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Economiser (English for preheater / saving system) is a heat exchanger for heat recovery by utilizing waste heat from a wide variety of industrial processes.

Boiler systems

Economiser in a large power plant, designed with enamelled pipes

In steam boiler technology , the economiser (ECO for short) is a "feed water preheater". The flue gas flows through the economiser at relatively low temperatures after it has previously given off its heat in the evaporator, superheater and reheater heating surfaces. The economizer uses the residual heat in the exhaust gases, which the steam boiler can no longer use for physical reasons.

Shell boiler

In shell boilers , the economiser is either attached to the boiler or it forms a separate component. The flue gas from the last boiler flue gas pass is fed into the economiser. The flue gas temperatures here at full load and unpolluted heating surfaces are around 50 ° C - 80 ° C above the saturated steam temperature in the boiler. The minimum permissible flue gas temperature behind the boiler depends on whether the chimney is resistant to dew point drops (acids). Therefore, a temperature-controlled bypass flap is often installed, which directs the flue gas past the built-in economizers if the temperature is too low.

In the case of a boiler that is regulated to a steam pressure of 10 bar, the temperature level that can be used in the Eco is between 240 ° C and 140 ° C; this corresponds to a fuel saving of approx. 5%. In this case, the temperature of the feed water would be increased from 105 ° C (after the feed water tank ) to around 135 ° C.

Water tube boiler

In the case of water-tube boilers , the economiser is integrated in the boiler and forms the last heating surfaces in the flue gas path before the air preheater, if installed . As the pressure rises, the saturated steam temperature of water rises and the enthalpy of evaporation decreases. Therefore, with increasing boiler pressure, the specific heat output that can be transferred to the heat transfer medium via the Eco increases. In the case of ultra-high pressure steam boilers that are operated at supercritical pressures, the designation economiser or evaporator is superfluous, since a saturated steam state is no longer defined here.

Refrigeration systems

In compression refrigeration systems economizer, to increase the coefficient of performance involved. The refrigerant is compressed by a compressor and then condensed in the condenser. A secondary stream is withdrawn from the main stream and is depressurized at an intermediate pressure. Main and secondary flow are fed to a heat exchanger. The refrigerant in the main flow is subcooled by the evaporating secondary flow. The bypass flow is pressure-regulated and fed back to the suction side of the compressor. The increase in the COP is very dependent on the refrigerant used and the pressure level of the refrigeration system.

An alternative to the Eco subcooler is the installation of a medium pressure bottle in a two-stage compression refrigeration system. The liquid refrigerant from the condenser is fed into the medium pressure bottle via an expansion fitting. The evaporated refrigerant is sucked off by the high-pressure compressor, which is adjusted to the intermediate pressure.

Web links

Commons : Economisers  - collection of pictures, videos and audio files
Wiktionary: Economiser  - explanations of meanings, word origins, synonyms, translations


  • Amiya Ranjan Mallick: Practical Boiler Operation Engineering and Power Plant . 4th edition. PHI Learning, 2015, ISBN 978-81-203-5139-4 , chapter 9.7.