FIG. 147.-Oil fired steam heating station, Pacific Gas and Electric Company, station S, San Francisco. B. & W. boilers are to the left, fuel oil pumps and heaters in the center background, feed water pumps and heater to the right.

For stoker fired boilers the design of furnace to be adopted will depend largely on the kind of stoker, and the arrangement of coal furnace and ashpit. If the use of coal is to be abandoned altogether the stokers should be removed, and an oil furnace installed of the general design indicated in Chapter XX. If the boilers are provided with basement ash pits advantage should be taken of this to increase the furnace volume by placing the furnace floor below the level of the fireroom floor, allowing the air to come up through the ashpit from the basement.

If oil is to be used only temporarily, and it is expected at some future time to go back to coal burning, it will be possible in many cases to leave the stokers in place, placing the oil burner at the rear of the furnace, and protecting the stoker from the heat by means of firebrick supported by structural material, leaving a dead air space between the stoker and the firebrick. The practicability of this arrangement will depend on the type of stoker, kind of boiler, and space available, and each case requires special study to secure the proper design.

The question whether steam atomizing or mechanical atomizing burners should be adopted will depend on local conditions. In general it may be said that steam atomizers should be used wherever they are applicable, as they have proved their practical value by years of application to stationary work. In special cases where steam atomizers are unsuitable the mechanical burner may be used to advantage. This would include plants so located that the waste of fresh water is a serious matter. Plants in which it is necessary to force the boilers up to 300 or 400 per cent. of their rated capacity may also find the mechanical atomizing burner more suitable, and this will be especially true if a steady load is carried and if the plant is already equipped with forced draft apparatus.

Number of Men Required for Operating Oil Fired Boilers.The number of men required to operate boilers fired by oil is much less than the number required to operate a coal burning plant. In an oil burning central station a fireman can operate six or seven large boilers having three oil burners each, and in addition attend to the feeding of the boilers with water. In other words, a plant having 26 or 28 boilers would require only four firemen on a watch besides a man to look after the feed pumps, oil pumps and keep records of oil consumption, temperatures, etc.

CHAPTER XXVI

THE GRAVITY OF OILS IN FUEL OIL PRACTICE

Fuel oil is classified, marketed, and designated by its gravity. Gravity is denoted in two distinct ways. The scientific method of notation is known as the "specific gravity," which is the ratio of the weight of a given volume of the oil to that of an equal volume of pure water. There has, however, grown up in practice an empirical method of representing the gravity of oil by what is known as the Baumé scale. This scale has two separate and distinct formulas for its conversion to specific gravity readings. One formula is for liquids heavier than water and the other for liquids lighter than water. In each instance the scale is graduated to 100 degrees and overlaps 10 degrees.

The use of the Baumé scale should be abandoned as it is not only unscientific, but confusing. However, as its use is universal in the oil industry, and it has obtained such a firm foothold among both producers and users of fuel oil, it is described and used freely throughout this book.

Antoine Baumé, a French chemist of the eighteenth century, distinguished for his success in the practical application of the science, was the inventor of the so-called Baumé scale now universally adopted in fuel oil practice for denoting the gravity of crude petroleum.

FIG. 148.-Baumé hydrometers.

The Scale for Liquids Heavier Than Water.Baumé hit upon a unique plan for the establishment of his scale. Certain fixed points were first determined upon the stem of the instrument. The first of these was found by immersing the hydrometer in pure water, and marking the stem at the level of the surface. This formed the zero of the

scale. Fifteen standard solutions of pure common salt in water were then prepared, containing respectively 1, 2, 3......15 per cent. (by weight) of dry salt. The hydrometer was plunged in these in order and the stem having been marked at the several surfaces, the degrees so obtained were numbered 1, 2, 3......15. The instrument thus adapted to the determination of densities exceeding that of water was called the hydrometer for salts.

Expressed mathematically in its relationship with the specific gravity S, the Baumé degree reading B becomes for liquids heavier than water:

The Scale for Liquids Lighter Than Water. Since practically all grades of crude petroleum are lighter than water, we are most interested in the method of expression for this latter phase of gravity denotation.

The original Baumé hydrometer intended for densities less than that of water, or the hydrometer for spirits, as it was called, was constructed on a similar principle to that for the hydrometer for salts above

described. The instrument was so arranged that it floated in pure water with most of the stem above the surface. A solution containing 10 per cent. of pure salt was used to indicate the zero of the scale, and the point at which the instrument floated when immersed in distilled water at 10°R. or 541⁄2°F. was numbered 10. Equal divisions were then marked off upwards along the stem as far as the 50th degree.

The Confusion in Expression for Specific Gravity and Baumé Readings. Modern gravities are expressed for liquid temperatures of 60°F. instead of 5412°F. as above set forth. This fact together with other inconsistencies and errors in observation have led to the invention of some seventeen different mathematical expressions, by various investigators and scientific bodies,