The purpose of any air-handling system is to provide air movement to ensure the comfort and safety of the occupants of the space. Notice the keyword “movement,” which indicates why using static (defined as lacking motion) pressure is incorrect. It is the movement of the air that is important. To provide the energy required to move the air, we must produce a pressure differential favoring motion in the desired direction.
Normally, the air is confined to move through ducts or spaces by the boundaries of these ducts or spaces. Resistance to this motion is caused by friction along the boundaries and by the energy lost in components that are intended to change some property of the air (such as filters, coils, sound attenuators and grills). When the system resistance is plotted as total pressure vs. distance, the total pressure will be decreasing everywhere except across the fan. Therefore, every non-fan component will have a total pressure loss associated with it. Special attention must be paid to those components in which a large or abrupt change in cross-section occurs, since the total pressure losses can be quite large. The long-standing practice of using static pressure leads to the use of misleading terms such as “static regain” or “static pressure recovery” to describe what is actually “conversion of velocity pressure to static pressure.”