# Specific values

Specific speed

The specific speed of a fan at a given duty is the speed at which a geometrically similar or homologous fan would have to run to give unit flowrate and unit pressure at the same point of rating (assumed same efficiency) when handling air or gas of unit density. Thus by manipulating the fan laws

NQ° 5 p° 75

Equ4.17

 PRESSURE COEFFICIENT T’M*) — P flV1 POWER COEFFICIENT X ilomodol — T, < . SPECFIC SPEEDS, • 9A. n 0“ tt

 D.=
 0 25
 P0 25 Q0 5 kp
 D„
 630mm
 710 800 900 1000 112) 1250 1400 1600 1800 2000 To obtain fan static efficiency or speed obtain curve value and multiply by factor Eg size 2000 mm selected at 55 % efficiency on curve and 1500 rev/min Therefore: Actual efficiency = 55 x 1 09 = 60-4% Actual speed = 1500 x 0-969 = 1454 rev/min
 (T.)’
 V
 If SI units were used then Ns (and N) should be in rev/s. Specific diameter Specific diameter Ds is the impeller diameter of the geometri­cally similar or homologous fan for which the specific speed has been calculated.

 2000 and above

 O .2 Ss O C 4) O ‘S O 2 Go

 Equ4.18

 Composite charts Reference to Figure 4.29 show that it is possible to plot all of a manufacturer’s product range on a single chart. Specific diam­eter and efficiency have been plotted against specific speed. It will be seen that the specific speed at maximum efficiency is a unique value for a particular design.

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 Figure 4.27 Effects on medium width centrifugal fan with backward inclined im­peller

 SPECIFIC SPEED N_

 Q » Votum*tr>c flowrgt« P * Fan ortMure Pa U — npciMr t« «pccti m/a *7TDN 0 — «nxHt*- «mitf m N — ftalatono) uMd r«v/« A• ini*» or цenawy kg/«3. [12 os wo)

 PLOW C06FFlCЂNT «jrf« — 0 ЬЦ5

 SuMcnet« si — •totie I — total

Figure 4.28 Reynolds number effects on the peak static efficiency of aerofoil Figure 4.29 Specific diameter and efficiency against specific speed for a range