Power Plant Engineering

STEAM POWER PLANT 157

steam demand. The boiler operation becomes more flexible and better efficiency of combustion is
achieved. This saves manual labour also.
Hagan system of automatic combustion control is shown in Fig. 4.19. Master relay R 1 , is sensi-
tive to small vanations in steam pressure and is connected to steam pressure gauge.

Boiler

Combustion

Chamber

Stoker

Draft

Connections

Fan Vanes

Fan

Vanes

Stoker

Motor Hand Regulator

Air supply

Air supply

Relay

Air supply

Master

Relay

Steam Pressure

Hand Gauge

Regulator

Secondary

Air Fan

I. D.

Fan

F. D. F a n

R 1

Fig. 4.19. Hagan System of Automatic Combustion Control.
A fall in pressure operates the master relay R 1 which in turn operates the servomotor coupled to
the vanes of the induced draught (LD) fan to open them slightly and simultaneously the secondary air
fan damper gets opened proportionately. By this readjustment of induced draught takes place and stabi-
lized conditions in the combustion chamber get changed. These changes operate relay R 2 to alter the
position of forced draught fan servo-motor to adjust the position of forced draught fan vanes so that
stable conditions in combustion chamber are maintained. This change causes more air to flow through
passage which in turn operates relay R 3. This causes stoker motor to supply extra fuel into the furnace.
In case of an increase of pressure of steam the above process is reversed. Hand regulators are provided
to servo motors and master relay for manual control of system.

4.7 Pulverized Coal

Coal is pulverized (powdered) to increase its surface exposure thus permitting rapid com-

bustion. Efficient use of coal depends greatly on the combustion process employed.

For large scale generation of energy the efficient method of burning coal is confined still to

pulverized coal combustion. The pulverized coal is obtained by grinding the raw coal in pulverising

mills. The various pulverising mills used are as follows:

(i) Ball mill (ii) Hammer mill

(iii) Ball and race mill (iv) Bowl mill.

The essential functions of pulverising mills are as follows: (i) Drying of the coal (ii) Grinding

(iii) Separation of particles of the desired size.

Proper drying of raw coal which may contain moisture is necessary for effective grinding.