Boiler and Steam Systems
This domain tests combustion efficiency, boiler efficiency, steam-system losses, and cost calculations. Questions often require converting boiler horsepower to fuel input or estimating savings from reduced excess air or improved condensate return.
Combustion and efficiency
- Stoichiometric air is the theoretical air needed for complete combustion. Excess air ensures complete combustion but carries heat up the stack.
- Combustion analyzers measure O2, CO, CO2, and stack temperature; high O2 indicates excess air, while high CO indicates incomplete combustion.
- Combustion efficiency is based on stack losses; boiler efficiency also includes radiation, convection, and blowdown losses.
- Gross vs net efficiency differs in how latent heat of water vapor is treated. Use the basis stated in the question.
- One boiler horsepower = 33,475 Btu/h of output, roughly 34.5 lb/h of steam at atmospheric conditions.
- Steam-system measures include blowdown control, condensate return, deaerator operation, steam-trap testing, insulation, and flash-steam recovery.
Worked example
A 500 hp boiler operates at 80% efficiency for 4,000 hours per year with natural gas at $8/MMBtu. Output = 500 × 33,475 = 16,737,500 Btu/h. Input = 16,737,500 ÷ 0.80 = 20,921,875 Btu/h = 20.92 MMBtu/h. Annual fuel = 20.92 × 4,000 = 83,688 MMBtu. Annual cost = 83,688 × $8 = $669,504. At 83% efficiency, input = 16,737,500 ÷ 0.83 = 20.17 MMBtu/h and annual cost is about $645,400, saving roughly $24,100. The reasoning: efficiency improvements reduce input for the same steam output, and savings scale with fuel cost and operating hours.
Traps that cost marks
- Confusing boiler horsepower with electric horsepower.
- Using gross efficiency when the question specifies net efficiency, or vice versa.
- Assuming excess air always improves efficiency; too much excess air raises stack loss.
- Ignoring blowdown, radiation, and condensate losses.
- Missing failed steam traps and leaks that waste steam without appearing in boiler efficiency.
- Using input fuel energy as output steam energy.