum to aluminum or copper to aluminum. It has a low melting point - therefore, it is suitable for these materials without deforming them with high temperature.

 
Posted in: Heat exchangers
Author: Adam Bert
Spezialist für Verteiler und Zubehör für Fußbodenheizungen sowie Auswahl von Heizgeräten für Fußbodenheizungen und herkömmliche Heizkörper, bei Nordic Tec seit 2017. Planer von Heizungsanlagen in Einfamilienhäusern und Gebäuden bis zu 500 Quadratmetern. Privat ein Fan guter Bücher.

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