Boiler Construction, Types and Terminology
Core concepts
A standard boiler licence candidate must be able to read a boiler like a diagram. Learn the pressure parts: shell or drum, furnace, tubes (firetube and watertube), tubeplates, stays, crown sheet and combustion chamber. Understand the difference between firetube (hot gases inside tubes, water outside) and watertube (water inside tubes, hot gases outside), and the consequences for heat transfer, water volume and response speed.
Terminology questions test precision. Design pressure, maximum allowable working pressure (MAWP), operating pressure and test pressure are not interchangeable. Evaporation rate, rated capacity, equivalent evaporation (from and at 100 °C) and factor of evaporation appear regularly. Know heating surface, primary and secondary heating surface, steaming area, water holding capacity and steam space.
Traps
- Confusing MAWP with design pressure or with the relief valve setting.
- Assuming a larger steam drum means faster steam response; watertube boilers with small water volume respond faster and are less tolerant of level errors.
- Mixing up fusible plug location (crown sheet) with stay failure symptoms.
- Forgetting that access doors, manholes and handholes are pressure parts when calculating strength.
Worked rule of thumb
Equivalent evaporation = actual evaporation × factor of evaporation, where factor = (h_steam − h_feed)/2257 kJ/kg. If a boiler makes 5 000 kg/h with feedwater at 20 °C and steam enthalpy 2 770 kJ/kg, factor ≈ (2 770 − 84)/2 257 ≈ 1.19, so equivalent evaporation ≈ 5 950 kg/h. Use this to compare boilers fairly.