Document RjKOOJj12x86qKbKRXe00zXOa

Heating Ventilating Air Conditioning Guide 1938 As more coal is fed from below the mass of coke continues to grow .forming a coke tree, plug or spar as it is variously designated. After a period of time, dependent upon the strength of the coke formed, pieces of the coke tree break off and fall upon the hearth surrounding the retort or within the retort itself where they are burned. While part of the ash fuses into particles at the surface of the coke as it is released, most of it is freed in unfused flakes or grains. The greater part of this unfused ash remains on the hearth or dead plates although a part may be expelled from the furnace with the gases. The 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 removalof ash as clinker and clinker tongs are provided to facilitate this purpose. Typical representations of underfeed bituminous stoker fuel beds- are shown in Figs. 8 and 9. The appearance of such fuel-beds is very ragged at times, and large masses-of coke build up, surrounded by blowholes with intense white; flame'indicating the presence of excess air.- There is a natural tendencyfor users-to disturb the fuel bed and make it conform to the conventional 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, free 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 thesebeing: -the type and design of stoker, the type arid characteristics of the. fuel, the method of stoker installation and the method of stoker operation. 210 Chapter 11. Automatic Fuel Burning Equipment 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. Table 1. Recommended Setting. Heights for Heating Boilers Equipped with Mechanical Stokers2 Fibebox Boxlsbs ` Actual Loadb 2500 5000 7500 10000 12500 15000 20000 25000 30000 A 18" 18" 20" 20" 22" 22" 24" 24" 24" B 42" 48" 54" 60' 66" 72" 78" 84" 84" A - Distance from bottom of Water Leg to floor. B= Distance from Crown Sheet to bottom of Water Leg. Compact Welded Boilebs Actual Loadb 2500 5000 7500 10000 12500 15000 20000 25000 30000 A 18" 18" 20' 20" 22' 22" 24" 24" 24" B 30" 33" 36" 42" 45" 48" - 54" 60' 60' A - Distance from bottom of Water Leg to floor. B = Distance from Crown Sheet to bottom of Water Leg! H. R. T. Boilebs . Hp 50. 75 . .100 125 150 175 200 225 A 5'-0" 5'-6" 6/-0'r 6'-6T. 7'-0" - 6* 7t6" 8'rO" A Distance from bottom of shell to floor. Hp = Installed horsepower. 250 ; 275 8';6" . 9'-0" " 300 9'-0" In the case of the Firebox or Compact Welded type boilers the desired setting height can be obtained by combining A and B dimensions. The load ratings shown for this class of boilers are actual developed loads in square feet of equivalent cast-iron steam radiation and are not manufacturers' ratings. The setting heights given for H. R. T. boilers may be used for developed loads up to 50 per cent above normal rating. ' ........... "From Data prepared by the Mid-west Stoker Association. ...... . ^Expressed as steam radiation, 1 sq ft = 240 Btu. ` _ The standards that have been rriost universally adopted for the. propor tioning of furnaces for bituminous coal stokers are those of the Midwest Stoker Association and the Steel Heating Boiler Institute. See Chapter 13. Table 1 gives recommerided setting heightsior:heating boilersequipped with mechanical stokers using bituminous coal. Furnace volume is not an important item in anthracite stoker instal lations.; ..Due care should be exercised for both anthracite and:bitutninous stokers. to: prevent intense heat application on the -metal surfaces -of The combustion chamber. The installation of a baffle orjadjustment.in setting height of the stoker.may be desirable in some cases. -- -. .-.-.5.7 2tt