Boiler designers used four drums and three drums for designing water tube boilers during the initial days. Later as the boiler technology advanced in capacity and pressure, they switched over to bi-drum and single drum design.
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.