Background and scope
Cryogenic coolers operating on regenerative cycles have been under modern technological development for at least three decades.
Many early devices operated on the Stirling cycle, with mechanical actuation by conventional (rotary) electric motor via a kinematic linkage. Noise and vibration levels were high, and service life was limited. Lifting 0.2 W from 77 K called for electrical input in excess of 100 W. The figure suggests a coefficient of performance (COP) of the order of 1 per cent of that of the Carnot cycle between comparable temperature limits.
The rhombic drive mechanism promised to mitigate vibration and wear, and on that basis the author contributed the thermodynamic and flow analysis, and the outline mechanical design, to the NAX-106 cooler shown in Fig. 1.1. The Hymatic Engineering Co. completed the detail design and manufactured two units to a high standard of machining for evaluation by the Royal Signals and Radar Establishment (RSRE – subsequently DERA and now dstl) at Malvern. Problems that persisted were brush wear of the direct current (d.c.) electric drive motors in the atmosphere of dry helium, together with friction, wear, and leakage at the ptfe-composite seals and bearer rings. Power consumption remained high.
Since that time, linear electro-magnetic actuation has become the norm for operating the piston, with the displacer driven in the same way, or spring-coupled to give appropriate phase-shift. Figure 1.2 ...