Unlike water, liquid refrigerants and other fluids that have a low surface tension spread quickly into a sheet when they come into contact with a surface. For many industrial processes, it would be better if the fluids formed droplets that could roll or fall off the surface and carry heat away with them.
Fluids such as those used in refrigeration systems, liquification, waste heat recovery, and distillation plants or materials such as methane in oil and gas liquifaction plants often have very low surface tension compared to water, meaning that it is very hard to get them to form droplets on a surface. Instead, they tend to spread out in a sheet — a property known as wetting.
When these sheets of liquid coat a surface, they provide an insulating layer that inhibits heat transfer, and easy heat transfer is crucial to making these processes work efficiently. That heat transfer is enhanced when the liquid quickly forms droplets, which then coalesce and grow and fall away under the force of gravity. Getting low-surface-tension liquids to form droplets and shed them easily has been a serious challenge.
By promoting droplet formation, it is possible to achieve a four- to eight-fold improvement in heat transfer. Because the condensation is just one part of a complex cycle, that translates into an overall efficiency improvement of about 2 percent.
The fluid-repelling effect was enabled using a very thin solid coating — less than a micron thick (one millionth of a meter). That thinness is important to ensure that the coating itself doesn’t contribute to blocking heat transfer. The coating, made of a specially formulated polymer, is deposited on the surface using a process called initiated chemical vapor deposition (iCVD) in which the coating material is vaporized and grafts onto the surface to be treated, such as a metal pipe, to form a thin coating.
The iCVD process was optimized by tuning the grafting of coating molecules onto the surface in order to minimize the pinning of condensing droplets and facilitate their easy shedding. The process could be carried out on location in industrial-scale equipment and could be retrofitted into existing installations to provide a boost in efficiency. The process is “materials agnostic” and can be applied on either flat surfaces or tubing made of stainless steel, titanium, or other metals commonly used in condensation heat-transfer processes that involve these low-surface-tension fluids.