Automatic Brazing Provides High-Quality Component Assembly
Air-conditioning core brazing automation is not new, but new specifications requiring higher quality, low leak rate assemblies demand more from the equipment. In the past, large, complicated and expensive brazing systems were able to meet the requirements of specifications existing at the time. However, tightened industry specifications and new refrigerants demand higher quality heat processing in air conditioner (AC) core-brazing automation. In spite of tighter specifications, readily available components have significantly lowered the cost and increased the quality of automated brazing, opening up its potential use in more applications.
Tube and fin heat exchangers used in residential air conditioners are composed of a series of long "U"-shaped copper (sometimes aluminum) tubes inserted into a densely arranged pack of aluminum fins. The formed tubes, called hairpins, are then circuited (joined) to each other at the top by a brazed-in-place return bend (Fig. 1). At low production volumes, the tubes are brazed manually. However, there is cost justification to automate the process when the production rates become high enough.
The tube-end design consists of a flare and bell for self-fixturing. The flare must be as large as the circumference of a preform braze-alloy ring at its thickest side dimension (the total diameter minus one half of the thickness of the preform on each side; in this case 0.070 in. (~2 mm). If the flare is too wide, the tube might tear, creating a path for the brazing alloy to flow out. On the other hand, a flare that is too narrow will cause the alloy to wick over the outsides of the tube, missing the joint.
Past practice not adequate today
In the past, brazing systems did not need to be as accurate in the way the core was heated. For example, earlier core manufacturing equipment did not need to control tube height and uniformity as carefully, and limiting the heat from the brazing operation from traveling away from the braze joint down the tube towards the fin pack was not a priority.
However, today's new refrigerants run at higher pressures than the R-22 type refrigerant used in the past, and these higher pressures require faster, more localized heating in the area around the joint to reduce unwanted annealing that can take place below the joint. Annealing of the tube below the joint can cause the tubing to burst under the new refrigerant's higher pressure conditions. Two other factors that complicate the bursting problem are the thinner wall tubing used as a cost-saving measure in the industry and the knurling enhancements made on the inside surface of the tubing to increase thermal efficiency.
Maintaining a system that brazes at a minimum uniform temperature is critical in today's autobrazing. Depending on the exact chemical composition of the brazing materials, the typical broad brazing temperature ranges for aluminum is 800 to 950F (430 to 510C) and 1375 to 1450F (745 to 790C) for copper. The brazing temperature spread for a given operation is I20F (I10C) for aluminum and I60F (I35C) for copper.
Control of the heating for the braze operation depends on several factors. The joint must be presented to the flame tip accurately both horizontally and vertically to each core row. Heating must be even or some parts of the core will be overheated causing overannealing, alloy degradation and top plate damage. The flame tip must be clearly defined. Fuzzy flame tips disperse too much heat around the part, slowing the heating and causing too much of the area to be annealed.
The time at heat must be kept to an absolute minimum. Fluxless phoscopper (P-Cu) preform rings allow little time (less time is always better, but under 20 seconds at heat is satisfactory) to braze before leak-forming oxides appear.
The burner gas system must be of bulletproof design. Burner output is based on gas and combustion airflow. This adjustment must be digital.
There should be no operator adjustable gas system settings other than burner Btu output. The system should be rigid and stable so changes made on one shift will not "blindside" another shift. This provides a baseline with which to diagnose brazing problems. The only setting required is the power; less power means less heat, more power means more heat. Problems with brazing can be then traced to other factors such as ambient-temperature changes, burner plugging, etc.
Calibrated flow meters must be present to verify correct gas flow. This is the main indicator used to diagnose the health of the equipment. Any deviation in the proper relationship between combustion air flow and fuel gas flow nearly always can be traced to clogged air intake filters and burner condition. These are simple maintenance issues that take very little time to remedy but need to be addressed.
