Thermodynamics HW Solutions 266

Thermodynamics HW Solutions 266 - o Steam pipe W 6974 C ) 8...

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Chapter 3 Steady Heat Conduction 3-156 An 10-m long section of a steam pipe exposed to the ambient is to be insulated to reduce the heat loss through that section of the pipe by 90 percent. The amount of heat loss from the steam in 10 h and the amount of saved per year by insulating the steam pipe. Assumptions 1 Heat transfer through the pipe is steady and one-dimensional. 2 Thermal conductivities are constant. 3 The furnace operates continuously. 4 The given heat transfer coefficients accounts for the radiation effects. 5 The temperatures of the pipe surface and the surroundings are representative of annual average during operating hours. 6 The plant operates 110 days a year. Analysis The rate of heat transfer for the uninsulated case is T air =8 ° C T s =82 ° C 2 m 77 . 3 m) 10 ( m) 12 . 0 ( = = = ππ L D A o
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Unformatted text preview: o Steam pipe W 6974 C ) 8 82 )( m 77 . 3 )( C . W/m 25 ( ) ( 2 2 = = = air s o T T hA Q & The amount of heat loss during a 10-hour period is day) (per ) s 3600 kJ/s)(10 974 . 6 ( kJ 10 2.511 5 = = = t Q Q & The steam generator has an efficiency of 80%, and steam heating is used for 110 days a year. Then the amount of natural gas consumed per year and its cost are therms/yr 2 . 327 days/yr) 110 ( kJ 105,500 therm 1 0.80 kJ 10 511 . 2 used Fuel 5 = = $196.3/yr = erm) )($0.60/th therms/yr 2 . 327 ( fuel) of cost fuel)(Unit of Amount ( fuel of Cost = = Then the money saved by reducing the heat loss by 90% by insulation becomes $177 = $196.3/yr 0.9 fuel) of (Cost 0.9 = saved Money = 3-109...
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