Specialized thin coatings developed by the MIT team cause even low-surface-tension fluids to readily form droplets on the surface of a pipe, as seen in this image, which improves the efficiency of heat transfer. Image courtesy of the researchers.
Specialized thin coatings developed by the MIT team cause even low-surface-tension fluids to readily form droplets on the surface of a pipe, as seen in this image, which improves the efficiency of heat transfer. Image courtesy of the researchers.

Unlike water, liquid refrigerants and other fluids with a low surface tension tend to spread quickly into a sheet when they come into contact with a surface. But 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.

Now, researchers at Massachusetts Institute of Technology (MIT) have developed a novel coating that can promote droplet formation and shedding in such fluids. This approach could lead to efficiency improvements in many large-scale industrial processes, including refrigeration, thus saving energy and reducing greenhouse gas emissions.

The researchers report their new findings in a paper in Joule by graduate student Karim Khalil, professor of mechanical engineering Kripa Varanasi, professor of chemical engineering Karen Gleason, and four others.

Over the years, Varanasi and his collaborators have made great progress in improving the efficiency of condensation systems that use water, such as the cooling systems used for fossil-fuel or nuclear power generation. But other kinds of 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. This makes it 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.

The problem is that when these sheets of liquid coat a surface, they form an insulating layer that inhibits heat transfer, and easy heat transfer is crucial to making these processes work efficiently. “If it forms a film, it becomes a barrier to heat transfer,” Varanasi says. But 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.

In condensing systems that use water, the overall efficiency of the process can be around 40%, but with low-surface-tension fluids, the efficiency can be limited to about 20%. Because these processes are so widespread in industry, even a tiny improvement in that efficiency could lead to dramatic savings in fuel, and therefore in greenhouse gas emissions.

By promoting droplet formation, Varanasi says, it’s possible to achieve a four- to eightfold 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%. That may not sound like much, but in these huge industrial processes even a fraction of a percent improvement is considered a major achievement with great potential impact. “In this field, you’re fighting for tenths of a percent,” Khalil explains.

Unlike the surface treatments that Varanasi and his team have developed for other kinds of fluids, which rely on a liquid material held in place by a surface texture, in this case they were able to accomplish the fluid-repelling effect using a very thin solid coating – less than 1µm thick. That thinness is important for ensuring the coating itself doesn’t contribute to blocking heat transfer, Khalil explains.

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. This process was developed at MIT by Gleason and is now widely used.

The authors optimized the iCVD process, by tuning the grafting of coating molecules onto the surface, in order to minimize the pinning of condensing droplets and facilitate their easy shedding. This 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,” Khalil says, and can be applied on either flat surfaces or tubing made of stainless steel, copper, titanium or other metals commonly used in evaporative heat-transfer processes that involve these low-surface-tension fluids. “Whatever material you come up with, it tends to be scalable with this process,” he adds.

The net result is that on these surfaces, condensing fluids such as liquid methane will readily form small droplets that quickly fall off the surface, making room for more to form, and in the process shedding heat from the metal to the droplets that fall away. Without the coating, the fluid would spread out over the whole surface and resist falling away, forming a kind of heat-retaining blanket. But with it, “the heat transfer improves by almost eight times,” Khalil says.

One area where such coatings could play a useful role, Varanasi says, is in organic Rankine cycle systems, which are widely used for generating power from waste heat in a variety of industrial processes. “These are inherently inefficient systems,” he says, “but this could make them more efficient.”

This story is adapted from material from MIT, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.