example 9-2 - Example 9-2 A duct carries air from the...

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Example 9-2 A duct carries air from the furnace of a residence along the basement ceiling and then executes a 90° turn to travel up through a wall. The basement portion of the duct is 20 ft long and loses heat to the room air at an average rate of 70 Btu/h per foot of duct length. Air leaves the furnace at 110°F, 17 psia, with a velocity of 13 ft/s. The duct is 7 in. in diameter. Calculate the air temperature at the 90° turn. Assume kinetic energy is negligible and use constant specific heat at 110°F. Solution: Apply the first law to the air in the 20 ft section of duct. For an open system 22 cv i e cv cv i i i e e e dE Q W m h gz m h gz dt ⎛⎞ =−+ + + − + + ⎜⎟ ⎝⎠ ± ± ±± VV In steady operation, the left hand side is zero. No work is done and there is no change in elevation. With negligible
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example 9-2 - Example 9-2 A duct carries air from the...

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