replaced by single drum boilers as the bi drum boilers are non-reheat units. So, due to the high heat rate of the plant a single drum boiler or a once through boiler is more feasible. In process steam generation the bi drum boilers are used commonly as they can adapt to the high load fluctuation and respond to load changes. Single drum boiler- used mainly for the power plants for power generation. The pressure limit for single drum boilers is higher than that of the bi drum boilers as the stress concentration is reduced to a greater extent. There exists only one drum and the downcomers are welded to it. Single drum boilers are suitable and can adapt to both reheat and non-reheat type of boilers. They can be designed as Corner tube boiler where the frame is not required as the downcomers itself serves the purpose of it and also they are designed as top supported where the whole boiler assembly needs an external frame and supported by top drum. See also Once-through steam generator, does not have a steam drum References Steam boiler components Boilers Steam generators