does steam flow from top to bottom or bottom to top in a heat exchanger
That would depend upon where it's injected.
Which depends on the type of heat exchanger and steam service. If there is a separate condensate return pipe, you probably want the steam injected high and the condensate returning low so the condensate doesn't pool-up anywhere. If it is a one-pipe system, the steam is injected low and the condensate just runs back down the same pipe, past the up-flowing steam.
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