nasa - james webb telescope - bei Linde Kryotechnik AG, Schweiz

K). Upon reaching the set-point, the MI and MO adjust the discharge pressure to maintainthe desired shield return temperature so that the refrigeration capacity matches the actualheat load.Liquid nitrogen (LN2) is used for pre-cooling the high pressure helium streamdown to 80 K. The LN2 pre-cooler implements nitrogen phase-separation using a thermosiphonand directs the total high pressure helium flow through the latent nitrogen heatexchanger section. The LN2 supply maintains a pre-determined liquid level set-point inthe phase-separator vessel (i.e., the ‘LN2 pot’). The usage of the LN2 is regulated by thehigh-pressure helium bypass between the large 300-80 K helium-helium heat exchangerand the smaller 300-80 K helium-nitrogen vapor heat exchanger. This valve uses thewarm-end helium-helium stream temperature difference as the primary process variable toaffect minimum LN2 usage. Cascade control is implemented on this valve to prevent thewarm-end helium-nitrogen vapor stream temperature difference from becoming tooextreme (say, a 240 K nitrogen vent temperature).FIGURE 5: Floating pressure control schematicSUMMARYThe system is designed to meet the needs of flexible load (both in meeting the actualload and the required operational temperature for the experiment) with good efficiencies.However, it is important for the supplier of cryogenic plants to receive as much