CHAPTER XXVIII

DETERMINATION OF HEATING VALUE OF OILS

To determine the efficiency of boiler operation it is necessary to know the heat producing value of the oil used in firing. Again, since oil is usually sold commercially by the barrel, the heat producing value of the product must be known in order that the

FIG. 154.-The Graphic Law for calorific value of fuels.

In this illustration is shown how a large number of experimental values often enable the engineer to ascertain an empirical law for setting forth experimental data. By plotting the heat determinations for fuel oil against their gravity expressed in Baumé readings, the experimenter deduced an equation for determining the calorific value of water free oil when its gravity Baumé is known.

engineer may ascertain the economic value the product may prove to his client in its use in the power plant for the generation of steam.

An Approximate Method Based on the Baumé Scale.-The heat producing value of oil is usually expressed in the number of heat units per unit of mass that the oil will give out when it is completely burned in a furnace. In engineering practice this is usually expressed in B.t.u. per pound of oil so burned.

There are various methods of ascertaining this value. An approximate method is that based upon the gravity of the oil. To establish this method a large number of samples with the gravities of the oil free from moisture expressed in Baumé readings were accurately determined as to their heating value. These values were plotted on a chart and it was found that the following relationship is approximately true in which H represents the heat units in B.t.u. liberated per pound of fuel burned, and B represents the gravity of the oil in degrees Baumé:

Thus in analyzing a composite sample of forty samples of Kern River oil, the United States Bureau of Mines found that its calorific value was 18,562 B.t.u. per lb. of oil, in which the oil had 0.5 per cent. moisture, and that the Baumé reading of this oil when free from water was 14.78°. According to the formula above, which was first announced by Professor Joseph N. LeConte of the University of California, the heating value of this oil when free from moisture should be

H

=

17680 +60 X 14.78

=

18,566 B.t.u. per lb.

In this instance then it is seen that this approximate method checks with considerable accuracy, since the water-free oil showed by actual test to have a heating value of 18,658 B.t.u. per lb. In the utilization of this formula, however, it must be remembered that the oil must be taken as anhydrous, or in other words that the oil sample is moisture free.

Dulong's Formula Based on the Ultimate Analysis.-The second method of arriving at the calorific value of crude petroleum is by means of Dulong's formula. This formula is based upon the ultimate analysis of the oil in which the heat value of carbon, hydrogen, and sulphur are taken into account.

In the burning with oxygen of one pound of carbon, one pound of hydrogen, and one pound of sulphur it has been established experimentally that 14,600, 62,000, and 4000 B.t.u. of heat energy are respectively given out. Hence it is evident that if a one

pound sample of fuel oil has C proportions by weight of carbon, H proportions by weight of hydrogen and S proportions by weight of sulphur, the total heat given out by the one-pound sample will be

In the chemical analysis of fuels a certain amount of oxygen (0) is always encountered. This of course kills, as it were, its combining weight of hydrogen. Since oxygen unites with oneeighth of its weight of hydrogen, the net hydrogen available for

heat generating purpose is (H).

Hence we have Dulong's formula

In this type of calorimeter the fuel sample is placed in the bomb, the bomb inverted, as shown in the sketch, and filled with oxygen which is accomplished by means of the spindle valve at the top of the bomb. After filling the calorimeter with distilled water and firing the sample by means of an electric circuit, the rise in temperature of the water in the calorimeter is ascertained, and the calorific value of the fuel thus determined.

For California oils, Dulong's formula seems to indicate a heat value per pound of about 5 per cent. in excess of the true value. In other words, it indicates a heating value of about 19,500 B.t.u. per pound of California crude oil, while a great number of calorific tests have shown that the average value is about 18,500 B.t.u. per pound.

The Fuel Calorimeter.-The most accurate method of determining the heating value of a sample of oil is by the employment of some form of calorimeter, wherein a sample of definite mass is burned and the heat given out ascertained. The fuel calorimeter is an entirely different instrument from the steam colorimeter used for measuring the moisture of steam, which was

described in an earlier chapter. The fuel calorimeter is a true instrument for measuring heat as its name implies. Calorimeters in general may be divided into two classes, the one known as the continuous method and the other as the discontinuous method. In the former instance a sample is continually burned, and the average results ascertained over a considerable period. This method is only applicable for gases and some unusual types of oils. The discontinuous process is on the other hand the most advantageous for the determination of the heating value of crude petroleum.

Several methods are employed in the FIG. 156. The Atwater- application of the discontinuous calori

Mahler bomb calorimeter.

This type of calorimeter is applicable to the highest scientific work. It permits of determining the exact amount of

water and carbon dioxide in

the products of combustion,

thus enabling the error due to the condensation of the water in the bomb to be overcome

and therefore making it pos

sible to calculate the exact

amount of heat the fuel should

produce under boiler conditions.

meter. Most forms of such calorimeters consist essentially of a strong combustion chamber with a crucible for holding the sample; valves for charging the chamber with oxygen in order to properly burn the sample; a method of igniting the sample; and a vessel of water in which the bomb or explosion chamber is immersed in order that the resultant heat may be absorbed by this water and thus carefully measured. This latter vessel is usually situated in a second compartment which serves as a jacket. The main principle upon which such calorimeters depend is based upon the fact that the burning of carbon, hydrogen, and sulphur with an artificial supply of oxygen presents the most accurate method of liberating the latent heat in the fuel and the ascertaining of its quantitative proportions. Types of this calorimeter familiar in the market are known as

the Mahler, the Hempel, the Atwater, the Emerson, and the Carpenter.

FIG. 157.-The Mahler bomb calorimeter.

This type of calorimeter represents one of the most accurate for the determination of the calorific value of fuel oil. The bomb is of enameled steel. The burning of the oil sample is accomplished by supplying an outside source of oxygen as in the Emerson Calorimeter.

The Parr Calorimeter.-In the commercial determination of the heating value of crude petroleum, however, it is often incon

FIG. 158. The Parr calorimeter unassembled.

In this type of calorimeter a carefully weighed oil sample is burned with a chemical agent without the use of free oxygen. The ease with which it may be manipulated commends its use for commercial application. For scientific work, however, a type of the bomb calorimeter is to be preferred.

venient to secure oxygen under the proper pressure required for the successful operation of this type of calorimeter. In