Q Drive
 
Thermoacoustic Stirling (pulse tube) Cryocooler Operating Principle
How does it all work?
Acoustic Stirling Technology.
Qdrive coolers work on a modified Stirling cycle.  The Stirling cycle has been long recognized as an efficient heat pump, but the mechanical complexity of prior embodiments have largely prevented its economical application.  Qdrive’s acoustic Stirling system eliminates ALL cold moving parts, valves, and contact seals.  Only the pistons of our resonant linear motors move, in balanced pairs to form our ambient-temperature Pressure-Wave Generator (PWG), the acoustic power source for our cooling cycle.  That pressure wave created by the PWG forces helium in the Stirling parts of the system to oscillate back and forth. (1) During the forward (compression) stroke of the pistons, any particular gas parcel in the region between the PWG and thermal buffer tube is compressed relative to the mean pressure of the system. And since an ideal gas (such as helium) will experience a temperature increase when compressed, said gas parcel rises in temperature while simultaneously moving towards the end of the coldhead. (2) The gas parcel is now hotter than its surroundings, so as it moves, it deposits heat (in the regenerator), which is ultimately removed from the system in the aftercooler. (3) At this point, the driven gas stops advancing when the pistons reach their stroke limit. However, the gas in the acoustic network (which consists of the thermal buffer tube, hot heat exchanger, inertance tank) continues to move in the same direction, driven by its own inertia. This acts like a mechanical piston, moving away from the cold exchanger, which expands the gas parcel. (4) As it expands, the gas parcel gathers heat from its surroundings (the area to be cooled). The pistons begin their backward (expansion) stroke and the gas parcel moves back to where it started. Again, delayed by its inertia, the gas in the acoustic network follows at a lag, and the cycle begins again.

Helium gas in the thermal buffer tube, hot heat exchanger, inertance tube, and reservoir does some, but only minor, heat pumping. Mainly, this acoustic tuning section acts like a mechanical piston, expanding the precooled gas parcel in the cold heat exchanger. This is one of the major benefits of acoustic Stirling technology, there are no mechanical parts operating in the cold region of the system (such as in traditional Stirling machines), only an acoustic network of compressed helium that acts like a piston.

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Other Cooling Technologies

Liquid nitrogen (LN2) has long been the standard cryogenic cooling method.  It is inexpensive and provides reliable unlimited cooling capacity.  However, LN2 can only cool to its liquefaction temperature of 77K, and the need to constantly replace the expendable cryogen leads to logistical inconvenience, cost, and delivery risk.

Because of its limited cooling temperature and the inconvenience — and in some applications the impossibility — of using LN2, users have frequently sought alternate ways to produce cryogenic cooling capacity. This has led to the development of many types of mechanical coolers including: displacer Stirling, Joule-Thomson, Gifford-McMahon, and acoustic Stirling type coolers. All cooler types have inherent strengths and weaknesses that make them better or worse for any particular applications.

Displacer Stirling cryocoolers use a mechanical displacer in the cold region of the cryocooler to expand compressed helium gas and create cooling. Typically these systems provide good efficiency and are fairly reliable and offer a range of cooling capacities at 77K. A major disadvantage is that the mechanical displacer motion creates vibration that is often at an unacceptable level for some sensitive applications.

Joule-Thomson (JT) systems use high-ratio oiled compressors that compress a specific mixture of refrigerant gas. The gas is expanded through a small orifice and cooling is produced. JT systems provide cryogenic cooling with a minimum of vibration because there are no moving parts in the cold region. The gas mixture that is expanded is specific to the desired cooling temperature, and JT systems often have poor cooling efficiencies across the cooling envelope. Also, they are unreliable as the small expansion orifice can easily become clogged with oil/particles and regular service, maintenance, and even unit replacement is common.

Gifford-McMahon (GM) cooler use a conventional oiled gas compressor to supply a steady stream of compressed helium to a coldhead through which the helium is expanded with mechanically driven valves. GM cryocoolers are preferred for temperatures well below 77K, but because of the coldhead valve wear and oil in the compressors they require regular maintenance and service.  Also, the compressors can only be used when resting still on a level surface, making them unsuitable for some military and aerospace applications. GM coolers produce significant noise and vibration in the coldheads, making them unusable in laboratory situations.

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Competitive Comparison
 

CompanyQdrive Brand 'S' Brand 'P' Brand 'C' Brand LN2
Cooler TypeAcoustic Stirling Displacer StirlingJoule-ThompsonGifford-McMahonLost Boil
Cooler Model 2s102Kexample: Cryotel example: Cryotigerexample: AL10/CP810  
      
Operating Characteristics     
Capacity @ 77K (Watts)8-105-15*<3.514Any
Power Draw (Watts) 27080-240*50012000
Standard Power Type110V-60Hz24 VDC110-50/60Hz  
Noise Level (dBA)<65<65<65Loud<65
System Weight (kg) 1963466Variable 
Maintenance IntervalNoneNone<8,000 hr8-12,000 hr None 
Orientation-independentYesPossibleYesNoNo 
Motion-tolerantYesYesYesNoNo
Cold Restart without delay YesNoYesYes 
Oil-freeYesYesNoNoYes
      
Cost Breakdown      
Base Cooler Cost ($USD)$15,400$9,500 ~$8,000~$16,000<$1,000 initial
(Required quantity for given price)120-50111 (dewar)
Heat Rejection Exchanger IncludedExtra $$IncludedIncludedIncluded (vapor)
Vacuum FlangeIncluded Extra $$ IncludedNo 
Capacity ControllerIncludedExtra $$NoNoNo 
Full Vibration Cancellation IncludedExtra $$   
Cold Temperature Sensor IncludedIncludedNo  Not Req'd 
TOTAL PURCHASE COST ($USD)$15,400$10-20,000$8-12,000~$16,000 <$1,000 +$$/use
      
Available Options     
Temperature Controller AvailableAvailable**NoNoNo 
Custom Voltage AvailableNoNoNo 
Custom InterfaceAvailableAvailable   
Custom HXAvailableNoNo No 
Remote ColdheadAvailableNoIncludedIncludedNo
50/60 HzAvailableAvailableIncludedAvailable 
      
* Various versions     
** T control only with 24 VDC input     

 
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