#55661, "RE: Air shafts for Engine Room and D deck galley." In response to In response to 0
It should be understood that the general arrangement plans are extremely simplified when it comes to ventilation.
The 'Engineering' profile is a bit more detailed (showing the galley flue and steam exhaust pipes rising up the funnel) but there would have been a number of smaller ventilating ducts running along the light and air shafts, that don't appear on the plans.
The 20-inch fan you mention was, on the Olympic, used to exhaust the nearby staterooms, and I expect it was the same on the Titanic.