Pulse Tube Cryocooler (Part 4) - Valve Controlled

This is the 4th part in my video series on designing/building/testing a pulse-tube cryocooler in an attempt to create low enough temperatures to liquify nitrogen (-196C or 77K). In this video, i'll be investigating an entirely different type of pulse tube that uses two valves - one connected to a compressor and another vented to the atmosphere - to create the pressure oscillations in the pulse tube. This is known as a "Gifford-McMahon" (GM)cycle, and is more commonly used in a refrigeration cycle with a moving displacer, but can also be found in use with pulse tubes.

The GM cycle is less efficient than using a piston (stirling-type) to generate pressure oscillations, but has the advantage that it's cheaper and easier to build, since it can be coupled with a variety of different compressors and matching acoustic impedance is not neccesary. GM-type cycles typically operate at a COP of under 5% of Carnot, whereas advanced stirling-type coolers can approach 30% of Carnot. Despite this, GM cycles are the primary type used in terrestrial applications, such as cryocoolers in labs and production
facilities
where modest quantities (under 10L per day) of LN2 are required. Despite the increased energy consumption, this configuration is typically cheaper to install/maintain than stirling cycle coolers, which are usually reserved for aerospace applications, like being mounted onboard sattelites or missiles to cool optics.

For the valve assembly, i used a pair of solenoid valves activated by MOSFETs which could have their timing controlled by an arduino with a front panel display, allowing me to know the exact timing I dialed in, which was very convenient for repeatability. Real GM cycle coolers use a rotary valve connected to a motor inside the pressurized assembly, which is more reliable than solenoid valves.

The entire assembly runs at around 0.5 hz. Typical commercial units range between 1-2 hz, in contrast to stirling cycle units, which can operate anywhere from 20 hz to 100 hz. However, pressure ratios are typically much higher on a GM unit, ranging from 2-3 with a baseline pressure typically from 10-30 atmospheres, whereas pressure ratios on stirling units are usually in the neighborhood of 1.1-1.3. Most GM units will have a high pressure side around 200-400 psi and a low pressure side from 50-200 psi, depending on the cold head and compressor being used.

The lowest temperature i achieved running on compressed air was -83C, but I didn't consider this a valid result, because I wasn't able to repeat it. The lowest repeatable temperatures were in the low 70's, basically matching the performance of my previous stirling-type pulse tube. Considering that the power input required was close to 1 kW, compared to the previous design's power of ~120W max., it's clear that this setup is far less efficient.

One of the major sources of inefficiency is the fact that my low-pressure side is simply connecting to the atmosphere, rather than being connected back to the compressor in a closed loop. This is because my current compressor isn't hermetically sealed, and not meant to operate with input pressures over 1 atm. With my high side pressure at 100 psi, theoretically, the most efficient low side pressure would be somewhere between 40-60 psi.

In the next part of this series, I'll be generating hydrogen and using it as a working fluid instead of air, with a closed-loop system running off a hermetically sealed fridge compressor. This will dramatically increase internal heat transfer, making the whole system more powerful and efficient, and should bring me much closer to my goal of -196C.

Part I:

Part II:

Part III:

Music Used:

Kevin MacLeod - Lobby Time
Kevin MacLeod - George Street Shuffle

[gkdresden]

I was one of the guys responsible for the 4-valve pulse tube cryocooler at the University of Jena. For GM-type pulse tube cryocoolers you have to avoid a too high pressure ratio. It is not wise to go over 3 to 3.5 because otherwise you get a superheating somewhere in the middle of the pulse tube length, because you cannot exchange about more than 30 to 40 per cent of the gas contained in the pulse tube across the cold and hot heat exchangers.

The heat exchangers are very crucial for the performance of such coolers. So for the heat exchange with the environment (aftercooler, cold hx, hot hx) we have used slittet copper parts manufactured by electro-erosion. For the regenerator we have used stainless steel mesh screens which were slidely over sized in diameter (0.2 mm) to get a dense package with no void passes at the rim of the regenerator tube.

The double-inlet version may be more powerful than the orifice or inertance tube version. But the adjustment of the hot end bypass valve is very tricky. In general it is almost impossible to avoid dc flow in the system which lead to enthalpy flow losses. So the 4-valve version is the most powerful one even in the case that it has comparable dc flow losses.

