The steel wheel for a freight car has three basic functions: (1) to act as a brake drum, (2) to support the weight of the car and its cargo, and (3) to guide the freight car on the rails. Freight car wheels are produced by either casting or rotary forging. They are subjected to complex conditions of dynamic thermal and mechanical stresses. Safety is of great importance, since derailment can cause loss of life and property. Develop a broad systems approach to the design of an improved cast-steel car wheel.

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?