16–40
16–68 An 800-W iron is left on the iron board with its base exposed to the air at 20°C. The temperature of the base of the
iron is to be determined in steady operation.
Assumptions 1 Steady operating conditions exist. 2 The thermal properties of the
iron base and the convection heat transfer coefficient are constant and uniform.
3 The temperature of the surrounding surfaces is the same as the temperature of the
Iron
800 W
16–42
16–70 A flat-plate solar absorber is exposed to an incident solar radiation. The efficiency of the solar absorber (the ratio of
the usable heat collected by the absorber to the incident solar radiation on the absorber) is to be determined.
Assumptions 1 Steady operating conditions exist. 2 Temperature at the surface remained constant.
Properties The absorber surface has an absorptivity of 0.93 and an emissivity of 0.9.
Analysis The rate of usable heat at the absorber plate can be expressed as
convradabsorbedusable QQQQ
4
0.472 2
2
solar
usable
W/m800
W/m5.377
q
q
16–43
16–71 A flat-plate solar collector is used to heat water. The temperature rise of the water heated by the net heat rate from the
solar collector is to be determined.
Assumptions 1 Steady operating conditions exist. 2 Specific heat of water is constant. 3 Temperature at the surface remained
constant. 4 Conduction through the solar absorber is negligible. 5 Heat loss through the sides and back of the absorber is
K])2535)(K W/m5(
2
W4.448
net Q
C21.4
K)J/kg kg/s)(4200 (0.005
W4.448
net
inout
p
cm
Q
TT
16–45
16–73E A flat plate solar collector is placed horizontally on the roof of a house. The rate of heat loss from the collector by
convection and radiation during a calm day are to be determined.
Assumptions 1 Steady operating conditions exist. 2 The emissivity and convection heat transfer coefficient are constant and
radconvtotal QQQ
16–47
Review Problems
16–75 Engine valves are to be heated in a heat treatment section. The amount of heat transfer, the average rate of heat
transfer, the average heat flux, and the number of valves that can be heat treated daily are to be determined.
Assumptions Constant properties given in the problem can be used.
16–49
16–79 An electric resistance heating element is immersed in water initially at 20°C. The time it will take for this heater to
raise the water temperature to 80°C as well as the convection heat transfer coefficients at the beginning and at the end of the
heating process are to be determined.
Assumptions 1 Steady operating conditions exist and thus the rate of heat loss from the wire equals the rate of heat
generation in the wire as a result of resistance heating. 2 Thermal properties of water are constant. 3 Heat losses from the
Analysis When steady operating conditions are reached, we have
W800
generated EQ
. This is also equal to the rate of
heat gain by water. Noting that this is the only mechanism of energy transfer, the time it takes to raise the water temperature
C W/m3185
C W/m1274
2
2
C)80120)(m (0.00628
W800
)(
C)20120)(m (0.00628
W800
)(
2
2
2
2
1
1
TTA
Q
h
TTA
Q
h
ss
ss
16–51
16–82 Electric power required to maintain the surface temperature of an electrical wire submerged in boiling water at 115ºC.
Assumptions 1 Steady operating conditions exist. 2
Convection heat transfer coefficient is uniform. 3 Heat
transfer by radiation is negligible. 4 Heat losses from the
boiler are negligible.
16–53
16–84 The rate of radiation heat transfer between a person and the surrounding surfaces at specified temperatures is to be
determined in summer and in winter.
Assumptions 1 Steady operating conditions exist. 2 Heat transfer by convection is not considered. 3 The person is completely
surrounded by the interior surfaces of the room. 4 The surrounding surfaces are at a uniform temperature.
W84.2=
]KK) (296273)+)[(32m )(1.6.K W/m1067.5)(95.0(
)(
4442428
4
surr
4
rad
TTAQ ss
(b) Winter: Tsurr = 12+273= 285 K
W177.2=
]KK) (285273)+)[(32m )(1.6.K W/m1067.5)(95.0(
)(
4442428
4
surr
4
rad
TTAQ ss
Discussion Note that the radiation heat transfer from the person more than doubles in winter.
Tsurr
Qrad
16–54
16–85 Prob. 16-84 is reconsidered. The rate of radiation heat transfer in winter as a function of the temperature of the
inner surface of the room is to be plotted.
A=1.6 [m^2]
epsilon=0.95
16–55
16–86 The base surface of a cubical furnace is surrounded by black surfaces at a specified temperature. The net rate of
radiation heat transfer to the base surface from the top and side surfaces is to be determined.
Assumptions 1 Steady operating conditions exist. 2 The top and side surfaces of the furnace closely approximate black
surfaces. 3 The properties of the surfaces are constant.
kW 340
W,660339
16–87 The power required to maintain the soldering iron tip at 400 °C is to be determined.
Assumptions 1 Steady operating conditions exist since the tip surface and the surrounding air temperatures remain constant. 2
The thermal properties of the tip and the convection heat transfer coefficient are constant and uniform. 3 The surrounding
surfaces are at the same temperature as the air.
Properties The emissivity of the tip is given to be 0.80.
W2.99 W45.1 W54.1
radconvtotal QQQ
16–57
16–90 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
qrad
W500
2
Q
qconv
T
, h