Every HVAC coil has fins. The fins increase the surface area available for heat transfer between the fluid inside the tubes and the airstream passing over the coil, and the material those fins are made from affects the coil’s performance, its durability in the specific application environment, and its resistance to the corrosion mechanisms that shorten coil service life.
Aluminum fins on copper tubes are the standard configuration for commercial HVAC coils. That combination covers the majority of applications correctly. But the standard configuration is not always the right configuration. Understanding when fin material selection matters and what the alternatives are is what allows engineers to specify a coil that will perform correctly for the life of the application rather than one that will require premature replacement because the fin material was not appropriate for the environment.
Why aluminum fins on copper tubes became the standard
Aluminum fins bonded to copper tubes became the dominant commercial HVAC coil configuration because the combination delivers an effective balance of thermal performance, weight, cost, and manufacturability for standard commercial applications.
Aluminum has high thermal conductivity, low density, and is easily formed into the thin fin profiles that maximize heat transfer surface area. Copper tubes provide the pressure containment, the corrosion resistance, and the thermal conductivity needed for the fluid-side of the heat transfer equation. The two materials form a galvanic couple that is generally stable in the water content and humidity levels found in typical commercial HVAC applications, though that galvanic relationship becomes important in certain corrosive environments.
For standard commercial HVAC applications in office buildings, retail spaces, schools, and similar environments, aluminum fins on copper tubes will perform correctly for the expected service life of the coil. The standard configuration is not a compromise. It is appropriate for the majority of applications.
When aluminum fins become a liability
Aluminum is susceptible to corrosion in environments with elevated chloride levels, acid gases, or chemical contaminants in the airstream. When an aluminum-finned coil is installed in an environment where those contaminants are present, the fin material degrades over time, the fin-to-tube bond weakens as the aluminum oxidizes at the contact point, and the coil’s heat transfer performance declines before the coil has reached its expected service life.
Coastal and marine environments are the most commonly cited application where aluminum fin corrosion is a concern. Salt air carries chloride concentrations that attack aluminum oxide surface film and accelerate corrosion. A coil with standard aluminum fins installed in a coastal facility within a few miles of the ocean may show visible fin degradation within a few years of installation. The same coil installed inland in a controlled office building environment may perform correctly for twenty years.
Industrial environments where the airstream carries acid gases, ammonia, sulfur compounds, or chemical process vapors present similar risks. Wastewater treatment facilities, food processing plants, chemical manufacturing, and facilities adjacent to industrial processes that generate acid gases all create airstream conditions where aluminum fin corrosion is a real risk rather than a theoretical concern.
Swimming pool environments are a specific application where aluminum fin corrosion is nearly universal without appropriate fin protection. The chlorine and chloramine gases in indoor pool air are aggressive toward aluminum. Coils in pool enclosures without corrosion-resistant fin protection fail at a rate that facility managers with pool HVAC experience find unsurprising and that facility managers installing their first pool HVAC system find expensive.
Copper fins as an alternative
Copper fins on copper tubes eliminate the galvanic couple between dissimilar metals and provide better corrosion resistance than aluminum in environments with elevated chloride or acid gas concentrations. Copper fins cost more than aluminum fins and add weight to the coil, but in corrosive environments the service life advantage over aluminum fins more than justifies the cost premium.
For coastal applications, indoor pool applications, and light industrial environments with moderate corrosive gas concentrations, all-copper coil construction is often the most cost-effective specification over the full service life of the installation, even though the first cost is higher than an aluminum-finned equivalent.
Coated aluminum fins
Epoxy and phenolic coatings applied to aluminum fins provide a corrosion barrier that extends the service life of aluminum fins in moderately corrosive environments without the cost and weight premium of all-copper construction. The coating covers the fin surface and the fin-to-tube contact area, isolating the aluminum from the corrosive elements in the airstream.
Coated aluminum fins are a middle ground between standard aluminum fins and all-copper construction. They are appropriate for applications where the corrosive challenge is moderate, where the coating can be specified to the specific corrosive agents present, and where the coating integrity can be maintained over the life of the installation. Heavy industrial environments or high-chloride coastal applications with very aggressive corrosion mechanisms may exceed what coatings can reliably protect against, in which case all-copper construction is the more appropriate specification.
Stainless steel fins for severe applications
Stainless steel fins are specified in applications where the airstream chemistry is severe enough that even all-copper construction does not provide adequate corrosion resistance. Chemical processing facilities, pulp and paper operations, and other heavy industrial applications where the airstream carries concentrated acid gases or aggressive chemical compounds may require stainless steel fin construction.
Stainless steel fins have lower thermal conductivity than aluminum or copper, which requires a larger fin surface area to achieve the same heat transfer duty. That requirement means stainless steel finned coils are larger and heavier than equivalent aluminum or copper finned designs, and the material cost is significantly higher. Stainless steel fins are the right specification in the applications that require them, but they should not be specified where copper or coated aluminum would perform adequately.
What to evaluate before specifying fin material
The fin material selection for a replacement or new coil should be based on an honest assessment of the airstream environment the coil will operate in. What contaminants are present in the airstream at the coil location? What is the distance from saltwater if the facility is in a coastal location? What chemical processes occur in or adjacent to the building? Has a previous coil at the same location failed prematurely, and if so, what did the fin degradation look like?
For replacement coils where the previous coil failed prematurely due to fin corrosion, replicating the original fin material specification produces the same result. The replacement specification should address the corrosion mechanism that caused the failure rather than simply replacing like for like.
For boiler plant applications where the heating coil serves a system supplied by GP Energy Products equipment, the GP Energy team handles the heating system side of the application across Pennsylvania, New Jersey, Delaware, and Maryland. Visit gpenergyproducts.com for more. For chilled water coil applications where the pump system serving the coil circuit affects the operating conditions, the Merion Pump Company team handles commercial pump selection and service. Visit merionpump.com for more.
HX Coils manufactures custom coils with aluminum, copper, coated aluminum, and stainless steel fin configurations for commercial and industrial HVAC applications across the Mid-Atlantic region. Reach out before the replacement is specified and we will confirm the right fin material for the specific application environment.
References
1. ASHRAE. HVAC Systems and Equipment Handbook, Chapter on Coils. Covers coil fin material selection, corrosion mechanisms, and application guidance for commercial HVAC. ashrae.org
2. AHRI Standard 410. Forced-Circulation Air-Cooling and Air-Heating Coils. Governs performance testing and certification for HVAC coils across fin material configurations. ahrinet.org
3. ASTM International. Standard Practice for Modified Salt Spray Testing. Covers corrosion testing methodology applicable to coil fin material performance assessment in corrosive environments. astm.org
4. Air Conditioning Contractors of America. HVAC Quality Installation Specification. Covers coil selection requirements including fin material for specific application environments. acca.org
