Cold head

Cold heading (radial) - Steel Cold heading (axial) - Steel... 

COLD HEADING PHOCE S CAPABILITY MAP FOH LOW TO MEOItJM CARBON STEELS... 

COLD HEADING PROCESS CAPABILITY MAP FOR LOW TO MEDIUM CARBON STEELS (AXIAL TOLERANCES ONLY)... 

Sometimes, the split-cycle cooler, partly shown in Fig. 5.17, is more suitable because of the separation of the cold head from the compressor. The compression is produced... 

The entire scope of a refrigerator cryopump is shown in Fig. 2.65 and consists of the compressor unit (1) which is linked via flexible pressure lines (2) - and thus vibration-free - to the cryopump (3). The cryopump itself consists of the pump casing and the cold head within. Helium is used as the refrigerant which circulates in a closed cycle with the aid of the compressor. 

Within the cold head, a cylinder is divided into two working spaces and Vj by a displacer. During operation the right space is warm and the left space Vj is cold. At a displacer frequency f the refrigerating power W of the refrigerator is ... 

The series-manufactured refrigerator cryopumps from LEYBOLD use a two-stage cold head operating according to the Gifford-McMahon principle (see Fig. 2.67). In two series connected stages the temperature of the helium is reduced to about 30 K in the first stage and further to about 10 K in the... 

Electric connections and cunent feedthrough for the motor in the cold head... 

Fig. 2.68 shows the design of a cryopump. It is cooled by a two-stage cold head. The thermal radiation shield (5) with the baffle (6) is closely linked thermally to the first stage (9) of the cold head. For pressures below 10 2 mbar the thermal load is caused mostly by thermal radiation. For this reason the second stage (7) with the condensation and cryosorption panels (8) is surrounded by the thermal radiation shield (5) which is black on the... 

Motor of the cold head with casing and electric connections... 

Cooldown time The cooldown time of cryopumps is the time span from start-up until the pumping etfect sets in. In the case of refrigerator cryopumps the cooldown time is stated as the time it takes for the second stage of the cold head to cool down from 293 K to 20 K. 

Q.2(20K) = Net refrigerafing capacity in Watts, available at the second stage of the cold head at 20 K. 

Single-stage cold heads are available with a refrigerating capacity of 50 W at 50 K. This application would require four such heads (and associated He compressors). From the point of view of both temperature and pumping speed, this is not an appropriate application for a liquid-cryogen-based cryopump. 

A possibility to reduce the number of cold heads/compressors would be to shield the 50 K panel with a LN2-cooled baffle. This, however, would reduce Xe owing to transmission restrictions, thus requiring an equivalent increase in Ac. Further, the cost of a LN2-cooled baffle and LN2 have to be considered. An unexpected problem is that, at 80K, ps Xe is 10 2 mbar. If the sticking coefficient of Xe is significant then this may limit the minimum attainable pressure in the system. 

Figure 4 Typical cryostat design for solution and powder samples. This design features an expanded berylhiun dome, which allows detection of the transmitted beam as well as the emitted X-rays that contribute to the NRVS signal. The Be dome and a thin polyethylene window on the front face of the sample cell minimize the absorption at 6.4 and 14.4 kev. A sapphire plate provides excellent thermal contact between the sample cell and the cryostat cold head, as well as allowing optical access for off-line Raman measurements to monitor sample integrity. Sample temperatures in the 20 - 30 K range are typically achieved for both solutions and powders...

Unfortunately, there were no solutions available on the market to characterise charcoal materials in the required temperature range. As a consequence, an own concept was developed, which evolved in the new experimental facility COOLSORP (see [2, 4] for technical details). COOLSORP combines a standard apparatus for sorbent characterisation at 77 K and a two-stage Gifford McMahon refrigerator (the cold head being in direct contact with the sample cell) so as to extend the operational range down to cryogenic temperatures. 

Figure 6. Schematics of the three common regenerative cryocoolers. The Stirling eryocooler (a) uses a valveless compressor or pressure oscillator and has a moving displacer operating synchronously with the piston. The pulse tube eryocooler (b) has no displacer in the cold head. The Gifford-McMahon eryocooler (c) uses a valved compressor with oil lubrication and oil removal equipment.

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