Specific values
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.)’ |
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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
Bladed fans of fans
Use of such charts is useful in both the selection and design of fans. The manufacturer can identify gaps in his range if adequate coverage of all duties is to be achieved.
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Posted in Fans Ventilation A Practical Guide