The evaporator section of a refrigeration unit consists of thin-walled, 10-mm-diameter tubes through which refrigerant passes at a temperature of −18°c. air is cooled as it flows over the tubes, maintaining a surface convection coefficient of 100 w/m2 · k, and is subsequently routed to the refrigerator compartment. (a) for the foregoing conditions and an air temperature of −3°c, what is the rate at which heat is extracted from the air per unit tube length? (b) the refrigerator is disconnected after the defrost unit malfunctions and a 2-mm-thick layer of frost has formed. if the tubes are in ambient air for which t∞ = 20°c and natural convection maintains a convection coefficient of 2 w/m2 · k, how long will it take for the frost to melt? the frost may be assumed to have a mass density of 700 kg/m3 and a latent heat of fusion of 334 kj/kg and a thermal conductivity of 0.4 w/m · k.