May I ask a question about the aircraft crew?
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What determines the model and size of the pre-evacuation pump in a vacuum system? Time? Air extraction volume? Why?
It should be time. The volume that needs to be evacuated in advance is fixed; once the time is known, the pumping capacity of the vacuum pump can be calculated, thereby allowing the selection of the appropriate model for the vacuum pump.
Does the pre-pump referred to here mean the preceding pump or something else? And what kind of structure does a vacuum unit refer to? All of these need to be clarified before an answer can be given