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Separate heat pipe radiator vacuum brazing

The separated heat pipe radiator is a new type of heat exchange equipment evolved from the heat pipe radiator, which can be installed on the flue, gas pipe and combustion air pipe of the hot blast stove respectively.

Advantages of separate heat pipe radiators

The separated heat pipe radiator consists of two relatively independent parts, and each part can be conveniently installed on the pipes that need to absorb and release heat, thus avoiding major changes to the pipe system. In a high-power waste heat recovery system, it is particularly convenient to apply a separate radiator. The radiator consists of two basically independent radiators, so the heat transfer area of each radiator can be changed according to the needs, and the tube bundle size, tube specification, arrangement, etc. can be independently selected.

The hot and cold fluids in the radiator are completely isolated, and the two fluids will not leak or mix with each other, avoiding possible safety accidents in the heat exchange process of flammable and explosive fluids. The infusion and exhaust process of the medium in the pipe can be carried out on site. During operation, the non-condensable gas generated is easy to discharge, and if the performance of the heat pipe is found to be degraded, it can be maintained on site.

The separated heat pipe radiator can transmit heat over a long distance, and the evaporator and condenser can be tens of meters apart and still work normally. The heat obtained from one hot fluid can be used to heat two different cold fluids, and conversely, the heat obtained from two hot fluids can be used to heat one cold fluid. In the same radiator, forward flow and reverse flow can be realized at the same time. The inlet tube bundle of the high-temperature fluid is connected with the inlet tube bundle of the low-temperature fluid to form a downstream heat exchange of the cold and hot fluids, while others can still retain the counter-current heat exchange. This unique arrangement can reduce the temperature of the steam in the tube bundle of the high-temperature fluid inlet tube bundle, keep the medium in the tube under the allowable temperature and pressure, and can also increase the tube temperature of the low-temperature fluid inlet tube bundle to avoid the adverse effects of too low temperature.

Radiator welding difficulties

The fins, headers and heat exchange tubes are all made of red copper, and the connections between the fins and the heat exchange tubes, and between the heat exchange tubes and the bottom plates of the upper and lower headers are welded. The structure of the heat pipe radiator is complex, the welds are densely distributed, and the welding volume is large. Most of them are thin-plate thin-walled pipes. The welding method or specification parameters are improper, and the deformation after welding is large. Among them, the post-welding deformation of thin fins not only affects the relevant structural dimensions, but also affects the heat transfer performance of the heat pipe.

In order to meet the use requirements of the separated heat pipe radiator, not only the welding between the fins and the tubes is required to be firm after welding, but also to meet the long-term work under vibration conditions, and the service life is not less than 3000h. According to the above characteristics, it is decided to adopt vacuum brazing for the heat pipe radiator, and the welding of the main brazing seam can be completed at one time.

Vacuum brazing process:

1.Surface preparation of weldment

All parts of the heat pipe radiator, including fins, heat exchange tubes and upper and lower header bottom plates, must be cleaned of surface oil, removed surface oxide film, neutralized gloss treatment and dried before vacuum brazing.

(1) Surface oil removal After soaking in acetone organic solvent for 7 minutes, wipe off the oil with clean cotton gauze, and then clean the treatment solution on the weldment with clean water.

(2) Remove surface oxide film Put the weldment in 12% HCl aqueous solution for 8 minutes, and then rinse the residual acid on the weldment with warm water at 60°C.

(3) Neutralize gloss treatment Put the weldment in 10% Na2CO3 aqueous solution and soak for 6 minutes, then rinse the residual liquid on the weldment with warm water at 60°C.

(4) Drying Put the weldment after neutralizing gloss treatment into an oven at 110°C to dry the moisture on the surface of the weldment.

The weldment after surface treatment should be stored and transported in a clean and dry closed container. The storage time should not exceed 24 hours. Vacuum brazing should be completed as soon as possible to reduce the possibility of re-contamination and oxidation of the weldment.

2.Furnace loading

Vacuum brazing was performed using a high temperature brazing furnace manufactured by SIMUWU. First put the stainless steel plate into the vacuum chamber, then put the high-temperature ceramic gasket on the stainless steel plate, and finally place the heat pipe radiator with complete assembly and brazing filler metal on the gasket horizontally. First start the mechanical vacuum pump of the vacuum furnace equipped with the radiator, and turn on the diffusion pump after the system reaches about 5Pa. When the vacuum degree is pumped to 150 MPa, start to heat up. The process curve is shown in the figure. After vacuum brazing, in order to speed up the cooling speed of the vacuum furnace, Ar gas can be introduced into the vacuum furnace. After the temperature of the vacuum chamber is lower than 60°C, the heat pipe radiator comes out of the furnace.

In conclusion

(1) For heat pipe radiators with complex structures, dense welds, large number of welds, and high welding deformation requirements, vacuum brazing is used to complete the main welds at one time, which can not only ensure the welding quality, but also greatly save working hours. Has greater economic benefits.

(2) In order to further reduce the deformation of the fins during vacuum brazing, a certain amount of lining blocks can be added to the peripheral position between the fins before vacuum brazing, and the lining blocks can be taken out after brazing.

In addition to adopting the correct technical process, the quality of the vacuum brazing furnace selected in vacuum brazing is also a crucial factor. The RHVB vacuum high-temperature brazing furnace produced by SIMUWU has the advantages of good temperature control accuracy and high heating uniformity, and is an excellent product for handling high-temperature brazing processes. Under the premise of selecting appropriate process parameters, high-quality vacuum brazing can be realized.