SELECTION OF STEAM GENERATING EQUIPMENT Steam generating equipment may be considered as comprising not only the boiler and furnace but all auxiliaries necessary or desirable from the point of delivery of the coal to the stop valve on the boiler. A proper evaluation of factors which determine the correct selection of the necessary or desired equipment can only be made by an engineer thoroughly familiar with all the factors, their relations and a knowledge of costs, values and economics.
In a modern utility plant the steam end may and frequently does include in addition to the boiler a water-cooled furnace— steam superheaters and re-heaters—mechanical stokers of various types ; burners for pulverized coal or oil ; economizers and air preheaters; induced and forced draft fans or blowers and their necessary motor or turbine drive ; ducts and flues for conducting the air and flue gas to desired points ; chimney and dampers ; cinder or dust catchers; boiler feed pumps and feed water regulators; soot blowers for the boilers; economizers and air-heaters; con densers ; blow down valves ; heat inter-changers to abstract heat from boiler blow down; open or closed feed water heaters; de aerators ; a feed water treating plant or evaporators.
The complete coal and ash handling system from the railroad siding to the coal bunkers may include a car dumper, track hopper, screens, crushers, magnetic separators, conveyors, skip hoists, locomotive-crane and all equipment necessary for storing and reclaiming coal. If coal is pulverized there will be dryers, pulver izing mills with piping collectors, conveyors, feeders, burners and necessary motors. Ash may be handled by cars, conveyors or by water in sluices or grab buckets. Other auxiliary equipment may include a combustion control system with elaborate inter-locking of the electrical equipment. Pressure and draft gauges, ther mometers, steam flow meters and gas analysers, all of the indicat ing or recording type, will also be found.
The problem of the selection of proper equipment may be very simply stated. Equipment should be selected that will pro
duce steam at the lowest total cost for the expected life of the plant, with the required reliability. The weight given to various factors will differ in various types of plants. In public utility plants where the fuel cost is about half the total cost of operation greater weight is given to economy, this factor being modified by the cost of fuel. In some industrial plants operating only ten hours per day with cheap coal, first cost may be a major factor. There are certain processes requiring continuity of steam pressure or power in which a failure of either or both for a few minutes may ruin material valued at thousands of dollars. In such cases reliability usually secured by duplication or excess spare capacity should be one of the major factors considered. The total Cost of steam generation is made up of fixed or capital costs and operating costs. The fixed charges or costs are made up of interest on first cost, taxes, insurance, depreciation or obsolescence. The operating costs include fuel; water ; maintenance material-repairs ; supplies of oil, tools and chemicals ; operating labour; maintenance labour; clerical salaries and operating supervision. Generally, by increas ing the capital cost, the operating cost may be reduced. High efficiency can be obtained by high capital expenditure. However, depending on the fuel cost and many other factors, a point is reached where the fixed charges of additional equipment equal the saving obtained by that equipment and any additions beyond that point result in fixed charges in excess of savings.
One of the important factors having a major bearing on economy and in capital costs is the plant capacity factor, some times named load factor. The capacity factor is the total load per year expressed in any suitable units divided by the rated or normal capacity of the units times 8,76o hours per year or the relation of the actual annual load carried to the annual load if units were operated at normal capacity every hour of the year.