Maestria

1-87E A 200-ft 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
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(b) The amount of heat loss per year is
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The amount of gas consumption per year in the furnace that has an efficiency of 86% is
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Then the annual cost of the energy lost becomes
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1-88 A 4-m diameter spherical tank filled with liquid nitrogen at 1 atm and -196C is exposed to convection with ambientair. 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 thin-shelled spherical tank is nearly equal to the temperatureof 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/m3, respectively.
Analysis The rate of heat transfer to the nitrogen tank is
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Then the rate of evaporation of liquid nitrogen in the tank is determined to be
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1-89 A 4-m diameter spherical tank filled with liquid oxygen at 1 atm and-183C 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 thin-shelledspherical tank is nearly equal to the temperature of the oxygen inside.
Properties The heat of vaporization and density of liquid oxygen at 1 atm are given to be 213 kJ/kg and 1140 kg/m3, respectively.
Analysis The rate of heat transfer to the oxygen tank is
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Then the rate of evaporation of liquid oxygen in the tank is determined to be
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1-90
"GIVEN"
D=4 "[m]"
T_s=-196"[C]"
"T_air=20 [C], parameter to be varied"
h=25 "[W/m^2-C]"

"PROPERTIES"
h_fg=198 "[kJ/kg]"

"ANALYSIS"
A=pi*D^2
Q_dot=h*A*(T_air-T_s)
m_dot_evap=(Q_dot*Convert(J/s, kJ/s))/h_fg

|Tair [C] |mevap [kg/s] |
|0 |1.244 |
|2.5 |1.26 |
|5 |1.276|
|7.5 |1.292 |
|10 |1.307 |
|12.5 |1.323 |
|15 |1.339 |
|17.5 |1.355 |
|20 |1.371 |
|22.5 |1.387 |
|25|1.403 |
|27.5 |1.418 |
|30 |1.434 |
|32.5 |1.45 |
|35 |1.466 |

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1-91 A person with a specified surface temperature is subjected to radiation heat transfer in a room at specified wall temperatures. The rate ofradiation heat loss from the person is to be determined.
Assumptions 1 Steady operating conditions exist. 2 Heat transfer by convection is disregarded. 3 The emissivity of the person is constant and uniform over the exposed surface.
Properties The average emissivity of the person is given to be 0.7.
Analysis Noting that the person is completely enclosed by the surrounding surfaces, the net rates of...