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Product Bulletin - Controlled Atmosphere Brazing Furnace Systems
ACCUBRAZE® Control Systems
Convection heating with accurate temperature control and uniformity are essential when running loads with varying masses and configurations one after another. The braze furnace is controlled by a PC/PLC based system utilizing ACCUBRAZE software developed by the SECO/WARWICK.
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| ACCUBRAZE Operator Screen for an Active Only® Semi-continuous CAB Furnace |
How It Works
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 set point. Another thermocouple is placed directly inside the muffle, in the nitrogen air stream between the muffle and recirculation baffle. This thermocouple provides set point 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 bottom thermocouple senses the temperature of the atmosphere being heated from the surface area of the muffle walls. The top 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 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).
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 indexed, the fan will run at a higher speed, distributing more heat to the load by recirculating the atmosphere through the load area. As the temperature difference sensed by the two thermocouples decreases, the fan will begin to slow until both thermocouples are at uniform temperature. At this point, the fan will continue to run, but at a minimal speed.
Results
The ACCUBRAZE computer control system ensures that the load area is heated in the shortest possible time and the temperature is constant throughout the load. The key result is that with a fixed soak time, the resulting total cycle time is adjusted according to the time required for a load to reach and become uniform at the set point 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".
In one experiment, the temperature profile for cores run through an Active Only furnace demonstrated that the convection heated system heated all parts of the cores at more uniform rates than by radiation heating. Both the fins and the headers may be heated to the same temperature by convection heating. Therefore, all parts of the core may be brazed using the same alloy, i.e. AA4343. This is different from continuous radiation brazing applications.
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. 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 an extremely fast response time 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.
R1, 2/2005
P. O. Box 908 180 Mercer Street Meadville, PA 16335-6908 USA
Phone (814)332-8400 - Fax (814)724-1407
info@secowarwick.com
©1995-2009 SECO/WARWICK Corporation All Rights Reserved.
The latest design, materials and equipment specifications should be obtained from the company before any reliance is placed on the enclosed since changes may occur due to product improvement.