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The Brazing Technology and Application in Refrigeration Equipment

Date:2013-08-26 14:23:35

With development of modern technology, brazing has received rapid development, no matter in brazing process and brazing materials. In a refrigeration device, the brazing method was introduced into joining the heat exchanger manufacturing and refrigeration circulation pipeline. The joint numbers, the diameter variation range and welding quality will directly affect the performance. Therefore, any company pays lots of attention to the brazing.

In the brazing process, a weak bonding strength could be caused in joining of copper and copper or copper to ferrous metals if not choosing right brazing technology. During brazing, the first thing is to dissolve and destroy the oxide film on the substrates, which could wet the substrates and decrease the surface tension. The above effect could make the brazing alloy spread fully and form the brazing seam after brazing. In order to ensure the quality of brazing joint and improve the sealing performance of the joint, it is significant to choose the proper brazing process.

When using gas flux during brazing. It is necessary to pay attention to cleaning, assembling, the right heating process, the proper way of adding brazing alloy, which is the key to guarantee the joint quality. During the process, we should make the joined materials heat uniformly to obtain the ideal capillary effect, and the brazing alloy could reach the root of inner part and fill all the gaps. The brazed joint of the pipeline system will not be corroded by using gas flux and it can avoid leakage accidents and also protects the substrates and the brazing alloy being oxidized and promotes the wettability. Then the final joint was obtained.

1. gas flux and brazing equipments

In manufacturing air-conditioning and refrigeration equipment, brazing is the key technology, and the earliest technology used the brazing alloy containing 45% silver and QJ102 flux. During brazing, the brazing alloy should be dipped with the flux and too much of the amount of flux could be forbidden. In order to avoid the contamination of the air condition system or corrosion of the joint and copper, it is necessary to clean the flux after brazing. The gas flux was introduced in brazing for improving the efficiency and quality. The kind of method was adding the flux in the flame, which is suitable for modern and economical high-speed welding method. The gas flux is a highly volatilized liquid compound without fluorine and corroded materials. During brazing, the flux reached the brazing areas through gas pipe and formed a protective film on the substrates, preventing oxidation on the substrates and improving the joint quality. In order to reduce the cost, the low silver brazing alloy could be used, such as 204. The brazing equipment was shown in Figure 1. Due the addition of flux, the flux with the "atomization" state can be uniform in the joint, improving the brazing quality and welding level, reducing the probability of leakage and ensuring high quality brazing seam.

 

Figure 1

 

2  Preparation before brazing

  2.1 Clean the brazing substrates and remove dirt

  The capillary effect can only be useful on clean surface. Therefore, it is necessary to remove all dirt (oil, grease, garbage, rust and oxides). The clean methods are showing as follows:

  (a)In order to remove dirt, rust, oxides on the joined materials, immerse the joined materials in a chemical clean solution. To remove dirt, rust, oxides or paint thoroughly, dispose the surface with sandpaper. After cleaning, remove the residues with cloth or sponge.

  (b)After cleaning, the joined materials should be thoroughly dried before brazing and need to be brazed quickly for avoiding further contaminated.

  2.2  Ensuring a good and proper assembly gap

  It is necessary to ensure the proper joint gap in order to obtain the capillary effect, easy assembly and easy to fill the brazing alloy. Figure 2 (a) shows the joint gap of brass at room temperature. Figure 2 (b) indicates the joint gap in a brass-iron joint. Since the thermal expansion coefficient of iron is smaller than that of brass, so the gap becomes small.

Figure 2

 

2.3  assembling and supporting

  The joined materials should be placed vertically and the nitrogen should go through inside the joint to protect them from oxidation.

3  Brazing components

  3.1  Prior to brazing

  a. Ensuring normal supply of nitrogen.

  b. When brazing service valves and four way valves, it is necessary to isolate for avoiding damage or leakage. If possible, the cooling device should be introduced. If not to use the cooling device, the area to be protected should be covered with wet cloth or sponge.

  c. To get a good brazing, the proper mixture of oxygen and acetylene is quite important.

  Ⅰ. Opening the acetylene valve;

  Ⅱ. Flaming;

  Ⅲ. Opening the oxygen control valves to obtain neutral or reducing flame;

  Ⅳ. Adjusting the flame of oxygen and acetylene properly, the neutral flame with pale blue is suitable for joining copper and brass or copper and copper. The reducing flame with light blue flame core can be used to all steel joints. Too little oxygen can cause insufficient burning and smoke, too much oxygen may lead to overheating or excessive oxidation.

