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.

Brazing Handbook - Brazing operation

Brazing operation

Because the amount of flux in the CuproBraze process should be zero or absolutely minimal, and the parts should be oxide free after brazing, an inert atmosphere is needed to prevent oxidation of the parent and filler materials. As the brazing temperature is much lower than the melting points for the copper and brasses, the temperature differences in the parts during the brazing process are not critical. As the CuproBraze process covers parts from around 100g up to more than 100kg it is not possible to advise exact settings of the furnaces.

Atmosphere

The primary function of the brazing atmosphere is to prevent oxidation. Furnaces for the CuproBraze process use high-purity nitrogen to displace oxygen from inside the furnace. The atmosphere of the furnace must have an oxygen content of less than 20ppm. The brazing powder is very sensitive when the binder starts to evaporate, If moisture and oxygen levels ar e higher than these levels, the powder and the base material have a risk of oxidation at temperatures exceeding about 200ºC the and the result is very poor joints. Thus the starting point of the brazing cycle is as sensitive for oxygen-content as the rest of the brazing cycle.

Mixing the brazing atmosphere with small amount of hydrogen (H ₂) is not generally recommended. Hydrogen can sometimes react with the organic binder forming products which can have an influence on the brazing result. Before using hydrogen or hydrogen mixed atmosphere, the paste manufacturer should be consulted.

Important : The oxygen content should be controlled from the time when the heating starts and no heat should be applied if the atmospheric conditions are not met.

Temperature and time

The difference in the melting points for the filler metal and the copper and brass materials is more than 300ºC, which means that there is no risk to destroy the parts by melting. The temperature above the melting point for the filler metal (600ºC) should be as short as possible but still gain a satisfactory brazing result. It means that the furnace must be able to heat up the load in the brazing zone with a steep ramp. A common value is more than 30ºC per minute. The furnace must be able to operate up to 700ºC. Figure 27 shows a principle temperature-time curve for the CuproBraze process.

Figure 27. Pr inciple temperature-ti me curve for the CuproBraze
process.

In the part A, the sample is slowly heated up and the binder evaporates and/or is decomposed. When the binder disappears, it leaves the particles in the brazing paste without any protection from oxidation if the oxygen content is too high. The oxygen content should therefore be controlled from the time when the heating starts and no heat should be applied if the atmospheric conditions are not me t. The brazing result will be poor if the brazing powder is oxidized before it starts to melt. Note that some big heat exchangers as well as “one shot” parts can include a lot of s mall half closed volumes, which could influence necessary time to reach satisfactory oxygen content. Good convection in the furnace is therefore recommended.
By the rather slow heating rate in this zone, the temperature differences in the samples are minimized, and the distortion of the sample due to heat expansion can also be minimized.

In part B th e whole co re will be p reheated to just under the melting point of the brazing metal. To minimize the temperature-difference in the core at brazing, the part B should be designed so the temperature in the whole sample is as equal as possible when the brazing part is entered. In some batch furnaces, there is no pre-heating and in that case furnace settings that minimize the temperature differences in the sample have to be used.

Part C is the brazing period. When the temperature exceeds 600ºC, the brazing filler metal (powder or foil) starts to melt. When it melts, metallurgical reactions (diffusion) starts and the extent of the filler-substrate interaction is the most important. The filler interaction on the fin material is when it starts to be alloyed with tin, forming a copper-tin alloy close to the joint. It does not influence the performance except for exceptional long brazing times, where some loss of thermal performance (up to 10 %) of the heat exchanger can occur. The governing factor for the brazing cycle in most cases is the brazing of the tube-header joints. In chapter 5 figure 11 it is shown that the brazing temperature has a big impact on the possibility to satisfactory fill gaps. In practice it has been found that to satisfactorily wet the surfaces and fill the joints, you should ensure that the temperature in the joints reaches 650ºC or for some cases even 670ºC.

As the alloying reaction starts when the molten filler metal wets the surfaces, the time above 600ºC should be as short as possible but long enough to reach complete brazing in tube-header joints., For small radiators, 3 to 4 minutes is typical. For bigger parts the time is guided by the tube-header brazing.

To reach short brazing times, usually the setting of the brazing part (A) of the furnaces is higher than 650ºC.
The effect of the brazing cycle on the tube-to-fin joints cannot be seen by the naked eye. During optimization of the brazing cycle, overshooting of the brazing temperature can sometimes happen, but it will not lead to any noticeable visual effect on the brazed heat exchanger as the melting point for copper and brass is far higher than the brazing temperature.

To minimize the risk for distortion of the joints during cooling (part D), it is recommended to have a low cooling rate down to around 550ºC, typical value 1ºC/s.

To prevent discoloration of the brazed parts, they should not leave the inert atmosphere until the part temperature is below 150ºC. In places where the ambient humidity is high the exit temperature should be even lower to prevent discoloration. Note: This discoloration is only a cosmetic effect and it will not deteriorate the brazed joint.

At least during optimization of the brazing cycle, it is highly recommended that some kind of measurement of the temperature with thermocouples mounted in the core should be used. To have full control on the brazing process, it is recommended to also have this equipment available to check the process every now and then during normal production. If it is not possible to use the thermocouples together with equipments outside the furnace, it is recommended to use them with a tracker following the sample through the furnace.

The brazed part should be cooled down as uniformly as possible at least in the first phase to prevent deformation. The temperature drop should be equal in the whole core. One way to achieve this is to slow down the cooling to around 550ºC. At that temperature, the filler metal in the joints is no longer molten.
To satisfactorily wet the surfaces and fill the joints, ensure that the temperature in them reaches 650ºC or for some new types of header pastes even 660ºC.