Power Plant Engineering

STEAM POWER PLANT 153

(i) The efficiency of a hand fired furnace is low.

(ii) Attending to furnace requires hard manual labour.

(iii) Study process of fuel feed is not maintained.

Cleaning of hand fired furnaces may be mechanized by use of rocking grate bars as shown in

Fig. 4.12. The grate bars loosen the slag and cause some of it to drop together with the ash into the

bunker without disturbing the process of combustion.

Lever

Grate bars

Fig. 4.12. Rocking Grate Bars.

4.5.2 Mechanical Firing (Stokers)

Mechanical stokers are commonly used to feed solid fuels into the furnace in medium and
large size power plants.

The various advantages of stoker firing are as follows :

(i) Large quantities of fuel can be fed into the furnace. Thus greater combustion capacity is

achieved.

(ii) Poorer grades of fuel can be burnt easily.

(iii) Stoker save labour of handling ash and are self-cleaning.

(iv) By using stokers better furnace conditions can be maintained by feeding coal at a uni-

form rate.

(v) Stokers save coal and increase the efficiency of coal firing. The main disadvantages of

stokers are their more costs of operation and repairing resulting from high furnace

temperatures.

Principles of Stokers. The working of various types of stokers is based on the following two

principles:

1. Overfeed Principle. According
   to this principle (Fig. 4.13) the pri-
   mary air enters the grate from the bot-
   tom. The air while moving through
   the grate openings gets heated up and
   air while moving through the grate
   openings gets heated up and the grate
   is cooled.
   Fig. 4.13. Stokers