Page:EB1911 - Volume 22.djvu/244

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230
[HYDRAULIC
POWER TRANSMISSION


75 % as the eliiciency of the motor through which the power is utilized, this rate would give I-83d. per brake or effective h.p. hour. This cost seems high, and it is difficult to believe that it is the best hydraulic power transmission can accomplish having regard to the well-established fact that the mechanical efficiency of a steam pumping engine is greater than any other application of a steam-engine, and that the power can be conveyed through mains without any material loss for considerable distances. Still, no other system of power transmission except gas seems to be better off, and there

A few gas jets judiciously distributed are of value where there is a difficulty in properly protecting the machinery by clothing. From the central station the hydraulic power must be transmitted through a system of mains to the various points at which it is to be used. In laying out a network of mains it is first necessary to determine what velocity of flow can be allowed. ms"'"'" Owing to the weight of water, the medium usually mm employed for hydraulic transmission, a low velocity is necessary in order to avoid shocks. The loss of pressure due to the velocity is

FIG. 6.-Half section and elevation at AB. Detail of Io” steel pipe.

FIG. 4. is much the same as the average rate charged for the supply of electrical energy to the ordinary consumer. Gas is undoubtedly cheaper, but in a large number of cases is mechanically inconvenient in its application. Hydraulic pressure, electrical energy and compressed air (with reheating) can all be transmitted throughout towns with approximately the same losses and at the same cost, because the power is obtained in each system from coal, boilers, and steam-engines, and the actual loss in transmission can be kept down to a small percentage. The use of any particular system of power does not, however, primarily depend upon the cost of running the central station and distributing the power, but mainly upon the independent of the actual pressure employed, and at moderate velocities of 3 to 4 ft. per second the loss per 1000 yds. is almost a negligible quantity at a pressure of 700 Tb per square inch. For practical purposes Box's formula is sufficiently accurate-2

Loss of head=gallOnS Xlenghh myafrds - There is a further (diameter of pipes in 1nches><3)5 loss due to obstruction caused by valves and bends, but it has been found in London that a pressure of 750 lb at the central accumulators is sufficient to ensure a pressure of 700 lb throughout the system. The greatest distance the power is conveyed from the central stations in London is about 4 m. The higher the initial velocity the more variable the pressure; and in order to avoid this variation in any large system of mains it is usual to place additional accumulators at a mechanical convenience of the system for the purpose to which it is applied. One form of energy is, in practice, found most useful A. for Ong puigicige, another form for another and no one can command / A -/

FIG. 5. %

When water is employed as the fluid in hydraulic transmission the eliects of érosthmust usuallyhbe provided iigainst. In London an ot er towns, the water, be ore being pumped ' P";'"?°”s into the mains, is passed through the surface condensers "";f os': s of tliiesngifnez io as tp raise its temperature.dThe mains are ai 3 t. eow the sur ace o the groun Exposed ' V, - pipes and cylinders are clothed, and means provided for draining / ' ' them when out of use. When these simple precautions are adopted damage from frost is very rare. In special cases oil having a low B freezing point is used, and in small p ants good results have been FIG. 6.—Half back elevation, half front elevation. Detail of Obtained by mixing glycerin and methylated spirit with the water. Io" steel pipe.