Within the last ten years the general introduction of the steam turbine and the high efficiency to which it has attained have brought about a great development in centrifugal machinery. It was formerly supposed that centrifugal fans were of no value for pressures above 1 lb. per square inch, and they were seldom or never compounded by putting two fans in series so as to increase the pressure they could attain. Instead, if a pressure of more than a few ounces was desired, the tendency was to pass to the use of the positive pressure blower.

The French engineer Rateau deserves the credit for working out into practical shape the use of the series fan capable of blowing pressures suitable for blast-furnaces. He introduced several of these machines in Europe about ten years ago.

Blast pressures in Europe are commonly not so high as those in this country, so these blowers were not suitable for American conditions. Moreover, it has been found to be universally the case that when a prime mover was introduced from Europe into America it had to be redesigned to meet American ideas and American conditions. This has been the case with the turbo blower. The first of these machines in this country was built by the General Electric Company for the Oxford furnace of the Empire Iron & Steel Company, and was started in 1910. A twin machine was started a few months later at the furnace of the Northern Iron Company at Port Henry, N. Y. Probably the largest single installation of these machines to date is that of the Iroquois Iron Company at Chicago, where the sole blowing power for two furnaces consists of three turbos with a capacity of 40,000 cu. ft. per minute each to a pressure of 30 lb.

More recently the Southwark Foundry & Machine Company has introduced the Rateau style of turbine into this country, and has already built several of them.

The Westinghouse Machine Company has as yet built no multistage turbine, but has built some single-stage machines for a pressure of 5 to 7 lb., these not being used in the iron industry, but under similar conditions in other industries.

The theory of the turbo blower is extremely simple. Air is admitted around the shaft, caught by the rapidly revolving vanes, or blades, of the fan, and has imparted to it a high velocity. This velocity is transformed into pressure, and the method by which this may best be done is a subject of some dispute among the designers of this class of machinery. The pressure obtainable in any one stage is limited by the speed which may be attained by the fan, and this is virtually the speed at which it is safe against the influence of centrifugal force.

Fig. 103. Southwark Rateau turbo blower.

The discharge from one fan is led around through an annular port in the casing to the center of the next, which increases its pressure by a similar amount, and then to a third, and so on.

The number of stages to be used is not definitely settled. The Rateau blowers use eight, the first General Electric blower used six, but in the more recent types of construction the latter company have diminished the number of fans in series to three by increasing the speed.

The pressure increases more rapidly in the later stages than in the earlier ones because the density is increased by the increasing pressure.

The work put upon the air in compressing it heats it exactly as it does in a piston-blowing engine, but to a greater extent because the efficiency of the operation is lower with greater losses of energy in eddy currents, etc., which are eventually converted back into heat. The casings are therefore cored out hollow and cooled by water circulation to keep down the temperature of the air.