Feedwater heaters in Combined Cycle power plants 2

[jgeorge747]

Please advise as to why combined cycle power plants (incorporating gas turbines and steam turbines) do not need or use feedwater heaters in comparison to conventional(rankine cycle) plants which use feedheaters to optimise the quality of water entering the boiler.

Many Thanks.

 

[metengr]

The quality of water has nothing to do with feedwater heaters. It is strictly cycle efficiency and cost. For a conventional steam power boiler, being able to preheat the feedwater entering the boiler saves on fuel costs. Per pound of water entering the boiler, the higher the feedwater temperature the less fuel is needed to generate steam. You try to capture and use every bit of energy from the cycle to improve overall cycle efficiency.

For HRSG's, I would think that the cost benefit analysis would probably not justify payback for the capital cost of purchasing and installing separate feedwater heaters in the cycle.

 

[jgeorge747]

Dear Metengr
Many thanks and appreciation for your help.

Regards
jgeorge747

 

[rmw]

Your HRSG stages in a sense are the feedwater heater string, except that instead of heating the FW with extracted steam from the ST, you are heating it with the energy already available in the GT exhaust.

The FWH's in a standard steam cycle are a penalty to the process, notwithstanding the benefit that they accomplish as outlined by Metengr, in that the extracted steam is unable to continue down through the steam turbine and produce power.

There is enough energy available in the GT exhaust to heat the incoming feed water to what it needs to be without taking any steam away from the ST.

rmw

 

[MJCronin]

rmw,

You hit it on the head......an excellent response !!

The best and briefest explanation is "Why extract that steam from the turbine (with the large capital cost, water chemistry and other expensive issues) when there is still all that hot gas available ?

My opinion only !!!

MJC