BOILER ENERGY RECOVERY
Boiler Economizer & Flue Gas Heat Recovery Selection Guide
Select a heat sink that can use the recovered energy. Then check flue-gas temperature, corrosion, fouling and draft before sizing an economizer.
QUICK ANSWER
Select the heat sink first, then set the safe flue-gas outlet temperature.
Measure the source
Use actual flue-gas flow, inlet temperature, oxygen, fuel chemistry, fouling and operating hours.
Define the sink
Feedwater, combustion air or process water must be available when the boiler is producing recoverable heat.
Protect the cold end
Dew point, sulfur, chlorides, particulates, corrosion, drainage and cleaning constrain the outlet temperature.
Screening formula: Q = ṁgas × cp × (Tin − Tout).
01 · RECOVERED-HEAT USER
An economizer only saves fuel when the recovered heat is useful.
Compare boiler operating hours with feedwater, makeup-water or process-water demand. Heat cannot produce a fuel saving when the selected sink is unavailable or already too hot.
Feedwater economizers recover heat for steam boilers. Air preheaters can serve boiler or thermal-oil combustion systems. Condensing recovery needs a sufficiently cool sink and corrosion-resistant equipment.
02 · RECOVERY OPTIONS
Feedwater economizer, air preheater and condensing recovery solve different problems.
| Option | Heat sink | Key design checks |
|---|---|---|
| Feedwater economizer | Boiler feedwater | Feedwater flow, steaming risk, bypass and outlet approach |
| Combustion-air preheater | Burner or furnace air | Fan pressure, burner limits, air leakage and controls |
| Process-water exchanger | Wash, makeup or process water | Demand coincidence, water quality and contamination barrier |
| Condensing economizer | Low-temperature water | Dew point, corrosion-resistant materials, condensate treatment and plume |
State LHV or HHV, operating load, auxiliary power and the efficiency measurement boundary.
03 · WORKED SCREENING EXAMPLE
Estimate sensible heat before detailed exchanger design.
For a sensible-heat estimate, assume flue-gas mass flow of 25,000 kg/h and average specific heat of 1.05 kJ/kg·K on the same composition basis. Cool it from 230°C to 160°C without condensation:
- Temperature reduction: 230 − 160 = 70 K.
- Gross sensible heat: 25,000 × 1.05 × 70 = 1,837,500 kJ/h.
- Convert to power: 1,837,500 ÷ 3,600 = approximately 510 kW.
- Annual-use example: if the heat sink is available for 80% of the evaluated operating hours, the time-averaged recoverable duty is 510 × 0.80 ≈ 408 kW. This is an availability factor, not another exchanger-efficiency factor.
The 510 kW result already represents the stated gas temperature drop. Confirm composition, moisture, fouling, heat-transfer area, approach temperature and pressure loss in exchanger design.
04 · COLD-END CORROSION
The minimum stack temperature depends on fuel chemistry and surfaces.
Condensation risk depends on gas composition and metal-surface temperature. Sulfur-bearing fuels can form acid condensate; biomass salts and deposits create additional corrosion mechanisms. Do not apply a natural-gas limit to every fuel.
For non-condensing equipment, establish a cold-end temperature limit. For condensing equipment, specify compatible materials, condensate drainage and treatment.
05 · BIOMASS AND COAL
Fouling, erosion and cleaning can dominate solid-fuel recovery design.
Match tube spacing and gas velocity to ash loading and erosion. Provide cleaning access, soot removal and ash discharge. Dense surfaces that foul quickly can erase the fuel saving.
Check seasonal moisture and ash variation. Estimate fuel input with the biomass boiler fuel-consumption calculation.
06 · DRAFT AND CONTROLS
Recovered heat must not destabilize combustion.
Add the economizer or air-preheater resistance to the complete gas-side pressure-drop calculation. Verify induced-draft and forced-draft fan curves, motor margin, altitude correction, damper authority and furnace-pressure control at clean and fouled conditions. A bypass may be required for startup, low load, maintenance or cold-end protection.
On the water side, check pump margin, control-valve behavior, thermal expansion, venting, steaming risk and low-flow conditions. Instrument inlet and outlet temperatures, pressures and differential pressure so fouling and performance can be trended rather than guessed.
07 · SAVINGS BASIS
Convert recovered heat into avoided fuel with the same efficiency basis.
Annual useful recovery equals usable recovered duty multiplied by coincident operating hours. Avoided fuel energy then depends on the baseline boiler efficiency. Include added fan or pump electricity, cleaning, water treatment, corrosion-resistant materials, downtime and maintenance in the lifecycle comparison.
The DOE reports that feedwater economizers can often reduce fuel requirements, but the actual result depends on stack temperature, load and the available heat sink. Request a proposal with inlet conditions, outlet guarantees, pressure drops, fouling basis, materials and measurement points.
Economizer and flue-gas heat-recovery RFQ checklist
- Fuel analysis, LHV/HHV basis and flue-gas composition;
- Boiler or heater load profile and annual operating hours;
- Measured gas temperature, oxygen and flow at representative loads;
- Candidate heat-sink flow, inlet temperature and operating coincidence;
- Minimum permissible outlet temperature and corrosion basis;
- Particulate, fouling, erosion, cleaning and ash-handling requirements;
- Allowable gas- and liquid-side pressure drops, fan and pump curves;
- Materials, bypass, controls, instruments and acceptance-test method.
TECHNICAL REFERENCES
Primary energy-efficiency references.
- U.S. Department of Energy — Use Feedwater Economizers for Waste Heat Recovery
- U.S. Department of Energy — Consider Installing a Condensing Economizer
- U.S. Department of Energy — Improving Steam System Performance
- JIELI — Complete boiler and thermal-oil system supply
All savings and outlet-temperature values require project-specific measurements and engineering. Do not infer a guarantee from a screening calculation.
FREQUENTLY ASKED QUESTIONS
Boiler heat-recovery FAQ
What does a boiler economizer do?
It transfers heat from outgoing flue gas to a useful sink, commonly steam-boiler feedwater, reducing the fuel required to reach the same output when the source and sink operate together.
How do I estimate recoverable flue-gas heat?
For preliminary sensible heat, multiply gas mass flow by average heat capacity and the proposed temperature reduction. Detailed design must include gas composition, water vapor, fouling, approach temperature, pressure drop and safe outlet temperature.
How low can boiler flue-gas temperature go?
There is no universal value. Fuel sulfur and chlorides, water vapor, excess air, materials, leakage and particulate deposits affect water or acid dew point and corrosion risk.
Is an air preheater the same as a feedwater economizer?
No. An air preheater warms combustion air, while a feedwater economizer warms boiler feedwater. They affect different fans, pumps, controls and heat-balance boundaries.
JIELI THERMAL ENGINEERING
Measure the source and heat sink before selecting recovery equipment.
Send fuel data, flue-gas temperature and oxygen, load profile, fan margin and candidate heat-sink conditions. JIELI can review the exchanger, bypass, controls and complete boiler or thermal-oil system.
Request a heat-recovery review