Book - School of Science and Technology

Pressure distribution in ducts 449Pressure+0Total pressureTP STP D0BFan RPDCStatic pressureVelocity pressure–ASP = –1 SP = –2 SP = 1 SP = 0TP = 0TP = –1TP = 2VP = 1VP = 1VP = 11 2 3 46 7 8 9FanSuction duct5Discharge ductTP=3 SP of Fan = 3 – VP(1)=2TP = 1VP = 1Figure 16.15 Total and static pressure changes at a fanvarying velocities. The velocity pressure is more generally not recovered, though sufficientmust remain at the duct termination to eject the air at the required velocity.Where, however, by careful design of the fan discharge expander, the velocity pressureis converted to static pressure (probably to the extent of about 75%), this additionalpressure may be reckoned as augmenting the static pressure of the fan.The pressure generated by a fan may be better understood by study of Figure 16.15:. The total fan pressure is defined as the algebraic difference between the mean totalpressure at the fan outlet and the mean total pressure at the fan inlet.. The total pressure on the suction side, as will be seen from this figure, is TP s , i.e. thenegative pressure AO minus the velocity pressure equivalent to AB.. The total pressure TP D on the discharge side is similarly the static pressure OC plus thevelocity pressure CD.It has previously been explained that we are concerned only with the resistancepressure set up by the fan and this, as may be seen, is the difference in pressure of pointsB and C.We can arrive at the static pressure by measuring the total pressure of the fan as by theU-tube 5, and deducting therefrom the velocity pressure as given by difference betweengauges 6 and 7. Such a method is valid only if the velocity in suction and discharge ducts isthe same.The other U-tubes indicate the pressures at the various points along suction anddischarge ducts, and their meaning will be apparent.If a fan has suction ducting only, the static pressure produced for overcoming frictionof ducting will be represented by OB (a negative pressure), since the discharge will be at