Civil Engineering Reference
In-Depth Information
Table 2.5
Parameters frequently used
Description
Other systems
SI system
kcal/kg
C
Btu/lb
F
Specific heat
kJ/kg K
Water
1
1
4.18
Superheated steam
a
0.5
0.5
2.09
Air
0.24
0.24
1
Iron
0.114
0.114
0.477
Copper
0.092
0.092
0.385
Mineral oil
0.486
0.486
2.034
Density
b
kg/m
3
Water 62.5 1,000
Air (standard conditions) 0.08 1.29
Mineral oil 57.75 925
Iron 490 7,850
Copper 557.5 8,930
Natural gas 0.047 0.750
a
Average value in industrial boiler. In air-water mixture (see Chap.
13
) the specific heat of
superheated steam equals 1.8 kJ/kg K (steam pressure
<
0.1 MPa)
b
Density is referred to standard conditions: 0.1 MPa (1.013 bar, 14.5 psi), 273.15 K (0
C; 32
F)
for air, and 288.75 K (15.6
C; 60
F) for natural gas
For an ideal gas, but widely accepted for most real gases, basic relationships (where
V
is the
volume) are:
lb/ft
3
pV¼
constant •
T
1
:
293
T
1
273
The density of air at
T
1
(K) and at standard pressure is: air density
TðÞ¼
ðÞ
Notice that in some countries and in some applications the standard conditions can be different
from the previous ones. For natural gas: 288.75 K (15.6
C; 60
F) and 0.1 MPa (14.5 psi). The user
should ascertain the reference conditions for each application
:
15
Tables
2.6
and
2.7
report values from international statistics and conversion factors.
Some definitions, which will be discussed in detail in later chapters, are
summarized below in order to allow a clearer understanding of Chap.
2
:
• Heating value. This is a measure of the heat any given fuel can release during the
combustion process.
Combustion of fuels consisting of carbon, hydrogen, and sulfur requires oxy-
gen, which normally comes from atmospheric air. It starts at different ignition
temperatures depending on the fuel. Typical values range between 573.15 and
973.15 K (300-700
C, 572-1,292
F).
Once ignition temperature has been reached, combustion continues until all
the fuel or oxygen has been consumed.
Hydrogen, combined with oxygen, produces water (roughly 9 kg of water for
1 kg of hydrogen), which is discharged as liquid water as well as water vapor into
the atmosphere together with combustion gaseous waste at the same temperature;
• Higher Heating Value (HHV), also called Gross Heating value. This is the
number of heat units measured as being liberated when unit mass of fuel is
burned in oxygen saturated with water vapor in a bomb in standard conditions,
the residual materials being gaseous oxygen, carbon dioxide, sulfur dioxide and
