Part of the main boiler of unit EC 20 in HDR

This is the bottom of the main boiler (with some processing in HDR).

It is hanging free in a steelconstruction, so it can expand freely when it is filled with water and being under pressure and at temperature. (between 120 and 180 bar and a steamtemperature of 540°C. (1004F).
At full load it expanded about 30 centimeter. (now it will never be filled with water and steam anymore....)
The part you're looking at were the former recirculation ducts; these boxes are connected to the furnace and two recirculating fans (huge) were pulling "hot" fluegasses from the furnace back to the inlet of the main boiler fans. This operation was done at low load.
This was all in use before the unit was transferred in 1987 to a so-called combi-unit with upstream gasturbine. At that moment the rec. fans were not necessary anymore and removed.
To give an idea of the dimensions: A man of two meters tall can just stand under the duct.

Better in LARGE....

Quiet shift; one unit running.