Working Principle of Cast Iron Finned Economizers
The cast iron finned economizer is installed in the boiler tail flue ahead of the induced draft fan and uses counter-flow heat exchange between flue gas and feedwater to preheat the feedwater with waste heat.
Overall heat exchange logic
Hot flue gas leaves the furnace and sweeps across the finned cast iron outer surface from top to bottom, while cold feedwater enters the bottom inlet flange and flows upward inside the finned tubes. The two flow in opposite directions, keeping a large temperature difference throughout and giving high heat exchange efficiency.
Water-side heat exchange inside the finned tubes
Cold feedwater, at ambient or low temperature after softening, enters the first single finned tube through the bottom header flange. Both ends of each tube are cast in one piece with curved connecting bends, and the water flows upward in series through the bends, moving slowly inside the closed cast iron cavity; the cast iron conducts heat from the flue gas side to the water, whose temperature rises steadily. Fully heated water leaves through the top outlet flange to the boiler steam drum or water walls for circulation.
Heat absorption on the fin side
Hot dust-laden flue gas sweeps at speed across the cast iron body and the dense, widened fins, and the integrally cast fins multiply the gas contact area, recovering 200-400℃ waste heat. Cast iron resists high temperature and wear, so coal ash and wood chip particles do not easily wear through the wall; at high temperature the surface forms a SiO₂ protective film against dilute sulphuric acid from condensing fuel sulphur. Wide fin spacing gives low gas resistance, so dust does not build up and block the passages and heat exchange stays stable; the cooled gas flows down to the dust collector and desulphurisation unit before release.
| Item | Parameter |
|---|---|
| Recovered flue gas temperature | 200-400℃ |
| Each drop in flue gas temperature | 5℃ |
| Rise in boiler thermal efficiency | About 1% |
Effect on the boiler heat cycle
Waste heat that would otherwise go up the stack is absorbed by the feedwater, which is preheated so the furnace need not burn large amounts of fuel to heat cold water; each 5℃ drop in flue gas temperature raises boiler efficiency by about 1%, directly cutting coal and biomass consumption. With much hotter feedwater, the temperature difference to the high-temperature heating surfaces shrinks, thermal shock on water walls and the steam drum from alternating hot and cold falls, and the risk of cracking and deformation in the boiler body drops.
Preheated feedwater also reduces the furnace heat load, keeps furnace temperature steadier and lets wet straw, sawdust and other high-moisture biomass burn more fully, cutting unburned loss and black smoke.
