FTFS Chap03 P107 - Chapter 3 Properties of Pure Substances...

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Chapter 3 Properties of Pure Substances Review Problems 3-107 A smoking lounge that can accommodate 15 smokers is considered. The required minimum flow rate of air that needs to be supplied to the lounge and the diameter of the duct are to be determined. Assumptions Infiltration of air into the smoking lounge is negligible. Properties The minimum fresh air requirements for a smoking lounge is given to be 30 L/s per person. Analysis The required minimum flow rate of air that needs to be supplied to the lounge is determined directly from 30 L/s person 15 smokers Smoking Lounge && ( VV air air per person No. of persons) = (30 L /s person)(15 persons) = 450 L /s = = 0.45 m / s 3 The volume flow rate of fresh air can be expressed as & (/ VA D == π 2 4 ) Solving for the diameter D and substituting, m 0.268 V = = = m/s) (8 ) /s m 45 . 0 ( 4 4 3 ππ V D & Therefore, the diameter of the fresh air duct should be at least 26.8 cm if the velocity of air is not to exceed 8 m/s. 3-108 The minimum fresh air requirements of a residential building is specified to be 0.35 air changes per hour. The size of the fan that needs to be installed and the diameter of the duct are to be determined. Analysis The volume of the building and the required minimum volume flow rate of fresh air are L/min 3150 = = = = × = = = L/h 189,000 h / m 189 ) /h 35 . 0 )( m 540 ( ACH m 540 ) m m)(200 7 . 2 ( 3 3 room 3 2 room V V V & The volume flow rate of fresh air can be expressed as & D 2 4 ) Solving for the diameter D and substituting, m 0.106 V = = = m/s) (6 ) /s m 3600 / 189 ( 4 4 3 V D & Therefore, the diameter of the fresh air duct should be at least 10.6 cm if the velocity of air is not to exceed 6 m/s. House 200 m 2 0.35 ACH 3-35
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Chapter 3 Properties of Pure Substances 3-109 The pressure in an automobile tire increases during a trip while its volume remains constant. The percent increase in the absolute temperature of the air in the tire is to be determined. Assumptions 1 The volume of the tire remains constant. 2 Air is an ideal gas. Properties The local atmospheric pressure is 90 kPa. TIRE 200 kPa 0.022 m 3 Analysis The absolute pressures in the tire before and after the trip are PP P P 1 2 200 90 290 220 90 310 =+ = + = = + = gage,1 atm gage,2 atm kPa kPa Noting that air is an ideal gas and the volume is constant, the ratio of absolute temperatures after and before the trip are PV T T T T P P 11 1 22 2 2 1 2 1 =→ = = 310 kPa 290 kPa =1.069 Therefore, the absolute temperature of air in the tire will increase by 6.9% during this trip. 3-36
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Chapter 3 Properties of Pure Substances 3-110 A hot air balloon with 3 people in its cage is hanging still in the air. The average temperature of the air in the balloon for two environment temperatures is to be determined. Assumptions Air is an ideal gas. Properties The gas constant of air is R = 0.287 kPa.m 3 /kg.K. Analysis The buoyancy force acting on the balloon is N 44,700 m/s kg 1 N 1 ) m )(4189 m/s )(9.8 kg/m (1.089 kg/m 1.089 K) K)(288 /kg m kPa (0.287 kPa 90 m 4189 /3 m) (10 4 3 / 4 2 3 2 3 balloon air cool 3 3 air cool 3 3 3 balloon = = = = = = = = = gV F RT P r V B ρ ππ Hot air balloon D = 20 m The vertical force balance on the balloon gives m cage = 80 kg P atm = 90 kPa T = 15 ° C g m m m W W W F B ) (
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This note was uploaded on 04/14/2008 for the course ME 320 taught by Professor Dr.kinne during the Spring '07 term at University of Texas at Austin.

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FTFS Chap03 P107 - Chapter 3 Properties of Pure Substances...

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