Practice Problem Solutions_Exam 3

Practice Problem Solutions_Exam 3 - A) Problem: Heat Loss...

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A) Problem : Heat Loss During Breathing Estimate heat loss during breathing. Assume that a normal person inspires about 6 L/min of bone dry air at 20 º C. Assume that expired air is saturated with warm vapor and is at 37 º C. The molecular weight of air is 28.9 g/mol and the density of air is 0.0012 g/cm 3 . The molal humidity for saturated water vapor at 37 º C is 6.7%; therefore, when saturated, there are approximately 0.041 g of water per 1 g of dry air. The enthalpy change involved with heating air from 20 º C to 37 º C is 494 J/mol and the enthalpy change associated with the vaporization of water to saturated air is 577 cal/g. Solution : The basis of the inlet strea ݉ 1 = ܸ ߩ = (6 L/min)(0.0012 g/cm 3 )(1000 cm 3 /L) = 7.2 g/min m is calculated using the inlet flow rate of 6 L/min: Energy balance equation: S p k W Q E E H = Δ + Δ + Δ Æ Q H = Δ Molar flow rates of air in and out: n 1,air = n 3,air = M V ρ = (6 L/min) (0.0012 g/cm 3 )(1000 cm 3 /L) / (28.84 g/mol) = 0.25 mol/min Ma ݉ 3,H2O = (0.041 g H 2 O/g air)(7.2 g air/min) = 0.295 g H 2 O/min ss flow rate of water in the outlet: vap O H heat air H m H n H ˆ ˆ 2 , 3 , 3 Δ + Δ = Δ = (0.25 mol/min)(494 J/mol) + (0.295 g/min)(577 cal/g)(4.184 J/cal) H Q Δ = = 836 J/min
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) Problem : Hydroelectric Power Suppose that water from a reservoir flowing into a hydroelectric power plant has a velocity of 1.0 m/s. After flowing through the plant, the water enters a river and its velocity drops to nearly 0 m/s. If water flows through the power plant at 2.8 m 3 /s and has a head of 9 m, how much energy can theoretically be produced? At this particular plant, only 190 kW of power is produced. The efficiency of the power plant is estimated as the ratio of the actual energy that is relayed from the plant relative to the ideal or maximum energy generation. What is the efficiency of
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Practice Problem Solutions_Exam 3 - A) Problem: Heat Loss...

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