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Choosing the Best Coatings for Heat Transfer Coils

Heat transfer coils are one of the most critical and delicate components in any air conditioning system. Chillers have them, heat pumps have them, and DX systems have them.

As the name implies, the coil is the heat exchanger in the air conditioner, transferring heat between two fluids. In a water-cooled chiller, the fluids are typically water or a water-glycol solution on one side and cooling water on the other side; air-cooled chillers would have water/water-glycol and reject the heat to air; in a water source heat pump, the fluids are refrigerant and water; and in a DX air conditioner, they are refrigerant and air.

Coils are also subject to a number of external and internal failure modes. Aside from mechanical damage – remember, they’re delicate, you can’t run a forklift into them and over time the walls will become thinner due to erosion – external failures are commonly leaks caused by corrosion. Internal coil damage may also be the result of corrosion and, in water-cooled equipment, can be caused by the formation of scale. Scale, which impedes heat transfer, and internal corrosion are controlled primarily by maintaining proper pH and reducing solids content as much as possible.

External coils, to operate properly and have a longer useful life, need to be kept clean and free of corrosion. One of the most effective ways to reduce or prevent coil corrosion is through the use of a protective coil coating. There are several type of coatings, with different application methods, that are commercially available:

  • The coatings are sprayed onto the coils;
  • The coils are dipped (immersed) in the coating material;
  • The coating is applied by electroplating.

The most common – and economical – method is generally going to be a sprayed-on coating, and that’s the type with which I am most familiar (in the interest of full disclosure, EnergyGuard, a leading spray-on coil coater, is a former client of mine). The other methods have the advantage of providing more uniform coverage, but they are more expensive and cannot be applied in the field (the sprayed-on coatings can be applied in a factory/shop or onsite). Cure time is an important consideration in selecting a sprayed-on coating that is going to be applied in the field, and – under ordinary temperature and humidity conditions – should not be longer than 15 minutes or so. Also, if the equipment being coated is operating in a coastal location, the coating should successfully pass the ISO 9227/ASTM B117, 10,000-hour salt spray test.

For optimum performance, coatings will employ a three-step process:

  1. Degrease;
  2. Apply a primer, which creates a conversion layer on the aluminum coil surface for better adhesion, and then...
  3. Coat.

One concern that came to mind, when I first learned about the coating, was if there would be a loss of heat transfer efficiency due to any insulating properties of the coating. Obviously, coating products containing aluminum solids, rather than silica or similar materials, will have a significantly better heat transfer coefficient (k). Depending on the amount of aluminum in the product, it seems reasonable to expect that the improved k will offset any loss of efficiency due to the insulating characteristics, if any, of the coating.

Finally, as a sustainability consultant, I strongly recommend selecting a product with low or no VOCs.

A regular contributor to HPAC Engineering and a member of its editorial advisory board, the author is a principal at Sustainable Performance Solutions LLC, a south Florida-based engineering firm focusing on energy and sustainability.

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