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Plate Heat Exchanger Vacuum Brazing

The plate heat exchanger is a high-efficiency heat exchanger composed of a series of metal sheets with a certain corrugated shape. Thin rectangular channels are formed between various plates, and heat is exchanged through the plates. Plate heat exchanger is an ideal equipment for liquid-liquid and liquid-vapor heat exchange. It has the characteristics of high heat exchange efficiency, small heat loss, compact and lightweight structure, small footprint, wide application and long service life. Under the same pressure loss, the heat transfer coefficient is 3-5 times higher than that of the tubular heat exchanger, the floor area is one third of that of the tubular heat exchanger, and the heat recovery rate can be as high as 90% or more.

Plate heat exchangers are used in the industry, and they have increasingly shown their excellent performance, and have been widely promoted and applied, becoming one of the compact and efficient heat exchange equipment. With the gradual improvement of the requirements for the density and working environment of heat exchangers in modern industrial production, higher and higher requirements are put forward for heat exchanger materials and their connection methods.

The types of plate heat exchangers mainly include frame type (removable type) and brazing type. Among them, the welded type has advantages in performance, its surface is smooth and not easy to scale, and the heat transfer efficiency is high. The body material is welded by brazing. It does not use any non-metallic sealing material, so it has high temperature resistance and pressure resistance. It can be applied to extreme working conditions with a working temperature of 300°C and a pressure of 3.0Mpa.

The process flow of plate heat exchanger brazing is: shearing plate→forming→surface pretreatment→assembly→vacuum brazing→welding inspection→finishing.

Specification material

Plate: 316 stainless steel, plate size is 389mm × 270 mm; solder: 99% pure copper solder, sheet (δ=0.3mm), brazing method: high vacuum brazing furnace. The vacuum degree is 1*10-2Pa, and the brazing temperature is 1130℃.

Selection of brazing temperature

For stainless steel, the brazing heating temperature should not be too high. When the brazing temperature is higher than 1150℃, the strength and heat treatment performance of stainless steel will be affected, and when it stays at 500~750℃, chromium carbide will precipitate along the grain boundary, which is prone to intergranular corrosion. When selecting solder and process parameters, prolonged heating above 1150°C should be avoided. Therefore, the brazing temperature was set at 1130°C.

Selection of vacuum solder

Vacuum brazing requires long-term maintenance of high vacuum, so the brazing filler metal with low vapor pressure should be selected. In the process of device manufacturing, the device is subjected to long-term high temperature (above 500 ℃) baking exhaust, and the brazed joints made of dissimilar materials are prone to large thermal stress. Therefore, the brazed joints are required to have sufficient mechanical strength and the ability to release stress.

Copper-based solder has low vapor pressure, good high temperature strength, good wettability to various metals, and moderate price, so it should be the first choice. It is worth noting that the pure copper solder must be made of oxygen-free copper, otherwise it is difficult to ensure the air tightness of the brazing seam. When pure copper is used as solder, it needs to be brazed in a protective atmosphere and vacuum, and the temperature is 1100~1150℃. At this temperature, the internal stress of the device has been eliminated, and because the device is heated as a whole, the thermal stress is small, and the weldment will not crack.

Copper-based solders are available in powder, filament and flake form. Among them, powdered solder has a large surface area, many surface oxides, and poor wettability during brazing, so it is not suitable for use. The sheet brazing filler metal can be well wetted in the molten state, and can ensure a certain flatness and density. Therefore, 99% pure copper, flake (δ=0.3mm) is selected as the solder.

Flux is generally not used during vacuum brazing. When the brazed joint is cooled with the furnace, argon is used as a protective gas at an appropriate time.

Preparation before brazing:

Surface preparation of stainless steel

Before brazing, the oxides and oil stains on the brazing metal surface should be pickled with hot liquid, the temperature is 20℃, and the pickling time is 10min. After pickling, carefully wash with hot water at 60~70℃ for 10min, and then dry in hot air at 60~70℃. Pickling should be carried out in strict accordance with the process regulations to avoid excessive corrosion.

Preparation of solder, die and fixtures

Cut the brazing filler metal into thin strips, wash with acetone, and air dry for later use. During vacuum brazing, in order to ensure the size and accuracy of the brazed components, special clamps are used to clamp the stainless steel and the brazing filler metal, which is also beneficial for the molten brazing filler metal to wet and fully spread to the surface of the stainless steel base metal.

Before starting the process, the fixture is subjected to a simulated brazing thermal cycle to ensure dimensional stability and stress relief.

Brazing process

During vacuum brazing, the heating temperature, holding time and vacuum degree of the workpiece are important factors affecting the quality of the brazed joint. Therefore, the brazing process is mainly to control the brazing temperature, holding time and heating and cooling speed. The brazing temperature is generally 25~60°C higher than the liquidus temperature of the solder. Selecting a higher brazing temperature can accelerate the interaction between the solder and the metal. However, when the temperature is too high, it is easy to produce brazing filler metal sputtering and corrosion of the base metal. When using copper brazing filler metal for brazing, argon gas is injected into the furnace when cooling with the furnace. The process flow is as follows:

As shown in the figure: (1) Before AO is evacuated. (2) OB, CD, EF, GH are heating time periods. (3) BC, DE, FG, HI incubation time periods. (4) During the cooling period of IJ, argon is added after the heat preservation at 1100°C. At this time, the vacuum degree is (7.8~8.0) × 10~2Pa, and argon is stopped at 1000°C.

The products under this process have passed the appraisal of Jinan Energy Technology Service Center and are excellent manufacturing processes for plate heat exchangers.

In addition to using the correct technical process, the quality of the brazing furnace selected for 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 processing high temperature brazing process. On the premise of selecting appropriate process parameters, high-quality vacuum brazing can be achieved.