Why does a ceiling fan blowing air downward cool in the summertime:
whereas a ceiling fan sucking air upwards heats in the wintertime:
?
Why isn't it the other way around?
Best Answer
In summertime, the ceiling fan blows air downwards and cools down your body using the wind chill effect.
In wintertime, if you have an active heating system at home, it will heat up the air in the room. Hotter air moves up and accumulate near the ceiling, colder air being down. A ceiling fan in reverse direction moves cold air up pushes hot air downwards to people's level. It can do this in forward direction but then you will have the wind chill effect again.
Why does the chill effect only occur in summertime mode ?
Consider this fan :
You can try it yourself: If you stand in front of the fan you will feel a strong air stream, if you stand behind you will feel a much weaker stream. This is because the fan collects air from all directions in the back and blows it in one direction in the front.
So, in wintertime mode, wind speed above the fan will be higher than that below, there will be a weak stream of air at your level so you will not feel the chill effect.
A ceiling fan pushes air downward, (or sucks it upward depending on the rotation direction), creating flow streamlines in the shape of a torus (the size of the room).
Basically then, the window is a cavity that has a flow moving past it. If the indoor air is moving vertically fast enough, there will be turbulent mixing between the inside/outside air. Just like when you open your car window while driving: the faster you drive, the more turbulent the mixing becomes, and the better the inside/outside air will exchange before being recirculated through the room.
these high speed particles hit us and hence evaporate our sweat
Indeed it's evaporation that cools us down, but the particles that hit us are not "high speed" particles in any sense (the warming effect of the fan is very small) and evaporation happens not because of their speed.
Evaporation always happens when there is a water surface with not enough water vapour next to it (nothing to do with non-water portions of air, only partial water vapour pressure matters).
To understand how a normal fan cools you down, consider evaporation rate with and without a fan. Without a fan you'll have a lot of water vapour next to your skin because of earlier evaporation, which slows evaporation down. A fan creates air flow that blows a lot of that water vapour away from your skin, therefore making evaporation go faster.
I understand some of the heat that the fan throws out is absorbed by
the water which in turn heats up and cools down the air.
By now it should be clear that that is not at all how it works. By injecting water drops into the airflow you create a lot of opportunity for evaporation by creating large water surface with (hopefully) not enough water vapour around it. Thus evaporation happens and air cools down.
The precise meaning of "enough water vapour" is given by a value called equilibrium vapour pressure. (see Vapour pressure of water) When water vapour pressure reaches that level we say that Relative humidity reached 100% and evaporation stops.
With this it's easy to understand the limitations of this DIY AC:
It won't work well if relative humidity in the room is high;
By evaporation it raises air humidity so it will stop working after a while unless you have a window open to let humid air out and bring dry air in.
Best Answer
In summertime, the ceiling fan blows air downwards and cools down your body using the wind chill effect.
In wintertime, if you have an active heating system at home, it will heat up the air in the room. Hotter air moves up and accumulate near the ceiling, colder air being down. A ceiling fan in reverse direction moves cold air up pushes hot air downwards to people's level. It can do this in forward direction but then you will have the wind chill effect again.
Why does the chill effect only occur in summertime mode ?
Consider this fan :
You can try it yourself: If you stand in front of the fan you will feel a strong air stream, if you stand behind you will feel a much weaker stream. This is because the fan collects air from all directions in the back and blows it in one direction in the front.
So, in wintertime mode, wind speed above the fan will be higher than that below, there will be a weak stream of air at your level so you will not feel the chill effect.