Backward and forward curved blades of centrifugal fan impellers have different performance characterisitics.
If you look at the velocity vector diagrams.
For the same tip velocity(Vb), they produce different air velocity (Va).
Vab is air velocity relative to the blade.
The effect on fan performance is shown in the different performace curves.
The forward curved type, often referred to as squirrel cage blowers basically run slower than the other types of centrifugal fans and are the quietest in operation.The forward curved (and radial) impellers use higher horsepower at low resistance than other types of impellers, and the least amount of horsepower at the higher pressures and low flows.
The backward curved type of centrifugal fan performance characteristic curve shows that for increasing delivery volume, they startout at a lower horsepower, rise to a peak on the horsepower curve near the point of highest efficiency on the fan performance curve and then drop off again.
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