High pressure boilers can be classified as single drum and bi-drum boilers. In early days, the Sterling boilers had four drums, one at the bottom and three at the top interconnected by a bank of tubes. Slowly the three drum and two drum designs evolved. The present day bi-drum boilers are designed for high pressure, flow, and temperature.
Modern day bi-drum boilers are used both for power generation and process steam generators. For power generation this type of boiler is used up to, say, 100 MW. These are essentially non-reheat units which are getting wiped out due to high heat rate of the plant. In process steam generating plants, this type of bi-drum boiler is preferred mainly because it can accommodate high load fluctuation and respond to load changes much faster than the single drum boiler.
The two drums in these types of boiler are connected by a large number of convective heat transfer tubes called the bank tubes or boiler bank tubes. These bank tubes in any bi-drum boiler will be around one thousand in number. These bank tubes and the bottom drum contain water, the level of which will be up to the normal working level of water in the upper drum. The large volume of water availability in the bi-drum boiler makes it adoptable for large load changes. The bank tubes toward the furnace side where flue gases enter the bank tubes act as raisers and contain steam water mixture. The number of rows of bank tubes along the flue gas path acting as raiser tubes will vary depending upon the load. The rest of the rows of bank tubes will act as down comer tubes between upper and lower drum. Another important feature of the boiler bank is that the flue gas temperature leaving the bank tubes remains almost constant at all boiler operating loads. During fast or slow load changes, the number of raiser tube rows increases and the down comer tube rows reduce thus ensuring more steam almost instantaneously in the steam drum as load surges upward.
The bank tubes are inserted in to the upper and lower drum holes, expanded to withstand the operating pressure. Some designers prefer to seal weld these with both upper and lower drum.
Bi-drum boiler can be of both bottom supported and top supported design.
Single drum boiler
Single drum boilers are used mainly for power generation. With supercritical pressure boilers coming into use, even the single drum gets eliminated. This type of boiler has the advantage that they can be designed for very high pressure, flow, and temperature. The down comers are welded to the drum and are pipes which are so selected in size and numbers two ensure the circulation in the furnace wall tubes. The single drum boilers are adopted for both non-reheat and reheat boilers. Only single drum boiler design can be adopted for high pressure natural circulation and forced circulation boilers. The single drum boilers are normally designed as top supported only.