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1-137 An electric heater placed in a room consumes 500 W power when its surfaces are at 120C. The surface temperature
when the heater consumes 700 W is to be determined without and with the consideration of radiation.
Assumptions 1 Steady operating conditions exist. 2 The temperature is uniform over the surface.
Analysis (a) Neglecting radiation, the convection heat transfer coefficient is determined from
W500
Q
A,
Ts
qconv
qrad
T
, h
Tw
e
W
1-75
1-138 An ice skating rink is located in a room is considered. The refrigeration load of the system and the time it takes to melt
3 mm of ice are to be determined.
Assumptions 1 Steady operating conditions exist in part (a). 2 The surface is insulated on the back side in part (b).
Properties The heat of fusion and the density of ice are given to be 333.7 kJ/kg and 920 kg/m3, respectively.
Analysis (a) The refrigeration load is determined from
( )
W156,300=−+−=
−+−=
−448
4
s
4
airload
273298)1067.5)(1240((0.95))020)(1240)(10(
)( TTATThAQws
(b) The time it takes to melt 3 mm of ice is determined from
min 47.2==
== s 2831
J/s 300,156
)J/kg 10)(333.7kg/m m)(920 )(0.003m 12(40 332
load
Q
hLW
tif
Tw = 25C
Qrad Qconv
Ice
Insulation
dT
4
dT
4
4
1-76
Fundamentals of Engineering (FE) Exam Problems
1-139 Which equation below is used to determine the heat flux for conduction?
dT
4
1-77
1-143 A 2-kW electric resistance heater submerged in 30-kg water is turned on and kept on for 10 min. During the process,
500 kJ of heat is lost from the water. The temperature rise of water is
(a) 5.6C (b) 9.6C (c) 13.6C (d) 23.3C (e) 42.5C
Answer (a) 5.6C
Solution Solved by EES Software. Solutions can be verified by copying-and-pasting the following lines on a blank EES
screen.
C=4.18 [kJ/kg-K]
1-144 Eggs with a mass of 0.15 kg per egg and a specific heat of 3.32 kJ/kgC are cooled from 32C to 10C at a rate of 300
eggs per minute. The rate of heat removal from the eggs is
(a) 11 kW (b) 80 kW (c) 25 kW (d) 657 kW (e) 55 kW
1-78
1-145 A cold bottled drink (m = 2.5 kg, cp = 4200 J/kgC) at 5C is left on a table in a room. The average temperature
of the drink is observed to rise to 15C in 30 minutes. The average rate of heat transfer to the drink is
(a) 23 W (b) 29 W (c) 58 W (d) 88 W (e) 122 W
1-146 Water enters a pipe at 20ºC at a rate of 0.25 kg/s and is heated to 60ºC. The rate of heat transfer to the water is
(a) 10 kW (b) 20.9 kW (c) 41.8 kW (d) 62.7 kW (e) 167.2 kW
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1-80
1-149 Steady heat conduction occurs through a 0.3-m thick 9 m by 3 m composite wall at a rate of 1.2 kW. If the inner and
outer surface temperatures of the wall are 15C and 7C, the effective thermal conductivity of the wall is
(a) 0.61 W/mC (b) 0.83 W/mC (c) 1.7 W/mC (d) 2.2 W/mC (e) 5.1 W/mC
1-150 Heat is lost through a brick wall (k = 0.72 W/m·ºC), which is 4 m long, 3 m wide, and 25 cm thick at a rate of 500 W.
If the inner surface of the wall is at 22ºC, the temperature at the midplane of the wall is
(a) 0ºC (b) 7.5ºC (c) 11.0ºC (d) 14.8ºC (e) 22ºC
1-81
1-151 A 10-cm high and 20-cm wide circuit board houses on its surface 100 closely spaced chips, each generating heat at a
rate of 0.08 W and transferring it by convection and radiation to the surrounding medium at 40C. Heat transfer from the
back surface of the board is negligible. If the combined convection and radiation heat transfer coefficient on the surface of the
board is 22 W/m2C, the average surface temperature of the chips is
(a) 72.4C (b) 66.5C (c) 40.4C (d) 58.2C (e) 49.1C
1-152 A 40-cm-long, 0.4-cm-diameter electric resistance wire submerged in water is used to determine the convection heat
transfer coefficient in water during boiling at 1 atm pressure. The surface temperature of the wire is measured to be 114C
when a wattmeter indicates the electric power consumption to be 7.6 kW. The heat transfer coefficient is
(a) 108 kW/m2C (b) 13.3 kW/m2C (c) 68.1 kW/m2C (d) 0.76 kW/m2C (e) 256 kW/m2C
1-82
1-153 While driving down a highway early in the evening, the air flow over an automobile establishes an overall heat transfer
coefficient of 25 W/m2K. The passenger cabin of this automobile exposes 8 m2 of surface to the moving ambient air. On a
day when the ambient temperature is 33oC, how much cooling must the air conditioning system supply to maintain a
temperature of 20oC in the passenger cabin?
(a) 0.65 MW (b) 1.4 MW (c) 2.6 MW (d) 3.5 MW (e) 0.94 MW
1-154 A room is heated by a 1.2 kW electric resistance heater whose wires have a diameter of 4 mm and a total length of
3.4 m. The air in the room is at 23ºC and the interior surfaces of the room are at 17ºC. The convection heat transfer
coefficient on the surface of the wires is 8 W/m2·ºC. If the rates of heat transfer from the wires to the room by
convection and by radiation are equal, the surface temperature of the wires is
(a) 3534ºC (b) 1778ºC (c) 1772ºC (d) 98ºC (e) 25ºC
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1-84
1-157 A 30-cm diameter black ball at 120C is suspended in air, and is losing heat to the surrounding air at 25C by
convection with a heat transfer coefficient of 12 W/m2C, and by radiation to the surrounding surfaces at 15C. The total rate
of heat transfer from the black ball is
(a) 322 W (b) 595 W (c) 234 W (d) 472 W (e) 2100 W
1-158 A 3-m2 black surface at 140C is losing heat to the surrounding air at 35C by convection with a heat transfer
coefficient of 16 W/m2C, and by radiation to the surrounding surfaces at 15C. The total rate of heat loss from the surface is
(a) 5105 W (b) 2940 W (c) 3779 W (d) 8819 W (e) 5040 W
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