Classification of boilers - SlideShare. Externally fired boiler [ all water tube boilers] [Babcock and Wilcox Boilers] 4. According to the pressure developed a. Low pressure boiler [Pressure … Classification of boilers - SlideShare. Externally fired boiler [ all water tube boilers] [Babcock and Wilcox Boilers] 4. According to the pressure developed a. Low pressure boiler [Pressure less than 80 bar] b. High pressure boiler [Pressure greater than 80 bar] 5. According … Oct 07, 2018· 4. Types of Boiler according to the number of tubes: Single tube boiler: In Single tube boilers, there is only one water tube or fire tube. Cornish Boiler and simple vertical … 4. Types of Boiler according to the number of tubes: Single tube boiler: In Single tube boilers, there is only one water tube or fire tube. Cornish Boiler and simple vertical boilers are single tube boilers. Multitubular boiler: Boilers having two or more water tubes or fire tubes are called Mulitubuler boiler.