No need to wonder -- The exciter provides DC power to the field coils on the rotor of the generator to create the magnetic field. When the rotor is turned by the turbine the magnetic field spins and passes through the coils (armature) of the generator to generate electricity.
Those tubes are too small to be those of a boiler. They are condenser tubes. What you are seeing here is the waterbox where the cooling water enters or leaves. The steam to be cooled would be on the other side of those tubes and tube sheets.
PCB's are Poly Chlorinated Biphneyls which are a class of chemicals that were added to electrical insulating fluids (and semi solids) to render them less flammable. They are toxic and when they burn (note I said less flammable) they would turn into dioxin which is even more toxic than PCB. They went by several trade names such as Askarel, Arochlor and Chlorhextol.
It appears that this is the same equipment as in the picture 'Rusted Ventricles' (Motts?). If it is thenthese are the motor drives for a set of draft fans (fans that move air through the boiler). There are a few types of fans FD which force air into the boiler, ID that remove combustin gasses from the boiler and Gas Recirc that move the air around in the boiler.
These are the phase markings for a three phase AC system. Most U.S. utilities label the phases A, B, C. This is how they keep track of what wiring to connect together. If the phasing is not followed through, a dangerous short circuit can be created. These picures depict what is known as cell type construction where the compartments containing the electrical conductors are part of the building.
With the dial marked as Motts describes it is a frequency meter. The US and Canadian power systems are operated at 60 Hz or cycles per second. A synchroscope is marked 'slow' and 'fast' and sometimes with the number of degrees out of phase. 0 degrees (pointer straight up) Is in sycnc and 180 (pointer straight down) is as far out of synch as you can get. Vinton is correct -- I have seen AC generating units where there has been a synchronizing accident. The damage can be anything from mechaincal damage to the turbine shaft & blades, to the generator, its circuit brekaer and transformer. Most modern plants have controls to do the synch and help prevent a synchroizing accident from occuring. I once added these controls to a generator about the same vintage as the one in this gallery.
Fire tube boilers each with two burners. The water to be heated is insde the tank and the fire / hot combustion gasses run through the tubes to the stack.
These are a motor generator sets. An AC motor spins a generator. The output of the generator could be AC or DC. If it is DC (which it probably is based on the appearacne of the brush riggings and commuator) the output could have been used to charge the control batteries or provide field current to the main generators. This was very common in power plants from this time period. There is also a possibility that this could be an ampllidyne (also part of the main generaator field circuit) althouhg this plant may be a bit too old for that.