01 / The opportunity
Why put an economizer after a boiler?
An economizer is a heat exchanger installed in a boiler’s exhaust path. It transfers energy from hot flue gas to a useful water stream—usually boiler feedwater, make-up water, or a separate process-water circuit. The recovered heat reduces the fuel required to bring that water to its target temperature.
A steam boiler cannot cool its exhaust all the way to ambient temperature within its own pressure vessel. Its heat-transfer surfaces are bounded by the temperature of the water and steam inside. Exhaust leaving the boiler can therefore still carry substantial usable energy, particularly when a cooler water stream is available outside the boiler.
02 / The fundamentals
How the heat transfer works
Flue gas passes over tubes while water flows inside them. The two streams remain separate. Heat moves through the tube wall because the gas is hotter than the water; as the temperature difference narrows, each additional unit of surface generally recovers less heat.
Fins add surface area on the gas side, where heat transfer is usually less effective than on the water side. More fins or tighter gas passages are not automatically better: surface area, gas velocity, fouling potential, and pressure loss must be considered together.
For example, a boiler making steam near 150 psig has saturated water around 366°F. Its exhaust may leave at a considerably higher temperature, while deaerated feedwater can enter an external economizer at a much lower temperature. The actual inlet and outlet temperatures must be calculated from the boiler, fuel, firing rate, water flow, and exchanger design.
03 / An operating constraint
Recover heat without overwhelming the fan
Higher gas velocity can improve heat transfer, but it also increases resistance through the economizer and downstream exhaust system. If the boiler’s fan cannot overcome that resistance at the required firing rate, combustion and boiler capacity can suffer.
The allowable gas-side pressure drop must be established for the specific boiler, burner, fan, existing stack, and any other equipment in the exhaust path. A generic firetube or watertube limit cannot replace that check. Water-side pressure drop also matters because the pump must deliver the design flow through the exchanger.
04 / Water and materials
Choose the heat sink and tube material together
Deaerated boiler feedwater is a common heat sink. Preheating it directly reduces burner duty, and suitably treated, deaerated water may permit carbon steel construction. A cooler make-up or process-water stream can create a larger temperature difference and potentially more recovery, but its chemistry and operating pattern may call for stainless steel or another material selection.
Dissolved oxygen, carbon dioxide, chlorides, treatment practices, and the possibility of stagnant water all affect corrosion risk. The water-side material should be selected for the actual service, not simply by assuming that every feedwater system behaves like a deaerator.
A water stream must also be available when the exhaust heat is produced. Large quantities of cold water do not help if demand occurs at different hours from boiler operation; the load profile matters as much as the nominal inlet temperature.
05 / Beyond sensible heat
When can the economizer condense?
A conventional economizer primarily recovers sensible heat as flue-gas temperature falls. If a sufficiently cool water stream is available, a properly designed condensing stage can also recover part of the latent heat released when water vapor in the exhaust condenses. The resulting condensate must be collected and drained without flowing back into equipment that is not designed for wet service.
Condensing operation calls for appropriate gas-side materials, drainage, access, and attention to fuel chemistry and flue-gas dew point. A second stage or HeatSponge Type III arrangement can be considered where the heat sink and boiler installation support it. Performance is application-specific; a second stage does not guarantee a particular efficiency gain.
06 / Long-term operation
Plan for flow, protection, and service
Water flow and exhaust flow do not always rise and fall together. If flue gas continues through an economizer while the water circuit stops, water may overheat or steam in the tubes. The piping and controls should establish the required minimum flow, interlocks, bypasses, or other provisions for the intended operating modes.
Provide for inspection, cleaning, condensate drainage where applicable, and tube repair. A design that is accessible after installation is easier to maintain through the life of the boiler plant.
07 / Next steps
What information starts a selection?
Begin with the boiler capacity and type, fuel, firing range, exhaust temperature, available fan pressure, intended water circuit, water inlet temperature, and expected flow. Add water chemistry, operating hours, installation envelope, stack arrangement, and any emissions equipment upstream or downstream.
Those inputs allow the economizer to be evaluated as part of the entire boiler system, including the amount of heat that can actually be used. For help entering the information into Bruce, see the online selection software tutorial.