ATTEMPERATOR - WATER & AIR COOLED

ATTEMPERATOR

Attemperators are fitted in order to ke ep the steam temperature in the final stages of the superhea ter within certain de sired limits.
This is done for two main reasons. The most important is the protection of the superheater tube materi al from the effects of overheating, as it is the steam temperature which governs the tube met al surface temperature rather than the temperature of the gas flowing over them. Thus, for example, mild steel superheater tubes should not be subjected to steam temperatures in excess of 455°C.
Two basic types of attemperators are -:

WATER COOLED ATTEMPERATOR


* These are constructed in a manner similar to that of a sur face type de-superheater. They are placed below the water level in the steam drum as shown or, as an alternative , placed in an external chamber connected by balance connections to the circulation circuit of the boiler.
* With this type of attemperator some of the steam leaving the primary superheater passes directly to the secondary superheater, while the remainder passes through the attemperator before re joining the rest of the steam to enter the secondary superheater.
* Control valves, fitted to the steam line leaving the primary superheater, regulate the final superheat temperature by varying the proportion of steam flowing direct to the secondary superheater to that passing to it via the attemperator.


AIR COOLED ATTEMPERATOR



* This is usually fitted in the double casing of the boiler, and consists of inlet and outlet headers connected together by a series of multi-loop tube elements, which enable the superheated steam to make several passes across the path of the combustion air flowing to the furnace .
* In this type of attemperator , all the steam leaving the primary superheater passes through the attemperator before entering the secondary superheater.
* The control over the final steam temperature is obtained by regulating the amount of combustion air flowing over the attemperator by means of control dampers and an air by-pass.

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