The highest level of quality can be assured when the proper protective coating is applied.  

CONDENSER COILS

Standard Coil Construction

The standard condenser coil has copper-tubes mechanically bonded to aluminum-fins with louvered enhancements.  Figure 6 shows a cross-section of a copper tube and several aluminum-fins.  High thermal efficiency is achieved through direct metallic contact between the tube and fin.  Fin louvers improve the fin’s heat transfer capabilities.  As a result, maximum thermal performance is achieved with this high efficiency coil design.

The standard coil generally provides the highest performance for non-corrosive environments.  Application of this coil in marine or industrial environments is not recommended due to the likelihood of visible deterioration resulting from corrosion.

Precoated Aluminum-Fin Coils

Precoated aluminum-fin coils have a durable organic coating applied to the fin.  This design offers protection in mildly corrosive coastal environments, but is not recommended in severe industrial or coastal environments.

Aluminum-fin stock is coated with a  baked-on organic coating prior to the fin stamping process.  Coating of the fin material prior to the fin stamping process is known as “precoating.”  The precoated fin material is then stamped to form a wavy fin pattern for optimum thermal performance.  The wavy design can be recognized by the vertical corrugation on the fin face.  Vertical corrugations increase the fin’s effective surface area and further enhance heat transfer properties.

The coil’s dissimilar metals are separated by a thin layer of inert organic precoating material.  As a result, the electrical connection between the copper and aluminum is insulated and thus inhibits the galvanic action.

In mild coastal environments precoated coils are an economical alternative to copper or postcoated coils and offer substantial corrosion protection beyond the standard uncoated coil.

Copper-Fin Coils

Copper-fin coils eliminate the bi-metallic bond found on standard and precoated fin coils.  A  copper wavy fin pattern, void of louvered enhancements, is mechanically bonded to the standard copper tube.  Copper-fin coils are priced higher than aluminum precoated fin coils, since material costs are greater than aluminum.  However, coastal corrosion durability is substantially improved over the standard or precoated coil construction, since the bi-metallic construction is not present. 

Copper-fin coils provide increased corrosion resistance in harsh coastal environments where industrial air pollution is not present.  Copper is generally resistant to coastal environments, since a natural protective film is formed to passivate the copper surfaces and a mono-metal bond exists between the tube and fin.  Uncoated copper coils are not suitable for industrial applications, since many industrial contaminants attack copper.  The use of uncoated copper in these applications is not appropriate and must be avoided to ensure long coil life.  Postcoated aluminum-fin coils should be considered for industrial applications.

 Postcoated Aluminum-Fin Coils

Postcoated aluminum-fin coils have a tough baked-on organic coating uniformly applied over all coil surfaces.  The coating prevents contact between the electrolyte or chemical and the metal fins and tubes.  This prevents contamination of the coil surfaces.  Fin enhancements have been replaced by a wavy fin design.  Fin spacing has been increased to prevent bridging of the coating material between the fin elements.

Postcoating is a multiple step, factory-applied process performed on complete coil assemblies prior to installation in the equipment chassis.  Coil assemblies are dip-coated in a total immersion bath and baked to ensure a high density, thin-film coating with an ultra-smooth surface.  This process consists of controlled cleaning, priming, coating and sealing applications with intermediate oven-baked curing.  This quality-controlled process ensures complete coating of the coil assembly to form a continuous barrier between the coil surfaces and the contaminated environment.

The combination of wavy fin design, optimum fin spacing, and a barrier between the coils and surrounding environment ensures a durable coil assembly with rugged corrosion resistance properties.  Aluminum-fins are lighter weight and lower cost than copper-fins.  Postcoated aluminum-fin coils offer economical protection and improved coil life in contaminated industrial environments.

Postcoated Copper-Fin Coils

Postcoated copper-fin coils have the same tough baked-on coating as the postcoated aluminum-fin coils.  Complete encapsulation of the coil surfaces ensures isolation from exposure to the contaminated environment.  When combined with the naturally corrosion-resistant all-copper coil construction, postcoating provides the highest corrosion-resistance of any coil option.

