Power to Drive Fans in Series

I take it the air flow of said fans is in series, not the electric motors (which should be wired in parallel through a single our multiple control contactors or variable speed drives)?

If the fans are drawing equal power then yes it would be three times that power.

Don't know why they would be in series. Electrical or flow makes no sense.

One fails they all do if electrical. And stacking them for flow would only see maybe minor improvement in flow.

What do you mean by series? Does the same air flow through all three fans, or the same current flow through the three fan motors? If it's the former, then it would obviously require less power as each fan would be "helped" by the other two. If they are electrically in series, then I don't see why the total power would not be three times the power required by one fan.

This sounds very familiar . . .

http://cr4.globalspec.com/thread/81836/Fans-in-Series

I'm thinking you're right. It isn't that convenient an answer unless you are also tripling the pressure differential.

First things first. Compute at the source- your hydraulic parameters

Fans in series would have the same Q -flow capacity but ΔP across it is definitely increase to some extent.

Hydraulic Power = ρgQH = Q*ΔP, all you need to do is divide it by 3, then check if motor rating of each fan is greater than the result. If so that it is greater, then no problem. That would be your maximum hydraulic power requirement for the fans, --no overloading with the motor rating. If hydraulic power is greater than motor rating -- then it wont work well and safe.

Q and delta P should be a design value as intended for the heat transfer capacity of the heat exchanger.

Hi PWSlack

Funny you say that, because that's what I did. Got a software tool from the fan vendor and it does all the math for you. With 3 fans, contrary to what I thought, the power remained pretty low. So it wasn't 3 times the power to drive one fan.

Thank you all. I think Rixter's answer hit the lightbulb in my head. No I don't mean to say electrically wired in series, the fans are stacked one behind the other on the same drive shaft. They are the same in design, so each fan has 6 blades and other similar geometry.

You should call it multistage series fan arrangement, When the output of first is input for the second and output of second is input for third the output, overall output will be different from each single fan output. Using even number of blades may not be acoustic and may cross the limits of db. Power will also be different for all three, ip/op, load rpm, fan load [even with same type of blades].

Multistage arrangements can be series or parallel, power calculation will differ in both the cases same way as in water pumps. If you have assess to either theory you can get answers easily. You can select either arrangement after deciding what you want more volume or pressure.