This chapter of the free Fluxtrol induction heating course walks through case story – induction brazing of aluminum heat exchanger in 9 slides. View the slides below, or skim the outline and slide text.
Chapter outline
- Problem Description
- System Geometry Description
- Step 1: Simulation of Existing Process
- Step 2: Development of Induction
- Step 3: Optimal Coil and Process Design
- Final Results
Slide text
Problem Description
- Part: Aluminum heat exchanger Operation: Brazing of a pipe to a short tube previously brazed to the heat exchanger header in a furnace Equipment: All equipment including 60kW,10-30kHz powersupply, part handling system and control system with IR pyrometers already existed Problems:
- Insufficient brazed joint depth
- Inconsistent process resulting in leakage and other defects Variables:
- Limited coil type modification
- Coil dimensions
- Coil positioning
- Magnetic controller dimensions Connection block Pipe coil with magnetic controller joint Tube Header Body Experimental brazing in Fluxtrol laboratory
System Geometry Description
- Existing inductors had a “horseshoe hairpin” shape for simple part loading. The coils consisted of two hair-pin sections connected in series with diagonal cross -over.
- Geometry is clearly 3D with multiple components (pipe, tube, header, coil copper, magnetic controllers). Due to planes of symmetry it was possible to simulate only ¼ of system.
- Non -symmetry due crossovers was neglected.
- t o Geometry prepared for simulation using Flux 3D program
Step 1: Simulation of Existing Process
- On a base of process analysis it was assumed that the main noncontrolled variable was an electrical contact between pipe and tube.
- These components are preliminarily coated with flux and electrical contact between them is unstable.
- When the joint is filled with molten filler metal, contact is good.
- Computer simulation confirmed this theory. With no electrical contact there i s a strong heat concentration on the tube edge closest to the coil. Overheating of this area is a common defect of brazing.
- With good contact current flows from tube to pipe resulting in heat pattern change. It is not possible to balance these two patterns achieving stable quality.
- Coil currents direction and magnetic generic field lines for horseshoe coil with diagonal crossover Current density distribution in joint area for diagonal crossover.
- Left – no electrical contact Right – good electrical contact
Step 2: Development of Induction
- Coil with Horizontal Crossover For a coil with horizontal crossover currents flow mainly inside of each component for good and no contact conditions.
- However, computer simulation showed that heating of pipe and tube is very weak compared to the heat exchanger header.
- Small heating of header is useful for support of temperature profile in the tube, but main power must be delivered to pipe and tube in a correct proportion.
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- Coil currents direction and magnetic generic field lines for horseshoe coil with horizontal crossover Current density distribution in joint area for horizontal crossover.
- Left – no electrical contact Right – good electrical contact
Step 3: Optimal Coil and Process Design
- Additional magnetic controller was placed between the coil bottom and header surface. Variation of magnetic flux controllers made of Fluxtrol “A” material allowed the process to achieve optimal power distribution between all three heat exchanger components.
- Laboratory tests confirmed stable process flow and good quality of brazed parts.
- Final coil design for one of the brazing joints Additional consideration:
- It was noticed that change of crossover resulted in much stronger electrodynamic forces.
- Coil tends to “open” when power is turned ON.To reduce effect of electrodynamic forces, an additional fiberglass connector was installed on the coil.
- Optimized power distribution in brazing joint components
Final Results
- Computer simulation helped to understand the factors causing brazing inconsistency and optimized coil design
- Combination of coil copper optimization and proper geometry of magnetic flux controllers solved a problem of brazing quality. There were no more part rejects caused by improper induction heating
- It was found that a variety of products may be brazed by the coils with the same copper by adjusting geometry of magnetic flux controllers
- Brazing cycle reduced by 15-30% when using new coils