If I reverse the direction of a centrifugal fan would the air go in the opposite direction?

A centrifugal fan? In a laptop? Do tell.

On large industrial projects we do use “centrifugal” fans. They’re a real thing. But that’s a huge fan (in our case, that is, in power boilers) mounted in duct moving air (or gas) in the direction of flow as indicated on the engineering drawings and required for the process. The fans we typically employ on a utility-class power boiler are bigger-than-room-size, meaning the fan itself is bigger than a room in a house, and the motor half as big as that. This is also commonly referred to as a blower fan. (Also common enough – though smaller! – in residential, industrial and HVAC and other such applications where air movement is through ducting. (Centrifugal pumps work the same way, in principal.)

But the fans that I’ve ever seen in desktop or laptop computers are not of that type. They are what is called an “axial type” fan (just like on a room or desktop fan). In this case the fan blades turn at right angles to the direction of the air flow. (Either forward or backward.)

In that case, turning an axial fan around (assuming there are no changes made to the drive system or electrical connectors!) would reverse the flow of air.

However, if you turn the fan around by also change the wiring leads, then – if the fan operates at all! – it may continue to turn in the same direction as before the reorientation, if the motor will run in reverse. (Probably not likely with a cheap laptop fan, but not to be fully discounted either.)

No it won’t. The air will always try to move away from the hub, regardless of the direct of rotation of the blades.

My father was a sheet metal worker, and did all kinds of HVAC work from the 1940’s through the 1970’s. He used to refer to these types of fans as “squirrel cage”. To be sure, it seems the impellers, operating in reverse, would force the air back toward the hub. But, as @thorninmud states, the centrifugal force of the spinning hub would tend to force the air away from the hub. You might have some net movement of air in the opposite direction, but it would be highly inefficient.

It seems that you would do better at reversing the air flow if the fan blades could be mounted facing the opposite direction. Lots of smaller motors are designed to allow you to achieve this through removing the armature and shaft from the stator, then installing them “upside down”. If your fan motor lacks this remedy, see if you can contact the the fan’s manufacturer about the availability of the same motor with the direction (clockwise or ccw) reversed.

No. It will still go in the same direction, just not as efficiently.

No the efficiency is the same. It’s my failure to adequately explain the thing that leaves you unable to visualize it. I have such a fan motor in front of me now, and I’ve installed and replaced dozens of them. Google “reversible evaporator fan” for some images.

@stanleybmanly If I understand the OP correctly, he’s talking about a different kind of fan. A centrifugal fan redirects the airflow perpendicular to the axis of rotation of the fan instead of just passing it through from one side to the other along the axis. They look like this.

In these fans, the orientation of the blades (how they’re pitched) won’t affect the direction of the airflow. In fact, different designs of centrifugal fans use blades pitched in the direction of rotation, away from the rotation, or even not pitched at all. They all still move air away from the axis toward the perimeter. So neither reversing the blades 180 degrees nor reversing the direction of rotation will change the direction of the airflow.

But the shape of the fan housing plays a role in shaping the air flow. If you reverse the rotational direction of the blades, the shape of the housing will be wrong. The outlet will be poorly aligned with the airflow as it comes off the blades.

But that said, I may have misunderstood either the OP, you or both.

@stanleybmanly AH! There’s the rub! “Reversible” evaporator fan! A normal squirrel cage type fan has angled or curved fan blades/slots so the fan must turn in only one direction for normal function. I’ll bet your fan blades/slots are straight.

Here’s a simple graphic showing that how the blades are oriented in a centrifugal fan has no effect on the airflow direction.

In a 12 volt automotive application, the heater blower motor has 2 wires, a black and a red. If you don’t get the wires hooked up correctly, the fan turns in the wrong direction and doesn’t produce the proper amount of air flow volume, and the blades are curved, so while correct directional air flow is maintained despite the fan’s rotation, the volume of air is not the same.

@thorninmud & @kritiper Thanks to both of you guys. I’d never heard of a “centrifugal fan”, (though I have worked on centrifugal pumps). I too called the things “squirrel cage” fans or “blowers”. I’m actually surprised that those tiny things in Thorn’s photos are pared down versions of the huge blowers noted by @Yetanotheruser.