Boiler-House Performance Gap in a Garment Factory
A garment factory operates a 2,000 kg/h saturated-steam boiler at 10 bar gauge. Feedwater enters at 20 °C and condensate return is only 25%; the rest is made up with cold softened water. The boiler runs 12 h/day, 300 days/year. Recent measurements give flue-gas temperature 230 °C, ambient 25 °C, flue-gas mass flow 2,400 kg/h, and dry flue-gas oxygen 7%. A portable gas analyser shows 350 ppm CO. The boiler is rated at 80% thermal efficiency on natural gas (LHV 36 MJ/Nm³), but the fuel bill suggests closer to 74%. There is no economizer, no oxygen trim, and the blowdown is manual, observed at 4% of feedwater. The steam plant has trap failures on two condensate lines and visible flash steam at the receiver. Management is considering a new boiler; the maintenance team wants to fit an economizer and repair traps instead.
1. Calculate the useful heat output and the stack heat loss, then assess whether the performance gap is explained by stack loss alone.
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At 10 bar gauge (about 11 bar absolute), saturated steam enthalpy is about 2,780 kJ/kg; feedwater at 20 °C is about 84 kJ/kg. Useful heat = 2,000 × (2,780 − 84) / 3,600 = 1,498 kW. Stack loss = 2,400 × 1.05 × (230 − 25) / 3,600 = 144 kW, using cp = 1.05 kJ/kg·K for flue gas. If the boiler were at 80% efficiency, fuel input would be 1,498 / 0.80 = 1,872 kW; the stack loss is then 144 / 1,872 = 7.7% of fuel input. At 74% efficiency, fuel input is 1,498 / 0.74 = 2,024 kW, so the extra fuel is about 152 kW. The stack loss of 144 kW is therefore close to the whole gap only if it is an excess loss over the design value; the rated 80% already includes some stack loss, so stack alone does not explain everything. Other losses must be present: 4% blowdown, radiation and convection from the boiler shell, failed traps and flash steam, and CO at 350 ppm. The conclusion is that the boiler is not necessarily worn out; the losses need to be separated and measured.
2. Recommend a prioritized set of improvements with approximate annual savings and payback logic, considering the 12 h/day, 300 day/year operation.
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Priority 1 is the steam and condensate system: repair the two failed traps and stop the visible flash steam. Returning more condensate raises feedwater temperature and reduces make-up water treatment; going from 25% to 60% return can save several percent of fuel and is usually low cost. Priority 2 is an economizer. Cooling flue gas from 230 °C to 140 °C recovers 2,400 × 1.05 × 90 / 3,600 = 63 kW, about 4.2% of the 1,498 kW useful output. At 12 h/day and 300 days/year this is 63 × 3,600 = 227,000 kWh/year, roughly 22,700 Nm³ of natural gas at 10 kWh/Nm³, before allowing for boiler efficiency. Priority 3 is oxygen trim to hold 3–5% O2; 7% O2 indicates excess air. Priority 4 is automatic conductivity blowdown instead of manual 4% blowdown. A new boiler is not justified until these measures are done, because the existing boiler can probably reach the mid-80s. Payback is annual gas saving times gas price compared with installed cost; the economizer and trap repairs should be evaluated first because they are lower cost and can be implemented without a shutdown.
3. The maintenance team proposes an economizer. Explain the operating constraints that could limit its benefit and the checks needed before installation.
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An economizer recovers sensible heat from flue gas into feedwater, but its benefit is limited if the boiler runs at low load or if feedwater flow is intermittent. The feedwater is only 20 °C, which is good for heat transfer, but the flue gas temperature must stay above the acid dew point; natural gas is low-sulphur, so the main risk is water/acid condensation at low load. Check the existing flue draught and fan capacity, because the extra resistance can affect combustion. Provide a bypass and temperature control so feedwater leaving the economizer does not approach boiling inside the economizer. Check that the economizer can be drained and cleaned, and that the stack temperature at low fire stays above the dew point. Also confirm the boiler feedwater pump head and the economizer pressure drop. If the boiler is cycling, the economizer may run dry or corrode; a bypass or a storage tank is needed. These checks decide whether the 63 kW recovery is actually available for 3,600 hours per year.