I know this is probably a really stupid question and I've asked it casually to some other people before... but tonight I was watching an old tape of the Suwanee Steam Special, and it sounds like the dynamo is spinning up and down? Is that it?

This is the cold water turbine spooling up and down by demand of a float valve on the hotwater (heater) tank. This then would be a Worthington SA feedwater system.

To go into a bit more detail for ya, the worthington feedwater system consists of a cold water pump with a connection from the tender that provides water to the heater unit mounted into the top front of the smokebox. Water is mixed directly with exhaust steam and a set level is maintained via the float valve. The water is then drawn from the heater by the hot water pump (piston style pump) and delivered into the boiler.
As a side note, the jet of steam seen coming out from under the left side cylinder is a vent to atmosphere from the heater unit itself.

With an SA type heater, does the water get added to the boiler automatically via the float valve? Or does the fireman continually have to add water like an unheated injection system?

What about with the Elesco system, how did it differ from the Worthington as far as how the fireman operated it?

The fireman manually controls the feed rate by varying the speed of the hot water pump with a valve in the cab. While running over the road the pump can be set to deliver water to the boiler at the same rate it is being used.

There is also a drifting control valve in the steam supply line to the system that restricts the speed of the pumps when there is less than about 90 psi. of steam chest pressure. This is to prevent cold water being pumped into the boiler when the engine is drifting or standing and there is not enough exhaust steam to properly heat the water being supplied to the heater from the cold water pump.

The Elesco is controlled in the exact same way as the Worthington SA, by a valve in the cab that controls steam flow to the pump. Depending on the heater style and the railroad's preference, there would also be a guage in the cab to give a visual display of the pump's operation. Elesco and Worthington S would typically have a guage on the delivery line before the boiler check. The Worthington SA used a pressure guage coming off the cold water pump (I want to say steam supply line to the pump, but it could have been the delivery line to the heater. Its been a while since I've been around it.) Indirectly, this proved your hot water pump was working. However, a guage is not really necessary, as watching the water glass and the boiler pressure gauge will tell you the same thing. Eventually.

As far as which is "better," that's a great topic for debate. Both had benefits and drawbacks. The closed system, such as the Elesco and the Coffin, allowed a higher feedwater temperature to be achieved. Since the feedwater was pressurized going into the heater, there was no concern about the feedwater flashing to steam if the exhaust pressure was decreased. Also, when equipped with an oil skimmer on the condensate return line, a closed system prohibits cylinder oil from entering the boiler. Whether or not this is a bad thing is a sub-debate topic. Before the advent of chemical antifoams, and in lieu of natural ones like tannins, valve oil was used to considerable success as an antifoaming agent.

In an open system, the feedwater temperature can never be affected by scaling, since the feedwater and exhaust steam are directly mixed together. The SP, with its hideously bad water, prefered Worthington equipment for this reason. It will also deaerate the feedwater, since the soluability of oxygen decreases rapidly with increasing temperature. That's the purpose of the little 3/8" or 1/2" line that was customarily plumbed in directly ahead of the stack on Worthington S and SA equipped engines. This allows the offgassed oxygen to escape the heater. A series of chokes prohibits an excessive amount of steam from escaping with it. This decreases both the amount of oxygen scavenger needed in the feedwater, and the TDS level in the boiler (if you're using a sulfite scavenger). For my money, I think the open heater has a slight advantage over the closed type, but they made plenty of both types.
-Chris Zahrt