Fig. 20 shows the application of a Wilgus oil burner to an upright boiler. A 3 in. tube is expanded through the water leg of the boiler, in which the burner is placed. A ring of fire brick about 14 in. in height is placed around the sides of the furnace ; fire brick are also placed about 1 in. apart on top of the grate bars. By this arrangement the boiler can read ily be used for wood or other fuel, by simply remov ing the burner. Many plants burning oil use this type of boiler for getting up steam required for pumping systems and burners operating the main boilers, after having been closed down on Sundays or holidays.
Much better furnace efficiency can be secured by having an oil burning expert inspect the furnaces when a change to oil fuel is contemplated. Such an expert will design a furnace front without expensive grate bars, bearing bars, or other fittings that a sales man might be anxious to install, and at the same time provide a furnace from which the highest efficiency may be secured. The vital importance of correct de sign and construction of furnaces will be further em phasized in a chapter on combustion.
The required height of chimney for fuel-oil plants is much less than is ordinarily supposed when the boilers are operating at or below their rated capacity, and considerably greater than is usually supposed when there are heavy over-loads. As pointed out by Mr. C. R. Weymouth, of San Francisco, Cal., in a paper read before the American Society of Mechan ical Engineers, a chimney of undue height will take an excessive quantity of air for combustion and per mit an excessive load on the boilers, both resulting in a large waste of fuel. When the chimney height is limited to that necessary for economical air supply at the desired boiler load, it will be impossible for the most careless fireman to cause serious waste of fuel, either by supplying excessive air, or seriously over loading the boilers. The chimney may thus become an important and inexpensive means of regulating boiler-plants, and an automatic safeguard against care less firing. Such service is, of course, most success fully secured only in plants operating at uniform load.
The San. Francisco earthquake of April 18, 1906, considerably reduced the height of most masonry chimneys, and resulted in an extensive collection of chimney data. Many of the results obtained appar ently were contradictory. Certain chimneys, reduced to a height of 30 ft., gave the usual boiler capacity ; and others, reduced only to a height of 75 ft., showed
under certain conditions of service a decrease in boiler capacity.
Altitude has an important bearing on chimney design. The error commonly made in the determina tion of stack capacities at high altitudes is to assume that a given grade of fuel at a fixed boiler rating will require at high altitude the same draft, measured in inches of water at the damper, as at sea level. It is evident that to develop a given boiler horsepower re quires a constant weight of chimney gases and air for combustion. As the altitude is increased, the den sity of the air is increased, and, correspondingly, its velocity through the furnace, the bed of coal, or the fire brick checkerwork. The boiler passes must there fore, be greater at high altitude than at sea level. The mean velocity for a given boiler horsepower and con stant weight of gases will be inversely proportional to the barometric pressure. And the velocity head, measured in column of external air, will be inversely proportional to the square of the barometric pressure.
For chimneys built at high altitude it is necessary to increase not only the height, but also the diameter. The increase in height causes an added frictional re sistance within the chimney ; this frictional loss must be compensated by a suitable increase in the diameter, and when so compensated it is evident that the chim ney height must be increased at a ratio inversely pro portional to the square of the normal barometric pres sure.
Based on 150 per cent as the ratio of actual boiler h.p. to rated boiler h.p. and assuming sea level atmos pheric pressure and 80 degrees F., the author pre sented the accompanying table of approximate maxi mum capacities measured in actual boiler h.p. These data apply to steel chimneys with short flues, the chim neys being centrally located over stationary B. & W. boilers. Other conditions are : Draft in inches at boiler outlet, chimney side of damper, 0.30; corre sponding excess air through boiler, per cent, less than 50; assumed excess air supply for determining boiler efficiency, chimney diameter and draft resistance of chimney and breeching, per cent, 50; assumed tem perature of gases leaving boiler, 525 degrees F.; as sumed temperature of gases entering chimney, 500 degrees F.; assumed boiler efficiency, working not test conditions, 73 per cent ; assumed pounds of chimney gases per actual boiler h.p., 54.6: