Fan and AC
Why is it that the cooling effect caused by an air conditioner is reduced when a fan is switched on?
P. K. VARADARAJAN
Chennai
The hot or cold feeling we have is derived from the temperature of air our body comes in contact with. A comfortable temperature ranges from 22 to 26 degree Celsius, which the air conditioner, normally programmed to, provides.
In an air conditioned room, in a hot place, the air is maintained at a prefixed temperature, normally lower than the temperature outside.
This is accomplished by making a stream of air, taken from the room itself, flow continuously against a large area of cold metallic surface and by subsequently blowing it into the room.
The temperature of air is monitored at the inlet of the stream and when it is noticed to be equal to the preset value, the cooling mechanism is turned off to be turned on again when the temperature rises by about one to two degrees.
This cold air is blown directly to the region of the room directly in front of the air conditioner unit. This air in turn, spreads into the room and cools the air. However, in the places near the blast of the AC unit the air is cooler than at places away from it.
Further, the cold air is denser than the relatively warmer air in the room. Thus, the air from the AC settles down towards the floor of the room. During this it loses energy and is more or less localized to the part of the room in front of the AC unit. In order to correct for this and distribute the cold air all over the room, the new AC units are fitted with swinging baffles throwing the cold air in different directions.
Normally our indoor furniture is positioned in a manner so as to receive most of the cold air. However, there are regions in the room, which are relatively farther from the cold region and the air there has a relatively higher temperature.
In a big size room, the temperature difference can be as much as 5 degrees Celsius, which can be easily perceived. If we switch a fan on, while positioning ourselves at the cold region of the room, the fan stirs up and mixes the air from all the regions of the room.
This mixed air would have a temperature, relatively higher than that of the air directly blowing from the AC. This gives us a feeling as if the air has become warmer by turning the fan on.
PROF. H. K. SAHU
Chennai Mathematical Institute, Chennai
