Steam Boilers, Steam Engines and Turbines
This section covers Steam Boilers, Steam Engines and Turbines. The exam tests boiler classification, mountings and accessories, steam properties, and the difference between reciprocating steam engines and turbines.
Core concepts
- A boiler generates steam at pressure. Fire-tube boilers pass hot gases through tubes surrounded by water; water-tube boilers pass water through tubes surrounded by hot gas. Water-tube designs suit higher pressure and larger capacity.
- Mountings are safety and control items: safety valve, water-level indicator, pressure gauge, fusible plug, blow-down cock and feed check valve. Accessories improve efficiency: economiser, air preheater, superheater and feed pump.
- Steam formation includes sensible heat, latent heat and, if heated further, superheat. Superheated steam has higher temperature than saturated steam at the same pressure.
- A steam engine is reciprocating; a turbine is rotary. Impulse turbines use pressure drop in nozzles; reaction turbines use pressure drop across moving blades.
Worked example
A boiler burns 2 kg of coal with calorific value 25,000 kJ/kg and produces 10 kg of steam with enthalpy rise 2,400 kJ/kg. Heat supplied = 2 × 25,000 = 50,000 kJ. Heat absorbed = 10 × 2,400 = 24,000 kJ. Boiler efficiency = 24,000 ÷ 50,000 × 100 = 48%. The reasoning compares useful heat in steam with chemical energy input.
Traps that cost marks
Do not list a superheater as a safety mounting; it is an accessory. Do not confuse fire-tube and water-tube arrangements. A safety valve is not for routine flow control. Impulse and reaction blading are not the same.