FTFS Chap09 P119 - Chapter 9 Gas Mixtures and...

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Unformatted text preview: Chapter 9 Gas Mixtures and Psychrometrics 9-119E Air enters a heating section at a specified pressure, temperature, velocity, and relative humidity. The exit temperature of air, the exit relative humidity, and the exit velocity are to be determined. Assumptions 1 This is a steady-flow process and thus the mass flow rate of dry air remains constant during the entire process ( ) m m m a a a 1 2 = = . 2 Dry air and water vapor are ideal gases. 3 The kinetic and potential energy changes are negligible. Analysis ( a ) The amount of moisture in the air remains constant ( ϖ 1 = ϖ 2 ) as it flows through the heating section since the process involves no humidification or dehumidification. The inlet state of the air is completely specified, and the total pressure is 1 atm. The properties of the air at the inlet state are determined from the psychometric chart (Figure A-33) to be h v 1 1 153 00030 12 9 = = = = . . ) . / Btu / lbm dry air lbm H O / lbm dry air ( ft lbm dry air 1 2 2 3 ϖ ϖ The mass flow rate of dry air through the heating section is ( . / ( ) . m A a = = × = 1 1 12 9 25 2 38 1 1 1 v V ft lbm) ft / s)( (15/12) / 4 ft lbm / s 3 2 2 π From the energy balance on air in the heating section, air dry Btu/lbm 9 . 16 Btu/lbm ) 3 . 15 lbm/s)( (2.38 = kW 1 Btu/s 0.9478 kW 4 ) ( 2 2 1 2 in =- - = h h h h m Q a The exit state of the air is fixed now since we know both h 2 and ϖ 2 . From the psychometric chart at this state we read T 2 = ° 56.8 F ( b ) air dry lbm / ft 1 . 13 3 2 2 = = v 30.8% φ ( c ) The exit velocity is determined from the conservation of mass of dry air, m m V v V v A v A v a a 1 2 1 1 2 2 1 1 2 2 = → = → = V V Thus, V V 2 2 1 1 131 12 9 25 = = = v v . . ( ft / s) 25.4 ft / s 9-59 50 ° F 40% RH 25 ft/s 1 atm AIR D = 15 in 1 2 4 kW Chapter 9 Gas Mixtures and Psychrometrics 9-120 Air enters a cooling section at a specified pressure, temperature, velocity, and relative humidity. The exit temperature, the exit relative humidity of the air, and the exit velocity are to be determined. Assumptions 1 This is a steady-flow process and thus the mass flow rate of dry air remains constant during the entire process ( ) m m m a a a 1 2 = = . 2 Dry air and water vapor are ideal gases. 3 The kinetic and potential energy changes are negligible. Analysis ( a ) The amount of moisture in the air remains constant ( ϖ 1 = ϖ 2 ) as it flows through the cooling section since the process involves no humidification or dehumidification. The inlet state of the air is completely specified, and the total pressure is 1 atm. The properties of the air at the inlet state are determined from the psychometric chart (Figure A-33) to be h v 1 1 550 00089 0877 = = = = ....
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This homework help was uploaded on 04/18/2008 for the course EML 3007 taught by Professor Chung during the Spring '08 term at University of Florida.

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FTFS Chap09 P119 - Chapter 9 Gas Mixtures and...

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