Census tracts are spatial areas with about 5,000 people each and varying areas. Using the data on Chicago census tracts for 2010 (included on the spreadsheet Census Tract Data, downloadable from Blackboard), estimate a density gradient for Chicago in 2010. In particular estimate γ, from the equation: lg Density = lg (D0) – γx, where x is distance in miles from the CBD.

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?