Stirling and Gifford-McMahon (GM) cryocoolers are two of the most commonly used cryocoolers in cryogenics. Both devices have a significant industrial base and operate at a wide range of temperatures and capacities.
The thermodynamic cycles for both of these cryocoolers are quite similar. The Stirling cycle consists of a compressor, regenerator and a cold displacer. The working fluid (typically helium) is compressed and the heat of compression removed at room temperature. Next the compressor and the displacer move together to transport the helium through the regenerator, where the cold regenerator cools the helium. Then the displacer moves, reducing the pressure on the cold helium and producing further cooling.
At this stage the helium absorbs heat from the item being cooled and stays at a constant temperature. Lastly, the displacer and compressor move together to pass the cold helium back through the regenerator, where the helium cools the regenerator and warms back up to its initial state, closing the cycle. Figure 1 illustrates this, both schematically and in a pressure volume (pV) diagram.
The GM cryocooler cycle is essentially the same, except that in this design a valve connected between the high and low pressure sides of the compressor drives the motion of the gas. The presence of the valve introduces irreversibilities into the cycle, but it allows the speed of the displacer to be decoupled from the speed of the compressor. This means that engineers can forgo custom built compressors and instead use reliable and relatively inexpensive commercial gas compressors that operate at power line frequencies for GM cryocoolers.
Care must still be taken to remove any compressor oil from the helium stream prior to entering the cryocooler and to ensure that the compressor chosen can work with helium, but the result is that GM cryocoolers can be very reliable and less costly than other cryocooler designs.Stirling and GM coolers have many applications. Engineers frequently use Stirling cryocoolers for aerospace or military applications while a major application of GM cryocoolers is the provision of 50 to 80 K cooling of the cryopumping panels that provide clean vacuum spaces in semiconductor production. Both Stirling and GM cryocoolers are also used in laboratory applications and frequently contain cold stations at multiple temperatures. Both types of cryocoolers have found applications in reliquefaction and zero boiloff systems. Figure 2 shows a split Stirling cryocooler in which the compressor and cold head are separated to reduce vibration while Figure 3 shows a typical GM cryocooler.
Good descriptions of Stirling and GM cryocoolers and their operating cycles are given by R. Radebaugh, “Cryocoolers: the State of the Art and Recent Developments,” Journal of Physics: Condensed Matter, Vol. 21, No. 16, 2009; A.T.A.M. de Waele, “Basic Operation of Cryocoolers and Related Thermal Machines,” Journal of Low Temperature Physics, Vol. 164, 2011; and G. Walker, Cryocoolers Part 1: Fundamentals and Part 2: Applications, Springer, 1983.Examples of Stirling cycle cryocooler applications include K. Nakano et al., “Development of High-efficiency Stirling Cryocoolers for High Temperature Superconducting Motors,” Advances in Cryogenic Engineering, IOP Conference Series: Materials Science and Engineering, Vol. 101, 2015; T. Nast et al., “Modular Approach to Spaceborne Stirling Cryocoolers,” Cryogenics, Vol. 34, 1994; and R. Boyle et al., “TIRS Cryocooler: Spacecraft Integration and Test and Early Flight Data,” Advances in Cryogenic Engineering, Vol. 59A, 2014.
Descriptions of GM cryocooler applications can found in C. Wang, et al., “A GM Cryocooler with Cold Helium Circulation for Remote Cooling: Advances in Cryogenic Engineering, Vol. 59B, 2014; M.A. Green, “Recondensation and Liquefaction of Helium and Hydrogen Using Coolers,” Advances in Cryogenic Engineering, Vol. 55A, 2010; H. Nakagone, “High Efficiency Cryocooler for Liquid Hydrogen System,” Advances in Cryogenic Engineering, Vol. 51B, 2006; and S. Fujimoto et al., “Cooling of SQUIDS using a Gifford-McMahon Cryocooler Containing Magnetic Regenerative Material to Measure Biomagnetism,” Cryogenics, Vol. 35, 1995. ■