A new surface treatment developed by researchers at Massachusetts Institute of Technology (MIT) could improve refrigeration efficiency.
Liquid refrigerants 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. These sheets of liquid provide an insulating layer that inhibits heat transfer.
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.
The new findings are described in the journal Joule, in a paper by MIT graduate student Karim Khalil, professor of mechanical engineering Kripa Varanasi, professor of chemical engineering and associate provost Karen Gleason, and four others.
By promoting droplet formation, Kripa Varanasi says it’s 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%. In large industrial processes that is considered a considerable improvement.
The surface treatment adds a very thin solid coating of less than a micron thick. The coating, made of a specially formulated polymer, is deposited on the surface using a process called initiated chemical vapour deposition (iCVD), in which the coating material is vaporised and grafts onto the surface to be treated, such as a metal pipe, to form a thin coating.
The process 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,” said Karim Khalil.
