Cryocooler heat exchanger

Mechanical cryocoolers are used either to liquefy a gas for use away from the machine or to provide a cold platform for a refrigerator. A cryocooler must be as efficient as possible, whilst taking account of any constraints there may be for particular applications. For this to occur, the maximum possible use must be made of any cold substance that is produced. It is important in a helium liquefier that the fraction of gas which was cooled but did not liquefy is used to precool further incoming gas. This leads us directly to consider the heat exchangers. The combination of a cyclical process with the need for efficient heat exchange led to the idea of a regenerator in which heat may be stored for a short time, so that heat output from one phase of the cooling cycle may be reinserted at some phase. 

Fig. 5.13. Schematic diagram of a Klimenko cycle cryocooler. E123 denote heat exchangers V12 3 n are capillary expansion valves S12 3in are liquid-vapour separators [53].

This cycle also uses continuous counterflow heat exchanger and is closely related to the Joule-Thomson and Claude cycles as shown in Fig. 5.15(a) [60], The cryocooling or reverse Brayton cycle derives from a reciprocating gas engine patented by G. B. Brayton in... 

Figure 5. (a) Schematic of the Joule-Thomson cryocooler showing the use of an oil-free compressor and a high-effectiveness heat exchanger, (b) The Joule-Thomson cycle shown on a temperature-entropy diagram. Dashed lines indicate the heat exchange process in the heat exchanger. 

It is interesting to note that Dh is independent of the thermal loss in the heat exehanger Qf . It is also independent of the size of the heat exchanger. For a minimum volume the hydraulic diameter is independent of the flow rate, volume, or refrigeration power of the cryocooler. According to equation (21) only the cross-sectional area varies proportional to the flow rate. For some common geometries the relation between the hydraulic diameter, defined by equation (8), and the characteristic dimension is given by... 

Equation (40) shows that the optimum hydraulic diameter decreases at lower temperatures. Thus, microscale effects may be more important in low temperature applications, such as in heat exchangers of cryocoolers. However, temperature has little effect on the relative importance of slip flow. The mean free path of gas molecules is given by... 

A few other examples are examined here to illustrate the range of geometries that may exist in optimized heat exchangers for cryocoolers. These examples are for Joule-Thomson cryocoolers, which are easily miniaturized. Table 1 lists the operating conditions and important input parameters for all of... 

The need for high effectiveness heat exchangers in cryocoolers is indicated by the low values of q, which gives the maximum ineffectiveness allowed for the heat exchanger, at which point net refrigeration is eliminated. With pure gases in a Joule-Thomson (JT) cryocooler starting from... 

Figure 18. Photographs of a miniature heat exchanger developed for the precooling stage of a helium Joule-Thomson cryocooler. The construction used photoetched stainless steel foil diffusion bonded together [9],...

Marquardt, E. D., and Radebaugh, R., (2003) Compact HighEffectiveness Parallel Plate Heat Exchangers, Cryocoolers 12, R. G. Ross (ed.), Kluwer Academie/Plenum Publishers, New York, pp. 507-516. 

Longsworth, R.C. (1993). Heat exchangers for Joule Thomson cryocoolers. In Aerospace Heat Exchanger Technology. Eds R.K. Shah and A. Hashemi. Elsevier, The Netherlands, 1993. 

---