Hydrophilic topcoat for microchannel heat exchangers

Condensation management is essential to ensuring the efficient function of heat exchangers and critical for the correct operation of evaporators.

Hydrophilic topcoats, surfaces tending not to adsorb water or be wetted by water, are effective in environments of excessive condensation, as they enable water to flow easily off the heat exchanger surface by virtue of lowered surface tension, thereby enhancing heat transfer performance in wet and frost conditions. At the same time, hydrophilic coatings protect heat exchangers from the corrosive effect of water and exhibit fine performance in providing protection in salty environments.

So far, hydrophilic topcoats were primarily available for traditional finned-pack coils. Observing growing interest in heat pumps based on microchannel heat exchangers, especially reversible models where MCHE coils are also engaged as evaporators, Kaltra proposed new coating chemistry and method of applying hydrophilic topcoats suitable for this type of coils.

A coating is considered hydrophilic in case the contact angle of a water droplet’s edge with a surface is less than 90 degrees. This refers to the angle that a droplet of water makes at the point of contact with the surface – a lower angle represents better coating performance. Field tests conducted on Kaltra’s microchannel heat exchangers treated with a hydrophilic agent have shown that a contact angle is fewer than 30 degrees with high critical surface tension.

Based on laboratory tests and validation exercises, we recommend hydrophilic treatment for coils exposed to wet and/or frost conditions, particularly evaporators, including those used in air-source heat pump applications, and for dehumidifying coils.

Laboratory experiments also have shown increased evaporation heat transfer rates of hydrophilic-treated coils, specifically at high Reynolds numbers, in the range of 25-55%.

In sum, the use of a hydrophilic polymer:

• Improves condensate drainage
• Increases performance in wet conditions
• Provides antimicrobial protection
• Enhances corrosion resistance of the heat exchanger, contributing to its extended lifetime

The chemical agent used for Kaltra’s hydrophilic coating has been designed by Nihon Parkerizing, a Japanese-based surface treatment industry leader.