Thermodynamics HW Solutions 21

Thermodynamics HW Solutions 21 - Chapter 1 Basics of Heat...

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Chapter 1 Basics of Heat Transfer 1-42E Air gains heat as it flows through the duct of an air-conditioning system. The velocity of the air at the duct inlet and the temperature of the air at the exit are to be determined. Assumptions 1 Air is an ideal gas since it is at a high temperature and low pressure relative to its critical point values of -222 ° F and 548 psia. 2 The kinetic and potential energy changes are negligible, . 3 Constant specific heats at room temperature can be used for air, C ΔΔ ke pe ≅≅ 0 p = 0.2404 and C v = 0.1719 Btu/lbm·R. This assumption results in negligible error in heating and air-conditioning applications. Properties The gas constant of air is R = 0.3704 psia.ft 3 /lbm.R (Table A-1). Also, C p = 0.2404 Btu/lbm·R for air at room temperature (Table A-15E). Analysis We take the air-conditioning duct as the system. This is a control volume since mass crosses the system boundary during the process. We observe that this is a steady-flow process since there is no change with time at any point and thus Δ Δ mE CV CV and = = 00 & , there is only one inlet and one exit and thus
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