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for this purpose and it should be so constructed as to thoroughly atomize the oil. Air should be admitted around the burner, giving complete combustion by passing through the flame.
The second type is called the "rotary" kiln, shown in figs. 96 and 97. This type had been universally adopted for the burning of Portland cement, and its proven efficiency showed its adaptability to lime burning when the lime is to be ground and hydrated. The lime produced in the rotary kiln is broken into fine pieces and consequently is not desirable for building lime. The size of the rotary kiln for lime burning is regulated to a great extent by the desired capacity, and there is a difference of opinion as to the proper diameter and length to secure the most economical results. The idea of the rotary kiln was first conceived by Crampton in 1877, but no practical application was made till Ransom patented his design in England in 1885. The rotary kiln is really nothing more than a plain cylindrical tube supported by four or five sets of heavy roller bearings and driven by a train of gear wheels The revolving speed is controlled by regulators and is from 1 to 21⁄2 R. P. M., depending upon the material to be burned. The tube is inclined towards the discharge end at an inclination of one to twenty-five. The rotation of the cylinder by reason of its inclination slowly advances the material toward and out of the lower end. The most popular sizes of rotary kilns for the larger plants are 7 to 71⁄2 feet in diameter by 100 to 125 feet in length.
The great Air Nitrates plant at Muscle Shoals, Alabama, built by the government, has 8 by 125 ft. rotary kilns for burning the lime used in the electric furnaces in the first step of the process. Repairs are very low in the moving parts of a rotary kiln and its life is about 20 years.
The disposal of the large quantities of lime sludge that are daily produced in the causticizing operation by those pulp mills using the soda or the sulphate process and by the alkali works, has long been a serious problem. The lime has so much actual value that it should not be thrown away. The price paid for lime probably averages $4 per ton f. o. b. plant, and the cost of disposing of the waste sludge is at least 25c per ton. This means that the waste sludge has a value of $4.25 per ton if burned back to lime. About 1900 the western beet sugar plants began to use the rotary kiln for re-burning their spent lime. The results have been perfectly satisfactory and today such installations are common. Lime sludge can be re-burned far more cheaply than new
lime can be bought. It is, of course, impossible to re-burn the same lime indefinitely, as it gradually becomes contaminated from constant use, mainly from the linings of the kilns. The customary practice is to introduce a certain quantity of new lime into the circuit periodically. This usually amounts to about 15% of the lime used. The quality of the recovered lime depends to a great extent on the quality of the original stone from which it was produced.
Portland cement is manufactured from a mixture of materials containing lime and silica in definite proportions. The raw materials are usually limestone in some form and clay or
FIG. 97. Oil-Burning Rotary Cement Kiln.
shale. The materials are pulverized raw and mixed either in the form of a dry powder or in a wet condition and are then delivered to the rotary kiln in which the required chemical changes take place. The temperatures required for burning cement clinker are from 2,800 to 3,000 degrees F. To withstand these high temperatures a lining having high refractory qualities must be employed. It must also have the quality of withstanding decomposition by the chemical action taking place within the kiln.
During the passage of the mixed material through the kiln there are two stages of physical and chemical changes. Water and carbon dioxide are driven off at an average temperature of
1,800 degrees F. in the first stage and in the second the burned mass is fused to clinker at high temperatures.
The rotary kilns used in cement plants vary in dimensions, but the tendency is toward greater length and diameter. In 1890 they were about 4 feet in external diameter and 40 feet long. At present they are 8 to 12 feet in diameter and 200 to 275 feet long. The economy of the kiln has been greatly increased by increasing its length and this is in part due to the carbon dioxide being driven off from the materials before they reach the combustion zone in the kiln and in part to the reduction of heat losses. With long kilns the average amount of oil required to burn one barrel of cement is 11 gallons.
In handling oil for fuel it is necessary to provide storage tanks of sufficient capacity to keep the plant running for a reasonable period of car blockade or other possible failure of the source of supply. They must also be erected far enough from the rest of the plant to avoid fire hazard and yet sufficiently near to eliminate long pipe lines. For unloading from tank cars it is usual to provide a steel or concrete sump, to which the oil is emptied directly and from which it flows by gravity or is pumped to the storage tanks. The latter in turn connect with, say, 1,000gal. "measuring tanks," from which the daily supply is taken into the plant. Further pumps, then, must be provided to send the oil to the kiln burners under pressure, or the same effect can be produced by gravity if a side hill unloading and storage sufficiently above kiln level is feasible. Where oil pumps are used, it is desirable to have them in duplicate, and also to have a duplicate or ring system of piping, so that any section can be cut out or bypassed if repairs become necessary.
Before being admitted to the burner spray nozzles, the oil must have its temperature raised sufficiently for atomizing in a steam heater designed for this purpose. The low-pressure system requires a blower, but the high-pressure system takes a compressor, about the same actual volume of free air being drawn through the intake in either case. The high-pressure system effects a much better atomizing of the oil but, of course, it costs considerably more for the compressor, motor, electric current and other running expense.
For each rotary kiln two oil burners or multiples thereof are ordinarily used, one being equipped with a round-point nozzle
and the other with a flat nozzle. The former is designed to throw the flame to the rear of the kiln and the latter to hold the flame near to the front. By this arrangement of nozzle units and proper regulation of the burner, the temperature can be accurately controlled at any point in the kiln.
The competitors of fuel oil at cement plants are natural gas, producer gas, and powdered coal, of which the last competes most actively. The advantages of oil over coal are obvious. It can be transported with much greater facility; no coal drying, grinding, or conveying machinery is necessary; the kiln can receive its supply of fuel in a minimum of time simply by turning a valve; and the supply can be regulated with the greatest ease.
HEATING PUBLIC BUILDINGS, HOTELS AND RESIDENCES
The increasing cost of coal and the uncertainty of obtaining a supply when it is most needed have brought about a steadily increasing use of fuel oil for the heating and lighting plants of public buildings, hotels, apartment houses, and private residences and for many domestic purposes.
A direct comparison of the cost of one million B. t. u. of coal and one million B. t. u. of oil is not an index to the desirability of burning oil under the boilers of these plants. It is necessary to consider also the freedom from smoke and dust which fuel oil burning insures and it is also necessary to remember that during the spring and fall months very little heat is required to provide a comfortable temperature in buildings. If coal is used, the consumption of fuel must continue after this temperature is attained, but oil burners can be shut off, stopping fuel consumption, when the desired temperature is reached. Table 24 gives the percentages of total fuel for the season required for the different months.
TABLE 24-MONTHLY FUEL REQUIREMENTS IN PERCENTAGES
The ease with which fuel oil burners respond to peak load demands for heat and light makes them especially desirable for office buildings. Elimination of the expense of ash removal when burning oil is also a point to be considered.
Fig. 98 shows the oil burner installation at the San Francisco Hospital. The San Francisco Hospital consists of ten buildings, costing $3,500,000, and is maintained by the City and County of San Francisco for the treatment of its sick poor. It has accommodations for 1,000 patients. It is the practice at the hospital plant to heat the oil to a temperature of about 270 degrees, forcing