Why Electric Power Decrease in Thermal Power Plant When Heater Out of Service?

Please try to use better diction, you will get better answers.

If a closed feedwater heater is removed from service in an operating thermal electric power plant, more steam will be passed through the turbine. This will enable the generation of more power.

However, when the feedwater heater is removed from service, this will cause the plant heat rate to INCREASE, thereby causing the plant owner to pay mote for fuel.

Review the Rankine cycle.

It is all explained here:

http://en.wikipedia.org/wiki/Feedwater_heater.

What exactly do you mean by "thermal power" ?.... do you mean thermal efficiency ?

Dear Mr.awaiscnpp

If the Feed Water Heater or De-Aerator, is taken out of service, even though the fuel supply and combustion remain same, the Enthalpy in the Feed Water is reduced while entering the Boiler for transforming to Steam. Hence the Steam Flow to the Turbine is reduced and hence the Electrical Power out put is reduced.

For example, in a 90 Tonne/Hr., 64 Kg/cm^2, 495 Deg.C Boiler, when the Feed Water Temp is reduced, from 120 Deg.C to 98 Deg.C,due to De-Aerator problem and taken out of service, Deg.C., 1 MW Power (Elec.Power Output) is Lost.

DHAYANANDHAN.S

If you have been advised to reduce load by the manufacturer when taking a heater out of service it is in the interest of reducing bucket/blade loading because more steam can be forced through the turbine than it was originally designed to pass, thus higher load capability that may affect thrust, buckets and generator windings etc.