Adrian Bejan | Ice Making, from Design in Nature
In this video, Adrian Bejan describes the design of the ice maker in a domestic refrigerator, where water freezes on a refrigerated surface called an evaporator, forming a layer of ice whose thickness increases over time, eventually reaching a point where it stops growing rapidly. He explains that the ice maker must then remove the manufactured ice to clean the surface, so the process repeats as a cycle with two time scales: a freezing time and a surface renewal time. He treats the surface renewal time as a known constant given by engineers and focuses on selecting the freezing time as the degree of freedom. The purpose is to maximize the time-averaged rate of ice production, which depends on how much ice is produced before removal and on the full cycle time. He connects this trade-off to familiar designs from domestic life to animal design and industry, where surfaces must be cleaned rhythmically to maintain high performance.
The heart of ice making is a cold, refrigerated surface where water at the freezing point accumulates a layer of ice. Over time, the process runs out of zip because the layer no longer grows quickly, so the surface must be cleaned.
Ice removal is a second time interval that renews the surface, and designs like ice trays with little cubicle alvoli help remove a whole bunch at once. This surface renewal time is treated as a constant, assumed to be known.
The object of the game is to determine the freezing time as the degree of freedom selected rationally. The purpose is to maximize the time-averaged rate of ice production over the cycle, including freezing and surface renewal.
The trade-off has two extremes: short freezing time on one side and long freezing time on the other. High values occur when the freezing time is determined by the time required to remove or clean the surface, which is good news for mechanical methods of scraping ice off.
The same idea appears in the windshield wiper that cleans the window and in the eyelids that lubricate and clean the eyeball rhythmically. He also points to heat exchangers whose ducts get clogged by debris or gunk called scale, like plaque on teeth, and to the cost of shutting down a power plant to clean the tubes internally.
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