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[Boeid]

I am trying to understand the reasons and justifications for the proposal by this one vendor among a few others on the design of attemperator for an HRSG with supplementary firing.

1) Single inter-stage attemperator vs. Multi inter-stage attemperator

Single inter-stage attemperator is an attemperator where the water injection is in between 2 superheater banks. The water injection is controlled by a temperature controller at the final outlet of the HRSG i.e.secondary or the last superheater bank. This is the one that I am more familiar with.

Multi inter-stage attemperator is - in my case - 2 attemperators where the water injection are in between first and second superheater banks, and between second and third/final superheater banks, respectively.
The water injection for the 1st attemperator between 1st and 2nd superheater banks is controlled by a temperature controller at the final outlet of the HRSG and for the 2nd attemperator is by a temperature controller at the inlet of the 2nd superheater bank.

What are the reasons and the advantages of the multi inter-stage attemperator?


2) Degree of superheat value of temperature above saturation point of steam at in between superheater banks

What are the reasons and the advantages of having lower degree of superheat for steam at in between of the superheater banks?
Our plant/company set a standard of 40 deg Celsius of superheat but the vendor proposed for only 28 deg Celsius.
(This was degree of superheat is set through design of the superheater banks and control of water injection at the 1st attemperator for multi inter-stage attemperator design)