Basic Documentation

Air Velocity
Air Velocity
As illustrated in Figure 18, pressure measurement instruments can determine both the total
pressure and static pressure of an airstream in a duct. Subtracting the static pressure value
from the total pressure value yields the velocity pressure, which, in turn, enables the velocity
of the air stream to be determined.
When air is rapidly flowing through a duct, it typically has a significant total pressure as
evidenced by the force exerted in the direction it is moving. When air flowing through a duct
is discharged into a room, its forward motion is significantly reduced while, due to the laws of
physics (conservation of energy), its static pressure is increased. Then the difference
between the total pressure and static pressure (the velocity pressure) becomes very small
and, practically speaking, becomes nearly zero. Thus, the static pressure and total pressure
become nearly equal as air enters a room, and for this reason, a room’s air pressure
component can normally be considered to be only the static pressure.
Units of Pressure Measurement
Pressure measurements of compressed air, steam, and water are commonly expressed as
pounds per square inch (psi)
19
. However, typical values for total pressure, velocity pressure,
and static pressures in ventilation system applications are quite small when compared to the
forgoing pressure measurements. Thus for ventilation applications, a smaller unit of pressure
measurement is used to avoid fractions or very small decimal numbers.
The unit inches of water column (typically expressed as inches w.c., inches of water, or
inches) is commonly used for this purpose. The term inches of water simply means that the
air's static pressure value is the same as the pressure exerted by a layer of water of the
stated height on a horizontal plane. In other words, a pressure value of 1 inch w.c. exerts the
same force over a given area as would a layer of water that is exactly 1 inch in depth. Note
that due to gravity, water can only exert a force in a downward direction. However, it is
always to be assumed that air at a given static pressure (that is, 1 inch w.c.) exerts its force
in all directions.
With reference to a room’s static pressure, typical values are much less than even 1 inch of
water. In fact, values of 0.01 to 0.05 inches w.c. are more typical.
NOTE: If psi units were used instead of inches of water, 0.01 inches w.c. and 0.05 inches
w.c. would have to be expressed as 0.000361 psi and 0.001805 psi respectively.
Such small numerical values would be very cumbersome to use.
20
19
In SI units the kiloPascal (kPa) is the normal unit of pressure measurement. (1.0 psi equals 6.89 kPa.)
20
A pressure of 1.0 psi is equivalent to the force produced by a column of water 27.72 inches in height over a given area.
Therefore, a column of water 1.00 inches in height would be fractionally equivalent to 1/27.72 of 1.0 psi or 0.0361 psi. A
pressure of only 0.01 inch of water then becomes 0.000361 psi.
Siemens Building Technologies, Inc. 47