Thermodynamics HW Solutions 18

Thermodynamics HW Solutions 18 - Chapter 1 Basics of Heat...

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Chapter 1 Basics of Heat Transfer 1-39 The resistance heating element of an electrically heated house is placed in a duct. The air is moved by a fan, and heat is lost through the walls of the duct. The power rating of the electric resistance heater is 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 -141 ° C and 3.77 MPa. 2 The kinetic and potential energy changes are negligible, . 3 Constant specific heats at room temperature can be used for air. This assumption results in negligible error in heating and air-conditioning applications. ΔΔ ke pe ≅≅ 0 Properties The specific heat of air at room temperature is C p = 1.007 kJ/kg· ° C (Table A-15). Analysis We take the heating duct as the system. This is a control volume since mass crosses the system boundary during the process.
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