From the viewpoint of efficiency the stirling-type pulse tube cryocooler is better. The best design is the active reservoir system, which I invented in 2010. In this design you can regain a lot of the expansion work with a second piston / cylinder system at the pulse tube hot end and you can exactly set the phase shift between pressure wave and mass flow, like in the case of a comparable alpha-Stirling cryocooler.

[oxoniumgirl]

Instead of using acid for H2 production, you may want to try Lye (NaOH) and Al foil; it generates copious amounts of H2 and there is no corrosive acid mist or vapors generated, and the sodium aluminate byproduct is much less corrosive than the lewis acid aluminium salts produced as byproducts from most readily available acids. If you want to dry the hydrogen produced then you can pass it through a drying tube packed with just more fresh NaOH, which you can then feed forward as more reactant for future cycles. If you use HCl for example, you're stuck with having to also neutralize the acid vapors and mists, which requires additional consummation of reagents without the potential for feeding forward the reagents as reactants.

[michaelandersen]

This is awesome! There are only two channels that make me enjoy looking at graphs and diagrams - this one and Huygens Optics :-)

[demandred]

When Applied Science, AND Nighthawk in light are following along, you know this is good stuff.

[elen]

this is inspiring stuff, "pulse tube cryo cooler in the basement" is now on my list of things I didn't know I needed

[jamesmorton]

This level of effort indicates that you are not married. Very very dense technical information exchange. Your work is STELLAR. Thanks for your sacrifice..❤❤

[beautifulsmall]

Pure science, with all the numbers. Its a voyage of discovery. Taking energy out of a thing is so much harder than putting energy in, and that's what makes it so interesting. 15 and 6% mesh fill, thats supprising. Nice winding the aly tube, I would be scared of kinking. Exceptionally well presented.

[Timestamp_Guy]

6% density is probably plenty. You should do it as a ratio of mass of the air you are regenerating vs mass of regenerator. Even at elevated pressure, you only have maybe 1 or 2 grams of air moving through that thing. I guess it depends on how many liters of air are passing through each cycle. 115 g of steel wool to 2 grams of air being regenerated... you might even go less to see if the reduced flow restriction is hurting more than a few extra grams is helping. Checking the pressure drop across regenerator vs mass through it per cycle could be enlightening.

[Neptunium]

Definitely some of the best project and video quality out here! Vastly underrated Chanel!

[marstrain]

This is brilliant, I was not expecting to see a 4 part series on this. I love the process you use to explain the design process, its a lot more transferable

[Rowow]

thanks alot for not giving up and continuing the project! cant wait to see it working, looking to make iron nitride magnets so this is definitely a project that will help a ton

[DEtchells]

I’m impressed that you managed a 100C delta-T with just air and a low side pressure of 1 bar. Mega-props, can’t wait to see more in this series!

[russoft]

I run a bunch of MRI systems and helium recovery systems all which use G-M cryocoolers, both mechanical and pulse-tube. This series is fascinating since I don't get the opportunity to really play with operating parameters of these systems and mostly just run them as-is from the manufacturer. Keep it up!

[andycrask]

Great video, it must be hard keeping track of results and having so many variables you can tweak.

[ATomRileyA]

Really loving these videos, great to watch along with your progress.

[lIII]

Very cool, pun intended. :). I want to get one when they become commercialized.

[lspcie]

I’m really looking forward to the next video. Ads a teenager i used to build single and two stage compressor coolers for CPUs. Using propane in stage one and ethene in stage two i got to -108C at 200W+ loads. That was using reciprocating (Danfoss) compressors but I think rotary or screw will be better for you in higher pressure applications.

[KU]

Pulse Tube Cryocooler (Part 5) - Staring at even more graphs!

This is like the most awesome project ever, turned into a crappy power point by that one guy at work.

[kenmagalnik]

Very cool project. Industrial helium compressors typically have a whole system to first introduce and then remove oils from the helium gas to lubricate the compressor.

[karlosss]

I'm in awe at how much time you must have put into this project. Its all very interesting... thx