# Refrigeration & Air Conditioning Technology, 8e - 32

```32

Section 1 Theory of Heat

decreases. Hydrogen has a specific volume of 179 ft3/lb under
the same conditions. Because more cubic feet of hydrogen
exist per pound, it has a higher specific volume, thus it is
lighter than air. Although both are gases, the hydrogen has a
tendency to rise when mixed with air. Natural gas is explosive when mixed with air, but it is lighter than air and has a
tendency to rise like hydrogen. Propane gas is another frequently used heating gas; it has to be treated differently than
natural gas because it is heavier than air. Propane has a tendency to fall and collect in low places and poses a potential
danger from ignition.
Specific volume and density are considered inverses of
one another. This means that specific volume 5 1 4 density
and that density 5 1 4 specific volume. If one knows the
specific volume of a substance, its density can be calculated
and vice versa. For example, the specific volume of dry
air is 13.33 ft3/lb. Its density would then be 1 4 13.33 ft3/
lb  5 0.075  lb/ft3. Notice that even the units are inverses
of each other. Substances with high specific volumes are
said to have low densities. Also, substances with high densities have low specific volumes. Since density and specific
volumes are inverses of each other, multiplying the density
of a substance by its specific volume will always yield a
product of 1.
The specific volume of air is a factor in determining
the fan or blower horsepower needed in air-conditioning
work. As an example, a low specific volume of air requires
a blower motor with higher horsepower, and a high specific volume of air can utilize a blower motor with a lower
horsepower rating. The specific volumes of various pumped
gases is valuable information that enables the engineer
to choose the size of the compressor or vapor pump to
do a particular job. The specific volumes for vapors vary
according to the pressure the vapor is under. As the pressure of the gas increases, the specific volume of the gas
decreases and its density increases. As the pressure of the
gas decreases, the specific volume of the gas increases and
its density decreases.

BOYLE'S LAW

2.6 GAS LAWS

Figure 2.6 Absolute pressure in a cylinder doubles when the volume
is reduced by half.

It is necessary to have a working knowledge of gases and
how they respond to pressure and temperature changes.
Many years ago, several scientists made significant discoveries about these properties. A simple explanation of some
the reaction of gases and the pressure/temperature/volume
relationships in various parts of a refrigeration system.
Whenever using pressure or temperature in an equation like
the gas laws, one has to use the absolute scales of pressure
(psia) and temperature (Rankine or Kelvin), or the solutions
to these equations will be meaningless. Absolute scales use
zero as their starting point, because zero is where molecular
motion actually begins.

In the early 1600s, Robert Boyle, a citizen of Ireland, developed what has come to be known as Boyle's law. He discovered that when pressure is applied to a volume of air that is
contained, the volume of air becomes smaller and the pressure greater. Boyle's law states that the volume of a gas varies
inversely with the absolute pressure, provided the temperature remains constant. For example, if a cylinder with a piston at the bottom and closed at the top were filled with air
and the piston moved halfway up the cylinder, the pressure
of the air would double, Figure 2.6. That part of the law pertaining to the temperature remaining constant keeps Boyle's
law from being used in practical situations. This is because
when a gas is compressed some heat is transferred to the gas
from the mechanical compression, and when gas is expanded
heat is given up. However, this law, when combined with
another, becomes practical to use.
The formula for Boyle's law is as follows:
P1 3 V1 5 P2 3 V2
where P1 5 original absolute pressure
V1 5 original volume
P2 5 new pressure
V2 5 new volume
For example, if the original pressure was 40  psia and the
original volume was 30 in3, what would the new volume be
if the pressure were increased to 50 psia? We are determining
the new volume. The formula would have to be rearranged so
we could find the new volume.
V2 5
V2 5

P1 3 V1
P2
40 psia 3 30 in3
50 psia

V2 5 24 in3

14.696
696 psia
LINDER
CYLINDER
LED AT
FILLED
PHERIC
ATMOSPHERIC
PRESSURE
SSURE

PISTON
STON AT
BOTTOM
OTTOM OF
STROKE

29.392
psia

AIRTIGHT
SEALS

PISTON AT 1/2
UPWARD
STROKE

```

# Refrigeration & Air Conditioning Technology, 8e

Contents
Refrigeration & Air Conditioning Technology, 8e - Cover1
Refrigeration & Air Conditioning Technology, 8e - Cover2
Refrigeration & Air Conditioning Technology, 8e - i
Refrigeration & Air Conditioning Technology, 8e - ii
Refrigeration & Air Conditioning Technology, 8e - iii
Refrigeration & Air Conditioning Technology, 8e - Contents
Refrigeration & Air Conditioning Technology, 8e - v
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Refrigeration & Air Conditioning Technology, 8e - Cover4
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