Can anyone give me a little insight into a steam boiler system with reference to whether or not an air separator and or expansion tank is always required.
With my experience as a system engineer at a nuclear power plant I can tell you that condenser air hoggers and good secondary chemistry is a very good substitute for maintenance-prone air-seperators.
There's confusion regarding air separators on hot water heating systems and air ejectors & hoggers used on steam turbine condensers.
Air ejectors and hoggers are used to remove non-condensibles from the steam/condensate side of turbine condensers.
In a hot water heating system, air separators function as deaerators to remove dissolved air from the loop water. Dissolved air that is not removed pops out of solution under system conditions and collects in high points preventing heat emitters from functioning properly. This problem can be dealt with by "bleeding" the air from rads, but a properly selected and located air separator will do the job much more effectively. This air can also result in objectionable system noise - the distribution piping sounds like it's full of BBs.
All of the steam cycle power plants that I have worked on have contained deaerators or air separators as you call them. I would imagine that there are other ways to do the same thing, but I think that a deaerator is usually the cheapest. Most of the time the deaerator is in the middle of the feedwater heater train. As TBP described above, the deaerator is used to get rid of all the air (oxygen more importantly) in the feedwater prior to sending it on to the boiler.
I could be mistaken, but I assumed the initial question was regarding a hot water heating system in a building, as opposed to a steam heating system in a building. I view air separators and deaerators as similar devices installed on completely different systems. Air separators in hot water heating systems are small, in-line devices. DAs in large steam plants are big tanks. Hot water systems have expansion tanks, steam systems do not.
My background is pretty much all utility and industrial powerhouses, but over the last few years, I've been reading up on residential/commercial steam & hot water heating systems. Some of the newer systems have very impressive technology. So did (do) some of the very old steam systems. A lot of the control devices that were invented prior to small electric motors being commercially available are nothing short of amazing.
Anyone who is interested should check out "The Lost Art of Steam Heating" by Dan Holohan. It covers steam heating technology from the 1850's through the decades that follow. He has also written several books on hot water heating. He has a very laid-back, informal style of writing - even funny. For a more formal view of hot water heating, check out John Siegenthaler's "Modern Hydronic Heating - For Residential & Light Commercial Buildings". Holohan writes for service techs, Siegenthaler from and engineering/design perspective. Both are worth reading.
