The specific volume (v) is expressed in cubic feet
per pound.
For any weight of a gas this equation maybe
modified as follows:
W = weight of the gas in pounds,
V = volume of W pounds of the gas in cubic feet.
The volume of 1 pound would then be V/W.
If we substitute this for v in equation 11-3, it then
becomes
Solving equation 11-4 for pressure,
In chapter 2 we defined density as the mass
per unit volume. In equation 11-5,
represents density. (Notice that this is the reverse
of the specific volume.) We can now say that
pressure is equal to the density of the gas times
the gas constant times the absolute temperature
of the gas. (The gas constant varies for different
gases.) From this equation we can show how
density varies with changes in pressure and
temperature. Decreasing the volume, with the
weight of the gas and the temperature held
constant, causes the pressure to increase.
NOTE: During the compression of the gas,
the temperature will actually increase; however,
the explanation is beyond the scope of this text.
a decrease in volume with the weight held constant
will cause density to increase.
TEMPERATURE
As indicated previously, temperature is a
dominant factor affecting the physical properties
of gases. It is of particular concern in calculating
changes in the states of gases.
Three temperature scales are used extensively
in gas calculations. They are the Celsius (C), the
Fahrenheit (F), and the Kelvin (K) scales. The
Celsius (or centigrade) scale is constructed by
identifying the freezing and boiling points of
water, under standard conditions, as fixed points
of 0° and 100°, respectively, with 100 equal
divisions between. The Fahrenheit scale identifies
32° as the freezing point of water and 212° as the
boiling point, and has 180 equal divisions
between. The Kelvin scale has its zero point equal
to –273°C, or –460°F.
Absolute zero, one of the fundamental
constants of physics, is commonly used in the
study of gases. It is usually expressed in terms of
the Celsius scale. If the heat energy of a gas
sample could be progressively reduced, some
temperature should be reached at which the
motion of the molecules would cease entirely. If
accurately determined, this temperature could
then be taken as a natural reference, or as a true
absolute zero value.
Experiments with hydrogen indicated that if
a gas were cooled to –273.16°C (–273° for most
calculations), all molecular motion would cease
and no additional heat could be extracted. Since
this is the coldest temperature to which an ideal
gas can be cooled, it is considered to be absolute
zero. Absolute zero may be expressed as 0°K,
–273°C,
or –459.69°F (–460°F for most
calculations).
When you work with temperatures, always be
sure which system of measurement is being used
and how to convert from one to another. The
conversion formulas are shown in figure 11-1. For
purposes of calculations, the Rankine (R) scale
illustrated in figure 11-1 is commonly used to
11-2
