COOK Laboratory Exhaust - Manual (Page 12)

4.0 Blowers
See the product specific I, O&M.
5.0 Air Balance
Velocity - W hen air moves at a given velocity in a duct, it creates a pressure corresponding to the velocity; this
velocity pressure (Vp) is a measure of kinetic energy in the fluid. Velocity pressure is always exerted in the
direction of air flow. The relationship between the velocity pressures may be expressed by the following
formulas:
Vp = V
4005
V = 4005 Vp
It is therefore a simple matter to determine the velocity (fpm) of an air stream if the Vp can be measured. For
example if a Pitot tube manometer hook up reads 0.250 in water, we substitute for the above equation:
(4005)(0.250) = 2002fpm
5.1 Static Pressure
Independent of its velocity, air, when confined within an enclosure such as a duct or tank, will exert itself
perpendicularly to the walls of the enclosure. This is the compressive pressure existing in the fluid, and it is
known as the static pressure (Sp). Unlike velocity pressure, which is always positive, static pressure, when it is
above atmospheric pressure will be positive, but when below atmospheric pressure will be negative. The
discharge side of a fan in a supply system will read a positive pressure, the inlet side of the fan in an exhaust
system will read a negative or minus pressure.
5.2 Velocity Pressure
The manometer does not sense the actual velocity pressure directly, but by using the Pitot tube hook-up with the
static opening connected to the low pressure side of the gauge and the total pressure opening connected to the
high pressure side of the gauge, the manometer will read the difference between the two, or the velocity
pressure.
Vp = Tp - Sp
5.3 Total Pressure
Static pressure is exerted whether air is at rest or in motion. Velocity pressure and static pressure change in the
duct work with every change in the duct configuration, but the total pressure, on the other hand, remains
constant. Hence, as the velocity pressure decreases, the static pressure increases and vice versa, because the
static pressure is always the difference between the total pressure and the velocity pressure. It should be
remembered, however, that in an actual duct system, the internal friction will cause a loss of total pressure. The
static pressure in an exhaust system is always below atmospheric pressure, and it is customary among
ventilation engineers to omit the minus sign affecting the static (gauge pressure). It is known, of course, that the
total pressure is higher than the static pressure by the amount of velocity pressure.
When the unit is designed for connection to a duct system and the installing contractor assembles ducts, elbows,
registers, grilles, etc. to the outer and/or inlet of the unit, the static pressure drop through the external duct work
is called external static pressure. Fans selected must be capable of moving the desired air flow through the
entire air moving system including the unit (internal Sp) and also the duct system (external Sp).
At a given flow rate the internal static pressure losses plus the external static pressure losses equal the system
static pressure or the summation static pressure. These pressures are of great importance when
troubleshooting for causes of reduced capacity, vibration, and noise.
Changes in the cross sectional are of duct (contractions or enlargements) cause changes in the velocity of the
air flowing through the duct. When velocity decreases, the velocity pressure also decreases. Some of the