Document zbMn0QGLkEw4ym9Kodj2LMdmB

Heating Ventilating Air Conditioning Guide 1939 iThe ash layer becomes thicker with time and that near the retort, being exposed to temperatures which are high enough at times, fuses into a clinker. The temperature attained in the fuel bed, the chemical compo- ! sition and homogeneity of the ash, and the time of heating are factors ; which govern the degree of fusion. Bituminous coal stokers of the Class 1 type operate on the principle | of the removal of ash as clinker and clinker tongs are provided to facilitate this purpose. Typical representations of underfeed bituminous stoker i fuel beds are shown in Figs. 8 and 9. The appearance of such fuel beds is very ragged at times, and large j masses of coke build up, surrounded by blowholes with intense white i flame indicating the presence of excess air. There is a natural tendency for users to disturb the fuel bed and make it conform to the conventional Fig.'8. Cross-Section of Fuel Bed with Weakly Coking Coal IFig. 9. Cross-Section of Fuel Bed with Strongly Coking Coal 3 representation or ideal fuel bed. Such attention should not be required, as usually the fuel bed tends to correct its own faults as the cycles of plasticity, coke tree formation and ash fusion recur. There are a number of factors, which materially affect the rate and type of combustion obtained in stoker usage, the most important of these being:- the type and design of stoker, the type and characteristics of the fuel, the method of stoker installation and the method of stoker operation. Furnace Design The burning of the fuel on the grate or in the retort will be influenced directly by the stoker design. The burning of the volatile gases above the fuel bed is a matter of furnace design! Proper care should be taken to provide furnaces sufficiently liberal in volume and with the grates or retorts at a sufficient distance from the heating surface to permit, proper combustion of the gases. Smoke and low efficiency will result if the furnace is too small to permit proper mixing of the gases and completion of combustion. 216 Chapter 11. Automatic Fuel Burning Equipment The standard that has been most commonly used for the proportioning t furnaces for bituminous coal stokers is the code of the Steel Heating gX lnsUtute (see Chapter 13). Furnace volume is not an important item in anthracite stoker instal' 1 tions. Due care should be exercised for both anthracite and bituminous tokers to prevent intense heat application on the metal surfaces of the combustion chamber. The installation of a baffle or adjustment in setting height of the stoker may be desirable in some cases. The prime essentials of good furnace design are: correct proportions, moderate combustion rate, adequate furnace volume and sufficient flame clearance. If these factors are properly compensated for and provision is made for the proper mixing of the gases bituminous coal stokers will operate smokelessly. In those stokers which are operated intermittently, however, some smoke may be produced during the off periods. Combustion Adjustments Satisfactory stoker performance may be secured by regulating the coal feed and the air supply so as to maintain, as nearly as possible, an ideal balance between the load demand and the heat liberated by the fuel. When the coal is consumed at about the same rate as that which it is fed this balance exists and uniform fuel bed conditions will be found. Under such conditions no manual attention to the fuel bed should be required other than the removal of clinker in those stokers which operate on this principle of ash removal. Since complete combustion is not'obtained in stoker furnaces receiving only the air theoretically required, it is necessary, even under the best of conditions, to supply from 30 to 50 per cent excess air to obtain desired combustion results. Due to the variable characteristics of solid fuels in burning, consideration must be given to a number of factors which affect the maintenance of the combustion conditions wanted. The specified rate of coal feed of a stoker may vary due to changes in the bulk density of the coal dependent upon: (a) the size of coal, (b) dis tribution of size in the coal, (c) segregation of coal in the stoker hopper, and (d) friability of the coal. The following factors may affect the rate of air supply: (a) changes in fuel bed conditions and resistance, (6) changes iii furnace draft due to a variety of causes, i.e., changes in chimney draft because of weather changes, seasonal changes, back drafts, failure or inadequacy of auto matic draft regulator, use of chimney for other purposes, possible stoppage of the chimney and changes in draft resistance of boiler due to partial stoppage of the flues, and (c) changes in air inlet adjustments, to the fan. Many domestic bituminous stokers now incorporate some method of automatic control which compensates for changes in fuel bed resistance. Since a secondary source of air due to leakage is present in most installa tions, the use of an automatic draft regulator to maintain the furnace draft at about 0.05 in. of water is desirable. This is quite important with intermittently operated stokers. Some fuel is burned by natural draft in the off periods, when fuel is not being fed, and it is essential that the burning in these periods be controlled. With excessive draft, due either 217