Along plastic rod of 20-mm diameter (k = 0.3 w/m ⋅ k and rhocp = 1040 kj/m3 ⋅ k) is uniformly heated in an oven as preparation for a pressing operation. for best results, the temperature in the rod should not be less than 200°c. to what uniform temperature should the rod be heated in the oven if, for the worst case, the rod sits on a conveyor for 3 min while exposed to convection cooling with ambient air at 25°c and with a convection coefficient of 10 w/m2 ⋅ k? a further condition for good results is a maximum–minimum temperature difference of less than 10°c. is this condition satisfied? if not, what could you do to satisfy it?

Air is to be cooled in the evaporator section of a refrigerator by passing it over a bank of 0.8-cm-outer-diameter and 0.4-m-long tubes inside which the refrigerant is evaporating at -20°c. air approaches the tube bank in the normal direction at 0°c and 1 atm with a mean velocity of 4 m/s. the tubes are arranged in-line with longitudinal and transverse pitches of sl- st 1.5 cm. there are 30 rows in the flow direction with 15 tubes in each row. determine (a) the refrigeration capacity of this system and (b) pressure drop across the tube bank. evaluate the air properties at an assumed mean temperature of -5°c and 1 atm. is this a good assumption?