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Desuperheaters remove the superheat in the steam that is to be used for auxiliary purposes, the reason is that if high temperature steam was used for auxiliaries then the materials used would have to be capable of withstanding high temperatures, this leads to increased initial cost. Desuperheaters are generally coils situated in the water spaces that are supplied with steam from the superheated section outlet; this type of desuperheater is generally called as internal desuperheaters. Such an internal desuperheater is shown down below.
If this type of desuperheater develops a leakage then water loss from the boiler occurs since boiler pressure is greater than steam pressure in the desuperheater coil. This water loss could result in water hammer and subsequent damage in the auxiliary system.
Spray type external desuperheater is shown in the diagram below, in which feed water of low solids content at 3.5 bar above steam pressure is supplied at a controlled rate into a vertically arranged vessel to mix with superheated steam. The excess water is kept to a minimum, collects at the bottom of the vessel and passes out through the drain and then the superheated steam moves up the annular space formed by the mixing compartment and the outer container to the auxiliary steam supply line.
Another type of external desuperheater is shown in the figure below. This is really part of the boiler externally arranged. It has a steam and water connection to the boiler drum so that the circulation through the desuperheater takes place, taking heat from the superheated steam in the solid drawn steel ‘U’ shaped tubes.
A blow down valve is fitted in order to remove any sludge that may accumulate in the lower portion of the vessel. This type of desuperheater is for large steam demands since the 3 pass U tube arrangement would be too large to accommodate in the water space of the steam drum.
"Steam Engineering Knowledge for Engineers" by Thomas D. Morton