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2300_les05

# 2300_les05 - II Energy and the First Law of Thermodynamics...

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1 Lesson 5, Geof Silcox, Chemical Engineering, University of Utah II. Energy and the First Law of Thermodynamics A. Energy Transfer as Heat Transfer. Energy is Conserved. 1. Energy can cross the boundary of a closed system by only two mechanisms: heat transfer and work transfer. 2. The change in energy of a closed system is equal to the net heat transferred to the system minus the net work performed by the system. , , sys in net out net E Q W = Closed system Net work out, W out,net = W out - W in Net heat in, Q in,net = Q in - Q out (2-34); (4-17) total system surroundings E E E 0 = + = system surroundings E E = − Lesson 5, Geof Silcox, Chemical Engineering, University of Utah II. Energy and the First Law of Thermodynamics B. Heat transfer 1. Heat is energy transferred across the boundary of the system due to a difference in temperature between the system and the surroundings . a. The heat flux, , is the rate of heat transfer per unit area of surface. The units are W/m 2 or kW/m 2 or Btu/(ft 2 h) b. The heat transfer rate (kW) is calculated from the heat flux as = A dA q Q ± ± A is the area over which heat transfer is occurring.

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