This preview shows pages 1–3. Sign up to view the full content.
This preview has intentionally blurred sections. Sign up to view the full version.
View Full Document
Unformatted text preview: Chapter 1 Basics of Heat Transfer 187E A 200ft long section of a steam pipe passes through an open space at a specified temperature. The rate of heat loss from the steam pipe and the annual cost of this energy lost are to be determined. Assumptions 1 Steady operating conditions exist. 2 Heat transfer by radiation is disregarded. 3 The convection heat transfer coefficient is constant and uniform over the surface. Analysis ( a ) The rate of heat loss from the steam pipe is 2 ft 4 . 209 ft) 200 ( ft) 12 / 4 ( = = = π π DL A s Btu/h 289,000 = F ) 50 280 )( ft 4 . 209 ( F) . Btu/h.ft 6 ( ) ( 2 2 pipe ° ° = = air s s T T hA Q ( b ) The amount of heat loss per year is Q Q t = = × = × ( , . ∆ 289 000 2 532 10 9 Btu / h)(365 24 h / yr) Btu / yr The amount of gas consumption per year in the furnace that has an efficiency of 86% is therms/yr 438 , 29 Btu 100,000 therm 1 86 . Btu/yr 10 532 . 2 Loss Energy Annual 9 = × = Then the annual cost of the energy lost becomes Energy cost Annual energy loss)(Unit cost of energy) = (29,438 therms / yr) therm) = = ( ($0. / 58 $17,074 / yr 188 A 4m diameter spherical tank filled with liquid nitrogen at 1 atm and 196 ° C is exposed to convection with ambient air. The rate of evaporation of liquid nitrogen in the tank as a result of the heat transfer from the ambient air is to be determined. Assumptions 1 Steady operating conditions exist. 2 Heat transfer by radiation is disregarded. 3 The convection heat transfer coefficient is constant and uniform over the surface. 4 The temperature of the thinshelled spherical tank is nearly equal to the temperature of the nitrogen inside. Properties The heat of vaporization and density of liquid nitrogen at 1 atm are given to be 198 kJ/kg and 810 kg/m 3 , respectively. Analysis The rate of heat transfer to the nitrogen tank is 2 2 2 m 27 . 50 m) 4 ( = = = π π D A s W 271,430 = ° ° = = C )] 196 ( 20 )[ m 27 . 50 ( C) . W/m 25 ( ) ( 2 2 air T T hA Q s s Then the rate of evaporation of liquid nitrogen in the tank is determined to be kg/s 1.37 = = = → = kJ/kg 198 kJ/s 430 . 271 fg fg h Q m h m Q 143 D =4 in 280 ° F L=200 ft Q Air,50 ° F 1 atm Liquid N 2196 ° C Q Vapor Air 20 ° C Chapter 1 Basics of Heat Transfer 189 A 4m diameter spherical tank filled with liquid oxygen at 1 atm and 183 ° C is exposed to convection with ambient air. The rate of evaporation of liquid oxygen in the tank as a result of the heat transfer from the ambient air is to be determined. Assumptions 1 Steady operating conditions exist. 2 Heat transfer by radiation is disregarded. 3 The convection heat transfer coefficient is constant and uniform over the surface. 4 The temperature of the thinshelled spherical tank is nearly equal to the temperature of the oxygen inside....
View
Full
Document
This homework help was uploaded on 03/19/2008 for the course ME 410 taught by Professor Benard during the Spring '08 term at Michigan State University.
 Spring '08
 BENARD
 Heat Transfer

Click to edit the document details