Fire-tube boiler efficiency and whether to buy an economiser
A textile processing unit runs a two-pass horizontal fire-tube boiler rated at 1.5 tonnes of steam per hour at 8 bar. During a six-hour trial the boiler evaporated 8,100 kg of dry saturated steam at 8 bar from feedwater at 32 °C, using 1,250 kg of washed coal of calorific value 24.5 MJ/kg. Flue gas leaving the boiler measured 330 °C, the CO₂ in the flue gas was 9 per cent, and a light haze was visible at the chimney top. There is no economiser or air preheater, the feed pump takes water directly from an overhead tank, and there is room at the chimney base for an economiser. The owner wants to know whether the boiler is performing as it should, and whether an economiser is worth buying at a coal price of ₹9,000 per tonne with the unit running 6 hours a day, 300 days a year.
1. Calculate the evaporation ratio and the boiler efficiency on the trial data, and compare with what this type of boiler should achieve.
Read the worked answer
Steam rate = 8,100/6 = 1,350 kg/h; coal rate = 1,250/6 = 208.3 kg/h.
Evaporation ratio = 8,100/1,250 = 6.48 kg steam per kg coal, measured from feedwater at 32 °C, not "from and at 100 °C". Converted to F&A (multiplying by the actual heat per kg, 2,635 ÷ 2,257) it is about 7.6 kg/kg - the figure a manager is likely to quote, so state the basis.
Efficiency. hg at 8 bar ≈ 2,769 kJ/kg; hf at 32 °C ≈ 134 kJ/kg. Heat to steam = 8,100 × (2,769 − 134) = 21.34 × 10⁶ kJ per 6 h. Heat in fuel = 1,250 × 24,500 = 30.63 × 10⁶ kJ. η = 21.34/30.63 = 69.7%.
A well-run fire-tube boiler of this size on washed coal should reach 78–80%. About 9–10 percentage points, roughly 3 × 10⁶ kJ per 6 h or 120 kg of coal per shift, are being lost. The boiler is not performing as it should, and the loss is large enough to justify investigation before any capital purchase.
2. Interpret the 330 °C stack temperature, the 9 per cent CO₂ and the chimney haze. In what order would you investigate, and what would you check?
Read the worked answer
Order of investigation.
- Air/fuel ratio. 9% CO₂ is far below the ~18% maximum for coal. Excess air ≈ (CO₂max/CO₂ − 1) × 100, so roughly 100% excess air. That multiplies the mass of gas up the stack and therefore the stack loss. Check damper setting, grate condition and coal size; target 12–14% CO₂, i.e. 40–60% excess air.
- Stack temperature. 330 °C is high for a fire-tube boiler with no heat recovery; 200–220 °C is realistic before an economiser, 170–190 °C after. Identify whether the temperature is caused by gas-side soot or water-side scale before blaming the burner.
- The haze. Visible haze together with high excess air points to poor fuel/air mixing, fine coal or a choked gas passage. If CO is present, combustion is incomplete. Look at unburnt carbon in the ash and fly ash.
- Water treatment and internal fouling. Scale and soot both raise stack temperature and cut efficiency, so check the treatment log, not only the fire.
Until the excess air is corrected, any heat-recovery estimate is based on an oversized gas flow and will not be met.
3. Estimate the saving from an economiser cooling the flue gas from 330 °C to 190 °C, using about 14 kg flue gas per kg coal and cp = 1.05 kJ/kg·K. Is the investment justified, and what precautions apply?
Read the worked answer
Flue gas flow ≈ 208.3 × 14 = 2,917 kg/h. Heat recoverable = 2,917 × 1.05 × (330 − 190) = 428,800 kJ/h ≈ 119 kW thermal. Fuel equivalent saved = 428,800 ÷ 0.70 (present efficiency) = 612,600 kJ/h → 612,600/24,500 = 25 kg coal per hour, about 12% of the fuel.
Annual saving: 25 × 6 × 300 = 45,000 kg, i.e. 45 tonnes, worth about ₹4.05 lakh at ₹9,000/tonne.
A finned-tube economiser for a 1.5 t/h boiler typically costs a few lakh installed, so payback is roughly one to one-and-a-half years - worth buying. Fix the excess-air problem first, otherwise the gas flow and the savings estimate both change.
Precautions: keep the gas outlet above the acid dew point (about 150 °C for this coal) to avoid cold-end corrosion; provide a bypass for cold start-up with 32 °C feedwater; check the feed pump pressure can overcome the extra economiser resistance; and fit drain points and a safety valve on the water side.