Explanation Express enthalpy of water. h w = h f @ 60 °F (I) Here, enthalpy of saturation liquid at temperature of 60°F is h f @ 60 °F . As the process is a steady flow and thus the mass flow rate of dry air remains constant during the entire process. m ˙ a 1 = m ˙ a 2 = m ˙ a Here, the mass flow rate of air at inlet is m ˙ a 1 , mass flow rate of dry air at exit is m ˙ a 2 and mass flow rate of dry air is m ˙ a . Express the rate at which liquid water is formed inside this pipe by using an water mass balance. ∑ m ˙ w , i = ∑ m ˙ w , e m ˙ a 1 ω 1 = m ˙ a 2 ω 2 + m ˙ w Here, mass flow rate of water at inlet and exit is m ˙ w , i and m ˙ w , e respectively, and the rate at which liquid water is formed inside this pipe is m ˙ w . Express an energy balance. E ˙ in − E ˙ out = Δ E ˙ system E ˙ in − E ˙ out = 0 E ˙ in = E ˙ out ∑ m ˙ i h i + Q ˙ in = + ∑ m ˙ e h e Q ˙ in = m ˙ a 2 h 2 − m ˙ a 1 h 1 − m ˙ w h w = m ˙ a ( h 2 − h 1 ) − m ˙ w h w Here, the rate of total energy entering the system is E ˙ in , the rate of total energy leaving the system is E ˙ out , the rate of change in the total energy of the system is Δ E ˙ system , initial and exit mass flow rate is m ˙ i and m ˙ e respectively, the required rate of heat is Q ˙ in , enthalpy at inlet and exit is h i and h e respectively and enthalpy at state 1, 2, water is h 1 , h 2 and h w respectively. Express the heat supplied to the system. q in = h 2 − h 1 − ( ω 2 − ω 1 ) h w (II) Express the mass of air that has to be humidified. m a = ν v 1 (III) Here, volume is ν and specific volume is v 1 . Express the required heat. Q in = m a q in (IV) Conclusion: Refer Table A-4E, “saturated water-temperature table”, and write the enthalpy of water at temperature of 60°F using an interpolation method. h f @ 60 °F = 28

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Chapter 14.7, Problem 129RP

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How much heat is required.

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Thermodynamics: An Engineering Approach

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How much heat is required.

Solution Summary: The author explains how much heat is required to express the enthalpy of water. The mass flow rate of dry air remains constant during the entire process.

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