How Air Conditioning Works

On the occasion three years ago of the 110th anniversary of the recognized invention of the modern form of air conditioning, I wrote this little historical/modern explainer about how heat-exchange and air-conditioning systems work for the Economist. Yes, the terrible title, "It's the Humidity," is all mine.

But I was trying to explain to Rex, age 8, this morning how air conditioning worked. I tried a bunch of explanations and metaphors, but this one stuck.

Imagine you have a pool that's 90°F. There is an endless line of swimmers who have a natural resting temperature of 60°F waiting to jump in. Each time one jumps in, they warm up to the pool temperature, and the pool, by necessity, gives up some of its energy to each swimmer, becoming cooler. The swimmer is then shot out through a chute, the friction of which warms them up even more. They stand in front of fan that cools them back to 60°F. Then they get back in the pool.

This is more or less it. A heat-exchange system has a coolant that has a low boiling point, so it's easy to manipulate it to absorb heat and shift from a liquid to a gas and vice versa, as well as varying pressure in gaseous and liquid states. The idea is that a fan takes in hot air, runs it across coils that absorb the heat, cooling the air, while circulating the heated coolant to an outside radiator or with a fan that helps vent the heat outside.

As with most things related to temperature change, it's a funny thing. Is "cold air" something real or is it reducing the temperature of air in its vicinity?