Basic Documentation

Chapter 5—Air Pressurization
Fundamentals
Chapter 5 presents the fundamentals of air pressurization, introduces ventilation system
pressure components, and summarizes important factors in relation to these components.
This chapter also discusses the following topics:
• Forces Exerted by Air
• Total Pressure
• Static Pressure
• Velocity Pressure
• Air Velocity
• Units of Measure
Forces Exerted by Air
Air can exert a force in two ways. Wind, which is air in motion, exerts a force as it strikes an
object. As wind speed increases, the greater the force that is exerted. Air that is compressed
and confined within a container, such as a compressed air tank, also exerts a force on the
inside wall of the tank. And, the more the air is compressed the greater the force that is
exerted.
The force exerted by wind (air in motion) as well as the force exerted by air that is under
compression can be expressed in terms of its effect on a given size or unit of area. When this
force is expressed in terms of force per unit area it is commonly referred to as air pressure.
Total Pressure
Air in motion occurs naturally as wind, but air can also be put into motion by mechanical
means, such as a fan. Ventilation systems move fresh air into building areas and also
remove contaminated air from areas by means of fans and ductwork. Any time air pressure
(force per unit area) occurs as a result of air that is in motion it is termed total pressure. A
piece of paper or tissue would be readily pushed through a ventilation system duct in the
direction of airflow as a result of the force or effect of the air stream's total pressure. The total
pressure of a moving air stream is at its maximum (and is always measured) in the direction
of the airflow. Figure 16 illustrates the concept of total pressure as the result of an air stream
moving through a ventilation system duct.
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