Postcoated copper-fin coils should be specified for environments with harsh coastal conditions, industrial contamination, or combinations of both.

 

COOLING/HEATING COILS

Standard Coil Construction

The standard cooling/heating coil (water, steam, or Direct Expansion) has copper-tubes mechanically bonded to non-lanced aluminum-fins with a wavy patter embossed on the fin face.  The fin pack is assembled with galvanized steel tube sheets and coil case.  This assembly has classic galvanic corrosion components with multi-metal bonds between the fin-and-tube and tube-and-tube sheet bonds.

In cooling applications, condensate accumulates on the coil surfaces when dehumidification occurs.  Wet coil surfaces, resulting from condensation, in the presence of a contaminated airstream will lead to galvanic corrosion if not properly protected.

Potentially corrosive airstreams may not be suitable for building occupants.  If a contaminated airstream can lead to corrosion, special consideration with respect to indoor air quality and potentially harmful side effects to building occupants is recommended.

 Copper-Fin Copper-Tube Coils

Much like the all-copper condenser coil, all-copper cooling/heating coils eliminate the bimetallic bond found on standard coils.  A copper-fin with wavy pattern is mechanically bonded to the standard copper-tube to ensure a single-metal assembly.  Most air-handling equipment is available with stainless steel tube sheets and coil cases to improve the corrosion durability of the entire coil assembly.   As a result, the potential for corrosion is reduced since bimetallic couples can be minimized within the coil assembly.

Postcoated Coils

Postcoated coils have a tough baked-on organic coating uniformly applied over all coil surfaces, including tube sheets and coil cases.  The coating provides a barrier between the coil surfaces and the corrosive effects of the atmosphere to prevent contamination of the coil surfaces.

In considering postcoated coils, it is important to also consider the effects of  moisture carryover.  Moisture carryover occurs when accumulated condensation is blown from the coil surface during cooling coil applications.  The extent of carryover is a function of airstream velocity across the coil, fin spacing, fin geometry and material of construction.  When postcoating is applied to a cooling coil, carryover will occur at lower oil face velocities.  The following recommendations should be considered when selecting chilled water or DX coils to ensure moisture carryover will be prevented.

MAXIMUM RECOMMENDED FACE VELOCITY (FPM)

FPI – Fins per inch

FPM – Feet per Minute

External fouling on cooling coils will adversely effect the maximum recommended face velocities.  Data based on clean coils with proper filtration and periodic cleaning of coil surfaces.

Field-Applied Coatings

Field-applied sprayed-on coatings generally do not provide sufficient protection in corrosive environments.  Possible reasons for poor performance:

• Coil cleanliness is crucial for proper adhesion.  Adequate field cleaning techniques are generally overlooked. 

• Field application cannot ensure continuous coating of coil surfaces on multiple row coils.

• Interior coil surfaces remain untreated when sprayed-on from unit exterior.

•  Inconsistent coating thickness minimizes coating protection.  Recommended coating thickness of 2.5 mils for standard units and 1 to 1.5 mils for units with enhanced fins and high fin density cannot be ensured with field application on multiple row coils.  Film thickness measurements are generally overlooked.

•  Coil must be void of corrosion prior to field application of the coating.  Encapsulation of existing corrosion makes the coating  ineffective or can lead to continued deterioration and coating delamination.

Postcoated Material and Chemical Resistance

Chemical resistance of the postcoating material is described in Appendix guide; “Postcoating Chemical Resistance Guide.”  Application of a postcoated coil should only be considered when the contaminant is listed in the Appendix guide.  If the postcoating is NOT resistant to the contaminant listed or if the contaminant is not listed in Appendix, application in this environment may not be recommended.  Please contact us for assistance.

Common industrial contaminants which are resisted by the postcoated coils include, but are not limited to:

NOTE:  Discoloration is an indication of potential problems.  However, identification of contamination sources based on color may be misleading.

Advantages/Disadvantages of Protection Options