The S.S Exxon Benicia had a regenerative heater exchanger. It transferred heat from boiler exhaust gas to incoming boiler combustion air. It did this using a cylindrical rotor formed from hundred of plates arranged in a radial direction. The rotor hot boiler gasses heating the plates on one half of the rotor, and a motor rotated the rotor such that the heated plated moved from the passage containing the uptake and boiler gases to the intake passage through which the combustion air flowed through. The combustion air picked up the heat from the plates. We had to clean the soot off plates every few months when the boiler was shut down by entering the combustion air passage through a hatch and using a water hose to wash them off. The water was diverted to drains so it didn't enter the boiler airbox below. Clever economizer unit!
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On the C-2 model below, the exhaust flue protrudes a bit, but that also indicates the diameter of the flue on a C-2. The hole for the exhaust flue is not shown on this model. Mixing fresh water with exhaust gases which contain sulfur creates a weak surfuric acid would would eat the uptake (exhaust) tube in a short amount of time. So, expelling the gases without them contacting water and having that contact carbon steel is a must. When the boilers are not fired in port, a flue cap was placed over the exhaust flue outlet to prevent ingress of rain. Perhaps this cover is shown on the model below, closing off the flue.
