Boilers
Boilers generate the steam that heats and powers process plants. Learn fire-tube vs water-tube boilers, steam properties and duty, efficiency (direct and indirect methods), fuel consumption, blowdown and water treatment, economisers, and boiler safety.
Steam is the universal heating medium of the process industry: it carries large amounts of energy (latent heat), condenses at a fixed temperature set by its pressure, and is safe and clean. Boilers turn fuel energy into steam.
Boiler efficiency and water treatment strongly affect plant energy costs and reliability.
An analogy
A boiler is a giant kettle with a very efficient burner — and strict rules so it never becomes a bomb.
Boiler types
Fire-tube (hot gas inside tubes surrounded by water) — compact packaged boilers up to ≈ 25 t/h, ≈ 20 bar. Water-tube (water inside tubes surrounded by flame) — large capacities, high pressures, power plants. Also waste-heat boilers (HRSGs), electric boilers and fluidised-bed boilers for solid fuels.
Efficiency
Direct method: steam energy / fuel energy. Indirect (loss) method: subtract stack loss (largest), radiation, blowdown and unburnt losses. Typical gas-fired efficiency 85–92 %; economisers and air preheaters recover flue-gas heat; excess-air control (O₂ trim) avoids waste.
Water treatment and blowdown
Dissolved solids concentrate as water evaporates; blowdown removes some boiler water to limit TDS. Softening/demineralisation, deaeration (oxygen removal) and chemical dosing prevent scale and corrosion. Condensate return saves water, chemicals and energy.
Safety and codes
Pressure parts designed to ASME Section I / EN 12952/12953; safety valves, low-water cut-outs, flame safeguard (burner management systems with purge, ignition and flame-failure trips). Boiler explosions from low water or furnace gas accumulation are prevented by interlocks and inspection regimes.
Formulas
- Boiler duty:
Q = á¹_s (h_g − h_fw) - Fuel input:
Q_fuel = Q / η_b, á¹_fuel = Q_fuel / LHV - Blowdown:
B = TDS_feed / (TDS_boiler,max − TDS_feed) - Evaporation ratio:
kg steam / kg fuel
Worked example: Boiler duty and fuel
Given: 10 t/h saturated steam at 10 bar (h_g = 2778 kJ/kg); Feedwater 105 °C (h = 440 kJ/kg), η = 88 %; Natural gas LHV 50 MJ/kg
Find: Duty, fuel input, gas consumption.
- Q = 2.778 kg/s × 2338 = 6494 kW
- Fuel input = 6494/0.88 = 7380 kW
- Gas = 7380/50 000 × 3600 = 531 kg/h
Answer: 6.5 MW duty, ≈ 531 kg/h gas
Worked example: Blowdown rate
Given: Feedwater TDS 200 ppm; Max boiler TDS 3000 ppm
Find: Blowdown.
- B = 200/(3000 − 200) = 7.1 % of steam
Answer: ≈ 7 % — better feedwater treatment reduces it
Common mistakes
- Wrong: Running with high excess air 'to be safe'. — Right: Use Oâ‚‚ trim — excess air carries heat up the stack.
- Wrong: Neglecting condensate return. — Right: Returning hot condensate saves 10–20 % fuel and water treatment.
Applications
- Process heating: Steam for reboilers, heaters, tracing and stripping.
- Cogeneration: High-pressure steam through turbines before process use (CHP).
- Waste-heat recovery: HRSGs behind gas turbines and furnaces.
Key takeaways
- Q = á¹(h_g − h_fw); fuel = Q/(η LHV).
- Fire-tube for small, water-tube for large/high pressure.
- Blowdown and water treatment protect the boiler.
- Economisers, Oâ‚‚ trim and condensate return save fuel.
Going further
- Burner management systems: SIL-rated purge, ignition and flame-failure logic (NFPA 85).
- Steam system optimisation: Trap surveys, insulation and pressure levels save energy.
- Electrification: Electrode and heat-pump steam generators for decarbonisation.