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Name: Kirk H.
Status: Other
Age: 50s
Location: N/A
Country: N/A
Date: July 2004


Question:
I am trying to determine the actual amount of "cooling" provided by ceiling fans in a residential application. I am particularly interested in the amount of energy saving which can be generated by use of a ceiling fan in an attempt to lessen the load on home air conditioners.

Is there a formula that could be used to calculate this information, basing the formula on the size of the fan, the rotational speed of the fan, the cubic measurement of the room, the size of the rooms windows and doors, the humidity present and the temperature setting for the air conditioner.



Replies:
I am afraid the ceiling fans do not cool. In fact, the electricity they use heats up the room.

The benefit comes because sweaty people evaporate more water when a breeze is blowing on them. It takes heat to evaporate the water and so the person and room are cooled. The same effect could be obtained by having the fan blow on a pan of water.

To plug some numbers in: A small room air conditioner (selling for $120) has a cooling capacity of 6,000 BTU (really 6,000 BTU/hour). A simple conversion (1 BTU = 1054 Joules, 1 hour = 3600 seconds, 1 Watt = 1 Joule/second) yields a cooling capacity of 1757 Watts. If one evaporates a kg of water (2.2 lb) every hour, 627 W is required. So you must evaporate about 3 kg/hr to cool a small room.

Once the air gets saturated with water, you cannot evaporate more water, so you have to exchange air with the outdoors or otherwise dry the air. This obviously warms up the room, esp[ecially on a hot day.

Evaporative cooling is sometimes used in cars, especially in very hot regions, though not much anymore since most cars are air conditioned.

I hope you find this answer useful. The most important point is that a fan is a source of heat, not cool.

Best, Dick Plano, Professor of Physics emeritus, Rutgers University


No doubt there is. I do not know what it is but it is going to be a very "messy" calculation because of the number of input variables and their interactions with one another. This is the type of calculation that an architect might have software to do.

Vince Calder


No formulas that I am aware of. It would not be terribly difficult to come up with some guidelines. To do that, one would take volunteers and put room with air conditioning, and also a room with a ceiling fan. Then tried different combinations of cool and fan speed until the researcher had some idea of which conditions gave an equally pleasant feeling.

Bob Erck



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