Boiler Fundamentals, Terminology, and Classification
Core concepts
A boiler is a closed pressure vessel in which water is heated to produce steam or hot water. Candidates must be fluent in the vocabulary that questions use interchangeably: working pressure (the pressure at which the boiler normally operates), design pressure (the maximum pressure the vessel is built to withstand), MAWP (maximum allowable working pressure, set by the weakest pressure part and stamped on the vessel), hydrostatic test pressure (typically 1.5 times MAWP for a new or repaired vessel), and accumulation of pressure (the rise allowed above the set point when the safety valve relieves, usually 10 percent).
Know the classification axes. By pressure: low (15 psig steam or 160 psig/250 °F hot water) versus high pressure. By firing position: internally fired (firebox or furnace inside the shell, typical of fire-tube) versus externally fired (furnace outside the pressure vessel, typical of water-tube). By circulation: natural circulation driven by density difference, forced circulation using a pump, and once-through designs with no drum. By tube arrangement: fire-tube, water-tube, and firebox. Also know heating surface, grate area, furnace volume, evaporation rate (lb steam/hr), equivalent evaporation (from and at 212 °F), and boiler horsepower (34.5 lb of steam per hour from and at 212 °F).
Traps
Questions love to mix "design" with "operating" pressure, to ask which boiler type suits high pressure and capacity (water-tube), and to test the boiler horsepower conversion. Another common trap is confusing equivalent evaporation with actual evaporation.
Worked example
A boiler evaporates 6,900 lb of steam per hour. Boiler horsepower = 6,900 ÷ 34.5 = 200 BHP.