Turbine exhaust pressure for Combined Cycle Operation

[swapanburo]

I am involved with the design of a Combined Cycle Plant.
The Steam Turbine exhaust is condensed in an air cooled condenser (ACC). The fan control of the ACC is done by measuring the turbine backpressure and comparing that with the turbine backpressure set point . The pressure controller adjust the number/speed of fans to reach the set point.
My question is how the turbine backpressure set point is selected for a combined cycle plant ? In winter the steam turbine exhaust pressure can be made lower for higher output from the steam turbine. Is this change in exhaust pressure set point with ambient condition done manually or automatically via an algorithm set up in the turbine control system ? What is the normal practise ?
Can anyone enlighten me on the above issue ?
Thanks,

 

[rmw]

The back pressure is going to be set by the condenser duty you select. Make the condenser large ($$$$) and the back pressure will be lower-maybe even too low in the winter time. Make the condenser too small, and your back pressure will never get low enough ($$$$).

You need to do some basic calculations regarding the heat rejection that you want (duty) and let the condenser manufacturers size a condenser according to the maximum/minimum ambient conditions you give them.

Also, check with the steam turbine manufacturers as to what their upper and lower back pressure limits are and ask the condenser manufacturer what provisions there are to keep the back pressure from getting too low in the winter.

rmw