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John O | May 2019

Surface treatment promotes droplet formation and enhances refrigeration efficiency


By Josh Perry, Editor
jperry@coolingzone.com

 

Researchers at the Massachusetts Institute of Technology (MIT) in Cambridge, Mass. demonstrated the effectiveness of a new surface treatment that promotes droplet formation even in fluids that have low surface tension (such as refrigerants) and enhances heat transfer.

 


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 here, which improves the efficiency of heat transfer. (MIT)

 

This surface treatment, according to a report from MIT, could benefit large-scale industrial processes, including refrigeration, reducing energy costs and greenhouse gas emissions.

 

The research team previously worked with condensation systems that used water, but other fluids have lower surface tension and formed sheets rather than droplets. These sheets acted as insulators, limiting the amount of heat transfer was possible. Forming droplets and shedding them quickly could boost heat transfer by as much as a factor of eight and an overall efficiency improvement in industrial processes of two percent. That would be a significant cost reduction.

 

Unlike previous surface treatments, “in this case they were able to accomplish the fluid-repelling effect 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 was composed of a specially designed polymer and is deposited on the surface using initiated chemical vapor deposition (iCVD), which was also developed at MIT. Researchers were able to fine-tune the process to boost the shedding of droplets.

 

The coating can be applied to flat or tubed surfaces and is applicable with a variety of materials, including stainless steel and titanium.

 

“The net result is that on these surfaces, condensing fluids like the hydrocarbons pentane or liquid methane, or alcohols like ethanol, 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,” the article continued.

 

The research was recently published in Joule. The abstract stated:

 

“A majority of the work devoted to surface engineering for promoting dropwise condensation heat transfer has focused on steam. Less attention has been dedicated to the condensation of low-surface-tension fluids such as hydrocarbons and refrigerants, which are used in several industrial processes including HVAC, refrigeration, LNG storage, and organic Rankine cycles.

 

“Durable low-surface-energy coatings that can promote stable dropwise condensation to increase the heat transfer coefficient of these liquids has potential for significant overall efficiency enhancements. However, this remains a challenge as these liquids wet all heat exchanger substrates. Even superomniphobic surfaces fail to promote dropwise condensation because of nucleation within roughness features.

 

“Here we show dropwise condensation of a variety of low-surface-tension liquids on covalently grafted polymer films deposited via initiated chemical vapor deposition and report improvements in heat transfer coefficients.”

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