CAB  Maintenance Tips

Nitrogen Adjustment Procedure for the Active Only Aluminum Brazing Furnace

In response to our reader's request for more information on managing the nitrogen flow in furnace, the following article presents a method for making the necessary adjustments for optimum performance.  The trade name Active Only® describes an indexing furnace designed to meet the needs of aftermarket manufactures or companies that have a wide variety of production needs from day to day. The furnace is designed to braze 3 to 5 load areas per hour on a semi-continuous basis.

The nitrogen flow into the Active Only aluminum brazing furnace must be sufficient to accomplish the following:

• Maintain less than 50 ppm of oxygen in the braze chamber.
• Provide flow towards the entry door to purge oxygen from newly charged cores.
• Make up for the atmosphere collapse as it is cooled in the cooling zones.
• Prevent oxygen infiltration into the cooling chamber through the exit door.

Adjustments

Adjustments are made with the furnace operating at temperature with a production recipe. No product is required for initial adjustments.  The soak time may be increased to make the cycle times close to what will be seen when the furnace is loaded with product.

The dampers on the scrubber exhaust ducts have a significant effect on the atmosphere in the furnace. The starting position of the dampers above the entry and exit doors should both be 10 degrees open.  If the damper on the entry door hood is open more than the rear damper, the atmosphere in the furnace will tend to travel towards the entry door, and vice versa. Experience has shown that these dampers should be open only as much as needed to pull the furnace gasses to the scrubber.  Blowing some smoke past the door openings can verify this adjustment. The smoke should gently roll under the opening into the scrubber pipe.

The nitrogen that flows through each of the inlets should be adjusted in the following sequence. Keep in mind that it may take a few trials to achieve the best combination of flows into each section.

• Drafts in the factory can cause problems with atmosphere quality in furnaces. If a draft problem is suspected, test the area by sprinkling a small amount of fine powder such as baby powder into the air. Any excessive drafts should be eliminated before proceeding with the nitrogen adjustment.
• Initial nitrogen flows to entry chamber, braze chamber, and cooling chamber can be set the same.  The braze chamber flows should then be increased until the oxygen level remains below 50 ppm oxygen.  After the braze chamber is stable with an atmosphere of less than 50 ppm oxygen, the cooling chamber should be adjusted. The atmosphere in the cooling chamber should then be monitored, and the nitrogen flow increased to eliminate post index oxygen spikes.
• To adjust the nitrogen flow to the entry chamber, test product should be cycled into the furnace.  The entry chamber nitrogen flow can then be slowly reduced until the there is an increase in the braze chamber oxygen level. The entry chamber nitrogen flow should then be increased to bring the braze chamber oxygen back down to less than 50 ppm.
• The exiting parts should have acceptable braze joints and color. If the braze joints are good, but the color is poor, the nitrogen flow to the cooling chamber should be increased and the product observed for improvement. Unless sufficient nitrogen volume is present, as the hot parts and atmosphere are indexed into the cooling chamber, the rapidly cooling atmosphere will contract and pull in air. Running the cooling chamber at a higher temperature can reduce this volume of nitrogen needed to minimize this problem. Many customers run their water jacket at a temperature of 120 degrees F.
• After the above adjustments have been made, the nitrogen flow to the braze chamber should be adjusted. Decrease the flow to the braze chamber slowly until the quality of the product begins to become objectionable. The flow should then be increased until the product quality improves to an acceptable level. This will be the minimum nitrogen flow for the braze chamber. The oxygen level that the braze chamber has reached for acceptable brazing should be noted, and the complete procedure should be repeated for this target oxygen reading in the braze chamber.

After the nitrogen flows have been fine tuned, the flow meter readings and exhaust damper positions should be recorded for future reference.

For more information on CAB Brazing, check out http://www.secowarwick.com/aluminumbrazing.html

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.