Air Supply.
—This completes the description of the special apparatus used in this system, but we have yet to say something regarding the machines that supply the air. In Paris the water from the city mains has been used to compress or exhaust the air used in small tubes, but to operate large tubes in most of our cities steam is the only available power. Except in isolated cases, an independent steam plant will be erected to supply the air for a system of tubes. This plant should be designed with a view to obtaining the maximum economy in coal consumption, labor, water, cartage, and incidental expenses. We might say that the same general rules of economy which govern the design and construction of electric-lighting plants should be applied to the plans and construction of air-compressing plants.
Three types of blowing machines are used,—viz., centrifugal fans, positive blowers, and air-compressors.
Fans.
—Very large tubes of moderate length can be operated by ordinary centrifugal fans. These fans are capable of supplying air under a pressure not exceeding ten or twelve ounces per square inch with very good efficiency. They are the simplest and most inexpensive of all blowing-machines.
Blowers.
—When tubes have a length and diameter that require a pressure from one to four pounds per square inch, some form of positive blower of the Root type can be used with economy. Their construction is familiar to nearly every one at all interested in machinery, so we need give no space to their description here.
Fig. 42.
THE STURTEVANT STEEL PRESSURE BLOWER.
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