Frequently Asked Questions
Brazing and furnaces
Question: What are the key factors to achieve good results in brazing?
Answer: The important factors are: 1) Use clean parts
2) Apply correct amount of filler metal
3) Do not overheat paste during drying
4) Use suitable fixturing during brazing
5) Check that the oxygen content in the furnace is less than 20 ppm
6) Apply correct brazing temperature on the parts.
More detailed explanations can be found in the Handbook.
Question: Do I have to clean the parts before assembling and brazing the core?
Answer:
In normal cases not. The fins are clean enough after forming and the tubes are clean enough for coating after welding. Normally the headers, tanks and side supports are clean enough, too. Sometimes, if the stamping oil or emulsion is dirty and causes staining on the parts, they have to be cleaned.
Question: Can vacum furnace be used for CuproBraze?
Answer: The furnaces are CAB-furnaces (Controlled Atmosphere Brazing Furnaces). Nitrogen is used as inert atmosphere. The reason why vacum furnaces cannot be used is that in vacuum zinc starts evaporating out of brasses at elevated temperatures. Why some people talk about vacum furnaces in conjunction of CuproBraze, is understanable but misleading. In the beginning of the furnace cycle air (oxygen) is evacuated normally by flushing with nitrogen or, alternatively, the air can by removed by creating vacum and then backfilling with nitrogen, before heating up and brazing. The heating and brazing never takes place in vacum.
Compatibility
Question: Can I use CuproBraze materials for soft soldered heat exchangers?
Answer: You can use the tube brass and header brass but not the fin copper. The fin copper requires treatment at the brazing temperature to reach the required thermal conductivity level.
Question: Can CuproBraze and aluminium products be brazed in the same furnace?
Answer: From brazing point of view it is usually possible as the protective atmosphere is the same and the brazing temperatures are close to each other. However, it is recommended that a furnace is dedicated to only one of the metals, because copper deposits on aluminium surfaces can cause pitting corrosion on aluminium. Therefore, switching between the materials is not recommended, unless the furnace is carefully cleaned between the runs.
Corrosion resistance
Question: How is the corrosion resistance of CuproBraze heat exchanger compared to soft soldered one?
Answer: It is generally better. On CuproBraze heat exchangers the materials are of equal nobility, so galvanic differences are practically eliminated. On soft soldered heat exchanger the solder is less noble than fin and tube materials and can suffer from galvanic attack in a corrosive environment. CuproBraze tube alloy contains 85% copper which gives high resistance against dezincification and stress corrosion cracking.
Question: How is the corrosion resistance of CuproBraze heat exchanger compared to aluminium one?
Answer: The corrosion mechanisms of copper alloys and aluminium alloys are generally different. Copper alloys tend to corrode more uniformly, over the entire surfaces at known rates, whereas aluminium is more likely to corrode locally by pitting mechanism, resulting eventually in holes. The corrosion of copper is thus more predictable. However, this answer is very much simplified. More in-depth answers can be found in the conference papers listed in the Literature-section.
Environmental
Question: Is CuproBraze a leadfree process?
Answer: Yes, the brazing alloy contains copper, nickel, tin and phosphorus.
Paste application
Question: What is the mimimum paste layer thickness on tubes to achieve a good joint?
Answer: The normal thickness of the dry paste film is about 0.100 mm, which is about equal to the maximum diameter of the metallic brazing powder particles. The coating is better characterized and measured by weighing than by thickness. The recommended quantity is typically 150 to 250 g/m2 of tube surface area. The better tolerances on tubes and fins, the smaller amount paste is required.
Repairability
Question: Can CuproBraze heat exchanger be repaired in the field?
Answer: Yes, repairability is one of the big advantages, especially in remote areas, where availability of spare parts is limited. You can repair it with leadfree soft solder (for example 97% tin, 3% copper) or with common silver containing brazing alloys.
Tolerances
Question: What are the tolerance requirements on the parts?
Answer: CuproBraze has similar behaviour to other brazing processes. The optimal gap width between the parts that are to be joined together is about 0.050 mm. At this gap size the capillary force, that sucks the molten brazing alloy to the gap, is at strongest. The maximal gap width is about 0.100 mm. At higher gaps the joints may not form.