  3.2  The brazing process

Figure 3

 

  a.When brazing brass or iron pipe, heating the outer parts with outer flame, which is 12 ~ 25 mm from the outside, as show in Figure 3. When joining iron and copper or brass with iron pipe in the inside, the external surface of the assembly should be heated since heat conductivity of copper is far better than that of iron. b. The outer flame should be used during heating and the inner flame should be places 12-25mm far from the outside. c. If the pipe is small, torch flame can simultaneously include the inside and outside parts for uniformly heating to temperature, which can prevent the occurrence of hot spots and overheating, as shown in Figure 4.

Figure 4

 

  3.3  Brazing temperature

  a. When copper becomes dark red; it shows that the joined materials have reached the brazing temperature. If turning bright red, it means the metal overheating.

  b. When brass is dark red, it shows that the joined materials have reached the brazing temperature. When the brass becomes bright red, it means overheating or nearly melting. While joining in the temperature, there will be a high possibility of high leakage.

  c. When the iron becomes dark red, it shows that the joined materials have reached the brazing temperature.

  3.4  Appling brazing alloy

  a. Keeping brazing alloy clean, any dirty or corroded brazing alloy may lead to inclusion in the joint.

  b. The molten brazing alloy will flow to the high temperature area, so it is necessary to applying brazing alloy close to the joint.

Figure 5

 

  c. Applying the brazing alloy when reaching the temperature, or the brazing alloy will not be attached on the brazing surface. When joining the pipe with small diameter, it is significant to move around to applying the brazing alloy, as indicated in Figure 5. When joining the pipe with large diameter, it should be applying the brazing alloy in two positions to avoid overheating. The brazing alloy will be flowing out when beyond the outer parts during brazing.

  d. It is important to apply the proper amount of brazing alloy to obtain a good joint and the added brazing alloy should be solidified in the joint gap, or it will be wasting brazing alloy and weakening the joint strength.

  e. During brazing, we should keep in mind that heating the joined materials not the brazing alloy. When the brazing alloy began to flow, it is necessary to move the torch around the joint to keep the uniform heating in the inner and outer parts, as shown in Figure 6. The above action could keep the favorable capillary effect.

  f. In order to obtain the ideal capillary effect, it should follow the right method and procedure.

Figure 6

 

  g. To heat the whole joint uniformly, the capillary effect of will induce the brazing alloy into joint gap.

4  Cleaning component and cooling

  (1)After brazing, the brazed assembly should be put in clean water for hardening, making the temperature decreasing since they can protect the joined being oxidized. For those brazed assembly that could not be quenched, the joint should be wrapped with wet cloth or sponge.

  (2)Careful cleaning of brass distribution pipe, valves and other components. Before cleaning, it should let the brass cool to a certain temperature. Cooling the brass immediately may cause crack after brazing.

  (3)Cleaning the joints thoroughly to ensure no flux or residue residing.

5  Results of brazing

(1)To cut the joint, a good joint should require that the brazing alloy spread the whole joint, and 80%~100% of the joint was covered and no porosity was found.

(2)Accurate heating method should guarantee the uniform heating. By observing the color of the joint, we could control the temperature and prevent overheating.

(3)In order to achieve the best mechanical properties of the brazed joint, we could see the uniform flowing of the brazing alloy and filling the gap quickly by observing the brazing process.

(4)The dense brazing seam would not produce any loose leakage phenomenon. Any residue should not be resided on the surface and the brazing area should be clean and bright without any clean-up procedure.

(5)The brazing process would not produce fluoride, be beneficial to the health of the operators.

(6)The gas flux and brazing equipment was used during brazing and the flux went into the flame in the form of gas, achieving the automation production.

(7)The corrosion protective layer formed at the surface of brazing seam can keep the brazing seam bright and smooth, no glass slag and oxidation residing, decreasing the hours and costs of clean-up processes.

6  Conclusions

(1)The following three methods are the key factors to ensure the joint strength: proper brazing clearance, right flame heating process and correct methods of adding brazing alloy.

(2)The brazing process should keep the joint heat uniformly to obtain an ideal capillary effect, which could make the brazing alloy reach the roots of inner parts.

(3)The gas flux used in brazing could protect the joint being corroded and oxidized during brazing. Meanwhile, they can also improve the production efficiency and save the cost.

TypeInfo: Brazing Knowledge

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