Flue gas temperature at the boiler outlet, when this is still clean or with partial loading, tends to decrease: it can however be mantained uniform with a simple device.
A thermo-sensor placed at the economizer outlet sends the measure of flue gas temperature to the control system.
This signal opens a three-way valve, which varies the water flow through the last economizer.
This solution has been designed to avoid evaporation effects of the water in the economizer.