Steam Boilers: how boilers function

A boiler is a closed vessel in which water is heated under pressure to produce steam for power generation, heating or industrial processes. The following describes the working of a modern water-tube boiler.

Main parts

• Furnace: the fuel (coal, oil, gas or biomass) is burned here.
• Water tubes and drums: tubes filled with water run through the hot gases; a steam drum collects the steam.
• Superheater: heats the steam further so that it is dry and at higher temperature.
• Economiser and air pre-heater: use waste heat of flue gases to warm feed water and air.
• Feed pump, safety valve, pressure gauge, water-level gauge and chimney.

Working

 Fuel + air --> FURNACE --> hot gases
                              |
   feed water --> water tubes (heated)
                              |
                       STEAM DRUM
                              |
                        SUPERHEATER
                              |
                    steam to turbine/engine

1. Feed water is pumped into the boiler through the economiser.
2. Fuel burns in the furnace; hot flue gases flow over the water tubes.
3. Water in the tubes becomes steam and rises, while cooler water descends, giving natural circulation.
4. Steam collects in the drum and passes to the superheater, where its temperature rises well above boiling point without a rise in pressure.
5. The dry, high-pressure steam goes to the turbine or engine. Exhaust steam is condensed and returned as feed water.
6. Flue gases leave through the chimney after giving up heat.

Safety and control

Safety valves release excess pressure; the water-level gauge and automatic feed control avoid overheating of tubes; pressure controllers adjust the fuel supply.

Modern boilers are efficient because they use a large heating surface, high pressure, superheating and waste-heat recovery.