Why do fans ...

Why do fans have three blades? And what are the optimal design characteristics for fans (I ask this on a day with 95F weather and humidex readings well into the 110’s).
Alex

Some of the big boy fans have four. I’m thinking of the big metal ones on pedastals they used to drag into the high school gym for strandardized tests in the summer. Before my family got AC, I remember having a huge window-sized fan with four blades that sounded like a Boeing prop engine.

Anymore than three or four would mean narrower blades. My guess is that skinny blades would not be as stable when whipping around at high speeds.
Alphagene

Also, why do ceiling fans go anti-clockwise and the table-fans clockwise?
anybody, any idea?

Most ceiling fans have the ability to go in both directions. One direction draws the air towards the ceiling, the other forces it down towards the floor.

How come fans seem to blow more than they suck?

Way back when my moral objection to AC was more imortant than sweating like a pig, I closed all the windows in my apartment except 2 at opposite ends. I arranged a large fan at each window, one blowing in, the other out. The air speed (and the comfort level) at the blowing-in fan seemed to be much higher than that at the blowing-out fan.
7th grade physics and common sense tell me that this couldn’t possibly happen or the building would explode, but it never did. What gives?

In response to the question about fans, I’ve just finished reviewing ceiling fans for a major publication and maybe I can shed a little light on the subject. I’m not promising much, but here goes. The number of blades are chosen basically on a cosmetic basis. A manufacturer would choose to have five blades instead of four simply because it makes the fan more aesthically pleasing as it rotates. Or in other words, as you pass out from a heat stroke and collapse under the fan, you can watch the blades magically appear to multiply. As far as guidelines go for optimum fan performance, blade pitch (or the angle of the blade) will help determine the cfm (cubic feet per minute or how fast a room cools). The sharper the angle, the more air it will move. You must also allow room for air to enter from one side. This means that you can’t stick a fan an inch from the wall or ceiling and expect the blades to produce much more than you could blow on a good day. A good motor is essential to move the blades and you have yourself a good fan. If my answer doesn’t help you at all, too bad. I have central air.

veera said

veera, this is because ceiling fans are manufactured in the Southern hemisphere whereas table fans are made in the Northern hemisphere.

falcon2 said:

Moral objections to AC? What are you talking about?


The mass of men lead lives of quiet desperation.
– Henry David Thoreau

Strainger: veera, this is because ceiling fans are manufactured in the Southern hemisphere whereas table fans are made in the
Northern hemisphere.

Veera: I dont see the connxn. Is this due to the sundials’s shadow?

He’s saying he was too cheap to by an AC unit :wink: Same as me. Two more months in this place then I’m moving to a joint with AC. So in the mean time, I’ll just pretend it my choice by mocking those with AC and calling them wimps. Wimps.


“I guess it is possible for one person to make a difference, although most of the time they probably shouldn’t.”

>Moral objections to AC? What are you talking about?
When I was lad Air conditioners used Freon, which has a nasty habit of leaking out and is in some manner responsible for the continent sized hole in th ozone layer.

Freon production is banned for quite some time, so I gather there using some other nasty stuff as refrigerant now, and the ozone hole hasn’t been getting much bad press lately, so the guilt isn’t as bad.

falcon2:

Near the incoming fan you sat (stood, lay) directly in front of the current of moving air. Near the exhaust, the fan drew air from many more directions surrounding it, lessening the experience of feeling the air “leave” from any one spot.


Tom~