Book - School of Science and Technology

448 Ductwork design+Pressure0Velocity pressureStatic pressureStatic pressure regainFanExpanderFigure 16.13 Velocity and pressure changes at an expander+Pressure0Velocity pressure lossStatic pressureVelocity pressure regainFanPlenumchamberFigure 16.14 Velocity and pressure changes at a plenum boxIf a fan discharges into an expanding duct (Figure 16.13), the velocity will obviouslydecrease as the distance from the fan increases, and at the same time the velocity pressurewill be converted into static pressure (not at 100% efficiency, but about 75% if theexpansion is sufficiently gradual). If the fan discharges into a large box (Figure 16.14),from which at some point a duct connects, the fan velocity pressure will be entirely lost ineddies, and at the duct entrance must be recreated by a corresponding reduction in staticpressure.Fan pressureIn all air flow considerations as affecting resistances of ducts, plant, etc., it is the staticpressure alone which is of importance, this being the pressure which changes with suchrestrictions. It is the static pressure set up by a fan which is, therefore, a criterion of itsperformance. The velocity pressure, if taken as supplementing the fan duty, may be moremisleading than useful, owing to the uncertainty of friction losses which occur at points of