Economiser

An economiser is a boiler heat-transfer section that uses remaining flue-gas heat to preheat feedwater before it enters the steam drum or evaporator circuit. It usually sits near the cold end of the convective pass, downstream of superheaters or reheaters and upstream of the air heater.

Operating role

Preheating feedwater reduces the fuel needed to raise steam and lowers stack temperature. In utility boilers, recovery boilers, biomass boilers and waste-to-energy plants, the economiser is also a pressure-drop and reliability boundary: it is one of the last dense tube banks the gas must cross before final heat recovery and particulate control.

Fouling mechanisms

Economiser ash is usually cooler, drier and more friable than furnace slag, but it can still bridge between tube rows. Large-particle ash can settle into hoppers or lodge across tube lanes. In biomass, waste and recovery service, alkali salts and chlorides can make deposits sticky; in sulphur-bearing service, cold-end surfaces may approach the acid dew point and corrode under deposits.

Typical symptoms include rising gas-side pressure drop, falling feedwater outlet temperature, higher stack temperature, increased fan power, uneven gas distribution and hopper pluggage. If ash bridges harden, gas channels around blocked regions and erosion can increase in open lanes.

Design and maintenance implications

Designers balance tube spacing, gas velocity, sootblower lanes, drainability and access. Operators track economiser inlet and outlet gas temperature, feedwater temperature rise, pressure drop, sootblower effectiveness and hopper discharge. Cleaning strategy depends on deposit character: steam sootblowers handle stronger deposits, while acoustic cleaning is better for dry, friable ash that should be prevented from consolidating.

Acoustic cleaning relevance

Sonic horns are often installed on economiser casings and associated ash hoppers because the geometry contains many deposit initiation points but the deposits are frequently responsive to pressure waves. Keeping the economiser cleaner helps preserve heat rate, reduces fan margin loss and limits downstream loading to air heaters and particulate controls.

Troubleshooting notes

Economiser fouling should be checked against both heat-transfer and gas-flow symptoms. A higher stack temperature with little pressure-drop change suggests insulating deposits or bypassing. A sharp pressure-drop rise suggests bridging or blocked lanes. If feedwater outlet temperature falls after sootblowing, the sootblower may be moving ash into a worse location or opening a gas bypass path rather than restoring uniform heat transfer. Useful inspections record deposit strength, colour, location by tube row, hopper condition and evidence of acid dew point corrosion.

Design and operating variables

Economiser design balances heat recovery against pressure drop, corrosion risk and cleanability. Important variables include feedwater inlet temperature, gas inlet temperature, gas velocity, tube pitch, fin geometry, sootblower or acoustic-cleaner access, ash loading, sulphur and chloride chemistry, tube material and the expected turndown range. The cold end is especially sensitive because low metal temperature can bring surfaces below the acid dew point and create sticky, corrosive deposits.

In boilers firing coal, biomass, waste or heavy fuel oil, the economiser often marks the transition from relatively hot, loose ash to cooler deposits that can become cohesive. Finned tubes increase heat-transfer area but also create smaller gas passages where fly ash packs between fins. Operators use gas-side pressure drop, feedwater temperature rise, stack temperature, oxygen, sootblower history and visual inspection to judge whether the section is clean. A rising pressure drop with falling heat pickup usually points to fouling rather than a water-side issue.

Failure and maintenance context

Common problems include tube erosion at high-velocity lanes, corrosion under deposits, acid dew-point attack, ash bridging between tube rows, vibration fatigue, header leaks and local overheating after flow maldistribution. Economiser tube leaks can introduce water into ash deposits, turning a manageable build-up into a hard blockage and increasing corrosion during shutdown.

Cleaning strategy depends on deposit character. Steam sootblowers, sonic horns, water washing during outages and manual lancing are all used in different services. Acoustic cleaning is most useful for dry, friable ash before it sinters or absorbs moisture; it is less effective on wet, acid-bound or fused deposits. Good access design also matters because an economiser that cannot be inspected safely will often be cleaned too late.