Aluminum Foil Fin Stock

Finned aluminum coils are used in a variety of heat exchange constructions where the basic function of the aluminum fin is to efficiently conduct (transfer) heat. Examples of these heat-exchanger applications are aluminum foil fins for evaporator and condenser coils in most residential, automotive, and commercial air conditioning units. In addition, foil fins are used in humidifiers, dehumidifiers, baseboard space heaters of various types, and other equipment.

Actual design of the fins varies from proprietary "spiral wound" or "spin fin" constructions to the more traditional plate fin assembly. Regardless of the design, function of the aluminum fin is essentially the same. 

The higher purity commercial aluminum alloys, principally 1100 and 7072, are generally employed. Smaller quantities of 1145, 3003, and 5005 alloys are used for specialized fin stock applications.

For many years, the tubing in "plate-fin" heat exchangers has been confined almost exclusively to copper, with the 1145 and 1100 aluminum alloys being the standard fin materials. In the past 15 or so years, however, less expensive aluminum tubing has replaced copper in numerous heat exchanger applications, with excellent results. 

The transition to aluminum tubing necessitated a change in fin stock material, from the pure aluminum series (1145/1100) to the more anodic aluminum-zinc 7072 alloy. This provides cathodic protection to the aluminum tubing (alloys 1060, 5052, 5454, and 6061) by serving as a sacrificial agent for any corrosion products that would normally attack the tubing.

Regardless of the design, whether commercially pure aluminum fins with copper tubing, or 7072 aluminum alloy fin stocks with aluminum tubing, the corrosion resistance is more than adequate to meet the standards required for heat exchanger constructions.

From a standpoint of economics, aluminum provides more heat transfer per unit of cost than any other available metal. Aluminum fin stock gauges typically range from 0.004 to 0.0059 in.

Fabrication

Most fins for "plate-fin" heat exchangers are drawn in a series of die progressions to produce collared holes, through which the heating or cooling tubes are passed. These formed collars serve as spacers in the assembly of a stack of heat exchanger fins and provide extra contact area for improved heat transfer.

The forming of the aluminum is frequently quite severe, ranging from shallow-drawn collar heights of 25 collars/inch to the more heavily deformed collars of 3-4 collars/ inch. These collars must be free of radial cracks, otherwise the collars will split further during the tube expansion operation resulting in a loss of thermal conductivity. Some of the other principal factors affecting severity of forming are alloy, fin geometry, hole size, hole spacing, fin stock thickness, tolerances, number of forming steps, lubrication, die alignment and clearances, and others. 

To meet the above requirements, fin stock must be capable of being deep-drawn to the desired collar heights yet have sufficient rigidity to withstand mechanical damage to the stacked fins after final assembly. Aluminum has excellent forming characteristics in the fully annealed -0 temper, as previously mentioned. Finstock is, however, available in other tempers to meet specific fabrication and end use requirements.

The light weight of aluminum is another important consideration, particularly in those applications where critical weight factors are involved, e.g. automotive air conditioning, room air condition units, and aircraft units. Also, aluminum alloys are selected for fin stock material because of their ability to resist corrosion. Since many heat-exchanger units are exposed to various corrosive environments, it is important that the fin material have good corrosion resistance.