342 Chapter 11 Temperature and Temperature-Related Parameters
Perform a search to obtain information about these
two types of calorimeters. Write a brief report dis-
cussing the principles behind the operation of these
calorimeters.
11.24. Refer to Tables 11.12 and 11.13 to answer this question.
What is the maximum amount of energy released when
a 10-lbmsample of coal from McDowell, West Virginia
is burned? Also calculate the amount of energy released
when 15 ft
3
of natural gas from Oklahoma is burned.
11.25. Contact the natural gas provider in your city and find
out how much you are being charged for each ft
3
of
natural gas. Also, contact your electric company and
determine how much they are charging on average per
kWh usage of electricity. If a hot-air gas furnace has an
efficiency of 94% and an electric heater has an effi-
ciency of 100%, what is the more economical way of
heating your home: a gas furnace or an electric heater?
11.26.Convert the results of Example 11.13 from Btu to
calories.
11.27. Calculate the heat transfer rate from a 1000 ft
2
, 6-in-
thick concrete wall with inside and outside surface tem-
peratures of 20 °C and 0 °C.
11.28. For Problem 11.27, calculate the reduction in the heat
transfer rate if a 2-in.-insulation batt with thermal con-
ductivity of is added to the wall.
11.29.The side walls of a refrigerator are made from two thin
layers of sheet metal, each 1/8 in. thick, and with ther-
mal conductivity of , and a 2-in. foam
insulation with. If the inside and
outside surface temperatures of the refrigerator wall are
35 °F and 70 °F, respectively, calculate the heat loss
through an area of 8 ft
2
.
11.30.Calculate the amount of thermal energy required to
raise the temperature of 20 gallon of water from 60 °F
to 120 °F. Express your answer in Btu, J, and cal.
11.31. In a commercial water heater, 20 gallons /min of water
is heated from 60 °F to 140 °F. Calculate the amount
of energy required per hour.11.32.A gas furnace puts out to compensate for
heat loss from a house located in Minnesota. What is
the equivalent value of thermal power (thermal energy
per unit time) output of the furnace in watts?
11.33.Calculate the heat loss from a double-pane-glass win-
dow consisting of two pieces of glass, each having a
thickness of 10 mm with a thermal conductivity of. The two glass-panes are separated by an
air gap of 7 mm. Assume thermal conductivity of air to
k1.3
W
m#K90,000
Btu
h1
Btu
h^2k0.027
Btu
h#ft#°Fk 42
Btu
h#ft#°Fk0.03
Btu
h#ft#°Fbe. Also, express the total Rand Uval-
ues in both SI and U.S. Customary units.
11.34.Determine the heat-transfer rate from an electronic
chip whose surface temperature is 30°C and has an
exposed surface area of 4 cm
2. The temperature of sur-
rounding air is 25 °C. The heat-transfer coefficient for
this situation is. Express your answer
in both SI and U.S. Customary units. What is the
R-factor (film resistance) for this situation?
11.35.Calculate the amount of radiation emitted for a unit
surface (1 m
2
) for the following situations: (a) a hot
pavement in Arizona at 50 °C (122 °F) and 0.8,
(b) a hood of a car at 40 °C (104 °F) and 0.9, and
(c) a sunbather at 38 °C (100 °F) and 0.9. Express
your answers in both SI and U.S. Customary units.
11.36.For Problems 11.11, 11.12, and 11.13, calculate the
U-factors.
11.37. For Problem 11.12, calculate the heat loss through the
frame wall if the R-19 insulation batt is replaced by
foam insulation having a R-value of 22.
11.38.For Problem 11.13, calculate the heat loss through the
ceiling, if the R-19 insulation batt is replaced by R-40
fiberglass insulation.
11.39. Nine, old, 12 ft
2
windows with were
replaced with new windows having.
Calculate the energy savings on a day during a 5-hour
period, when Tin68 °F, Toutside10 °F.
11.40.For Problem 11.39, calculate the savings in ft
3
of nat-
ural gas (from Louisiana). Assume the furnace has an
efficiency of 92%.
11.41. A family uses 80 gallons of hot water per day. The water
is heated from a line temperature of 55 °F to 140 °F.
Calculate the amount of natural gas (Oklahoma) that
is required to heat the water in a heater with an effi-
ciency of 80%.
11.42. For Problem 11.17, determine the thermal conductiv-
ity of the unknown sample for the set of data given in
accompanying table.
U0.3
Btu
h#ft^2 #°FU1.2
Btu
h# ft^2 #°Fe
e
e
h 25
W
m^2 #Kk0.022
W
m#KThermocouple Temperature
Location (C)1 125
2 105
380
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