Normally, there is no phase change in the economiser, meaning the water does not change into steam in the economiser. However some economisers are designed as “steaming economisers”, where the final rows of economiser heat exchange result in some steam production.
As economisers face colder flue gas temperatures and flue gas to economiser tube temperature difference is much less than in the boiler proper, extended surfaces are often used to enhance the heat transfer. These extended surfaces can range from flat fin tube in line with the gas path and parallel to the tube, rectangular or “H” fin that is perpendicular to the tube or a spiral wound helical fin. In power boilers, bare tube economisers are also common as fouling from entrained gas particles could cause blockages in a finned economiser.
As the flue gas temperature drops, acids that might be in a vapour state in the flue gas begin to condense onto the colder tubes. It is a fine line between extracting maximum heat from the flue gas stream and preventing cold end corrosion of economiser tubes. Sometimes cast iron gills (finned castings) are shrink fitted on to carbon steel tubes to form a protective barrier, and at the same time enhance the surface area of economisers operating at lower temperatures.