The idea of the rotary positive machine was not a new one. Many earlier examples are known, the most famous being the Pappenheim Engine, perhaps dating to the 17th century. The German engineer Franz Reuleaux described this engine as well as a number of other early examples in his Kinematics of Machinery. These devices all used a similar principle, that of two opposite rotating wheels geared together by teeth or lobes. They were used as pumps and ventilators among other purposes.
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Reuleaux also states that shortly before the Roots invention almost identical blowing devices were constructed in England. He cites one made by George Jones of Birmingham in 1859 and an earlier one of 1853. it is not impossible that the Roots simply copied recent British work and secured an American patent for it. However, as far as it is known, there would have been no way for the Roots to know about the British work, and the examples cited by Reuleaux preceeded their work only by a year or two at any rate. Thus, although the idea was an old one, the Roots should be credited with perfecting the design and first putting it into operation on a large scale.
This is the conclusion of the London-published Engineer of 1867 reporting on the exhibit of the Roots company at the Paris exhibition: "Mr. Roots has the merit of having brought the machine independently to such a state of perfection as to render it superior in many respects to the ordinary fan, and to make it an aspiring competition with the blowing cylinder."
The first major use of the Roots blowers was in cupola furnaces. The first two experimental machines were tested in foundries and large numbers of blowers were manufactured for this purpose. A large blower for the West Cumberland Hematite Iron Works in Workington, England, was described in 1872. At the Philadelphia Centennial, Roots blowers were praised by the Scientific American Supplement as efficient and economical devices for use in foundries:
In a force-blast blower there are two great benefits: economy of power and certainty of result. The blowers exhibited by the Messrs. P.H. and F.M. Roots, of Connersvi1le, Indiana, seem to come up to these requirements.
In reference to economy of power, they, operating by a regular displacement of air, which is forced forward in constant quantity at each revolution use all the power applied either in driving the machine or forcing the air forward. Friction is slight. The internal parts do not come in contact during running, simply approaching each other as close as possible without an actual contact; this renders the blower practically air-tight. The power applied is absorbed but slightly in running, and is therefore almost entirely applied to the work of forcing the air forward. Results are as far as possible certain with these blowers...In securing the best possible results, the proportional amounts of iron to be melted, fuel used, and of air supplied in a given time, should be fixed and unvarying. As these blowers measure and force forward a definite quantity of air at each revolution, constancy of melting conditions are secured."
In 1917 industrial Management magazine reported that rotary blowers were still largely used for cupola work. It also stated that they were most economical for pressures between 1/2 pound per square foot and 8 or 9 pounds per square foot.
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Another major use of the blowers was in mine ventilation. This was a relatively new field and Roots blowers constituted some of the largest installations in the 19th century. Already by 1870 several Roots blowers were in use in mines of the Comstock lode. Two of the largest Roots blowers ever made were installed at the Chilton Colliery near Ferryhill, Enqland in 1877. The impellers each had a diameter of 25 feet and were 13 feet wide. The maximum capacity of the two machines was 200,000 cubic feet per minute. The ventilator was housed in an engine house with a perforated roof. The Engineer of London, reporting on the results of tests run on various ventilators, concluded that the Roots machines were the most efficient that had been installed in mines. The other ventilators compared were those of Cooke, Waddles, Rammell, Leeds, and Guibal.
Of the many uses to which the blowers were put, certainly the most spectacular was in the underground subway constructed by Alfred Beach in 1867 under Broadway in New York City. This "aeolar" had an iron shell 21 1/2 feet high and impellers 16 feet long. At 60 revolutions per minute it produced 100,000 cubic feet of air a minute. Costing $20,000, the machine was transported on five large platform cars. The "Western Tornado", as it was called, was pictured in a contemporary pamphlet with the figure of a man inside to show the huge scale. The "aeolar" provided the power for a 22 seat passenger car which ran on tracks from Murray to Warren streets. The car was literally blown to one end and then sucked back by the action of the machine. Other uses were in pneumatic tubes, aeration and agitation, supercharging and scavenging on diesel engines, and in vacuum processes.
Most of the patents which the Roots brothers took out after 1860 were modifications of their first scheme. In almost every case these alterations were intended to allow a closer fit, less friction in operation and cheaper means of construction.
The case was modified In a series of patents. Originally sheet metal bent to the proper shape was used. In 1864 the Roots patented a shell made of two pieces of cast iron joined together by metal plates. In 1868 they further modified the design by placing packing strips in the case which formed the contact surface for the impellers. This was done to cut down on friction and to allow the cast iron case to be made rough since the impellers were no longer in contact with it. This reduced the costs of boring and polishing. Later in the same year a patent was obtained specifying soft metal or plaster of Paris for this purpose.
The shape of the impellers was also modified several times. In 1886, the Roots patented a design for impellers which were constructed of a metal core adjusted to the desired shape by wood strips. This was more economically constructed than solid iron impellers and allowed them to be renewed without entirely reconstructing the entire impeller. In the same year, the Roots also modified slightly the shape of the impellers creating the Figure-8 shape which became very common thereafter. The design of the impellers was subsequently modified in 1868, 1881, and 1882.
The form of the impeller varies in the number of the lobes, twin-lobe or three-lobe, according to the purpose. Available bore size ranges from 20A to 700A.
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