Soldering and pipe work — hard soldering, nitrogen purging and pressure testing

The education's competency goals are concrete on this point: you must be able to perform hard soldering on copper pipe installations with solder material containing silver and phosphorus, in a quality that complies with applicable standards for refrigeration system installation, and you must be able to work according to a given soldering procedure. It is not by chance that it is stated so practically—poor soldering is one of the most common reasons a refrigeration system leaks refrigerant and needs to be serviced too early.

§Copper pipe — cleaned and prepared for the purpose

The pipes in a refrigeration system are typically made of phosphorus-containing, deoxidized copper (often called Cu-DHP), which is specially cleaned and sealed from the factory, precisely because the pipes must be completely free of moisture, oil and impurities when put into use. Calculations of how much pressure a given pipe dimension can withstand follow standards such as EN 14276, while the installation itself and pressure classes relate to DS/EN 378.

§Hard soldering with silver and phosphorus

Hard soldering differs from soft soldering in that the filler material melts at a high temperature and creates a much stronger and more pressure-tight joint. For copper pipe installations in cooling systems a filler material containing silver and phosphorus is typically used — the phosphorus allows copper to be soldered to copper without separate flux while the silver content lowers the melting point and strengthens the joint. The work requires even controlled heating with a flame so the filler material is drawn into the joint by capillary action instead of just sitting on top as a blob.

§Nitrogen purging — the invisible detail that means everything

When copper is heated to soldering temperature in normal air, a layer of fire scale (oxide) forms inside the tube because oxygen in the air reacts with the hot metal. These scales later come loose and circulate around the system, where they can clog filters, expansion valves, and the compressor's lubrication. The solution is to let a weak stream of nitrogen (an inert gas) flow through the tube while soldering, so oxygen is displaced and the internal surface stays clean and bright. It's a small extra effort that prevents errors that otherwise don't show up until months after commissioning.

§Tightness test before evacuation

When the pipes are assembled, the entire system must be pressure-tested for tightness before it's evacuated and filled with refrigerant. This is typically done by filling the system with nitrogen to a set test pressure and then checking whether the pressure stays stable over time, and by looking for leaks at the connections with foam liquid or an electronic leak finder. Only when the system passes the tightness test does it make sense to spend time on thorough evacuation with a vacuum pump—otherwise you just suck air back in through a leaky connection.

• 01Clean and prepare the pipes, and keep the joints free from moisture and dirt.
• 02Solder with silver/phosphorus added material according to an established procedure.
• 03Backfill with nitrogen while soldering to avoid internal glowing scales.
• 04Pressure and leak test the entire system before evacuation and filling.

§Why it pays off to take your time

Plumbing work is rarely what the customer notices when the system is finished — it is hidden behind panels and insulation. But that is precisely why it must be right the first time: a leaking joint that is only discovered after months of operation is far more expensive to fix than spending the extra minutes on nitrogen purging and leak testing during installation.

“A good joint is not seen from the outside. It is seen by the fact that the system never leaks.”