Refrigerator cryopump

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. 

Today refrigerator cryopumps are being used almost exclusively (cold upon demand). These pumps operate basically much in the same way as a common household refrigerator, whereby the following thermodynamic cycles using helium as the refrigerant may be employed ... 

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... 

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. 

Crossover value The crossover value is a characteristic quantity of an already cold refrigerator cryopump. It is of significance when the pump is connected to a vacuum chamber via an HV / UHV valve. The crossover value is that quantity of gas with respect to T =293 K which the vacuum chamber may maximally contain so that the temperature of the cryopanels does not increase above 20 K due to the gas burst when opening the valve. The crossover value is usually slated as a pV value in in mbar I. 

The maximum pV flow at which the cryopanels are warmed up to T = 20 K in the case of continuous operation, depends on the net refrigerating power of the pump at this temperature and the type of gas. For refrigerator cryopumps and condensable gases the following may be taken as a guide ... 

Crossover pressure. If a refrigerator cryopump is attached to a vacuum system via a valve and the pump is already cold, the crossover pressure... 

Pumping system of the BAK 1200 consisting of rotary vane pump. Roots blower and refrigerator cryopump. 

Referring to Fig. 5.6, the 3He refrigerator [25] contains a pump P and an evaporator E. They are connected by a stainless steel tube T internal to the copper support C. The latter is in good thermal contact with the working plane B of a pumped 4He cryostat (for example that of Fig. 5.3) not shown in figure. The tube is connected to a charcoal cryopump P linked to the 4lie bath by a thermal connection L. A thermometer Th monitors the temperature of the pump. A thermal shield (not shown), at the temperature of the 4He bath, surrounds the refrigerator. 

This effect has been utilized for a long time in condensers (see 2.1.5) mainly in connection with chemical processes previously the baffle on diffusion pumps used to be cooled with refrigerating machines. Also in a sealed space (vacuum chamber) the formation of condensate on a cold surface means that a large number of gas molecules are removed from the volume they remain located on the cold surface and do not take part any longer in the hectic gas atmosphere within the vacuum chamber. We then say that the particles have been pumped and talk of cryopumps when the pumping effect is attained by means of cold surfaces. 

CRYOPUMP. 11) An exposed surface refrigerated to cryogenic temperature for the purpose of pumping gases in a vacuum chamber by condensing the gas and maintaining the condensate at a temperature such that the equilibrium vapor pressure is equal to or less than the desired ultimate pressure in the chamber. 

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. 

The refrigeration capacity required to cryopump a specified rocket nozzle flow is the sum of the gas-phase heat content and the heat of sublimation for the particular exhaust gas ... 

Thus for a rocket impulse I of 250, approximately 10.5 kw of refrigeration capacity are required per pound of rocket thrust. However, this constant-flow approach may not be necessary in most applications since the rocket flow is usually of short duration and thus amenable to a heat sink type of cryopump, as described by Wallace and Rogers [2]. 

Warm-up tests of the cryopump cold plate demonstrated a low refrigerative requirement. As a result it becomes possible to supply the refrigeration with dense helium gas from sources other than closed-cycle refrigerators. A simple source of the gas is accelerated boil-off gas from a liquid-helium storage dewar. 

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