STEAM BOILERS 105 



lining of the boiler front, and a resting place on which bitumi- 

 nous coal may be coked before it is placed on the fire. To 

 support the grate bars, the inner edge of the dead plate is 

 either beveled or a lip is provided, as at a, Fig. 4. 



Objection to Stationary Grate Bars. The greatest objec- 

 tion to stationary grate bars is that with them the furnace 

 door must be kept open for a considerable length of time 

 when the fire is being cleaned. Cleaning fires when the boiler 

 has a stationary grate not only severely taxes the fireman, but 

 the inrush of cold air chills the boiler plates, thus producing 

 stresses that in the course of time will crack them. 



Shaking Grates. There are on the market many designs of 

 shaking grates for large steam boilers, differing chiefly in detail 

 and arrangement. Usually the grate bars are hung on trun- 

 nions at each end and are connected together with bars to 

 which are attached shaking rods that extend forwards through 

 the furnace front. Levers or handles are attached to the 

 shaking rods, and by working them back and forth the grats 

 bars receive a rocking motion that breaks up the bed of coal 

 on the grate and serves to shake the ashes through into the 

 ash-pit. The fires may thus be kept clean without the neces- 

 sity of opening the fire-doors. 



Classes of Mechanical Stokers. A mechanical stoker is a 

 power-driven rocking grate arranged so as to give a uniform 

 feed of coal and to rid itself continuously of ashes and clinkers. 

 The principal designs of mechanical stokers and automatic 

 furnaces may be divided into two general classes, overfeed and 

 underfeed. 



Overfeed Stoker. In overfeed stokers the fixed carbon of 

 the coal is burned on inclined grates. The coal is pushed on 

 to these grates, which are given a sufficiently rapid vibratory 

 motion to feed it down at such a rate that practically all 

 the carbon is burned before reaching the lower end, where 

 the ashes and clinkers are discharged. In Fig. 5 is shown 

 a sectional view of a stoker of this class. The coal is fed into 

 the hopper a, from which it is pushed by the pusher plate b 

 on to the dead plate c, where it is heated. From c it passes 

 to the grate d. Each bar is supported at its ends by trunnions 

 and is connected by an arm to a rocker bar i, which is slowly 



