15 in. high by 26 in. wide inside, and 9 to 10 ft. long if single-ended, or 18 to 20 ft. long if double-ended. The retort walls are about 4 in. thick and each retort is connected with a pipe that allows the gases to escape as fast as formed. After passing through various devices to remove the ammonia, tar, and sulphur, the gas passes into a gas holder and is ready for distribution.

Analyses of typical gas coals from the Pittsburg, Pa., field are given in the following table.

Under ordinary conditions 1 T. of such coal should produce about 10,000 cu. ft. of gas of 17 c. p., 1,400 lb. of coke, 12 gal. of tar, and 4 lb. of ammonia. The following may be considered as the average composition of purified coal gas:

The use of illuminating gas as fuel for steam-raising is limited by its cost which, while sometimes as low as 80 c. per 1,000 cu. ft., is usually about $1. A certain amount is used directly in small gas engines in the larger cities, but the larger amount is used for illumination or in cooking ranges, domestic heating stoves, and the like. Coal gas made from gas coals in retorts is being largely displaced by water gas.

Water Gas.-Water gas contains the same combustible constituents as coal gas but not in the same proportions. It is made commercially by the contact of steam with incandescent carbon, in the form of anthracite or coke, which decomposes the steam separating the hydrogen from the oxygen. The oxygen takes up carbon from the coal or coke and forms carbon monoxide, along with a small amount of carbon dioxide. The resultant gases therefore are mainly hydrogen and carbon monoxide mechanically mixed together. This is what is called blue, or uncarbureted, water gas. It burns with a non-luminous flame and is consequently useless for lighting purposes, except in incandescent lamps of the Welsbach type. In actual practice, this water gas is always enriched with oil gas, which furnishes the hydrocarbons necessary to make a luminous flame. The oil gas was made separately in many of the older forms of apparatus, but it is now commonly produced in the same apparatus in which the water gas is made.

The only impurity that must be removed from water gas is hydrogen sulphide, which is formed from the sulphur that is always present in varying amounts in the coal or coke and sometimes in the oil. The hydrogen sulphide

is removed by purification with lime or iron oxide in the same way that the purification of coal gas is accomplished.

Carbon dioxide, which is formed either by imperfect contact of the steam with the incandescent carbon, or because the temperature of the carbon is too low, is not a dangerous impurity, but is merely an inert gas incapable of combustion. It, however, absorbs heat when the gas is burned, and is consequently injurious to the heating and lighting power. It can be removed by purification with lime, but this is not necessary if the generating apparatus is handled properly, as the quantity made will be very small. No ammonia is produced.

The following is a volumetric analysis of purified water gas:

Hydrocarbon vapors..
Heavy hydrocarbons.
Carbon dioxide, CO2.
Carbon monoxide, CO.

Marsh gas, CHA.

Oxygen, O...
Hydrogen, H.

Nitrogen, N.

Total..

Per Cent.

1.2

12.6

3.0

28.0

20.2

.4

31.4

3.2

100.0

Water gas requires from 30 to 40 lb. of coal or coke per 1,000 cu. ft. of gas made, and from 4 to 5 gal. of oil, depending on the candlepower required. Usually between 5 and 6 c. p. is obtained from each gallon of oil used. The specific gravity of 24 c. p. water gas is about .625, air being taken as unity. Pure uncarbureted water gas has no perceptible odor, but the carbureted gas has an odor fully as strong as coal gas. This is mainly due to the hydrocarbons from the oil that is used for enriching. It should be noted that these hydrocarbons are not added if the gas is to be used for heating or in gas engines. Producer Gas.-Producer gas is made in a cylindrical riveted shell of boiler plate, lined with firebrick. A thick bed of fuel is maintained in the bottom of the producer and through this is passed a moderate supply of air, with or without water vapor or steam. By properly regulating the air supply, a partial or incomplete combustion of the fuel is maintained, resulting in the gradual consumption of all the combustible matter. The coke, instead of remaining as a by-product, as in the manufacture of coal gas or in by-product or retort coke ovens, is all consumed in making the gas. When dry air alone is forced through the fire, the resulting gas is known as air gas and is the true producer gas; when the air is mixed with steam or water vapor the resulting gas is called mixed gas, and is frequently made in producers; and when air is not used at all and steam alone is forced through the fire, the product is called water gas as previously explained.

The quantity of producer gas derivable from 1 T. of fuel will vary according to the fuel used, the type of producer plant, and the method of operating. The United States Geological Survey, at its Fuel Testing Plant at the Louisiana Purchase Exposition, held in St. Louis, in 1904, made an exhaustive series of tests of American coals used in gas producers, and from its reports the following tables, etc., are taken.

QUANTITY OF GAS PRODUCED PER POUND OF FUEL IN AN UP-DRAFT PRESSURE PRODUCER

The yield of gas, in cubic feet per pound of dry fuel, which may be expected in the up-draft producer from various fuels is, roughly, as follows: Coke or charcoal, 90; anthracite, 75; bituminous coal, 65; lignite, 46; and peat, 38. On the basis of the Survey's tests the yield of gas and the heat value of the gas per ton of fuel as fired are approximately as in the table here given.

