In my last article, I explained some of the basic science of phase changes, referencing the Wicked Witch of the West and the phase change she underwent when Dorothy threw water in her face in The Wizard of Oz. (I’ve since added to the article the clip of that scene from the movie.) I listed some phase changes important in building science and promised to come back and talk about latent heat, one of the most interesting concepts in heating and cooling, so here ya go.
It’s not the heat; it’s the humidity
But wait—there’s heat in humidity!
Air conditioner capacities always give two numbers: one for sensible capacity and another for latent capacity. The sensible cooling capacity is the heat removed by an air conditioner as it lowers the temperature of the air. The latent cooling capacity of the air conditioner is the amount of heat absorbed when water vapor condenses on the evaporator coil (see photo at right).
Condensing water vapor is a really important job for air conditioners in humid climates. Those 2260 kJ of heat that went into each kilogram of water to boil it off (see previous article) turn into energy of the water vapor molecules. That energy has to come back out of the water vapor to condense it. For each kilogram of water that condenses on the evaporator coil then, the AC has to remove 2260 kiloJoules of heat.
To lower the temperature of the air by 20° F requires the removal of only about 10 kJ per kilogram, far less than the 2260 kJ/kg for condensing a kilogram of water vapor. The sensible load is always higher than the latent load, though, because there’s much more air than water vapor and, more important, most of the heat that comes into the house (by conduction through walls, ceilings, windows, and floors, for example) is sensible heat, not latent heat.
The Wicked Witch of the South
Latent heat is in the air when there’s water vapor in the air, 2260 kiloJoules for each kilogram, as a matter of fact. But if it’s just water vapor in the air, why do we call it heat? One of the ways our bodies cool off is through sweating. (Well, that’s the case for men. Women perspire, of course. Or glisten.)
As our bodies push water to the skin, it evaporates, taking heat with it (2260 kJ for each kilogram). The more water vapor that’s already in the air, the harder it is for that sweat to evaporate. So, just as higher temperatures make it harder for our bodies to cool off, so does higher humidity. In the former case, we call it sensible heat; in the latter, latent heat.
The Wicked Witch of the West melted. The Wicked Witch of the South causes us to melt, or at least appear to melt.