A30-mm-diameter shaft transmits 700 kw at 1500 rpm. bending and axial loads are negligible. (a) what is the nominal shear stress at the surface? (b) if a hollow shaft of inside diameter 0.8 times outside diameter is used, what outside diameter would be required to give the same outer surface stress? (c) how do weights of the solid and hollow shafts compare?

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?