Boilers and Steam Generators

The joints to be welded are carefully machined before welding and submitted to X-ray examination after welding under rigid inspection. The drums are then annealed to remove stresses.

In the United States the steel for riveted and forge-welded boiler drums must conform with the chemical and physical char acteristics given in the A.S.M.E. boiler code. Alloy steels with varying percentages of chrome, titanium, molybdenum, and other elements are used to take care of the higher temperatures and to keep plate thicknesses within reasonable limits for high pressure.

Large Units.

For large capacity units, the bent tube boiler has been found to give the most economical results. At low ratings, below 30o%, the straight tube boiler has been found to give lower exit temperature but above these ratings the bent tube boiler gives approximately the same and in some cases lower exit temperatures.

At high ratings the draft loss of the bent tube boiler is much less. It is customary in designing large boilers to place two bent tube boilers in the same setting and facing each other. With this method of construction, firing is usually done from two sides or from the four corners. The radiation losses are low, the gases are most thoroughly mixed before entering the boiler surface, the unit is compact and the drums are practically one-half the length that would be required for a single unit with firing from one side. Sin gle units to deliver 750,000 lb. of steam per hour and double units to deliver over i,000,000 lb. of steam per hour at 1,40o lb. per square inch pressure, and over 900° F. steam temperature, were in operation in 1939.

(See BOILER-MAKING ; BOILERS.) (J. B. C.) A mechanical stoker is a device for stoking or firing a furnace by mechanical means. In its broadest interpretation mechanical stoking would include the mechanical feeding of any kind of fuel to a furnace, but in common usage the term is limited to the firing of solid fuels such as coal and coke. The earliest form of mechanical stoker was an endless conveyor with two endless chains situated near the sides of the furnace and engaging sprock ets mounted on shafts situated at the front and rear. Transverse grate bars were attached to these chains to form the fuel supporting surface, one shaft was provided with means for driving the chains and grate so that the top surface through the furnace from front to rear. A hopper fixed at the stoker front

supplied fuel to the grate and natural or chimney draft provided air for combustion which was admitted through the grate and fuel bed. The speed of the grate was regulated so that the fuel was burnt out during its travel through the furnace and the resultant ash and refuse was discharged into a pit as the grate made the return bend around the sprockets on the rear shaft. A natural draft chain stoker is applicable to non-coking or free burning bituminous coals.

A modification of this type of stoker provides for the use of forced draft supplied by a fan or blower for admitting the air for combustion. With this provision, higher rates of combustion are made possible, and a wide variety of fuels may be burned successfully including anthracite fines, coke fines, lignite, and free burning bituminous coal.

The overfeed, inclined grate natural draft stoker is used to a limited extent for burning bituminous coals at moderate ratings under small boilers. The fuel is fed from the hopper usually at the front, to stepped grates which are rocked by mechanical means to impart a downward travel of the fuel during combustion and to deposit the ash and refuse at the rear, on dump trays which are lowered by hand at intervals to discharge the ash into a pit below.

In overfeed stokers of the spreader type the coal is fed from a hopper into the path of rapidly revolving paddles which throw the coal into the furnace where the fines are burnt in suspension and the coarser particles on a grate using forced draft.

Underfeed stokers, as the name suggests, introduce the fuel below the surface of the fuel bed. The volatile constituents of the coal are distilled off and pass up through the incandescent fire where they are rapidly ignited and burned without producing smoke. Forced draft is used with this type of stoker. This meth od of firing finds its greatest field for use with those bituminous coals which during combustion tend to form coke masses that must be broken up to insure uniform fuel bed conditions. Under feed stokers may be broadly subdivided into two general classes— single retort stokers and multiple retort stokers.