Increased efficiency on the combustion side

Flue gas temperature or flue loss

The flue gas temperature is a crucial factor when it comes to determining efficiency. However, in order to operate a boiler in a technically sensible manner, there must be a minimum temperature difference between the flue gas and the heating water in the boiler for the heat transfer to take place. To ensure that this is also economically viable, hot water boilers are usually designed with a flue gas heat exchanger downstream of the boiler heating surfaces.

For hot water boilers with flue gas heat exchangers, the flue gas temperature at the boiler outlet is therefore still approx. 20 – 30 °C above the flow temperature. For example, the flue gas temperature of a 2.5 MW boiler with a flow temperature of 80 °C is around 115 °C. This corresponds to a flue loss of approximately 2.2 %.

The graph “Efficiency curve for boiler and Eco” also clearly shows that the available return temperature has a significant influence on the efficiency. Cooler return temperatures allow the flue gas to be cooled further without changing the design of the boiler system. By setting back the return temperature by 20 °C, the flue gas temperature can also be reduced by approx. 20 °C. This reduces flue loss by around one percentage point and increases boiler efficiency accordingly.

As can also be clearly seen, efficiency can be improved significantly by a further 1 – 2 % if the heating surface is enlarged at the “cold” end of the heating surfaces, i.e. at the flue gas heat exchanger.

These larger heating surfaces in the flue gas heat exchanger allow the flue gas temperature to be cooled down to almost the return temperature entering the boiler, making it the most suitable system, as shown in the “Flue gas temperature” graph.