Cast Iron Finned Economizer Tubes
Economizer tubes are the core heat-exchange elements of a boiler economizer. A cast iron finned economizer is built from several separately cast curved finned tubes assembled into one heat-exchange box, and it is used for tail-end heat exchange in coal-fired, biomass chain-grate or reciprocating-grate boilers.
Structure and water circuit
A single cast iron finned economizer tube is cast in one piece, with dense heat-dissipating fins outside the tube to enlarge the flue-gas heat-exchange area. Both ends of the tube body are curved connecting bends, and several tubes in series form a complete water circuit.
During assembly, inlet and outlet flange headers are fitted at top and bottom, and single finned tubes are connected in series vertically and arranged side by side to form a standard heat-exchange module. The number of tubes can be increased or reduced according to boiler capacity.
Low-temperature feedwater enters through the bottom flange and flows upward layer by layer through each finned tube. After absorbing flue-gas waste heat, the feedwater flows from the top flange into the drum, while hot flue gas scrubs the outer fin surface crosswise to complete heat exchange.
Material and performance
The main material is high-silicon heat-resistant cast iron RTSi5 or HT200 grey cast iron. The cast iron matrix has high hardness; compared with 20G steel tubes, its resistance to wear by fly ash, wood chips and straw fragments is more than 3 times higher, so biomass boilers with high dust content are not easily worn through and leaking.
The surface of silicon-series cast iron forms a silicon dioxide protective film at high temperature, which resists the dilute sulphuric acid dew-point corrosion produced by the sulphur in the fuel. This material suits high-sulphur coal and wet biomass fuel boilers, and in 130~180℃ low-temperature flue-gas duty its life is longer than that of ordinary carbon steel tubes.
The whole finned body and the curved bends are cast in one piece, so there is no tube weld seam that can crack and leak, and the pressure rating is stable at ≤1.6MPa, which suits most industrial low-pressure boilers. Wide-spaced fins keep flue-gas resistance low, wood chips and coal ash do not stick easily, and a soot blower can clean the surface quickly. The overall cost is lower than that of ND steel or alloy steel tube economizers, and a damaged single piece can be replaced on its own.
Energy saving
The economizer recovers waste heat from 220~400℃ flue gas at the boiler tail and preheats the boiler feedwater. For every 5℃ drop in flue-gas temperature, boiler thermal efficiency rises by about 1%, and coal or biomass consumption falls by 8%~15%. With higher feedwater temperature the furnace temperature difference shrinks, thermal shock on the water wall and drum is reduced, and boiler body life is extended. A stable furnace combustion temperature also helps wet biomass fuel burn out fully and cuts heat loss from incomplete combustion.
Suitable duty and selection
The economizer suits 1~10t/h chain grates, biomass reciprocating grates and small coal-fired hot-water or steam boilers. Suitable fuels include wood chips, straw, rice husk, palm kernel shell, class II bituminous coal, high-ash poor coal and high-sulphur fuel. It is not suitable for medium- and high-pressure power station boilers above 1.6MPa, nor for gas condensing boilers.
| Item | Parameter |
|---|---|
| Suitable furnace type | 1~10t/h chain grate, biomass reciprocating grate, small coal-fired hot-water or steam boiler |
| Suitable fuel | Wood chips, straw, rice husk, palm kernel shell, class II bituminous coal, high-ash poor coal, high-sulphur fuel |
| Pressure rating | ≤1.6MPa |
| Flue-gas temperature | 130~180℃ |
| Recovered flue-gas temperature | 220~400℃ |
| Standard module tube count | 30 tubes, 60 tubes, 80 tubes, 100 tubes |
The standard single heat-exchange unit is a uniform curved finned casting, and units can be combined into sets of 30 tubes, 60 tubes, 80 tubes or 100 tubes according to boiler capacity. The set is lifted vertically into the boiler tail flue, the inlet and outlet flanges are standard and connect directly to the boiler feedwater pipe. Height and tube count can also be customised to the on-site flue size.
