Thermodynamics HW Solutions 581

Thermodynamics HW Solutions 581 - Chapter 7 External Forced...

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Chapter 7 External Forced Convection 7-52 A steam pipe is exposed to a light winds in the atmosphere. The amount of heat loss from the steam during a certain period and the money the facility will save a year as a result of insulating the steam pipe are to be determined. Assumptions 1 Steady operating conditions exist. 2 Air is an ideal gas with constant properties. 3 The plant operates every day of the year for 10 h a day. 4 The local atmospheric pressure is 1 atm. Properties The properties of air at 1 atm and the film temperature of (T s + T )/2 = (75+5)/2 = 40 ° C are (Table A-15) 7255 . 0 Pr /s m 10 702 . 1 C W/m. 02662 . 0 2 5 - = × = ° = υ k Analysis The Reynolds number is [ ] 4 2 5 10 632 . 1 /s m 10 702 . 1 m) (0.1 m/s 1000/3600) (10 Re × = × × = υ = D V Wind V = 10 km/h T = 5 ° C Steam pipe T s = 75 ° C D = 10 cm ε = 0.8 The Nusselt number corresponding this Reynolds number is determined to be [] [] 19 . 71 000 , 282 10 632 . 1 1 ) 7255 . 0 / 4 . 0 ( 1 ) 7255
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