55
Appendix B. Actual vs. Standard Air Velocity and Flow
Measurements
Actual Air Velocity
is also known as the “local” air velocity. It can be thought of as the speed of a
microscopic dust particle being carried along in the air stream.
Actual Air Velocity is measured with a Pitot tube using the basic formula:
air
ACT
VP
7
.
1096
V
ρ
×
=
Eq.
(1)
where:
V
ACT
is Actual Air Velocity, in units of (Actual Feet per Minute)
VP is Velocity Pressure, measured in units of (in. H
2
O)
ρ
air
is the air density, in units of (lb
m
/ft
3
)
Air density,
ρ
air
, can be calculated from the equation:
460
T
P
325
.
1
ACT
ACT
air
+
×
=
ρ
Eq.
(2)
where:
P
ACT
is the atmospheric pressure, in units of (in. Hg)
T
ACT
is the local air temperature, in units of (°F)
Standard Air Velocity
is also known as the mass velocity of air. It can be thought of as the local air
velocity corrected to standard conditions of air. Standard Conditions are defined as air at 70°F and P
STD
=
29.92 in. Hg.
Substituting Standard Conditions into Eq. (2) above we can see that:
3
m
air
ft
lb
0748
.
0
460
70
92
.
29
325
.
1
STD
=
+
×
=
ρ
Eq.
(3)
Standard Air Velocity and Standard Air Flow are often the preferred units of measure when evaluating
Heating, Ventilating and Air-Conditioning (HVAC) system performance because the heat-carrying
capacity (and cooling capacity) of air is directly related to Standard, not Actual, units of measure.
Converting between Standard and Actual units of measure simply involves an inverse air density ratio, as
follows.
From the continuity of mass equation, we can state the following:
STD
air
ACT
air
V
V
STD
ACT
×
ρ
=
×
ρ
Eq.
(4)
