Economizer Fouling and the Cost of Deferring Cleaning
A 12 t/h packaged water-tube boiler is running at 70% load on natural gas (LHV 35.8 MJ/m³). The bare-tube economizer was last cleaned 18 months ago. Design data: feedwater enters the economizer at 105°C and leaves at 140°C; flue gas leaves the economizer at 160°C. Today the feedwater leaves at 118°C and the flue gas leaves at 210°C. The stack is clear, there are no water leaks, the forced-draft fan is at 78% speed, and flue gas oxygen is 3.5%. Fuel flow is 980 m³/h. The boiler is meeting steam demand, but the operator proposes increasing firing to restore the feedwater outlet temperature. The plant has a planned shutdown in 10 weeks but can isolate the boiler earlier at a production cost.
1. Estimate the fuel penalty caused by the fouled economizer and decide whether waiting 10 weeks is justified.
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Use the stack-loss comparison, not the feedwater temperature alone. The flue-gas temperature rise is 210 − 160 = 50 K. At 3.5% O₂, excess air is roughly 20%. For natural gas, flue gas mass is about 21–22 kg per kg of fuel at that excess-air level. Fuel mass flow is approximately 980 × 0.72 = 706 kg/h, so flue gas flow is about 15,000 kg/h. With cp ≈ 1.05 kJ/kg·K, the extra heat lost up the stack is:
15,000 × 1.05 × 50 ≈ 788,000 kJ/h ≈ 219 kW.
Fuel input is 980 × 35.8 = 35,084 MJ/h ≈ 9.75 MW. The penalty is therefore about 219/9,750 = 2.2%, or roughly 22 m³/h of gas at this load. Over 10 weeks at, say, 4,000 operating hours, that is about 88,000 m³. That is usually enough to justify early cleaning if isolation can be arranged safely.
The decision should also consider fan margin and flue-gas temperature limits. At 78% fan speed there is some margin, but a load increase or colder ambient air will reduce it. Waiting is defensible only if stack temperature, fan current and economizer outlet temperature are trended daily and a trip point is set; otherwise clean at the first safe opportunity.
2. Why is increasing firing rate not an acceptable way to restore feedwater outlet temperature?
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Raising firing adds heat input at the burner, but the economizer is limited by gas-side fouling, not by heat availability. The deposit adds a gas-side resistance, so the overall heat-transfer coefficient falls. More firing raises the gas temperature entering the economizer, but the same fouling layer still restricts heat flow to the water. The result is a higher stack temperature, higher excess-air fan load and greater fuel consumption, not a return to the design feedwater temperature.
There is also a control interaction: if the feedwater control valve is already near fully open, more firing will not increase the water-side flow; it will simply raise steam pressure and flue-gas temperature. At high firing the boiler may reach its high-pressure trip or the forced-draft fan may reach its current limit. The correct response is to accept reduced economizer performance, maintain excess O₂ around 3–4%, keep load within the fan and stack-temperature limits, and clean the economizer at the earliest planned or opportunistic shutdown.
3. If cleaning is deferred, what secondary risks should the operator monitor and what limits would trigger immediate action?
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Monitor four things: economizer outlet flue-gas temperature, stack temperature, forced-draft fan current/speed, and economizer water outlet temperature. A rising flue-gas outlet temperature at constant load confirms progressive fouling. Fan current approaching its motor rating or fan speed above about 90% removes the margin needed for load swings and should trigger a load reduction or early cleaning.
Also watch for localized corrosion under deposits, especially if the boiler is cycled and the metal temperature drops below the acid dew point. Fouling can create cold spots and acid condensation on the gas side even though the average stack temperature is high. If the economizer outlet gas temperature falls close to the acid dew point at low load, or if water-side flow becomes unstable, stop the economizer or reduce load. A sudden rise in stack temperature with unchanged firing can indicate tube leakage or bypassing, not just fouling. Set a hard operating limit, for example stack temperature 30 K above the current baseline or fan current at 95% of rated, and clean or reduce load before reaching it.