Longer life in the harshest industrial environments
When it comes to long life under tough industrial conditions, Armstrong is all you need to know about coils. Even in the most severe environments, where coil leaks and corrosion are costly problems, Armstrong coils maintain high efficiency and output.
Choosing a tube material
The choice of tube material depends on several important factors:
- The corrosive quality of the steam or liquid medium
- The ability to pipe, trap and vent steam coils effectively
- The size and service requirements of the installation
- The external corrosion to which the coils are likely to be subjected
Generally speaking, the heat-transfer characteristics of the tube material are of little consequence. The fin area constitutes the vast majority of the heat-transfer surface, so it is the most important factor determining heat-transfer effectiveness. Therefore, the choice of tube materials should be based on service requirements, not heat-transfer efficiency.
Internal corrosion. The base material found in Series 6000 coils is steel. The minimum wall thickness is 0.109" for steam and liquid coils. This thickness is sufficient to provide both strength and corrosion resistance. All Armstrong coils are of monometallic design, which means that all wetted parts are made of the same materials. This precludes the likelihood of galvanic corrosion often experienced in coils made of dissimilar materials. For most applications, steel will provide very satisfactory service. However, coils must be carefully piped, trapped and vented to ensure good condensate and non-condensable gas evacuation.
There are many cases where the steam cannot be conditioned enough to be non-corrosive, or it is impossible to pipe, trap and vent the coils properly. In these cases, Armstrong recommends stainless steel wetted parts. Making a choice between steel and stainless steel depends on the degree and type of problem, as well as the steam pressure involved.
External corrosion. When it comes to external corrosion, the decision between steel and stainless steel depends on the corrosiveness of the airstream and the compatibility of the materials with the airborne contaminants. In addition to the base materials available, Armstrong offers these coatings: hot-dipped galvanizing, epoxy powder, baked phenolic, or Teflon®. These coatings are frequently used when only external corrosion is a consideration.
Service requirements. These may be as important as the factors above. While many factors can cause coil failure, the most prevalent cause is failure of the tube-to-header joints. This failure occurs because of defects in the coil design, insufficient material at the tube-to-header joint, or the method of connecting the tube to the headers.
Armstrong Series 6000 coils are designed to accommodate the service requirements of a particular installation. They are built with enough material at the tube-to-header joints to make them strong. When differential expansion between tubes in steam coils is likely to over-stress the joints, centifeed types of coils are recommended. Finally, Armstrong coils are always of welded construction—the best method of connecting the two parts.
The best combination of coil materials is the one that delivers maximum heat transfer and service life. Tubes, regardless of material, contribute little to heat transfer in extended-surface coils. It is the fins, fully exposed to the airstream, that provide the greatest contribution to heat transfer. Therefore, choose tube material on the basis of application.
- Keyfin. The keyfin is the standard design for Armstrong’s most popular coils. Keyfin coils are manufactured by forming a helical groove in the tube surface and winding the fin into the groove. The displaced metal from the groove is then peened against the fin. This means a tight fit between the fin and the tube, ensuring efficient operation over wide temperature ranges. The keyfin is the superior design for dissimilar fin tube materials. (See drawing.)
- L Fin. The L fin has a “foot” at its base and is tension-wound on knurled tube material. The L-shaped base provides a large contact area between the tube and the fin, ensuring effective, long-lasting heat transfer. The L fin is recommended when tubes and fins are of the same material. (See drawing.)
- Overlap L Fin. The overlap L fin is simply an L fin with an extended base. Each fin overlaps the foot of the previous fin, completely covering the tube surface. The overlap technique makes it possible to create a completely aluminumized coil for applications where exposed steel would be vulnerable to corrosion. (See drawing.)