Air Conditioning

Aug 1st, 2009 | By | Category: How it Works

Air conditioners aren’t cold machines; they’re pumps. After this past week in Seattle, several of us are a bit more interested in how to move heat around.

Science can help. First up, the ideal gas law: PV = nRT. (It’s pronounced peev-nert. Say that to your friendly local mechanical engineer and you’ll likely receive a ‘8.31, yo!’ back in return. Plus or minus a fist pump, or high five. Know you’ve done well.)

You probably know this law, at least in a practical sense. Think of the last time you’ve used an aerosol spray can. After a while, it gets cold. Decompressing a gas (from high pressure inside the can to relatively low pressure in the air) requires adding some heat to the gas. The heat is taken from the surrounding environment. The can gets cold. On the flip side, think of the last time you filled up a tire with air. As you compress air into the tire, it gets hot. Compressed gasses need to give off some heat, making the surroundings hotter.

You can think about the molecules in a gas as being like a class worth of kindergartners. At low pressure, it’s like they’re at recess–each running around like a hyperactive lunatic. When compressed, gasses are more like the kids sitting quietly around at story time; some fidgeting is going on, but each must be calmer and closer together. Pressurizing a gas from recess to story time requires a whole bunch of hyperactivity energy to be blown off.

Compressing gasses gives off energy as heat. Decompressing gasses absorbs heat. Air conditioners do this in a constant loop. Inside your house, a gas is depressurized, absorbing heat from the room in the process, cooling the air. The low pressure gas is then pumped outside. Outside, the gas is compressed, releasing the heat energy taken from inside. Then it heads back in. Over and over again.