Economisers, also referred to as flue gas heat exchangers, belong to the standard equipment of a boiler system as they can be used in more or less any system.

Although the investment costs of an integrated economiser are roughly 7 – 15% of a boiler, it also increases the efficiency of the system by up to 7% and therefore normally pays for itself within several months of operation.

Info on Flue gas temperature or flue gas loss

The flue gas flow emerging from the steam boiler is still at a temperature of 200 – 280°C. The economiser is equipped with highly-efficient heat exchangers for dry flue gas heat recovery which makes use of this thermal potential at a high temperature level. The heat extracted from the flue gas is normally used for feed water heating and therefore improves the boiler efficiency.

Flue gas condensing via flue gas heat exchangers, air preheating or a feed water cooler can be used to further improve efficiency.

Info on Condensing heat exchanger

Info on Air preheater

Info on Feed water cooler

Various types of economiser are available for the system-specific mode of operation of the flue gas heat exchangers which is optimised for each individual plant.

Integrated economiser

The fully integrated economiser which is directly mounted on the boiler offers benefits especially for new boiler systems. The specially developed heat exchanger bundle with variable size and highly efficient finned tubes is installed as an integral component of the boiler in the flue gas collection chamber, fully insulated and connected directly to the boiler on the water side. Integrated economisers are available for the U-MB, CSB, UL-S, ZFR and HRSB boiler series.

An integrated economiser has significant benefits compared to a conventional boiler with separate economiser.

Integrated economiser in UL-S

Integrated economiser in UL-S

Integrated economiser in UL-S


  • Highly efficient spiral finned tube system for gas and fuel oil “EL”
  • Integrated in the flue gas chamber on the boiler
  • Increase in efficiency of up to 7% points
  • Assembled at the plant, piped and ready for connection, tested and thermally insulated
  • Low space requirement
  • No additional foundation required
  • No local installation
Economiser without shut-off facility, uncontrolled (left) and economiser without shut-off facility, controlled on the water side (right)

Economiser without shut-off facility, uncontrolled (left) and economiser without shut-off facility, controlled on the water side (right)


Temperature indicator



Temperature controller




With the uncontrolled variant, all flue gas always flows though the heat exchanger pipes to ensure the maximum amount of energy is recovered from the flue gases in every load position. In addition to achieving the best efficiency, this is also the most cost-efficient and therefore most commonly used variant.

To prevent the flue gas temperature downstream of the economiser from falling too far, if a minimum flue gas temperature requirement exists for masonry chimneys for example, the variant with water-side control is used. With this variant, some or all of the feed water flow is routed via a three-way control valve past the economiser until the preset flue gas temperature downstream of the economiser is established.

Standalone economiser

Standalone economisers can be installed independently of the boiler which means they can be very conveniently retrofitted to existing systems. They are also used if the economiser needs to be bypassed on the flue gas side from time to time, as is the case when using a second fuel containing sulphur (e.g. heavy fuel oil).

The flue gases enter the lower section of the economiser and flow through the heat exchanger in the upper section where the heat recovery process takes place. The flue gas bypass including motorised flue gas damper, the pipework for the connections and valves are ready assembled and belong, together with thermal insulation, to the scope of delivery supplied ex-works. The standalone economiser features a special double finned tube which makes the heating surfaces easier to clean and also makes it suitable for heavy fuel oil or fuels with a high soiling tendency.

The standalone economiser is available for all CSB, U-MB, UL-S, ZFR, HRSB boiler series and also boilers by other manufacturers.

Standalone economiser in combination with a dual combustion system (gas/heavy fuel oil)Standalone economiser in combination with a dual combustion system (gas/heavy fuel oil)