Cryocooler

HO-LY-FA-ZO-LI. I'm on the brink of accomplishing one of my long-time desires - having a device capable of creating liquid nitrogen OUT OF THIN AIR (or 253-foot-above-sea-level air at least). Why? Why not, of course! How? Let's go there.

I'm now the proud owner of a Stirling cycle refrigerator (or cryocooler) capable of cooling something down to the neighborhood of 77 Kelvin, or 320 degrees Fahrenheit below zero. Assuming you're reading this on Earth, your atmosphere consists of something like 78% nitrogen, 21% oxygen, and 1% argon and other gases. By cooling down air to somewhere between 77 K (nitrogen's boiling point) and 90 K (oxygen's boiling point), air will begin to condense into a liquid mixture of nitrogen and oxygen. Trouble is, the coldest day in the chilling depths of South Texas winter is about 280 K, or about 200 degrees K or C too hot. Luckily, at the intersection of cell phones and superconductors, there was reason for a company called Superconductor Technologies to manufacture a fine piece of thermodynamics known as a Stirling Cycle Cryocooler.

Now that everybody has fourteen cell phones and smart watches apiece blasting out radio waves constantly, cell towers must be able to filter out all of the noise in order to provide service for each device. It turns out, for reasons I won't even pretend to give a hilariously naïve explanation of, that superconductors can help do a fantastic job of filtering only the target radio signal and amplifying it. Back in the 80's, some sharp grad students discovered that certain materials lose all resistance at temperatures as high (hah) as liquid nitrogen's boiling point. This finally culminated in the purpose for the folks at Superconductor Technologies to attempt to create a compact, reliable, low-ish cost refrigeration device capable of getting down to those temperatures, in order to cool superconducting filters. They did it so well that today you can buy ten year old devices on Ebay for $600 or so that are still perfectly functional. The ghost of Carl von Linde is probably beaming with joy. That these pieces of surplus telecom equipment had within them these cryocoolers was discovered by tinkerer extraordinaire Ben Krasnow and documented over at his blog, which is where I got my inspiration.

I bought a Superlink 850 superconducting filter off of ebay, which includes a Stirling cryocooler rated up to 150W input power. Like any refrigerator, it has a hot end and a cold end. The working fluid is high pressure helium. The piston and displacer are shuttled back and forth by a solenoid linear motor. Here's a paper that describes the workings.

The Superlink - some disassembly required

When I first received the cooler, I powered it up with a variable power supply (it needs about 27VDC to run) but the unit didn't cool. The seller was able to put me in touch with another cryocooler enthusiast who was able to help teletroubleshoot the problem down to a blown power transistor on the control board - shout out to Andrew for graciously donating his Friday night helping out a total stranger.

The toasted transistor kindly left a chalk outline around its dead body to identify himself.

After the seller shipped a new board, the cooler worked just as intended. Besides the troubleshooting, Andrew also offered some valuable tips about the cooler's operation, including a suggestion to use a 36V power supply adjusted down to the acceptable range of the cooler. I bought one off ebay capable of 11 amps output and able to adjust down as low as 28V out of the box, which is in the acceptable input voltage range.

I have some work to do to get it into a functioning air condensing machine, but it'll be fun along the way!

(Edit: Here's Part 2 and Part 3 of the Cryocooler Saga. Part 2 is pretty darn cool!)