The manufactuer designed it that way, as a used equipment dealer we work with what is there or replace them for the test.

By putting pressure in the shell, the tubes are pressurized from the outside within the shell and any leaky tube would have had pressurized air screaming out of it on the open ends... the tubes are press fit and flared into the end plates which are welded to the shell, so there's no chance of leaks through the end plates... it's the most simple and easy way to do it

Sorry but I'm not sure

This unit was pressure rated only and not vacuum rated...hence we test only for pressure