Synopsis The scenario of transformation of the stored energy of fossil fuels into mechanical energy through the steam route has changed beyond recognition since the age of its first engineered application culminating in the stream engine, functionally though the boilers continue to be heat transfer units. Modern boilers are capable of producing >1000 t/hour of high pressure high temperature steam of high purity to drive a single turbo-generator of appropriate capacity. In comparison, modern boilers are of gigantic dimensions suitably engineered through the use of appropriate grades of steels to ensure continuous operation involving its first conversion into saturated steam at elevated temperatures and its uperheating to sufficiently high temperatures pre-established at the design stage with an eye to achieve both high thermal efficiency and high plant availability at the same time.