Application Guide

63-7062 44
Boilers and Forced Draught Fan (See Fig. 53)
Boiler
M
Pilot Gas
Gas
Control
Valve
Pressure
Regulator
Manual
Valve
Main Gas
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Fig. 53. Boiler with forced draught fan application.
Application Description
On large boilers modulating control is used to regulate the boiler output to match the building load. The steam pressure or hot
water temperature is controlled by varying the volume of fuel (gas or oil) fed to the burner.
To maintain efficient combustion the volume of air supplied to the burner is regulated, relative to the amount of fuel. A simple
combustion control system adjusts the air flow by means of a damper and linkage connected to the same modulating motor
that adjusts the fuel supply. Fitting a VFD to the motor of the forced draught fan allows the fan speed to be varied relative to the
volume of air required, giving more precise control and ensuring efficient combustion is achieved. Only when the boiler is at
maximum load will the forced draught fan be running at full speed.
Other Applications
— Process furnaces.
— Incinerators.
VFD Features Used
— Power failure recovery.
— Auto restart on trip.
— Over speed.
Benefits
— Reduced electrical energy.
— Improved combustion control.
IMPORTANT
Careful commissioning is required to ensure efficient combustion and that emissions are kept under control.