A thin aluminum sheet with an emissivity of 0.38 on both sides is placed between two very large parallel plates, which are maintained at uniform temperatures T1 = 905 K and T2 = 650 K and have emissivities ε1 = 0.5 and ε2 = 0.8, respectively. Determine the net rate of radiation heat transfer between the two plates without the shield. (Please provide an answer before moving on to the next part.)

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?