SWAMBO wants a ceiling fan for our lounge. We already have 3 in other smaller rooms, they are all the same model with 6 short blades each. The question is do we stick with many but short blade models or do we go for fewer long bladed models. The ones we have have quite a fast top speed but what I have seen of long bladed models they all turn slower. The other factor is that the lounge is a much larger area and we do not want more than one.
So which way do we go?
Richard
large slow rotors move more air, but slower
Experience with fan blown heatsinks suggests that once air velocity gets to a certain point, conduction doesn't improve. And I would guess that the evaporation rate doesn't improve either.
Most fans in India imitate the punkah wallah,big and just enough to keep the air moving.
Don't have a lot of experience with specifying ceiling fans*, but for regular fans:
Larger => slower => quieter for the same volume of air Larger fans are also more complicated to build as the tips move faster and suffer higher stresses.
More blades push more air - feel it further away Curvier blades make higher static pressure - overcomes obstacles like heatsinks
You might want to listen to them - a fan with a high top speed on paper might be a jet engine, and you could only ever use it on a lower speed.
I get the impression many ceiling fans are designed to be decorative and not for decent aerodynamics - can't imagine this would get your Spitfire very far:
Theo
I have 5 bladed* ceiling fitted in the bedroom. It has a remote with 3 speed options. The low speed is ideal for creating a downward draft to sleep under, medium speed is tiny bit too much for sleeping, the fast speed is too noisy (both motor noise and airflow noise from the blades). At least with a 3 speed model I have the choice and when the bedroom is too hot a blast at high speed an hour before bed can help push some of the hot air out through the window.
I seem to remember that with a wiring change it could either push air downwards or upwards but its not a trivial job one installed to change directions for the seasons.
My fan is mounted approx 1.5feet down from a 9 foot high ceiling and the downward draft covers an area of a double bed but not much beyond, although it will stir up the air a bit in the room.
I've found that a pedestal fan set at head height whilst seated provides better comfort air flow in a living room because it can be directed to the seating position. A ceiling fan is often placed centrally on the ceiling and most people don't tend to sit in the middle of the room.
The remote is possibly an early version of this
The remote came with a holster for wall mounting. I see that the remote above now comes with a fan direction reversing facility.
Edit: The remote I have and purchased 10+ years ago is more like
I have an early model of this
Do they actually make much difference though, as in theory, the hotter air will be above the cooler air. Used to have one in an office I visited, total waste of time and dust gathering device. Brian
it makes sod all difference at the sort of speeds those fans run it
You are not trying to generate thrust, you are trying to move air.
Large and slow is always more efficient and blade design scarcely matters
That's why spitfires gave geared props that are as large as the airframe can stand - and even then the torque and p factor on the later hawker tempests was unbelievable.
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