City of Chicago Boiler Practice Exam

In a watertube boiler, water runs through the tubes while steam drums sit at the top and a bottom drum, called the mud drum, collects any sludge or sediment that comes out of the feedwater. This bottom drum is part of the circulating water path: solids settle there and are can be removed through blowdown, helping keep the heated water clean and the tubes from fouling.

That bottom-mud-drum arrangement is a hallmark of many watertube designs, where the flow and circulation bring heavier impurities down to the mud drum for collection. In contrast, firetube or Scotch marine boilers have their own arrangement around the shell and tubes, and electric boilers don’t use a mud drum at all since there’s no combustion-derived sediment in the same way.