PERFORMANCE TRADE-OFFS

The purpose of any protective coating is to insulate the material being coated from the environment. A potential disadvantage to coating condenser coils is that there may be a negative effect on the performance of the condensing unit when compared to the performance of the same unit with an uncoated coil. Consequently, there may be a bit of a trade-off between performance and protection.

This trade-off can be limited by selecting a coating that either enhances the coil’s ability to function, or that only affects it minimally.

If a coating is too thick, airflow over the coils will be restricted, affecting pressure drop. Coatings that are too thick or that contain organic pigments may also inhibit thermal transfer. The optimum coating is thin, so it does not significantly inhibit heat transfer.

There are two general categories of protective coatings, pre-coats and post-coats.

Pre-coat processes coat sheet aluminum before it is manufactured into coils. According to Richard Griffin, marketing manager, Blygold America, Stevensville, MD, pre-coats are not a very good long-term coating solution because the manufactured edges are not coated and are therefore exposed to corrosive forces. “This results in a rapid deterioration of the coil. In addition, the application of a pre-coat prior to manufacture precludes the effective application of any other type of coating to the coil.”

There are numerous types of post-coats available in the United States: polyurethane, epoxy poly-urethane, phenolic, epoxy phenolic, polyelastomer- and silica-based coatings, and many others. Each has its own strengths and weaknesses.

WHAT TO CONSIDER

When choosing a protective coating for condenser coils, the important points to remember (in addition to corrosion resistance and complete coverage of the coil), are that the coating should be thin, flexible, impact resistant, and UV resistant.

Thinness: As mentioned before, coating that is too thick restricts airflow over the condenser coil. As fin spacing on condenser coils gets denser, this becomes more important. Some coatings are so thick that fin bridging occurs, severely restricting the efficiency of a condensing unit.

Flexibility and impact resistance: A coating that is not flexible or impact resistant will crack or flake when the coils get hit or bent during normal operating conditions. Once any portion of uncoated metal is exposed, corrosion will form and begin to corrode the underlying metal. “I have had contractors tell me of coated coils that appeared to look fine from a distance, but upon closer inspection [they] had almost completely disintegrated, leaving only the exterior shell of the coating,” says Griffin.

UV resistance: Any condenser coil that will be exposed to sunlight should have a coating that is resistant to UV radiation. Several of the popular post-coats do not have inherent UV resistance and offer a polyurethane “topcoat” to prevent rapid deterioration of their base coating. Polyurethane is UV resistant. “A negative aspect is that the extra layer of polyurethane adds undesirable thickness to the coating,” notes Griffin.

DIP AND SPRAY

There are two methods for applying post-coat coatings: dipping or spraying.

All coatings applied by a dip process must be applied at a factory. Products are immersed into product baths a set number of times and for specific durations, then either baked or allowed to cure naturally.

Spray processes can be done either at the factory or in the field. Factory-applied processes are generally more reliable, since coils in field installations may present access problems. In addition, factory-applied spray processes use different methods of air-assisted spraying to guarantee penetration of the coil. Some processes have developed protocols to coat coils uniformly without removing them from the equipment, while others must have the coils removed.

There are advantages to the factory process that can coat the coil while it is in the unit. One is not having to cut and resolder tubes and remove/replace refrigerant. Obviously, the potential for damage to the integrity of the condenser unit is lessened if the coil does not have to be removed. Another advantage of in-place coil coating is quicker turnaround times. Contractors working on tight deadlines appreciate this.

Field-spray applications range from air-assisted product pumps producing high-pressure penetration to aerosol cans. These applications are popular, but have varying degrees of success in terms of coverage and penetration. The general opinion is that some protection is better than none; bare aluminum coils just don’t last long in corrosive environments.

As with any mechanical process, it is possible to perform a coating process incorrectly. Most coating failures are probably due to improper preparation of the substrate. The success of any coating depends heavily on good adhesion. If the condenser coil is not properly cleaned and prepared before coating, failure is likely to occur.

Contractors are a very important part of the protective coating-selection process, even though they may not always have a say in which type of coating is used.

“Contractors definitely have the local field experience to recommend to a customer whether or not a protective coating should be used,” says Griffin. “They can provide a valuable service by educating end users of protective coating options when it comes time to purchase a new unit or replace a coil.”

The first Blygold coating system was specifically designed to resist the corrosive attack on condenser coils by the combined marine and industrial environment found in the Netherlands. For more information, contact Blygold at 410-643-6080; 410-643-6092 (fax); or info@blygoldamerica.com (e-mail).

Publication date: 02/18/2002