The oxy-acetylene welding process is the best known of the hot-flame systems and is based on the combustion of oxygen and acetylene at the tip of a torch as the source of heat. This process has apparently been developed to the highest possible degree and is probably the most efficient of the various hot-flame systems in general use, the flame having an approximate temperature of 3500 degrees centigrade. The practical value of this process was overestimated, when it was first introduced, and there are still many limitations to be overcome but improvements in torches, valves, generators, and storage tanks are being made and should result in improving its status. Owing to the low first cost, there are thousands of oxy-acetylene plants in use for welding and cutting, in spite of the high cost of operating, and reductions in the cost of the gases will eventually bring down the cost of operation to a more reasonable basis.
The principal elements of an oxy-acetylene installation are: the oxygen generating or storing apparatus; acetylene generating or storing apparatus; and the burner or torch with its connections. For large plants it will pay to install oxygen generating plants as well as the acetylene plants, but in other shops the oxygen is usually purchased in steel tanks and the acetylene is generated in small-sized outfits. For moderate sized shops a portable outfit can be used, consisting of an oxygen tank and a small acetylene generator, or an oxygen tank and an acetylene tank on an ordinary hand truck.
The acetylene generator is a comparatively simple device, usually a single steel receptacle for holding the gas, with various attachments for controlling the action of the water on the carbide. There are two general systems of acetylene welding in use, the high pressure system and the low pressure system, both of which have their advocates. As a matter of fact, the so-called high pressure system, used in this country today, is a medium pressure system, the true high pressure system being used principally in France and not yet having been introduced commercially into this country.
Acetylene generators are made in five types, one of which only is used to any great extent for making acetylene for welding use. These are the "dip", the "drop" or "plunge", the "overflow", the "rescission", and the "spray" types, the drop type being the one most used.
The dip generator is so arranged that the calcium carbide is suspended in a sort of basket inside the gas holder and, as the gas is used, the basket is lowered until the carbide comes in contact with the water. More acetylene is then generated and this causes the pressure to raise the holder and basket, thus stopping generation until the gas is used.
Fig. 112. 100-Pound Oxy-Acetylene Welding Outfit Courtesy of Davis -Bournonville Company.
The rescission generator is similar in principle to the dip type, the only important difference being that the gas pressure forces the water away from the carbide basket instead of lifting it.
The overflow generator has the carbide in a series of compartments, the water filling the first before overflowing into the second, then into the third, etc-., until all of the carbide has been flooded.
The spray generator is one of the oldest types and has a pan of carbide located so that the water may drip into it from above. The supply of water is cut off when the gas pressure becomes too high and steam is frequently formed by the high temperature of the reaction. These machines are wasteful and dust from the carbide may clog the pipes and burner.
The drop or plunger type of generator is the most economical and satisfactory and has practically superseded all of the others. This type of generator is arranged so that the carbide falls a few lumps at a time into a large vessel of water, the feeding being done by suitable mechanism; the water absorbs the heat so rapidly that the gas is kept cool and the temperature of the entire outfit is much lower than with other types. The gas is washed by bubbling up through the water and the lime remains in the bottom and is frequently removed. This lime makes a good fertilizer. It has been claimed that this type of machine is inefficient because the water will take up gas at the rate of 1 cubic foot per cubic foot of water; but as the water becomes saturated with the lime, it causes the gas to pass out, so that only about three per cent of the gas is actually lost by this action. Theoretically, 1 pound of carbide requires ½ pound of water but, in practice, it takes about 1 gallon to the pound for the best results and should produce 4½ feet of gas.
The best known acetylene generator on the market in America today is the Davis generator made by the Davis-Bournonville Company, New York, for use in connection with their welding apparatus, Fig. 112, made under the Bournonville patents. The carbide is fed into the machine through a hopper at the top in the form of lumps, because ground carbide will produce less gas, the carbide being dropped into the water as required. The feeding mechanism is on top and operates through variations in the gas pressure and through its effect on heavy weights and a moving gas bell. Attached to the tank are also a filter, flash-back chamber, drainage chamber, water filling tube, blow-off valve, and such other devices as are required for the safety and operation of the generator. The pressure of the gas may be varied and can be made to run up to 15 pounds if desired. These machines are made in sizes from 20 pounds carbide capacity up to 300 pounds capacity, a small-sized portable outfit being shown in Fig. 113.
Fig. 113. Portable Welding or Cutting Oiy-Courtesy of Davis-Bournonville Company.