Making Cold from Heat: How a Propane Refrigerator Works

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How can a propane powered refrigerator possibly work? It’s impossible! Propane burns, producing heat; there’s no cold involved. This is ridiculous. It makes no sense.

So sayeth my stepson, Zach, who stares at our 19.3 cubic foot refrigerator every time he’s here at the Border Fort. He knows it works because he’s checked it out; the refrigeration section keeps food plenty cool and the freezer takes that compartment’s temperature all the way down to ten below zero Fahrenheit. Sometimes even colder.

His head-scratching puzzlement is completely logical. We live off grid, where propane powered fridges are the rule, not the exception–but I never can remember the theory, either. Yeah, I’ve looked it up a number of times over the years, more or less grasped the concept…and then forgotten it completely a few hours later.

Well. You know what they say: If you want to learn, teach.

At this precise moment, typing this sentence, I have no clue. By the end of the page, maybe–hopefully–I’ll have figured propane refrigeration out well enough to let the reader “get it” and to retain the knowledge in my own long term memory as well.

Man, if I could only apply that technique to learning Spanish….

Our 19.3 cubic foot E-Z Freeze propane refrigerator, complete with a full complenent of dirt-enhanced fingerprints all over the doors.

Our 19.3 cubic foot E-Z Freeze propane refrigerator, complete with a full complenent of dirt-enhanced fingerprints all over the doors.

The Propane Doesn’t Cool Anything Directly

Obviously, the first step would be to find a YouTube video that really spells this out nicely, right?

Ri-i-ight. Except for one thing: There doesn’t seem to be even one such vid out there that doesn’t leave me just as confused (or bored) as I was before.

Hm…Plan B….

Looks like we’ll have to stumble along, see if we can comprehend the first step. Maybe with that percolating through our systems like another cup of warmed-over coffee (which I’m about to pour, thank you very much), we can gear up mentally for step two.

Hey, a guy can dream.

Okay. Step one: The propane burner doesn’t cool anything at all.

Huh?

No. It really doesn’t. What it does is heat up a little storage tank full of a fancy mixture: Ammonia, hydrogen, and water, don’t ask for the proportions or the chemistry involved. You clean windows with a mixture of ammonia and water. Dump that into a tank, add however much hydrogen under pressure, and there you have it–either really clean, wet laughing gas or the secret to propane refrigeration.

A little tank full of those three goodies, under pressure (don’t ask the psi, ’cause this writer has no clue, but they print stern warnings on the generator tanks, so….) is the base point for all fridging and freezing systems of this type (also called “absorption” refrigeration).

Got that? It’s a wet yet volatile mix. You heat that mess up with the propane burner. The ammonia boils off…and stuff starts to happen. Just what sort of stuff, we’ll get to in a minute.

The round tank aka generator on our fridge. Everything starts when this thing gets hot--wow, I really should dust back here more often.

The round tank aka generator on our fridge. Everything starts when this thing gets hot–wow, I really should dust back here more often.

Hot Gas, Cooling Down!

I’ve got it. I really think I’ve got it.

And I’m going to simplify it. All those “other guys” out there who’ve posted wonderfully precise explanations of the process have covered all the bases, starting with this chemical doing this and separating that and recombining yonder and diddy-bopping the chemo-hop over here. It’s enough to turn a real person’s brain to overcooked oatmeal mush. Engineers, mathematicians, and chemists no doubt love going that route.

Not me. In college, I had to bust my butt just to pull a B in Physics and a C in Organic Chemistry. Calculus? Don’t even go there.

Okay, then. Dumbing it down enough for this redneck to comprehend his own explanation, it goes something like this:

    1. Annie Ammonia is floating around down there in the generator tank, minding her own business, mixing it up with her homies, Wayne Waters and Henry Hydrogen.

    2. Some yahoo lights up the burner, boils the frog in the tank. Annie Ammonia yells, “Ye-owch! You bleepity-bleep!” All hot and bothered, she rockets up through the tubes, a** on fire, leaving those slowpokes Wayne W. and Henry H. in the lurch.

    3. By the time she hits the top of the pipe set, Miz Ammonia is all alone, fried and fuming and entirely gaseous, dry as a popcorn fart. She’s wishing for a way to cool off, chill out, get back to her mellow liquid state. That Fire Demon down there is such an a**hole!

    4. Aha! A radiator! There’s a nice, expansive radiator up here on top of the refrigerator world! She’s no hothead by nature, not our girl. Annie Ammonia begins to chill out rapidly, dissipating all that hateful heat in one helluva hurry.

    5. Oh, dang. Now she’s done went and caught herself a chill. What to do?

    6. Look! Another miracle! There’s a little tube thingie here, and a little bit of heat, not too little, not too much, she’s…she’s…she’s going to suck up some of that heat as she slides through the freezer tube (where she sucks heat the hardest and makes the freezer the coldest). Then on down through the fridge (where she sucks heat again, but this time not as much, ’cause hey, she’s warming up a little).

    7. All relaxed and content, her little Annie Ammonia molecule temperature absolutely perfect, she drops all the way back down into the generator, meeting up with Wayne Waters and Henry Hydrogen once more….

    8. …and that *&%#!! Fire Demon is still cranking the burner! She tries to stay with her heavier, slowpoke, less volatile buddies–she really does love them–but she can’t control herself. Off she goes again, rising up through those pipes, hotter’n Hell in a handbasket, looking desperately for that radiator up top….

And thus the cycle goes, ’round and ’round and ’round. Makes you feel sorry for poor little Orphan Annie Ammonia, but her cyclical suffering at the hands of the Propane Fire Demon does keep the food cold.

Hot little Annie Ammonia's cyclical salvation: The radiator near the top of the fridge.

Hot little Annie Ammonia’s cyclical salvation: The radiator near the top of the fridge.

Summary

It appears that “propane refrigerator” is a misnomer. The absorption unit is really an ammonia refrigerator…and the ammonia really does work every bit as hard to keep our food from spoiling as the trials of Annie Ammonia illustrate. Basically, simplifying everything down to the nittiest gritty, it’s a matter of overheating the ammonia…and then, as the ammonia cools, it draws heat from the freezer and fridge boxes…rinse and repeat.

Before closing, a wee bit about ammonia per se. From Wikipedia:

Ammonia, as used commercially, is often called anhydrous ammonia. This term emphasizes the absence of water in the material. Because NH3 boils at −33.34 °C (−28.012 °F) at a pressure of 1 atmosphere, the liquid must be stored under high pressure or at low temperature. “Household ammonia” or “ammonium hydroxide” is a solution of NH3 in water.

Bottom line, it turns out the propane refrigerator is simple enough. It’s just one female molecule, Miz Annie Ammonia, going through a neverending series of hot flashes and chills.

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