Cryosurfaces/-pumps

Fig. 6a. Cryosurface pumping speed as a function of time 70°F nominal CO2 total temperature.

For approximately one-half of each run, observed cryosurface pumping speeds remained near the value determined by other experimenters in the free-molecular flow regime even though pressures were in the continuum flow regime. 

A steady decrease in cryosurface pumping speed during the latter part of each run was attributed to an increase in the cryosurface temperature due to cryodeposit build-up. Data were correlated by a steady-state conduction heat transfer analysis which also gave good correlation between the hot and cold CO2 data. 

The pumping speed of a cryosurface under free-molecular conditions can be predicted from kinetic theory if the proper capture coefficient is known. The capture coefficient, as defined by the ratio of the actual pumping speed to the theoretical maximum pumping speed [ ], is a measure of the fraction of molecules that stick or condense on the first collision with the cryosurface. The capture coefficient is related to the energy of the gas molecules being pumped as well as to the cryosurface temperature. If this relationship were known for a particular gas, the pumping speed at other temperatures could be readily predicted. 

The apparatus used in this study consisted of a vacuum chamber containing a spherical cryosurface and an independent gas addition system. The gas addition system, shown in Fig, 1, comprised a surge tank, a barometer for measuring the upstream gas pressure, and a series of five calibrated standard leaks, with individual valves, arranged in parallel, permitting individual use of each leak or any combination of leaks. The entire gas addition system has a separate pumping station. 

The vacuum chamber (Fig. 2) is a 20 x 39-in. stainless steel cylinder of all heliarc-welded construction. The chamber is suspended in an insulated aluminum tank. The chamber volume after correction for the pump lines and cryosphere was calculated to be 202 liters. This chamber size makes possible the use of commercially available standard leaks, without introducing significant errors in the time measurement when calibrating the gas analyzer by the rate of pressure rise method. The 7-in.-diameter cryosphere has a calculated surface area of 995 cm . The coolant transfer lines to the cryosurface are vacuum-jacketed to prevent possible condensation. 

The capture coefficient at each cryosurface temperature was calculated from the pumping speed data. The capture coefficient is defined as... 

Answer by Author Using the maximum black-body radiant heat load on the cryosurface, the temperature increase across the in. thickness of stainless steel was not found to be significant. It has also been noted experimentally that there is no effect of cryodeposit thickness on the pumping speed until the thickness approaches 1 cm. Therefore, the temperature increase across the 10ft maximum thickness of the cryodeposit in these experiments must also be very small. 

Cryopanels (Meissner traps) are cryocondensation surfaces in the deposition chamber that use large areas of cooled surfaces to freeze out vapors, particularly water vapor and solvent vapors. They are cooled by LN2 at — 196°C or refrigerants to about — 150°C from a closed-cycle refrigerator/compressor system. The vapor pressure of water at these temperatures is very low, as shown in Table 3.4. It takes about 780 watts to freeze one kilogram of water per hour and 11 kilograms of LN2 to freeze one kilogram of water. The ideal cryosurface should pump about 10 liters per second per square centimeter. 

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