Production Flexibility Increased using an "Intelligent" ACTIVE Only®, Semi-Continuous CAB Brazing Furnace

The brazing requirements of aluminum automotive heat exchangers vary widely between large OEM manufacturers, after-market manufacturers, and small radiator shops. The requirements are driven by differences in products and production levels among these three entities: large runs of specific product for the OEM, mid-size production runs of variety of product for the after-market suppliers, and short runs of a wide variety of product for the radiator shops.

Controlled atmosphere aluminum brazing is usually associated with high production rates and with the large, continuous furnaces of the OEM manufacturer. Clearly, a different type of aluminum brazing furnace is required for smaller heat exchanger manufacturers who need to braze a wide variety of product in short-to mid-range runs, while maintaining OEM quality.

For this purpose, a semi-continuous controlled atmosphere aluminum brazing furnace (ACTIVE Only) has been designed by SECO/WARWICK with the flexibility that enables a growing manufacturer to accommodate small run, special-order needs while assuring OEM-quality results.

CAB Aluminum Brazing Process

A typical aluminum heat exchanger may be manufactured using a base aluminum alloy such as AA 3003 with a cladding of AA 4343, 4045, or 4047. While the base alloy melts at about 630 degrees C(1166 degrees F), the clad material can melt between 577 degrees C and 613 degrees C (1070 degrees F and 1135 degrees F). Therefore, the ideal furnace temperature is somewhere in the middle of the melting range of the cladding material.

A basic non-corrosive flux composition such as potassium fluoro aluminate, KALF₄, must be used to break down the oxide layer of the aluminum and provide a surface for capillary action to draw the molten cladding into the joints. A controlled atmosphere that has an oxygen content of less than 100 ppm and a dewpoint of less than -40 degrees C(-40 degrees F) must be provided. These atmosphere conditions are accomplished through the use of nitrogen which is readily available.

Once a part reaches the set point temperature in the brazing chamber, successful brazing requires only that the parts be held uniformly at the required brazing temperature for the required soak time as determined by the melting requirements of the clad materials. In practical terms, the benefit of using an ACTIVE Only semi-continuous CAB Furnace is that a load of radiators could follow a load of charge air coolers, followed by a load of double-layered condensers followed by a load of heater cores and the quality of the brazed products would remain the same from load area to load area. The furnace setpoints would not have to be adjusted for the different masses being conveyed through the furnace. The "intelligent" control system would ensur the load area meets the temperature requirements for brazing.

Furnace Description

The semi-continuous controlled atmosphere aluminum brazing furnace is an indexing furnace with an alloy mesh belt conveyor system (Fig. 1). A typical furnace load area can measure 915mm x 1830mm x 203mm  (3 feet W x 6 feet L x 8 inches H).

Custom designed load area have been provided to meet particular customer requirements. One or several heat exchangers can be loaded in this area for processing and brazing.

Cores may be placed in single layers or may be stacked in multiple layers to increase production rates. Once loaded, the furnace automatically indexes the load into an inline dry off oven to remove any water that may be present from a wet-type fluxing process.

The dry off oven also serves as a thermal degreaser to vaporize (depending on oxidation temperature) any lubricating oils that can be present from previous forming operations.

Flux Application

Electrostatic fluxing (dry fluxing, Fig. 2a) is a viable alternative to wet fluxing (Fig. 2b).  Electrostatic fluxing eliminates the need of a dry off oven from the complete system as water is t present from the fluxing operation.

Fig. 2a Manual electrostatic fluxer	Fig. 2b Wet Fluxer

The furnace mesh belt is indexed at a time interval which is based on data feedback from the control system of the furnace. The index cycle time is defined as the amount of time required for the load area to reach a preset soak temperature set point in the braze chamber plus the desired soak time at that temperature.

The concept utilizes a patented convection muffle design, which gives excellent temperature uniformity throughout the load area.

After drying/thermal degreasing, the load is conveyed into an insulated entrance purge chamber prior to the brazing chamber. The purge chamber maintains the load temperature and removes oxygen from the chamber by purging it with nitrogen. This ensures that the atmosphere in the braze zone atmosphere will not be contaminated with oxygen once this load is indexed.

Once purged, the load is indexed into the brazing chamber, which again utilizes the nitrogen atmosphere. The load area will then be heated to a preset brazing temperature determined by the alloys being used. When the brazing temperature is achieved and uniform throughout the load a preset soak time has elapsed, the load is then indexed into a water jacketed cooling / purging chamber to solidify the braze joints. The load is then indexed from the water-jacketed cooling chamber into an air blast station to cool the load for handling.

