Difference between Attemperator and De-superheater

Attemperator are fitted in order to keep the steam temperature in the final stages of the superheater within certain desired limits .
This is done for two main reasons .

1. The most important is the protection of the superheater tube material from the effects of overheating.
*Mild steel superheater tubes tubes should not be subjected to steam temperatures in excess of 455℃.

2. The formation of bonded deposits on superheater tubes can be reduced if steam and gas streams within certain temperature ranges are prevented from coming into contact of each other.

De-Superheater is used to reduce the temperature of a quantity of superheated steam and make it suitable for used in auxiliary services requires low temperature or saturated steam .

*The reason for doing this way rather than by taking the saturated steam direct from steam drum , is to protect the superheater , these must be protected against overheating under all conditions of load by circulating steam through them .

*Although attemperators and de- superheators are both used to protect the superheater from overheating, they do this in different ways . Attemperators are fitted to control the temperature passing through while de- superheater helps to provide necessary circulations.

De-super heater are fitted after the final superheater stage and the de-superheated steam is not raised again in temperature .

Reference: MARINE BOILERS
third edison G.T.H FLANAGAN