Forward curved fan can backspin??

[Waramanga]

Hi all,
Does a forward curved centrifugal fan backspin with air flow being pumped through it in the direction that it would ususally move air? I have heard that backward curved fans do this, but i dont know about forward ones!
cheers

 

[AbbyNormal]

The direct drive forward curved blowers, such as found in residential equipment are very prone to this happening

The way we build has a far greater impact on our comfort, energy consumption and IAQ, than any HVAC system we install

 

[willard3]

So are large fans with FC blades. It will do it with any blade configuration.

If air is going through it forward with the motor on and correct rotation, air going backward with motor off spins it backward....'tis the nature of the beast.

 

[Waramanga]

willard3,
so just to get this 100%, forward curved with air forced (by another fan say) in the usual direction of flow (with power off the fan motor) the resulting fan rotation will be??? (normal or reverse to usual spin?)

 

[willard3]

If the air enters the fan from the inlet without swirl and the fan is off, rotation will be in the normal direction.The radial blade fan may not do this, all other blade types do.

If the air enters from the scroll outlet without swirl and the fan is off, rotation will be backward on all fan types.

Backward = reverse of normal flow.

 

[ChasBean1]

Waramanga, it's a good question and I think I'd need to do an experiment to tell you for sure. Willard3 sounds pretty confident in his answer however... My thoughts:

If it was a plug fan (no shroud/volute) it would be simple. It will act like a turbine with air forced into the inlet. It would spin backwards.

But a centrifugal fan with the volute forcing air only out the top ~90° of the fan's full rotation might cause a different effect and could drag the fan in the forward direction.

Hopefully I didn't just add to the confusion.

 

[Waramanga]

thanks guys,
so who is gong to do the experiment to confirm the theory???? Nothing beats empirical proof.
Anyone already done this experiment?

 

[AbbyNormal]

for forward curved? been there done that

The way we build has a far greater impact on our comfort, energy consumption and IAQ, than any HVAC system we install

 

[Waramanga]

ok, "been there done that" and the result was....???? (drum roll)

 

[AbbyNormal]

the result was "The direct drive forward curved blowers, such as found in residential equipment are very prone" to spinnng backwards when the fan is not engaged.

I suppose with enough air forced through a unit with a larger belt driven blower, The inertia of the heavier wheel and the belt tension would be over come and these wheels would spin backwards as well.



The way we build has a far greater impact on our comfort, energy consumption and IAQ, than any HVAC system we install

 

[Waramanga]

Ok, so to finish the thread off conclusively, an unpowered forward curved fan with air pushed through it with another fan in the normal direction of air flow will spin backwards, yes or no?

 

[AbbyNormal]

yes

The way we build has a far greater impact on our comfort, energy consumption and IAQ, than any HVAC system we install

 

[ChasBean1]

Second that. I realized we have a Powercat blower (small, forward curved centrifugal fan) and a leaf blower. Just put them together, leaf blower blowing into Powercat suction and voila, mega backspin...

 

[Waramanga]

thanks chasbean etc. Put that one in the bank!