A thin-walled, uninsulated 0.4 m diameter duct is used to route chilled air at 0.07 kg/s through the attic of a large commercial building. The attic air is at 37oC, and natural circulation provides a convection coefficient of 4 W/m2K at the outer surface of the duct. If chilled air enters a 16 m long duct at 7oC, what is its exit temperature and the rate of heat gain? Properties of the chilled air may be evaluated at an assumed average temperature of 300 K.

The temperature of air decreases as it is compressed by an adiabatic compressor. a)- true b)- false

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?

During a steady flow process, the change of energy with respect to time is zero. a)- true b)- false