superheater design

[JHJ]

we have to design superheater in a waste heat boiler where gas inlet temp is constant at all loads .
superheater to perform from 100 % load to 15 % load .
100,000 kg/hr 45 bar saturated to 440 oc [normal load ]
to perform at 20 % load also
gas temp is constant at all load
gas flow reduses at 15 % load
at 15 % load area is too large ,hence control is problem ,
what design /control you suggest

 

[davefitz]

conventional low cost solution is to use a spray attemporator , but one must be certain that the water purity is adequate to avoid downstream turbine deposits.

If spray water attemporator use is not desireable, one can use a steam bypass of part of the superheater. This lowers the heat trasferred to the banks with the lowered mass flowrate. At the location where the steam flows will mix, there will be generated thermal stresses in the tee junction that need to be managed.

 

[athomas236]

hello,
never seen anyone try to control temperature over such a wide range of loads. normal custom is 100%mccr t0 70%.
athomas236

 

[hacksaw]

this very same question came up several months ago,

steam bypass is the usual approach,but at sufficient turn down you need to be looking for a way to dump steam to lower pressure headers or to dump it to atmosphere.

be careful of attemporating because you are creating more steam down stream of the superheater. this has the effect of reducing SH duty and compounding your problems.

depending on your process, you may have to consider hot gas bypasses. This will buy you a little flexibility.

you'll need a great deal more instrumentation, controllers and valves. not a cut and shoot design problem, but one that requires careful integration with the mechanical set up and operations. Good luck