Wednesday, March 31, 2010

Why do fans have three blades?


Now, just try to figure this out.. Why do most fans have only three blades? why could it be so?? Hmm...Any idea??

Is there any aerodynamic law that supports the efficiency of using only three blades in electric fans? Think.....! Are you thinking of any kinda complicated theorems in fluid mechanics? Are you trying to solve this query using Bernoulli's principle?

All I can say is that it's a vain attempt!

Actually an electric fan can have any number of blades depending upon certain factors. But, why is it so that most fans have only three blades? Let's see here.

Firstly, the main purpose of an electric fan is to create air-flow. So, a good fan must create a good air-flow with minimal noise. Usually an electric fan has a shaft which is rotated at a moderate speed by an electric motor. The speed of electric fans is chosen in a moderate level for safety purpose and to reduce the noise produced when a fan is operated. There are blades attached to this shaft which rotate along with the shaft.

The number of blades and their shape may vary depending upon the model and aesthetic sense. Even there are certain fans with only one blade. These are usually meant for aeroplane propellers. (see bottom)

Fans may have any number of blades placed at any position. But the weight of the blades must be balanced so that there is maximum efficiency. For that reason, fans with only one blade is not often preferred, as the weight of the blade tend to bend the blade down.
Coming to the arrangement of fans with two blades, it is obvious that these two blades must be oppositely arranged to each other. Similarly in the case of having three blades, each of them must make an angle of 120 degrees with the next blade. 


Now coming to the structure of the blades, each blade is moderately long, thin and bent a little up. Usually metal/alloy blades are preferred for their rigidity and ability to withstand mechanical stress. But, nowadays, there are plastic(polymeric) blades having a comparable strength to metal blades, are used.

Each blade is inclined a bit so that it forces air ahead of it forward. When an electric fan is switched on, the rotation of the blades increases the velocity of the air along the blades (in circular direction) Generally hot air rises upwards. This hot (less dense) air comes to contact with the whirl(rotation) of the blades and are forced back with a little higher pressure. Thus air-flow is created.

An interesting point to note is that an electric fan never creates a cooling effect, by itself. Try switching on a fan in an insulated room. After some time, the temperature will rise a bit. This is because, the electricity used by fan is not completely converted into mechanical energy, as per the second law of thermodynamics. Some amount of the electric energy is converted into heat energy due to the resistance of coil and several other effects and thus the temperature rises. But every-time when we are laid down under an fan, we feel cool breeze, isn't that true? How it happens?

The cooling effect produced by a fan is due to the fact that the air it pushes down makes our sweat evaporate at a faster rate thereby decreasing our temperature. Thus, we feel chill! So, forgot your tension, sit back and relax!

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Images in the article are taken from the web.





10 comments:

  1. Crappy article. It says "These are usually meant for aeroplane propellers. (see bottom)" but this is never elaborated on in the end of the article. Plus, it does not really answer why fans have three blades.

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  2. In this why fan have three blands? is not clearly explained.

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  3. Lovely blog! According to me, it's really too informative blog about electric fans. I really enjoyed while reading your blog. I also would like to share something that if someone want to buy fans then try to buy designer ceiling fans for better cooling air. Thanks for sharing this blog with us.

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  4. Three blade design is the most efficient design for a ceiling fan . It will require the smallest motor and the transverse load on the shaft will be the lowest . Four blade , or five blade fans are more for aesthetic reason than anything else .

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  5. You should concern the color combination between the text and background. Your text is grey and the background is black. It is hard to read!! Btw, good article.

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