Micro Stirling Mechanical Cryo-cooler teardown by Fraser

[Fraser]

In the past I have been asked whether I could do a Stirling Cryo-Cooler from a cooled thermal camera. The problem with this request is that such cryo-coolers are not common and if you have one that works, you generally do not dismantle it as such renders the cooler unuseable.

One of my cooled cameras that was awaiting my assessment is a FLIR SC3000 Quantum Well LWIR camera. Sadly that cameras cooler proved to be dead and it had clearly been dismantled by the previous owner. This presented me with the opportunity to fulfill fellow forum members requests for a teardown of the cooler 

Before we start, a little about these Stirling mechanical cryo-coolers. Some thermal imaging sensors and arrays require cooling to very low temperatures in order to work as an imaging sensor. When I say "low temperatures" we are dealing with similar temperatures to that of Liquid Nitrogen. Many thermal cameras operate their Stirling Coolers at 77K (-196 Celsius) but some operate at different temperatures. This QWIP unit operates at 70K (-203 Celsius). These are the low temperatures needed to lower the internal noise of a semiconductor thermal detector to a level where it can detect the thermal energy found in thermal imaging camera systems.  At room temperature such a detector would be swamped with its own thermal noise. There are special semiconductor detectors that are also cooled but can operate satisfactorily at "only" -70 Celsius but they tend to use Peltier Stack coolers and not a Stirling mechanical cooler so will not be covered here. The Micro Stirling Mechanical Cryo-cooler is a precision piece of mechanical engineering that has an operation al life before it requires a service. The predicted operational life of a rotary Micro Cro-Cooler used to be around 2000 hours. This has improved over the years and a 10,000 hour life is not unusual now. Servicing a Micro Cryo-Cooler, such as we are discussing here, is a specialist task that requires the correct service tools, techniques, spare parts and Ultra Pure Helium gas (NOT Helium balloon gas!). This results in Cryo-Cooler servicing being expensive and difficult to source. A cryo-cooler service can easily cost $5000 per cooler.

The cryo-cooler that we are going to see in this thread is a Rotary type but there is another type that is called a Linear Cryo-cooler. There are also split cooler systems. I will not go into the different types of Cryo-Cooler here as there is plenty of information available on the topic via Google. Are such Cryo-Coolers still made and used ? Yes they are. High performance thermal imaging systems often use a Stirling Cryo-cooler to produce low noise imagery. You will also find such Micro Stirling Cryo-coolers flying on satellites where they are used in all manner of applications including Imaging, Sensor arrays and specialist science systems that require operation at very low stable temperatures.

OK, enough of the background, on with the pictures. I have included some images that show the design of a Micro Stirling Cryo-cooler to help readers understand the content of the images. 

[Fraser]

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So what was wrong with the SC3000 Micro Stirling Cryo-cooler ?

Tests showed that the brushless motor controller/driver has failed. This means that the field coil is not receiving the correct drive but the whole integrated coil and controller PCB can be replaced without breaching the coolers Helium gas seals. Sadly someone decided that opening the coolers sealed casing and releasing the Ultra High Purity (UHP) Helium was justified. The moment the seals were breached, the cooler effectively became scrap. The failed controller and drive PCB is potted into the top of the motors field coil housing and so would not be a simple repair. The QWIP detector end of the assembly also has a very unhealthy rattle so something has broken loose inside. This Integrated QWIP detector and Cryo-cooler assembly is beyond economic repair. Hence the teardown.

In the provided images the reader will see that the motor shaft incorporates a couple of offset sections that drive the Compressor Piston and Displacer connecting rods. It looks a little like piston connecting rods on the crank shaft of an Internal Combustion Engine. The offset sections of the motors drive shaft provide the correct phase relationship between the compressor Piston and Displacer movements for the Stirling Cooling effect. Ultra High Purity Helium is used as the gas within the coolers sealed casing as it it greatly improves performance and will not freeze at 77K. Any air contamination of the Helium Gas fill can lead to 'piston lock' as a result of freezing.

The cryo-cooler that I have disassembled for this thread showed signs of high run time hours so it was likely nearing the end of its operational life when it eventually suffered an electronics failure. There is solid material contamination within the motor magnet area and this may be the result of the high run time hours or the previous disassembly. If it is due to run-time hours, something was wearing and releasing black material into the ultra clean environs of the Compressor Piston and Displacer   I have another Micro Stirling Cryo-Cooler that sounds like its has bolts rattling around inside it when running. I suspect a bearing or connecting rod pin failure. Such a failure would likely distribute solid material around the ultra clean areas of the cooler and lead to its self destruction over time. Machines suffer from friction that casuses wear so it is not surprise to see a Rotary Stirling Cooler suffering as a result of its lack of liquid lubrication.

I trust that this teardown has been of interest to those who know of the technology but have not seen the internal workings of a unit before.

[coppercone2]

what are the highest tolerances required for the parts in the cryo cooler?
and a university particle physics department with a accelerator might be a source of the UHP helium gas, I have seen big walls of filters / objects that had something about high helium purity on it (but the requirement for the feed gas was already some purer form of helium).

Looked like you need to send it through like 10 different canisters to get it cleaner.

I would not be surprised if they had additional surface finish steps in something like that, I doubt its just ground. Like burnishing or electro polishing.

[Fraser]

I regret that I have no inside knowledge on these coolers and their tolerances. The UHP Helium was a challenge for me to obtain when I went searching for it from BOC. They would not even entertain supplying it to me and the cost was high for small amounts anyway. The Helium pressure inside the cooler is around 40 Atmospheres (~588 psi) and you need specialist equipment to purge the air out of the cooler, fill it with UHP Helium and then insert the Helium seal and grub screw, all without disconnecting from the coolers single fill port. I soon gave up on the idea of refilling my old Micro Cryo Coolers with UHP Helium. 

[Spirit532]

what are the highest tolerances required for the parts in the cryo cooler?

H6/h7 or thereabouts for the motion parts, minus the crankshaft. Gas dynamic bearings(running in helium, obviously) in operation, until it crashes and starts rubbing(like in this teardown).

I doubt its just ground. Like burnishing or electro polishing.

Fine ground or lapped(honed), depends on which part.

Looked like you need to send it through like 10 different canisters to get it cleaner.

You just buy UHP 6.0 helium from a supplier. Cleaning it is quite difficult, way cheaper to just buy. UHP isn't too expensive.

I soon gave up on the idea of refilling my old Micro Cryo Coolers with UHP Helium. 

Should've sent them to me