Brazing Handbook - Selecting a furnace

Selecting a furnace

Factors to consider when select ing a suitable furnace are production volume, part size, available floor space, capital expense, and operating cost. Based on these specifications, the CuproBraze heat exchangers can be processed in batch, semi-continuous or continuous furnaces.

Selecting a suitable furnace requires knowledge of the temperature, time and atmospheric conditions of the process. All of the furnaces have heating and cooling sections. Batch furnaces and semi-continuous furnaces are suitable for any part size but limited with respect to production volume. Continuous furnaces are suitable for high volume production.

Batch furnace

A batch furnace uses the same door to load and unload the part. These furnaces can only produce one batch at a time. A load is purged with nitrogen then moved into the brazing chamber; after brazing, the load is moved back into the purge chamber where it is cooled.

Semi-continuous furnace

In a semi-continuous furnace parts are indexed from the loading area to the purge chamber, where the part is purged with nitrogen and then moved into the next chamber. The furnace simultaneously moves the purged part into the brazing chamber and a new part into the purge chamber. This type of furnace is suitable for large parts or intermediate volume production.

Continuous furnace

A continuous furnace uses a conveyor-belt to continuously move parts through the furnace where they are continuously purged with nitrogen, brazed and then cooled. This type of furnace is for high-volume production. A continuous furnace is not recommended for parts longer than 1000 mm because when the front of the part enters the heating zone, it conducts heat to the rear of the part. As a result, the trailing section of the part is held at temperature for a much longer time than the leading edge of the part.

Heating source

It is possible to heat all three types of furnaces with electricity, natural gas, or propane/butane. In many countries, natural gas and propane are a cheaper source of energy than electricity, but they require more maintenance and have a higher initial cost. Gas burners also require a gas-tight barrier between the combustion products and the brazing atmosphere. Such a barrier can be radiant tubes or a muffle.

Process emissions

Process emissions are generated when the binder is volatilized during the first part of the heating cycle. These emissions must be properly managed to prevent contamination of the atmosphere in the furnace. The constant flow of nitrogen normally expels the vapour from the brazing atmosphere. As there is no oxygen available to burn the gas products in the furnace, the gases mu st either be burned outside the furnace or diluted with ambient at mosphere, according to the local regulations, requiring, in most cases , some kind of afterburner. The laws vary from country to country and state to state, so one must check with the local authorities before designing a furnace. The generated emission is influenced by the binder system and could be totally different between paste ma nufacturers. Further information regarding the emissions, and further handling of them, are obtainable from paste manufacturers.

Important: Most of the binders form emissions. Contact the paste manufacturer to check if any kind of afterburner must be used.