Eco is a gas-water HX to heat the incoming fw with flue gases before fw reaches the steam drum. In case of SC boilers, water from Eco is led to the water walls as there is no drum. It is so named as it ‘economises’ on the use of fuel by cooling the hot flue gases to acceptable economical levels. It is the last water-cooled HS in a boiler. Being a low-temperature HX, Eco needs a lot of HS, which, in turn, demands more space, supporting structure and soot blowing, making bulky.