Chapter 1 Basics of Heat Transfer1-122 The duct of an air heating system of a house passes through an unheated space in the attic. The rate of heat loss from the air in the duct to the attic and its cost under steady conditions are to be determined. Assumptions1 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 Steady operating conditions exist since there is no change with time at any point and thus ΔΔmECVCVand ==00. 3 The kinetic and potential energy changes are negligible, ΔΔkepe≅≅0 . 4Constant specific heats at room temperature can be used for air. This assumption results in negligible error in heating and air-conditioning applications. PropertiesThe gas constant of air is R= 0.287 kJ/kg.K (Table A-1). The specific heat of air at room temperature is Cp= 1.007 kJ/kg·°C (Table A-15). AnalysisWe take the heating duct as the system. This is a control volumesince mass crosses the system boundary during the process. There is only one inlet and one exit and thus
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