A water district wishes to compare alternative plans for additional water supplies, using a forty-year period of analysis and an interest rate of 6%.
 A gravity plan would require an investment of $5,000,000 for construction and
$20,000 per year for operation and maintenance.
 A pumping plan would require $3,000,000 for initial construction, an additional
investment of $500,000 in the tenth year, and $50,000 per year for operation
and maintenance. The pumping plan would also incur power costs of $100,000
per year for the first ten years and after connection to another source of demand
additional power costs of $100,000 per year.

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?