Brochure

In order to evaluate air management of a
specific application a better understanding
of how tempera ture and pressure affect the
solubility of air in water is required. Air is a
mixture of gases, approximately 80% nitrogen,
20% oxygen and trace amounts of others.
Within a short time after the initial fill, the air
in a prop erly operating closed system begins
to lose its oxygen content through oxidation.
Unless fresh water, with its 2% air is added, the
gas remaining becomes nitrogen, an inert gas.
The chart in Figure 2 illustrates the effects of
pressure and temperature on the solubility
of air in water.
With the air out of solution, the air separator will
isolate the air and move it to the air vent for air
elimination, or send it to the compression tank
for air management. The principles governing
the design of air separators are simple. One
is by reducing the water velocity to ½ ft/
sec or less. Water will not carry along free air
bubbles at that velocity. Stokes’ Law explains
the separation process. Separation occurs when
the buoyancy of the air particle exceeds the
gravitational force and the friction force created
by the system water. The Rolairtrol enhances
the separation process by using centrifugal
force to separate the air from the water rather
than relying on gravity alone. Gravity is the only
variable that can be improved in the formula.
The centrifugal force created by the Rolairtrol
pushes the heavier water toward the shell and
the lighter air toward the air collector where it
will move to the top of the Rolairtrol. This is the
most efficient method of separating air in large
commercial HVAC applications.
How the Rolairtrol Works
Stokes’ Law
ν = (2gr
2
)(ρ1-ρ2)/9η
Where:
ν = velocity of rise (cm sec-
1
),
g = acceleration of gravity (cm sec-
2
),
r = “equivalent” radius of particle (cm),
ρ1 = density of particle (g cm-
3
),
ρ2 = density of medium (g cm-
3
), and
η� = viscosity of medium (dyne sec cm-
2
).
Fig. 2 Solubility Versus Temperature and Pressure for
Air/Water Solutions
(Coad 1980a)
PRESSURE, psig
AIR BY VOLUME, %
TEMPURATURE, °F
0
40
60
80
100
120
140
160
180
200
220
240
20 40
2 4 6 8 10
12
14
16
18
20
60 80 100 120 140
2004 ASHRAE HANDBOOK © American Society of Heating,
Refrigerating and Air-Conditioning Engineers, Inc., www.ashrae.org
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