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#BoilerManual #AirAndGasFlow #Section3 #Page26

maintenance of safe clearances, it must be regulated within permissible limits by some means of accurate control.

Due to these turbine requirements, steam temperature must be controlled to correct fluctuations caused by operating variables. Probably the most frequent cause necessitating the need for control in regulating steam temperature is the deposit of ash in some form on the surface of saturated and steam heating elements. This condition can usually be corrected by simply changing the operating sequence and frequency of sootblowing.

The removal of feedwater heaters for servicing may be cited as another instance off variable operation. To maintain the same load, with the feedwater heaters out, requires an increase in the heat input to the boiler unit, with a corresponding increase in superheater or reheat absorption. Unless some means of control is available for regulating steam temperature to cover this condition, a drop in load might be required to protect both thte boiler tube metals and the turbine.

Flue Gas Recirculation for Steam Temperature Control

One method of steam temperature control is by the use of flue gas recirculation. Flue gas exiting the boiler is recirculated with a suitable fan and flue to the furnace. The flue gas is introduced in such a manner that furnace heat absorption is reduced by shielding a substantial part of the furnace wall area with a layer of recirculated gas. This results in making more heat available for superheater and reheater absorption.

In order to minimize fan erosion, specific requirements have been set as to location, direction and velocity of the recirculated flue gas takeoff. This will minimize the carryover of large ash particles into the recirculated gas stream and hence into the recirculated gas fan.

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#BoilerManual #AirAndGasFlow #Section3 #Page26

maintenance of safe clearances, it must be regulated within permissible limits by some means of accurate control.

Due to these turbine requirements, steam temperature must be controlled to correct fluctuations caused by operating variables. Probably the most frequent cause necessitating the need for control in regulating steam temperature is the deposit of ash in some form on the surface of saturated and steam heating elements. This condition can usually be corrected by simply changing the operating sequence and frequency of sootblowing.

The removal of feedwater heaters for servicing may be cited as another instance off variable operation. To maintain the same load, with the feedwater heaters out, requires an increase in the heat input to the boiler unit, with a corresponding increase in superheater or reheat absorption. Unless some means of control is available for regulating steam temperature to cover this condition, a drop in load might be required to protect both thte boiler tube metals and the turbine.

Flue Gas Recirculation for Steam Temperature Control

One method of steam temperature control is by the use of flue gas recirculation. Flue gas exiting the boiler is recirculated with a suitable fan and flue to the furnace. The flue gas is introduced in such a manner that furnace heat absorption is reduced by shielding a substantial part of the furnace wall area with a layer of recirculated gas. This results in making more heat available for superheater and reheater absorption.

In order to minimize fan erosion, specific requirements have been set as to location, direction and velocity of the recirculated flue gas takeoff. This will minimize the carryover of large ash particles into the recirculated gas stream and hence into the recirculated gas fan.

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#BoilerManual #FluidCirculation #Section2 #Page26

From the second pass inlet header, flow is straight up the second pass side wall and discharges into the side wall outlet headers located in the penthouse (Figure 21).

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Alt = Labeled Figure 22 Upper furnace circuitry. Simplified line drawing of the topmost part of the furnace but stopping where the Secondary SuperHeater (SSH) ends, and the Screen tubes of the SSH are pointed out where the furnace top meets the beginning of the SSH.

#BoilerManual #FluidCirculation #Section2 #Page26

From the second pass inlet header, flow is straight up the second pass side wall and discharges into the side wall outlet headers located in the penthouse (Figure 21).

------------------------------------------------- 26 ------------------------------------------------------
Alt = Labeled Figure 22 Upper furnace circuitry. Simplified line drawing of the topmost part of the furnace but stopping where the Secondary SuperHeater (SSH) ends, and the Screen tubes of the SSH are pointed out where the furnace top meets the beginning of the SSH.