WHEN TO DEFROST A REFRIGERATOR, AND WHEN TO REMOVE THE SCALE FROM THEHEAT-EXCHANGER OF A POWER-PLANT

This paper demonstrates the existence of an optimal on/off sequence fo r operating a household refrigerator that accumulates ice on its evapo rator coils. Experimentally it is shown that the rate of ice formation is constant in time. The optimal (intermittent) regime of operation d etermined in this paper minimizes the power required by the refrigerat or, maintains the prescribed temperature of the cold space, and remove s intermittently the ice layer. The second part of the paper proves th at a similar strategy can be used for dealing with fouling in heat exc hangers, i.e. for maximizing the power output of a power plant that is diminished by the formation of scale on its heat exchanger surfaces. These optimal on/off sequences of operation for refrigerators and powe r plants are proven based on pure heat transfer and themodynamics grou nds. The refrigerator and power plant models employed are the simplest models possible. Consequently, the optimal on/off sequences demonstra ted in this paper are fundamental features that will be present (and d eserve to be identified and exploited) in the design of actual refrige rators and power plants, no matter how complicated these designs may b e.