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Unformatted text preview: Chapter 3 Properties of Pure Substances Review Problems 3107 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 ( V V 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 ( / ) V A D = = V V 2 4 Solving for the diameter D and substituting, m 0.268 V = = = m/s) (8 ) /s m 45 . ( 4 4 3 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. 3108 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 . )( 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 ( / ) V A D = = V V 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. 335 Smoking Lounge 15 smokers 30 L/s person House 200 m 2 0.35 ACH Chapter 3 Properties of Pure Substances 3109 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. Analysis The absolute pressures in the tire before and after the trip are P P P P 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 P V T P V T T T P P 1 1 1 2 2 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. 336 TIRE 200 kPa 0.022 m 3 Chapter 3 Properties of Pure Substances 3110 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....
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 Spring '08
 Chung

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