Dilution refrigeration cooling power

Figure 6.3 shows the comparison between the cooling powers supplied by a 3He evaporation refrigerator and a DR. The cooling power produced by the evaporation of 3He from the concentrated to the diluted phase is ... 

Fig. 7.3. Comparison of the relative cooling powers Qlh of a Pomeranchuck refrigerator and of a dilution refrigerator. In the former case, die liquid is converted into solid at a rate of 10 fxmol/s in die latter case, 3He is removed from the mixing chamber at the same rate. 

From Fig. 10.13, we see the latter condition is fulfilled in the first three cases, but not in the fourth case. The most stable situation is obtained with Rx. The choice R = RcosL is however usually adopted when the power supplied to the resistor must be measured. The control of temperature in the real (dynamic) case is much more complex. The problem is similar to that encountered in electronic or mechanical systems. The advantage in the cryogenic case is the absence of thermal inductors . Nevertheless, the heat capacities and heat resistances often show a steep dependence on temperature (i.e. 1 /T3 of Kapitza resistance) which makes the temperature control quite difficult. Moreover, some parameters vary from run to run for example, the cooling power of a dilution refrigerator depends on the residual pressure in the vacuum enclosure, on the quantity and ratio of 3He/4He mixture, etc. 

The delivery of SCUBA-2 instrument to the JCMT is due for the end 2006. SCUBA-2 cryogenics is based on a dry dilution refrigerator (DR) (see ref. [7]) the main cryostat cooled by pulse tube coolers with 60 and 4 K stages, and the 1K box and detector arrays with DR for 1 K and millikelvin stages. The PT system has a cooling power of 50W at 45K on the first stage and 1.0W at 4.2 K on the second stage. The coolers were chosen for their inherently low vibration levels. 

This system has two gas lines for mixture He-" e gas line (line 1) and " He gas line (line 2). The " He is supplied from liquid He tank by the vacuum power of the Pump2. This mixture He-" He gas is cycled in the dilution line by the Pumpl. The " He gas is used to cool the IK pot in the dilution refrigerator below 2K because the " He gas is cooled by the reduction in pressure in the line2. The mixture gas is cooled 1 K pot, still, and the mixing in that order. 

For areas where cooling water is either scarce or not available, direct liquid injection may be a possibility. The liquid coolant should be injected near the discharge end of the compressor to minimize lubricant dilution. Alternatively, the liquid can be flashed in a separate exchanger and used to cool the lubricant. While the cooling may appear to decrease the power to the compressor, the net effect is an increase in the power due to the additional weight flow of the extra refrigerant needed to perform the cooling. 

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