CALORIFIC VALUE In order to make a comparison between fuels, it is necessary to knOw the amount of heat which a given quantity of the fuel will give off when burned. The amount of heat which a given quantity of a fuel gives off is known as the calorific value or heat value. The standard measure of heat in this country is the British thermal unit. One British thermal unit is the amount of heat necessary to one pound of pure water from 62° F to 63° F. It is possible k to calculate the calorific value of a fuel from its elementary composition, but calculations which are based upon the ultimate analysis of a sample may be very misleading because the heat of combustion is dependent upon the state of combination of the elements in the substance, and is never equal to the sum of those of elements taken proportionately.
Determination of the calorific value of fuels is made by means of a calorimeter. In a calorimeter a weighed amount of fuel is completely burned, and the heat generated by the combustion is absorbed by a fixed weight of water, the amount of heat being calculated from the increase in the temperature of the water. A calorimeter, which has been accepted as the best for such work, is one in which the fuel is burned in a steel bomb filled with compressed oxygen. The function of the oxygen, which is ordinarily under a pressure of about 25 atmospheres, is to cause the rapid and complete combustion of The fuel sample. The fuel is ignited by means of an electric current, allowance being made for the beat produced by such currents and by the burning of the fuse wire. Among the standard calorimeters used are the Atwater, Mahler and Kroeker bombs. Fig. 9 shows the Mahler calorimeter. The apparatus consists of : A water jacket, A, which maintains constant conditions outside of the calorimeter proper, ,and thus makes possible a more accurate computation of radiation losses ; the porcelain-lined steel bomb, B, in which the combustion of the fuel takes place in compressed oxygen ; the platinum pan, C, for holding the fuel ; the calorimeter proper, D, surrounding the bomb and containing a definite weighed amount of ovater ; an electrode, E, connecting with the fuse wire, F, for igniting the fuel placed in the pan, C; a support, G, for a water agitator ; a thermometer, I, for temperature determination of the water in the calorimeter. The thermometer is best supported by a stand independent of the calorimeter, so that it may not be moved by tremors in the parts of the calorimeter, which would render the making of readings difficult. To insure accuracy readings should be made through a telescope or eyeglass ; a spring and screw device for revolving the agitator ; a level, L, by the movement of which the agitator is revolved ; a pressure gage, M, for noting the pressure of the oxygen admitted to the bomb. Between 20 and 25 atmospheres are ordinarily employed ; an oxygen tank, 0; and a battery or batteries, P, the current from which heats the fuse wire to ignite the fuel.
The description of the operation of one bomb calorimeter is typical of all of thema. The lower half of the bomb is placed in the cast iron Abdut one gram of the oil is weighed to the nearest 0.0001 gram into the fuel pan and is placed in the bomb on the fuel pan holder. If the oil is volatile it is not advisable to pour the fuel directly into the fuel pan. For this purpose small gelatine capsules weighing .1 gm. are used and may be filled with ignited asbestos and into this the light oil is dis charged from a weighing pipet. The capsule is immediately closed, leaving a minimum amount of air space. A similar capsule has been previously weighed and its calorific value determined. A, stock of standardized capsules should be kept on hand in an air tight receptacle(. The platinum fuse wire is cut, equal in length to the taper pin wrench, which is connected to the terminal, being careful that it does not touch the pan. The wire is bent down
so that it is covered by the oil or by the lips of the capsule. The tipper half of the bomb is carefully fitted on the lead gasket to the lower half. - The nut is screwed down over the upper half, being careful not to cross the threads. The bomb nut is now tightened by the use of the long wrench, being careful to cause no sudden jerking or vibrating which will throw the oil from the pan. The bomb is now carefully lifted out and placed on the table and connected with the oxygen piping. The valve in the top of the bomb is opened about one turn and the valve in the oxygen cylinder is carefully and slowly opened so that the pres sure in the bomb as shown by the indicator rises to 300 pounds. The bomb valve is now closed and the oxygen cylinder valve is closed. Exactly 1,900 grams of water at a temperature of about 4° below room temperature is weighed into the calorimeter water bucket. This is placed in the calorimeter container. The bomb is connected with the electric wire and is introduced into the water, being careful to place it in the center of the bucket. Two 100 watt lamps placed in parallel are in series with the fuse wire when a 110 volt circuit is used for firing. The stirring motor is placed in series with a 60 watt lamp on a 110 volt circuit. The cover is put on, the connections to the bomb wire are made and the stirrer is introduced as far down as it will go. It should not -touch the bomb. The thermometer is introduced and stirring is continued for about 5 minutes. The temperature is.read and the stirring continued for exactly 5 minutes and the temperature is again read and the charge is fired by quickly throwing in the. switch and withdrawing it. The stirring is continued for 5 min utes, the temperature being read at minute intervals or at the end of 5 minutes, unless extreme accuracy is required. The stirrer is then run for an additional 5 minutes and the temperature is again read. The thermometer is corrected in accordance with the cor rections furnished by the Bureau of Standards. The radiation corrections may be applied to each one-minute interval, but for ordinary purposes 1/5 of the radiation for the 5-minute period before firing is applied on the 5-minute period immediately after firing and 4/3 of the radiation in the third 5-minute period is applied on the 5-minute period immediately after firing. The calorimeter constant (usually about 2,400) is determined by a blank test using exactly 1 gram of benzoic acid. This constant always remains the same with the same calorimeter, but must be determined each time a change is made in the calorimeter. In the case of oil in which it has been necessary to use the capsule the correction made must' be applied for the calorific value of the capsule. This is most conveniently applied to the corrected net rise in temperature of the thermometer. To convert British thermal units per pound to calories per grain, multiply by ,5/9. To obtain the water evaporative power, multiply the B.t.u. per pound by 1.035. To obtain the B.t.u. per gallon, multiply the B.t.u. per pound by the weight per gallon. An approximation of the heating value of fuel oil can be obtained by the following • formula : B.t.u. in lbs. per gallon = 18700 4- 40 (° Be — 10).
A standard of 18500 B.t.u. to the pound of pure fuel oil is a good figure to be taken as a -basis if the fuel oil is to be purchased on calorific determinations. A bonus may be paid for calorific value in excess of this figure and deductions made if the heat value of the fuel is below 18,500 B.t.u.'s per pound. The heat value of fuels is measured by the number of British thermal units contained in one pound of the fuel and this statement fur nishes a direct comparison between fuels. Table 10 gives the calorific values of various oils.a