There are several different configurations that a cryocooler system can take: users can choose between Gifford-McMahon or Pulse Tube cold head assemblies, and connect the cold head to an air- or water-cooled compressor unit. The following guide should help shed some light on the advantages and disadvantages of the different available types.
Gifford McMahon (GM) and Pulse Tube (PT) are two different kinds of closed-cycle cryocoolers that employ different operating principles. The base temperature achieved with either is about the same (guaranteed to be <4K, typically under 3K). The table below compares GM and PT cryocoolers.
|Maintenance Interval||10,000hrs (may be
higher in practice)
|Base Temperature||~ 3 K||~ 3 K|
GM and PT cryocoolers come in different cooling capacities, i.e. the power that can be dissipated whilst still keeping the temperature below 4.2K. It is important to calculate how much heat your experiment will generate, including the effect of thermal radiation, when choosing a cryocooler.
The cooling capacity also translates into how fast the cryocooler on its own (i.e. with no thermal mass attached) gets down to 4.2K, which can vary from a few hours for small capacity to less than 1 hour for large capacity cryocoolers. Higher capacity cryocoolers are more expensive and have higher power requirements than lower capacity cryocoolers. For example, it may require 8kW of power to provide 1W of cooling capacity at 4.2K, but only 1.5kW to provide 0.1W of cooling capacity at 4.2K for a smaller cryocooler.
The amount of time to get to base temperature increases once a thermal mass, such as our Freeze4 sorption fridge is added on. When attached to a small 0.1W GM cryocooler, it initially takes about 10-14 hours to cool from room temperature to 0.8K (depending on the capacity of the sorption fridge). After the fridge stays cool for its hold time (e.g. 12, 24 or 36+ hours), it only takes 1-3 hours to cycle (i.e. recharge) the fridge. With a larger cryocooler, the cooldown and cycle time can be made shorter.
GM and PT cryocoolers come with associated compressors that continually operate to keep the system at the base temperature. Heat from the system is dumped either into water or into the air, thus the need for either a water-chiller, or ample air-conditioning. Air-cooled compressors have large fans which typically have a louder ambient noise than water-cooled compressors. If desired, the compressors can be located some distance away (e.g. in an equipment chase area) by using longer helium flex-lines between the compressor and cold-head without impacting performance. The table below compares air- and water-cooled compressors.
|Compatible cryocooler systems||GM or PT|
|Power, if with PT cryocooler||
220V-1ø for 0.25W @ 4.2K PT
208V-3ø for 0.5W @ 4.2K PT
220V-1ø for 0.25W @ 4.2K PT
|Power, if with GM cryocooler||208/230V (~0.25W GM)
3-phase 208V above 0.5W GM
110V or 208/230V (0.1-0.25W GM)
|Space requirements||smaller, but need cooling water||more for air intake and vent|
|Minimum heat generated for smallest available cryocooler||water chiller must be
|6000 btu/hr (for 0.1W)
11,600 btu/hr (for 0.25W)
20,000 btu/hr (for 0.5W PT)