Fans, breeze at a distance

[Mr Skeptic]

OK, so I decided to buy a new fan, and got a physics related question. For the breeze of a fan, should I be looking at the volume per second, at the velocity blown, both of those, or perhaps something like the kinetic energy of the air expelled (ie, (1/2) * volume/second * velocity^2)? Assume I want a good breeze.

 

On a related note, I recently bought (but may return) a "high velocity" fan (Lasko 4940), though from looking it up it only has about 350 cubic feet per minute (CFM) on high. However I noticed it produces a decent breeze even at a distance. I think the kinetic energy of the blown air is catching more air along the way and draging it along, producing a stronger breeze at a distance. Anyhow, anyone know how the distance will affect the breeze on a fan with a given volume/second and velocity? Also, anyone know any good fans to buy?

[michel123456]

The best fan is a ceiling fan.

For best comfort, speed is not good. What you want is a large volume of air displaced at moderated speed. so you need a very large fan in diameter working at low speed (and at low noise). Generally speaking.

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Haha, now i looked at what you bought. Exactly the contrary.

your lasko looks like a hair dryer.

 

Anyway, if you look for something unusual, they are fans without blades from [no commercial]. Google it.

 

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Edit.

you may be interested in human thermal comfort and psychrometry.

Edited June 5, 2011 by michel123456

[lemur]

The best fan is a ceiling fan.

For best comfort, speed is not good. What you want is a large volume of air displaced at moderated speed. so you need a very large fan in diameter working at low speed (and at low noise). Generally speaking.

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Haha, now i looked at what you bought. Exactly the contrary.

your lasko looks like a hair dryer.

He could probably position such a fan in a way that would make a nice breeze. It's just a question of distance, isn't it? His fan might even be better because it will be further away, along with the noise it makes. I like ceiling fans for the quiet but they do blow warm air back down. Box fans are my favorite but any fan that generates a horizontal breeze is good, imo. I think a little air-speed (pressure) is good, too. After all, it is the wind-chill effect you're going for. It just becomes a problem when your hair and papers start blowing around.

Edited June 5, 2011 by lemur

[John Cuthber]

Haha.

The bladeless fans work by shifting a relatively small volume of air at high speed and using that to move lots of air.

Doing so is not very efficient (and an inefficient fan warms the air more that necessary which is not what you want.

[lemur]

Haha.

The bladeless fans work by shifting a relatively small volume of air at high speed and using that to move lots of air.

Doing so is not very efficient (and an inefficient fan warms the air more that necessary which is not what you want.

Could such a fan best be positioned in a window or elsewhere to blow warm air outside and create air-movement inside via the resulting pressure-decrease?

[PerpetuallyConfused]

I don't see why not, but then you'd have to consider where the intake would be to 'rebalance' that pressure decrease. If for example, you had a fan pushing air out into the sunward side of a house, and you had an intake from the opposite direction, the air immediately outside that wall should be cooler (by a few degrees). Computers work on the same principle, closed system, pull air in from cooler area, make it hotter with internal components, expel it again.

[michel123456]

Computers work on the same principle, closed system, pull air in from cooler area, make it hotter with internal components, expel it again.

 

For fans acting upon humans, the fact that the air moves is more important than the temperature of the air. If you put a fan in a closed room, without air intake or exhaust, it will still make you feel colder. Even if the temperature hasn't changed at all.

[SamSneed]

What a fan is rated at doesn't mean it actually does what it claims. I bought a 14K BTU AC but it feels like a 8K BTU AC.

 

Too many variables to use physics in buying a fan. What if your body temp is different than mine? What if the part of the country you live in differs from mine? What about elevation? Air is thinner at higher elevations than lower elevations. However, the fan is the exact same. Obviously that will produce a different result.

 

Buy what makes you feel comfortable. If that doesn't work, take it back and buy another.

[Phi for All]

I've used this one before. It's a bit pricey but it's belt-driven so the motor is quieter than a direct drive, and it's got a huge blade to displace a lot of air without the steep bevel that makes the blades loud. On low it can be used indoors to move the air from room to room. On high you better make sure everything is secured to the walls. But it's great if you set it so it can circulate air from upstairs to downstairs, or in an unused bedroom aimed down the hall towards the main part of your habitat.

 

Fans work best if they've got a lot of clearance behind them, say 18" or so. That's one of the things I don't like about ceiling fans in an 8' tall room. They work better on cathedral ceilings, but they're better than nothing. Much better than cooling with air conditioning, imo. I like the breeze that michel123456 mentioned, and he's right, it may be psychological but it seems cooler to feel it.

[CaptainPanic]

The most efficient heat transfer takes place in highly turbulent systems, so to cool down, a high air speed is good (it makes for a thinner stagnant and insulating layer of air around you)... but not very comfortable.

 

Unfortunately, the theory of heat transfer does not take "comfort" into account... so this problem is difficult to optimize.

[michel123456]

The most efficient heat transfer takes place in highly turbulent systems, so to cool down, a high air speed is good (it makes for a thinner stagnant and insulating layer of air around you)... but not very comfortable.

 

Unfortunately, the theory of heat transfer does not take "comfort" into account... so this problem is difficult to optimize.

 

Difficult but not impossible. I couldn't find anything in English, sorry. Don't ask me to explain the following diagram, it is only to give you an idea.

from http://www.google.gr/url?sa=t&source=web&cd=15&ved=0CEYQFjAEOAo&url=http%3A%2F%2Fhal.archives-ouvertes.fr%2Fdocs%2F00%2F24%2F52%2F22%2FPDF%2Fajp-rphysap_1984_19_7_513_0.pdf&rct=j&q=courbe%20de%20confort&ei=AwqHTtWONObl4QS_3JjRDw&usg=AFQjCNF2_CO9SPh0vtDfxJsDOIrMfh3OWg

[CaptainPanic]

Difficult but not impossible. I couldn't find anything in English, sorry. Don't ask me to explain the following diagram, it is only to give you an idea.

I was going to ask you what the units are on the axes - but then I remembered that I started about comfort... and comfort is definitely going to be a dimensionless number.