particle inertia, causing the contaminated air to flow into the hood. The capture velocity
can be calculated for a distance (x) from the hood, using Figure 4-4.
Duct velocity. The air velocity found within the duct.
Face velocity. The air velocity is at a point parallel with the face of the
(4) Transport velocity. That velocity required to prevent the settling of a
contaminant within the duct.
TYPES OF PRESSURES
a. Velocity Pressure. Air in motion exerts a pressure called velocity pressure,
which maintains air velocity and which may be thought of as kinetic energy. This
pressure exists only when air is in motion, and it acts in the direction of air-flow. It is
always a positive pressure.
b. Static Pressure. Another important aspect of airflow principles is static
pressure. Static pressure actually produces initial air velocity; it also overcomes the
overcomes turbulence and shock caused by a change in direction or velocity of air
movement. Static pressure may be thought of as potential energy; it exists even when
there is no air motion and acts equally in all directions.
c. Total Pressure. The driving force for airflow is actually a pressure difference.
Pressure is required to start and maintain flow. This pressure is called total pressure
and has two components: velocity pressure and static pressure. Static pressure,
velocity pressure, and total pressure are all interrelated, as shown by the formula:
SP + VP = TP