Gases and their sources form a very important part of the study of the gas-welding processes, because a knowledge of these will often be of great value in determining the best process for particular purposes. This is becoming more necessary because so many concerns are installing their own generating plants in order to get their gases cheaper and more promptly than is otherwise possible. The gases used in hot-flame welding today are acetylene, blau gas, coal gas, hydrogen, oxygen, pintsch gas, and water gas, and various methods are in use for their production and storage.
About one hundred years ago, considerable experimenting was done by the leading physicists to determine the best methods of obtaining the various gases on a commercial scale, and to discover how they might be compressed or even liquefied, with the result that many of the gases in use today for welding were produced on a commercial scale and their value for heating purposes demonstrated, many years ago.
Acetylene is a colorless gas with a very disagreeable odor, very largely due to the impurities present. It was first obtained by Davy in 1837 when making potassium. Ber-thelot produced acetylene in 1858 by passing hydrogen through an electric arc, and Wohler produced calcium carbide in 1862 by fusing lime, zinc, and carbon together and then obtained acetylene by adding water to the carbide. Acetylene was first liquefied by Cailletet in 1877 and the use of compressed acetylene was developed by Claude and Hesse. Today, acetylene is obtained almost exclusively from calcium carbide and water, and great care must be exercised to see that pure carbide is used in order to prevent the generation of phosphureted hydrogen along with the acetylene.
Calcium carbide is a dark gray slag formed by fusing lime and coke in the intense heat of an electric furnace; it possesses a great affinity for water. When calcium carbide is combined with water (H2O), in the proportion of 2 parts water to 1 part carbide, a chemical reaction takes place which heats the mass and forms acetylene (C2H2) and lime (CaOH20) in the form of ashes. In other words, the carbon combines with the hydrogen to form acetylene and the calcium combines with the oxygen to form lime. One pound of carbide will yield about 4½ cubic feet of acetylene.
Calcium carbide alone is not an explosive and it will not explode even when exposed to the highest heat but, unless it is kept dry, it will absorb moisture and generate acetylene, which is explosive. This is why it is best to store calcium carbide in air-tight tins.
When mixed with air, acetylene is explosive over a long range of proportions and this makes the gas very troublesome. It is explosive over the limits of 2 per cent gas and 98 per cent air up to 49 per cent gas and 51 per cent air and, when mixed with oxygen, it burns with a tremendous heat. Acetylene dissociates at 780 degrees centigrade into carbon and hydrogen and, when under a pressure of two atmospheres (30 pounds) or more, it is tricky and liable to explode; so it is sometimes stored in specially prepared tanks. Acetylene is readily soluble in liquid acetone, which is cheap, inert, and incombustible; so storage cylinders or tanks are filled partly full of it and then the acetylene gas is compressed into it. Acetone at atmospheric pressure and a tem-perature of 15 degrees centigrade will dissolve 24 times its own volume of acetylene and, at 12 times atmospheric pressure (180 pounds), it will dissolve about 300 times its volume of acetylene and expand about 50 per cent. The cylinders are partly filled with asbestos fiber to carry the acetone and, to fill the cylinder, it is merely necessary to charge it with compressed acetylene. The various types of acetylene generators will be described later.
Blau gas is liquefied illuminating gas and is produced by the distillation of mineral oils in red-hot retorts. It contains the same elements as ordinary coal gas but in different proportions; it is free from carbon oxide and is therefore not poisonous. It contains carbon and hydrogen in the proportion of about 5 parts of carbon to 1 part hydrogen and will develop about 20 per cent more heat units than acetylene. Blau gas, named after its inventor, can be compressed and liquefied; when liquefied it occupies but 1/400 part of its gaseous volume and is usually sold under a pressure of 00 atmospheres, in steel cylinders. It is very inert and therefore difficult to explode, the range of explosiveness being from 4 per cent gas and 96 per cent air up to only 8 per cent gas and 92 per cent air. This gas is already used quite extensively abroad, and is beginning to be used more and more in this country.
Coal gas, or illuminating gas, is produced by the destructive distillation of coal and its discovery dates back to 1727. It is made by heating coal to the point where it decomposes in a closed retort in order that the gas, tar, and other constituents may be saved. Bituminous coal is better for gas making than anthracite because it softens or fuses at a temperature much lower than that required for combustion and this fusion is the commencement of the destructive distillation which forms the solid, liquid, and gaseous compounds from the coal. The operation takes place in specially constructed furnaces and the gas is carried to storage tanks after being washed to remove impurities. One ton of coal will produce about 10,000 cubic feet of gas, 1400 pounds of coke, 12 gallons of tar, and 4 pounds of ammonia, the operation lasting about 4 hours. The gas contains about 5 per cent of hydrocarbon vapors, 13 per cent of carbon oxides, 31 per cent margn gas, 46 per cent hydrogen, and 5 per cent nitrogen with traces of oxygen, and has a heat value of about 40 per cent that of acetylene. Its use for welding is limited to metals of low melting points and is gradually being superseded by other gases.