336 Chapter 11 Temperature and Temperature-Related Parameters
Example 11.17 How much thermal energy is released when 5 lbmof a coal sample from Emroy, Utah is burned?
The total amount of thermal energy,Ethermal, released when some fuel is burned is deter-
mined by multiplying the mass of the fuel,m, by the heating value of fuel,HV.Example 11.18 Calculate the total amount of thermal energy released when 60 ft
3
of natural gas from Louisiana
is burned inside a gas furnace.11.7 Degree-Days and Energy Estimation
With the current energy and sustainability concerns, as future engineers, it is important for
you to understand some of the simple-energy-estimation procedures. For example, we use
degree-days to estimate monthly and annual energy consumptions to heat a building. A
degree day (DD) is the difference between 65°F (typically) and the average temperature of
the outside air during a 24-hour period. For example, for Mankato, MN, on October 10,
2009, the low temperature was 28°F and the high temperature on that day was 38°F. Then,
the degree day for October 10, 2009 for Mankato, MN was: DD65°F((38°F
28°F) /2)32°F. Now, if we were to add up the degree days for each day in a month, we
would get the total degree-days for that month, and similarly, if we were to add up
the degree-days for each month, we would then obtain the annual degree-days. In
engineering practice, historical degree-day values (based on 30-year-averages) are used to
estimate monthly and annual energy consumptions to heat buildings from the following
relationships:(11.29)
-Days) (11.30)
Q (^) yearly 1 QDD 21 Yearly Degree-Days) (11.31)
Q (^) monthly 1 QDD 21 Monthly Degree
Q (^) DD
Heat Lossa
Btu
h
b24 hrs
Design Temperature Difference 1 °F 2
Ethermal 1 60 ft
3
2 1002 a
Btu
ft
3 b60,120 Btu
EthermalmHV 1 5 lbm 2 13,560a
Btu
lbm
b67,800 Btu
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