Titus 2008 - 2009 Product Catalog | www.titus ... - Texas Air Products

ENGINEERING GUIDELINES
B
B60
TERMINAL CONTROLS AND ACCESSORIES
Engineering Guidelines | Terminals, Controls and Accessories
Some Basic Concepts-Pressure Measurement
Three categories of pressure are
connected with air handling:
1. Static pressure may be thought of
as the pressure in a tire or storage
tank. It is exerted in all directions
equally.
2. Velocity pressure, as its name
implies, is entirely a function of
air velocity and its direction. It is
the pressure you feel against your
hand if you hold it outside the
window of a moving car.
3. Total pressure is the sum of static
pressure and velocity pressure.
It and static pressure are the
pressures actually sampled by
velocity sensors in terminals and
by commonly used measuring
devices, as described next.
The interaction of static, velocity, and
total pressures is illustrated by
(Figure 105). The Pitot tube, which
is used to measure velocities and
pressures, is really a tube within a
tube. The inner, or impact, tube senses
both the velocity pressure and static
pressure combined (total pressure).
The outer tube, which communicates
with the airstream through small holes
in its wall, avoids the impact of the
air movement and senses only static
pressure.
The U-tube manometer, connected to
both parts of the Pitot tube, has the
effect of subtracting static pressure
from total pressure to give a reading of
velocity pressure.
Once the velocity pressure is known,
the velocity can be calculated easily:
where V = Air Velocity
P = o
v V p
4005
and
P v = Velocity Pressure
Knowing both the velocity and the
cross-sectional area of the duct, the
flow rate is then:
cfm = Area x Velocity
Titus 2008 - 2009 Product Catalog | www.titus-hvac.com
2
or V = 4005√P v
P T
P S
P S
Pitot Tube
Connecting
Tubing
P V Difference in Liquid Level
P T minus P S = P V
Glass Manometer Tube
P T
Figure 105. Static, Velocity and Total Pressures Interaction