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Saturday, August 1, 2009

Science The Miracle of Air Conditioning

Posted by Jonathan Golob on Sat, Aug 1, 2009 at 1:21 PM

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.

Comments (19) RSS

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you know what else is a "miricle" of modern science? spellcheck.

Posted by yay! leftovers on August 1, 2009 at 1:25 PM

@1: Got it. Thanks

Posted by Jonathan Golob

on August 1, 2009 at 1:29 PM

As an architect, I seriously desire to impress my engineers - plus, I tend to crush on geeky guys - so I cannot wait to use "peev-nert" on them at a meeting and see what happens! Thanks, Jonathan!

Posted by Donna on August 1, 2009 at 1:52 PM

I think I learned this *IN* kindergarten.

Posted by veo_ on August 1, 2009 at 2:13 PM

needs more pics of hot girls in lab coats

Posted by Swearengen on August 1, 2009 at 2:27 PM

You could have been a bit more precise talking about the phase change involved (it's not a gas for half of the process), but not a bad summary for a short post.

Posted by Matt the Engineer on August 1, 2009 at 2:59 PM

Maybe it's a Southern pronunciation, but I always heard it:
Puhv - nert.

Posted by Ackham on August 1, 2009 at 3:07 PM

Is that why I blow icicles out my ass?

Posted by Vince on August 1, 2009 at 4:08 PM

So this is why the air pressure in my bike tires changes when the temperature outside changes, right?

Posted by TVDinner

on August 1, 2009 at 6:34 PM

@7: Yeah, that's the way I learned it too in California.

If you say "aldol condensation" to your friendly local chemist, you get invited to the secret Chem Keg Party.

Posted by Max Power on August 1, 2009 at 7:02 PM

As a mechanical engineer who studied at a proper school, I can tell you we had no such saying. Don't listen to to his witch doctor (or witch engineer?) science! Lies! Lies, I tell you!

Posted by Trioculus on August 1, 2009 at 7:26 PM

Of course that doesn't explain my Supentown portable air conditioner which I can proudly say, after 3 years of ownership, I finally mastered!

Portable air conditioners have a 6" hose (don't get excited, SLOG) like a dryer. It ferries the hot air from the expander outside.

Now, what are the secrets? First of all, I had been positioning my hose with a lot of bends. This severly limited the ability of the exhaust fan to pump out the air. I also had the device in a place where I used every inch of the length of the hose. Also bad for two reasons: 1) it was hard to push the air out of the hose (2) the hose got so hot it was transferring the heat back into the apartment!

Therefore, I repositioned the air conditioner from a window location to the living room and run the exhaust hose out my sliding door! I also make sure that I'm using a minimal length of hose (it's collapsible like a Slinky) so that the exhaust fan doesn't have to work so hard to push the air out of the hose.

Before I was averaging just a few degrees cooler than outside, now I've got it up to 20 degrees cooler and DROPPING. I bought a space age material at Home Depot -- bubble wrap covered in mylar -- to fill the gap of the open door on the patio.

It took me hours of introspection to get this right, sort of like Richard Feynman finally getting inspiration from tossing paper plates in the CalTech cafeteria...and now, like Mr. Freeze, I will shut the blinds and immerse myself in coldness...

Posted by Mr. Freeze on August 1, 2009 at 7:32 PM

Donna@3: "peev-nert" is a new one on me, and I hang with geeks. Golub's a doctor type, not an engineer. Your jargon-dropping strategy will leave your crushee cold. I suggest instead buying a battered old HP-15C on eBay, and learn how to use its retro-chic revere-polish notation. Keep it in the breast pocket of your blouse. Then lean over the conference table to point to a key detail in the HVAC drawings. Look up real quick. The guy you catch trying to peak down your pocket is yours for the taking.

Posted by Eric from Boulder on August 1, 2009 at 8:06 PM

@3-- I'm a mechanical engineer who designs HVAC for buildings. Maybe its just a regional thing, but I don't get the impression that "peev nert" is a commonly-used expression for the ideal gas law. I typically hear it spelled out when spoken: "pee vee equals in are tee." Regardless, we definitely don't use that equation on a day-to-day basis.

If you really want to make a splash with your HVAC guys, use this one: "A bad delta T makes me HOT!" This refers to the difference between the room temperature setpoint and the temperature of the air being supplied via the HVAC system. If asked to explain further, tell them that if the "delta T" is out of whack, then the space will likely become unfomfortable, but be prepared to admit that "a bad delta T" could just as easily cause a person to be uncomfortably cold as well. If pressed even further, tell them it was "just some stupid bullshit I read on the internet."

Posted by dangerkitty5000

on August 1, 2009 at 8:22 PM

I'd always wondered why compressed air cans gather frost after a few minutes use. This is an awesome explanation that explains many other things as well.

Followup question, though: why is freon the preferred particle for use in cooling systems? I mean, this effect is obviously not specific to any one molecule. Is freon just good at being compressed or something?

Posted by Lee on August 1, 2009 at 8:57 PM

Awesome explanation. Now, where is that working air conditioner ... address please ;)

Posted by Ryan-T

on August 2, 2009 at 3:58 AM

Nice explanation, but I can certainly imagine that anyone who isn't a scientist doesn't remember what P, V, n, R, and T are. (yeah you can google it, but then, why explain in the first place?)
P= pressure, V=volume, n is essentially the amount of gas, R= an empirical constant, T= temperature.
Just means that the pressure and volume of a gas are proportional to how much of it there is and how hot it is, and that they are inversely proportional to each other. Fun Times.

Posted by zapfizzle on August 2, 2009 at 8:59 PM

By the way, one thing to note is that air conditioners make the outside air hotter. So for those who have, yay you, stay inside; for those who don't, sucks to be you, try not to wilt.

Also, air conditioners use motors to compress the gas and motors generate heat, so there’s that added heat. Also, they use a lot of electricity, which translates to a lot of added heat…

Not that cars are free and clear of blame, but let’s keep in mind the glorious contribution A/C has had on keeping our planet cool…

Posted by Allyn on August 3, 2009 at 8:02 AM

A heat pump is basically a reversible air conditioner. When you run it one way, you chill the outdoors and heat your house. When you run it the other way, you chill your house and heat the outdoors. (Not including waste heat.)

Posted by Greg on August 3, 2009 at 12:03 PM