YIELD AND HEAT VALUE OF GAS PER TON OF FUEL AS FIRED IN AN UP-DRAFT PRESSURE PRODUCER

It will be noted from this table that while the inferior fuels yield less gas per ton, as might be expected, the heating value of the gas, in British thermal units per cubic foot, is greater than in gas made from high-class coals.

Gas Producers.-There are three general types of gas producer in use. In the suction type, the drawing of the air and steam through the fire and, consequently, the generation of the gas, is accomplished by the suction in the engine cylinder in which the gas is used. While the gases are scrubbed, etc., to get rid of the tar that otherwise would clog the cylinders, the absence of storage tanks for the gas and the fact that the suction of the engine causes the operation of the producer, renders absolute separation of the tar difficult if not impossible. Hence, only low volatile coals are adapted to use in this type of producer, and because the price of such coals is always high, suction plants, though numerous, are of comparatively small power, few exceeding 300 H. P. each, and most of them not exceeding 100 H. P.

The up-draft pressure producer is the common American type in which the gas is developed under a slight pressure due to the introduction of the air and steam blasts, and the gas is stored in holders until required by the engine. As the generation of the gas is independent of the suction stroke of the engine, tar and other impurities may be removed by suitable devices and hence the use of bituminous coal, lignite, and peat is possible. This form of producer is offered in many types, some of which are without gas holders and are proving eminently satisfactory. If the holder is omitted, automatic devices must be introduced for controlling the pressure and the supply of gas to the engine. TYPICAL ANALYSES BY VOLUME OF PRODUCER GAS

In both the foregoing types of producer, the extraction of the tar removes a large part of the heat value of the gas. If the tar can be sold at a good price this may not make much difference, but where the tar is thrown away the loss is sufficient to warrant the attempt to devise some means of converting this tar into gas of suitable quality for engine use. To this end down-draft producers are coming into general use and in them the gases are drawn down through the bed of coal and the tar is thereby decomposed into fixed, combustible gases. Typical analyses of gases made from the same fuels in the up-draft (U. D). and down-draft (D. D.) producer, the percentages being by volume, are given in the preceding table.

In the matter of steam raising it is questionable if better results are obtained by using the gas made from the coal than by firing the coal directly under the boilers, especially in the case of good grades of coal from subbituminous to anthracite, but many fuels, notably peat and some of the true lignites, that give indifferent results when fired directly under a steam boiler, give most excellent results when fired as gas. Further, almost any material containing carbon will yield fuel gas in the producer, even bituminous shale, saw-dust, wood pulp, cornstalks, and the like. It is practically impossible to predicate the yield of gas and the quality thereof of a coal from its analysis. Tests in the various types of producers are required for this purpose.

Strictly speaking, the gaseous fuels previously described are all producer gases, except natural gas, coal gas (illuminating gas), and the waste heat gases from beehive coke ovens. These various producer gases are not commonly used for steam raising under boilers, but are a direct source of power in internal combustion engines. It must be noted that as carbon monoxide is one of the most important heat-producing constituents of all these gases, extreme caution must be observed in inhaling them owing to their highly poisonous nature.

BOILERS

STEAM

PROPERTIES OF STEAM

Saturated Steam. If water is put in a closed vessel and heat is applied until boiling occurs and steam is given off, the pressure and the temperature of the steam will be the same as those of the water. The steam thus produced is known as saturated steam; that is, saturated steam is steam whose temperature is the same as that of boiling water subjected to the same pressure. Its nature is such that any loss of heat will cause some of the steam to condense, provided the pressure is not changed. Saturated steam that carries no water particles with it is called dry saturated steam; if it contains moisture it is called wet steam. At every different pressure, saturated steam has certain definite values for the temperature, the weight per cubic foot, the heat per pound, and so on. These various values, collected and arranged in order, form the table of the Properties of Saturated Steam, more commonly termed the Steam Table, which is given on the following pages.

The various properties of steam, with their symbols, as given in the Steam Table, are as follows:

The temperature, of the steam, which is the boiling point of the water from which the steam is formed.

The heat of the liquid, q, which is the number of British thermal units required to raise the temperature of 1 lb. of water from 32° F. to the boiling point corresponding to the given pressure.

The latent heat of vaporization, r, often termed the latent heat, which is the number of British thermal units required to change 1 lb. of water at the boiling point into steam at the same temperature.

The total heat of vaporization, H, often termed the total heat, which is the number of British thermal units required to raise 1 lb. of water from 32° F. to the boiling point for any given pressure and to change it into steam at that pressure. It is the sum of the heat of the liquid and the latent heat.

The specific volume, V, which is the volume, in cubic feet, of 1 lb. of steam at the given pressure.

The density, w, which is the weight, in pounds, of 1 cu. ft. of steam at the given pressure. It is the reciprocal of the specific volume.