Filters should be changed according to need, because different plants have different air quality. Filters are inexpensive and should be changed often, generally once a month or more. Changes in filter air flow result in changes in flow meter settings. These should be monitored daily and compared with the historical baseline of the system. Burners also can plug and deteriorate. Visual inspection is sufficient to determine their health. The flame cone should be well-defined and consistent from one burner to the next. Flow meter settings should be compared with every filter replacement to match against the baseline. Deviation from the baseline should trigger the need to replace burner tips, typically 6 months to two years, depending on use.
Changes in ambient pressure in the plant must be kept to a minimum. A change in plant pressure is one of the most overlooked contributors to erratic brazing behavior. The exhaust hood over the brazer is vented to the outside. If too much air escapes (such as when a loading-dock door opens, lowering the static pressure in the plant), then the braze will go cold. If too little plant air escapes, the plant can fill up with the products of combustion.
Changes in ambient temperature also can affect the braze. If the plant temperature varies, then changes in braze settings should be noted. This can be monitored and changed automatically if desired. Each plant must empirically determine the effects of pressure and temperature changes on the process. It is possible to place a thermocouple in the exhaust hood, which displays the exhaust stack temperature. Changes in the temperature reflect changes in ambient conditions.
Autobraze equipment design considerations
The tube height (up/down) must be controlled to within 0.062 in. (1.5 mm) to ensure that the proper pattern of heating takes place. Heat too high or too low can result in the alloy not melting or flowing in the wrong direction (molten alloy follows the heat, sometimes defying gravity). Often, technicians attempt to overcome this problem by overheating, which exchanges one problem for another.
The core must be kept between the burners (left and right) within a 0.062 in. to ensure equal heating. The more equal the heating, the greater is the probability that no area of the core will see significant overheating.
The velocity at which the assembly passes the burners must be fast enough to liquefy the alloy and make the brazed joint before excessive oxidation occurs. Oxidation will increase the number of leaks found after brazing. Speeds can be as slow as 6 fpm (~2 m/min), but the best systems travel at 18 fpm (5.5 m/min). Faster speeds also limit the annealing of the tubes, thereby raising the burst pressure.
Most cores are made of a galvanized top sheet, and newer designs use thinner gage sheet. Care must be taken to design the core so the tubes extend far enough above the end sheet to prevent the brazing heat from burning the galvanize coating off the end sheet. Burners must be angled and focused from the side on the area very close to the braze joint to avoid heating the top sheet (Fig. 2).
Economical core designs cannot be used when the core increases beyond 4 rows wide (5 in some cases) due to the difficulty of aligning burners to the joint to be brazed. Down-fire style burners are successfully used, but core designs must contend with the depletion of zinc (from the galvanize coating) and warping of end sheets. Also, reduction in burst pressures require more expensive, thicker wall tubing.
Figure 3 shows a system that incorporates a single "Kanban" system, allowing cores of various heights and row thickness to be processed one right after another. What that means is that one core measuring 3 ft (915 mm) can run right after one measuring 4 ft (1,220 mm) without the operator having to make any manual adjustments. The operator need only scan the bar code from the work ticket and the machine does the rest.
Advancements in the art of autobrazing AC cores include automatic burner height and row width adjustment and a multitude of conveyor methods. Brazing-specific functional improvements to the autobrazing system are not the only improvements.
There have been many other value-added factory automation devices included to make the systems more complete including:
- Automatic return bend placement (Fig. 4 and 5)
- Automatic inert gas purging
- Gas flux conservation features and application options for reduced maintenance and increased reliability
- Electronic flow meters to track fuel flow over time
- Gas quality monitoring devices to measure incoming Btu values in the gas
- Stack temperature sensors that feed back data to the PLC control in the event of changes in ambient temperature and building pressure
- Machine-vision measurement to feed back flare-size quality and consistency
- Flare resizing
- Laser height measurement to ensure proper tube height
- Modem-accessed PLC control for remote machine programming and troubleshooting
- Server uploads for data control
- Automated leak testing