Spending time in the Penthouse sounds a lot more appealing than it is, although as boiler spaces go, it is fairly comfortable.
Boilers are built so that water and steam circulate from the hottest spots, to where the work needs to be done. Forced circulation boilers have big circulating pumps that move the boiler water around. These are common in supercritical plants. Supercritical plants are not plants that are just really grouch, and always complaining about something. They are plants that operate at such high pressures that steam never forms - the water turns to something that isn't water and isn't steam. Then at a pressure drop over a valve, this fluid will flash to steam and drive a turbine.
We have a natural circulation boiler. There is a big fireball area in the cavity of the boiler. It is surrounded by water filled tubes- the water walls. Which is different than the walls of water that you put around your tomato plants. The water boils in the tubes, and the steam that is formed rises, carrying a lot of water upwards with it. The water and steam flows to the steam drum and is separated there. The water falls back down to the cool bottom of the boiler in large pipes called 'downcomers', and the steam starts a path through a series of tubing loops where it becomes progressively hotter and dryer. At each stage there is a really big pipe or 'header' that has all the looping tubes welded to it. The pressure and temperature of each small tube sort of equalizes out in the header, and then the steam is redirected to another set of tubes where it gets hotter yet, until finally it comes out of the secondary superheat header, into the main steam line and goes down from the 19th floor to the 2nd floor where the turbine is.
The place above the boiler roof where all these headers are, and where all these connections are made is called the penthouse. Why I don't know. But there is a lot of stuff going on there. Lots of tubes, lots of pipes. It is really surrealistic to look at, and it takes some time to figure out where everything goes. I think that is enough of a lesson tonight. Here are a few pictures of some of the headers. I end up crawling around in the insulation dust quite a bit, so it is a bit itchy. Still, interesting. I didn't think I would be doing this when I was a little boy.