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Cooling of Cooling Fans
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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