Controlled atmospheres increase furnace-brazing productivity.
We specialize in designing and manufacturing electronic and electromechanical control devices that regulate temperature, pressure, time sequencing, current, fluid flow, and humidity for industrial, home, and automotive heating and cooling systems. Brazed assemblies are used in the majority of our products.
Reversing valves used in heat-pump systems, for example, have four external connecting tubes and one internal seat joined to the assembly by brazing. This is a critical operation; all joints must be hermetically sealed.
The assemblies are brazed in three Sergeant & Wilbur belt-type furnaces. Another S&W furnace at the Plain City plant is used for brazing beryllium-copper bellows assemblies and other copper subassemblies. Our Delaware plant uses a fifth S&W unit for brazing and annealing in a controlled nitrogen atmosphere.
Brazing is time and temperature dependent. Brazing-zone temperatures and conveyor speeds are set as high as possible to maximize furnace output. For reversing valves with brass and copper components, brazing-zone temperature is set just below the melting point of brass (1540 F). Brazing is accomplished with fluxless filler metals (low phosphorous/ silver alloys), usually in paste form, with melting temperatures between 1300 F to 1500 F.
Until two years ago we generated nitrogen /hydrogen furnace atmospheres by burning ammonia. Each brazing furnace had its own gas generator located in a building outside the plant.
Brazing was done in a nitroneal (burned ammonia) atmosphere (75 to 77 percent nitrogen, the balance hydrogen), which was blended with air and dried in molecular sieves to reduce the dew point. The furnace atmosphere was maintained at a slightly higher pressure than the outside atmosphere.
Unreliable performance of the gas generators placed a severe constraint on brazing-furnace productivity. The generators required 2 hr to 3 hr of daily maintenance.
Further, excessive moisture in the furnace atmospheres had an extremely adverse impact on brazed joint quality, i.e., it oxidized the metal surfaces. We aimed for a moisture content below 420 ppm (dew point -20 F), but were lucky to get it below 1260 ppm (dew point OF). The high moisture level necessitated rerunning many parts. Moreover, uncertain joint quality, caused by varying gasgenerator performance, dictated 100 percent testing of the reversing-valve assemblies immediately after brazing.
A better way
Furnace downtime and reruns limited net production of the valves. When forecasts showed demand would soon exceed capacity, we immediately studied ways to increase output.
We decided to rebuild the line and increase capacity by acquiring a new, larger conveyorized brazing furnace. Rather than buying another nitroneal generator and molecular sieves for this furnace, we elected to eliminate atmosphere generators for the entire operation.
The furnace-atmosphere gas would be produced from a bulk supply of vaporized liquid nitrogen and from hydrogen (derived from cracked ammonia at one dissociator). The gasses would be delivered through a central-distribution system and blended at each furnace through a flow-control panel.
This approach obviated a $135,000 capital investment for new atmosphere-generating equipment. And it eliminated the troublesome existing generators, while saving floor space.
Before making the conversion, we had to be sure the concept worked in our operation. Testing was conducted in our Delaware brazing furnace in collaboration with Airco Industrial Gases (division of The BOC Group), Murray Hill, NJ, which supplied bulk nitrogen and hydrogen from tank trailers. The two gasses were blended manually be adjusting the flow rates to maintain correct proportions.
Nitrogen doesn't react with copper and brass at brazing temperatures; its primary function is preventing oxidation. On the other hand, hydrogen reacts with oxides on a metallic surface, reducing them to produced the metallurgically clean area required for high-integrity joints (molten brazing alloys can flow smoothly over the clean base metal).
The tests were a success. They demonstrated it was both practical and economical to use pure nitrogen/hydrogen mixtures. We decided to convert.
We've been using the nitrogen-based controlled atmospheres at both plants for nearly two years, with no problems. Furnace downtime due to generator problems and varying product quality caused by suboptimal control over the atmospheres are now history. Because our furnaces continuously operate during working hours--and there's no need to rerun parts--operation productivity has improved greatly. For the reversing valves, daily brazing furnace output significantly increased, with additional production capacity remaining. This was attained without any changes in brazing alloys, brazing temperatures, or conveyor speeds.
In our present setup, nitrogen is delivered as a liquid and stored in insulated tanks outside the plants. The liquid is automatically vaporized as needed, and the gas is piped to each furnace.
To avoid storing large quantities of hydrogen, we generate the gas as needed from ammonia dissociators at the Plain City and Delaware Facilities. The dissociators perform reliably; brazing-furnace downtime is rare.
For close control over quality and added safety, we specified an automatic gas-flow control system, which was designed and built by Airco. The first time a job runs, optimum nitrogen/hydrogen ratios are determined. Nitrogen and hydrogen flow rates are then set on the system's control panel. Correct ratios are maintained automatically through solenoid-actuated valves.
Different nitrogen/hydrogen ratios are used in the entry, brazing, and exit zones of the furnaces. In the brazing zone, an optimum nitrogen/hydrogen ratio (88/12) is maintained; in the entry and exit zones, pure nitrogen is introduced, which acts as a barrier against outside air.
A number of safety features are built into the system. For instance, if furnace temperature falls below the combustion temperature of hydrogen, this gas is cut off and nitrogen flow is increased to purge all traces of hydrogen. This eliminates the possibility of an explosion. Moreover, if we lose nitrogen pressure or flow, pressure-actuated switches immediately cut off the hydrogen.
Controlling moisture in our furnace atmospheres is no longer a problem. In the morning, the moisture content reaches 16 ppm to 17 ppm (dew point -70 F). Since some moisture is created as the hydrogen scavenges oxides, moisture level plateaus in the afternoon at around 240 ppm (dew point -30 F). This is well below the concentration that causes brazing defects.
We maintain a lower flow of nitrogen in the furnace during idling periods. The nitrogen, which is spiked with a small amount of hydrogen (10 cfh) to scavenge oxygen, precludes moisture buildup.
Maintaining a controlled atmosphere during idling also helps improve furnace productivity. It now takes 30 min to get a furnace into equilibrium when changing from an idling mode to a production mode. If a furnace was wet, it would take several hours to get it into equilibrium, thereby costing valuable production time.
Life is now easier for furnace operators as well. They no longer experience the frustration of production delays while gas generators are maintained. Further, they no longer must cope with quality problems caused by poorly controlled furnace atmospheres.
Currently, we're looking at ways to make the furnaces even more productive. Because of the purity and low dew point of nitrogen-based atmospheres, we've obtained greater reducing power than generated atmospheres. Now it's possible to increase furnace belt speeds, thereby increasing output. Closer control over furnace atmospheres gives us the ability to perform improved annealing and stress-relieving operations in the furnaces, which was difficult with generated atmospheres.
While the improved productivity of our furnaces enables us to presently keep up with the growing demand for heatpump controls and other products, we know that additional units must be installed eventually. The fact that we won't have to purchase a gas generator for each new furnace will reduce our capital requirements and shorten the payback period. We'll be buying predictable production capacity, not problems.
Photo: Bright and clean. These heat-pump reversing-valve assemblies are ready for furnace brazing in a controlled nitrogen/hydrogen atmosphere that is free of moisture and other contaminants. Also, the clean, uncluttered working environment helps increase efficiency and maintain quality.
Photo: Touch control. Valve assemblies are loaded onto a brazing-furnace conveyor manually. The operator carefully checks each assembly, making sure fit-up is correct.
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|Publication:||Tooling & Production|
|Date:||Sep 1, 1984|
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