An actual question my five year old asked me and I couldn’t answer, please help!
In: Physics
Good answers here already. I will add that some fans don’t look like what you consider to be a fan at all, if you look online at pictures of industrial exhaust fans you’ll see what I mean, but here’s what I want to say about that:
As others have said, how much air a fan moves depends on a few things: diameter of the fan, the angle of the blades in relation to the fluid you want to move, number of blades, the horsepower of the motor that’s turning the fan, and how many rotations the fan makes per minute.
In a factory with, say, a very dusty process, if you want to control that dusty air by sucking it away from the work area, you would need one kind of fan. Generally speaking a very big one with a big motor, because sucking is very expensive (lol). Especially if you are sucking large volumes of air heavy with dust you want to move from one place to another.
Try blowing a small piece of paper across a table with a straw. Easy. Now try sucking the same piece of paper up from a distance of an inch or more. Not so easy.
So part of what determines how many blades a propellor or fan will have is, as others described, what’s it for? How much fluid does it have to displace, and in what timeframe? What is the density of the fluid? Air? Water? Wet concrete?
Think about/look at the motor on a celling fan. Then go outside and look at the outside unit for your air conditioner. It has a fan that you can see and hear up close. (There’s also a compressor that makes it’s own noise) Would you want that fan in your living room? Why not?
Big, loud, blows too much air.
Can you propel a 4000lb boat with a celling fan? No. Why? Well, the motor for one. But also the orientation of the blades relative to the fluid being displaced. A boat propeller is shaped more like a screw, or like an auger.
That’s about the direction you want to move, and is beyond what I can eli5.
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