Example 11.3 Using the units of energy and time, show that 1 watt (W) is equal to 3.4123 Btu /h, as shown
in Table 11.2.Example 11.4 In many parts of the United States, in order to keep a house warm in the winter months, a gas
furnace is used. If a gas furnace puts out 60,000 Btu /h to compensate for heat loss from a
house, what is the equivalent value of the thermal power (energy per unit time) output of the
furnace in watts?
From Table 11.2, you know that: 1 Btu 1055 J; you also know that 1 h 3600 s. Sub-
stituting for these values,Or you could have used the direct conversion factor between Btu /h and W, as shown:
Now that you know heat transfer or thermal energy transfer occurs as a result of tempera-
ture difference in an object or between objects, let us look at different modes of heat transfer.
There are three different mechanisms by which energy is transferred from a high-temperature
region to a low-temperature region. These are referred to as themodesof heat transfer. The
three modes of heat transfer are conduction, convection, and radiation.Conduction
Conductionrefers to that mode of heat transfer that occurs when a temperature difference (gra-
dient) exists in a medium. The energy is transported within the medium from the region with
more-energetic molecules to the region with less-energetic molecules. Of course, it is the inter-
action of the molecules with their neighbors that makes the transfer of energy possible. To bet-
ter demonstrate the idea of molecular interactions, consider the following example of
conduction heat transfer. All of you have experienced what happens when you heat up some
soup in an aluminum container on a stove. Why do the handles or the lid of the soup container
get hot, even though the handles and the lid are not in direct contact with the heating element?
Well, let us examine what is happening.q60,000a
Btu
h
b°
1 W
3.4123a
Btu
h
b
¢17,583 W17.583 kW
q60,000a
Btu
h
ba
1,055 J
1 Btu
ba
1 h
3,600 s
b17,583a
J
s
b17,583 W17.583 kW
1 W 1 a
J
s
ba
1 Btu
1055 J
ba
3600 s
1 h
b3.4123
Btu
h
316 Chapter 11 Temperature and Temperature-Related Parameters
Copyright 2010 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s).圀圀圀⸀夀䄀娀䐀䄀一倀刀䔀匀匀⸀䌀伀䴀圀圀圀⸀夀䄀娀䐀䄀一倀刀䔀匀匀⸀䌀伀䴀