A production rate of three to five loads per hour can be achieved depending on the mass of the load area.Furnace Control SystemConvection heating with accurate temperature control and uniformity are essential when running loads with varying masses and configurations one after another.

ACCUBRAZE Control System

Fig. 3 ACCUBRAZE Control Screen

The ACTIVE Only braze furnace is controlled by a PC/PLC based system utilizing ACCUBRAZE© software developed by the SECO/WARWICK (Fig. 3).The brazing furnace utilizes four fixed thermocouples.

• One is located outside the muffle for over temperature control, strictly a safety measure for preventing the heating chamber from exceeding a preset temperature setpoint.
• Another thermocouple is placed directly inside the muffle, in the nitrogen air stream between the muffle and recirculation baffle. This thermocouple provides setpoint temperature control data for the ACCUBRAZE computer control system.
• Two additional fixed thermocouples, one directly above the load and one beneath the load, read the actual temperatures near the load area. When a load is indexed into the brazing chamber, the top thermocouple senses the temperature of the atmosphere being heated from the surface area of the muffle walls.
• The bottom thermocouple senses the temperature of the recirculated atmosphere after passing through the cold load. When a load is first indexed into the braze chamber, the bottom thermocouple can record a temperature much lower than the top thermocouple depending on the mass of the load (e.g., the higher the load mass, the greater the temperature deviation).

PID Control Loop

Top-to-bottom temperature uniformity is dramatically improved by the development of a PID control loop that controls the fan speed by actively sensing the temperature difference between the top and bottom thermocouples and adjusting the fan speed accordingly. This uniformity is especially crucial when the load reaches the melting temperature of the clad material. When a load is initially charged, the fan will run at a higher speed, distributing more heat to the bottom of the load by recirculating the atmospherefrom the top of the load area to the bottom. As the temperature difference sensed by the two thermocouples decreases, the fan will begin to slow until there is no temperature difference between the thermocouples. At this point, the fan will continue to run, but at a minimal speed.

Heating the Load in the Shortest Possible Time

The ACCUBRAZE computer control system ensures that the load area is heated in the shortest possible time and that the temperature is constant throughout the load. The key result is that with the soak time fixed, the resulting total cycle time is adjusted according to the time required for a load to reach and become uniform at the setpoint temperature. Thus, the mass of back-to-back loads can vary while the same results will be achieved. Providing the products are of the same alloy from load area to load area, operator intervention is not required once the automatic cycle has begun, thus the denotation of the "intelligent furnace system".

Addressing Uniformity

Experiments and temperature profiles have proven that the convection heated system will heat all parts of the cores at more uniform rates than by radiation heating. Both thefins and the headers may be heated to the same temperature at the same time by convection heating. Therefore, all parts of the core may be brazed using the same alloy, i.e. AA4343 or AA4045 cladding. This is different from continuous radiation brazing applications.

To accommodate for the fact that radiation heating is not as uniform as convection heating when considering thick header material, the header material is typically brazed using a lower melting temperature material such as AA4045, while the fin is brazed with a higher melting temperature material such as AA4343.The brazing results described above are not only attributed to the control system, but also are intrinsic to the design of the braze chamber itself (see Fig. 4).

The brazing chamber is insulated with lightweight ceramic fiber insulation and is heated with high velocity natural gas burners. This combination of low heat storage insulation with maximum heating potential gas burners allows extremely fast response times for heating or cooling. The recirculating fan provides convection in the furnace atmosphere for fast heating and uniformity distribution of temperature. Since the patented convection muffle design is low in volume, atmosphere integrity (less than 50 ppm oxygen in most cases) is maintained with low nitrogen consumption.

Other considerations were made to improve part quality and appearance. High velocity nitrogen in a furnace is known to create a matte or dull looking part. This furnace system overcomes this problem and achieves a brighter, cleaner looking part by reducing fan speed to as low as 90 rpm when the filler material is molten.

CONCLUSION

By monitoring the temperature differential through the load and adjusting the fan speed (i.e., heating rate) and cycle time accordingly, the semi-continuous controlled atmosphere brazing furnace system has the ability to braze parts of various dimensions or loaded in different configurations successfully under a single set of parameters. Part-to-part temperatures can be maintained at the recommended temperature range of  600 degrees C-620 degrees C(1112 degrees F and 1150 degrees F) for three to five minutes, with a temperature uniformity of plus or minus 5 degrees F.

R1 12/7/2004 Updated source code
R2 12/12/2005 Removed author reference, updated source code, added photograph in lead paragraph
R3 1/09/2008 Updated source code, format
Feature Article, Vol. 59, Issue 2 December 2001, updated 1/9/2008

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