Book - School of Science and Technology

Pressure distribution in ducts 447StaticpressureTotalpressure(a) Side gauge(b) Facing gaugeFigure 16.12 Air pressure gaugesPressure distribution in ductsStatic pressureWhen air is moved in a duct or through a filtering, heating, cooling or humidificationplant, a resistance to flow is set up.The air is slightly compressed by the fan on its outlet side, so setting up a static pressurein the duct or plant. This pressure is tending to `burst' the duct, and may be read by meansof a U-tube partly filled with water, connected at right-angles to the air stream at anypoint in the duct: this is called a side gauge (Figure 16.12(a)). On the suction side of the fanthe static pressure is negative with respect to the surrounding atmosphere, tending tocollapse the duct.As the air proceeds along the duct from the fan, the compression is released graduallyuntil at the end of the duct open to atmosphere, the air is at atmospheric pressure. Thisfalling away of the static pressure proportionately with the length of travel is called theresistance of the duct. Similarly all obstructions, such as heaters, filters, dampers, etc.,cause a loss of pressure when air is passing through them.It should be noted that as the static pressure becomes reduced, the air in effect expandssuch that pressure volume ˆ a constant (or nearly so, as explained earlier). Thisexpansion therefore signifies an increase in velocity of the air if the size of the duct isunchanged.Velocity pressureA fan, in addition to generating static pressure, supplies the force to accelerate the air andgive it velocity. This force is termed the velocity pressure, and is proportional to the squareof the velocity (see Figure 16.1). It is measured by a `U' tube connected to a pipe facing thedirection of air flow in a duct, etc., which is called a facing gauge (see Figure 16.12(b)).But, obviously, the pressure so measured will in addition include the static pressure whichoccurs throughout the duct, as mentioned earlier, and the reading so obtained will thus bethe total pressure. Thus, the velocity pressure alone may be found by deducting the staticpressure from the total pressure reading, or by connecting one side of the U-tube to thefacing gauge and the other to the side gauge, provided that the two gauges are at the samepoint. A Pitot tube, as illustrated in Figure 16.32, combines a side gauge with a facinggauge in one standard instrument.