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axis instead of obliquely as in the case for producing these pressures, but its of the disc fan, and which revolve in a economy falls off very fast if the speed is circular or spiral casing surrounding the made so great as to produce a pressure fan and opening into a large discharge per square inch of much over one pound. pipe that leads off at a tangent in any This machine seems peculiar in its bedesired direction—up, down, or side havior owing to the fact that it requires ways. Air enters around the shaft of the less power to maintain a certain speed fan through openings in the center of when the discharge passage is partly

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the casing, and, being received by the closed than when it is fully open. It is blades, is whirled around and thrown evident that when there is no discharge outward into the surrounding chamber opening, the fan, together with the air with a pressure proportionate to the between its blades, will spin around freesquare of the speed of the fan. The ly like any wheel. If the discharge gate pressure thus produced is sufficient for be opened, air will flow about the fan blowing fires of all kinds, for removing blades to their outer ends, and so into the small fragments of wood as well as finer casing and thence to the pipe system; particles from wood-cutting machines, while air from the center of the fan folfor delivering illuminating gas to hold lows out into the space between the ers, and for other purposes requiring a blades, its speed increasing as it appressure of not much over one pound proaches the ends of the blades, whereper square inch. The centrifugal blower upon it is hurled into the casing. It is is the most efficient contrivance known the work done in accelerating the motion

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one or

of these particles of air, which explains the resistance to the movement of the fan; and this work is evidently in proportion to the quantity of air that passes along the fan blades, being zero when, by reason of the closing of either suction or discharge opening, no air is discharged, and being greatest when, with full opening, the volume of air passing is the maximum. The blades of the blower would fly in pieces before the speed could be increased sufficiently to produce an air pressure much over one pound per

more blades, revolving in connection with an "abutment" performing the office of the partition just alluded to. A well-known form consists of a cylinder carrying two radial blades, and revolving in contact with a cylinder of half the diameter which is provided with a gap of proper form to receive the pass


square inch.

An apparatus identical in principle is the centrifugal pump, which, handling far heavier fluids, produces by centrifugal action pressures measured by pounds instead of ounces.

The Positive Blower In direct competition with the centrifugal fan is the positive blower. Consider a fan having opposite blades, revolving in a closed casing, and carrying around with it the air confined between the blades. If a radial partition could be slipped in just after one of the blades had passed, the following blade would compress the air between it and the partition, and would force it out through a discharge pipe if one were provided near ing blades. As each blade travels around the supposed partition. This partition the casing, it drives the air before it might be imagined to be momentarily up to the smaller cylinder, whose gap withdrawn as each blade passed, so that comes around at the proper instant to successive portions of air would be trap allow the blade to pass, while interceptped and compressed. The positive ing the air continuously. blower consists of a fan of this sort, with The faults of these blowers are two


namely, weight and leakage. The cylin- above the valves. They close at the end ders, blades, and casing-which, on ac of the intake stroke, just as the piston count of their peculiar shapes and accu comes to rest. The outlet valves lift rate fitting, are almost necessarily made during the compression troke, at about of cast iron—are excessively heavy for the time the rising pressure in the cylinuse in connection with air at low pres der becomes equal to that in the outlet sures, while pressures sufficient to utilize passage above the valves; and they close the full strength of the apparatus could when the flow of air ceases as the piston not be attained because of the excessive completes its stroke.



leakage that would occur along the loose Perfectly correct action of automatic rolling contact of blades and abutment, valves is not realized in practice. The and between blade and casing after a valve must evidently be larger than the little wear had occurred. The mass of opening in its seat, so that the upper surthe moving parts, contrasted with the face is larger than the area underneath lightness of the working fluid, suggests reached by the lower pressure; consethe clumsiness of using a two-inch plank quently the valve will not open until the for a palm-leaf fan. The positive blower, pressure below is greater than that above. limited to moderate speed by the noise To prevent destructive slamming of the and shocks of the heavy members of the valves, springs must be provided to force apparatus, can produce pressures but them to their seats just as the flow ceases little greater than those obtainable by and before the reverse stroke of the pismeans of the centrifugal blower.

ton can cause much backward flow of

air. The pressure of the springs acts The Air-Compressing Engine

to choke the flow through the valves, inThe air-compressor proper is a cylin- creasing the resistance they offer to the der-and-piston machine like the common passage of air. Large compressors are steam engine. It comprises two sets of therefore often provided with mechanicvalves, usually designed to be opened ally-actuated valves which are opened automatically by excess of pressure under and closed smoothly at the proper mothem and to be closed by gravity or by ment by eccentrics and valve rods. Any the action of springs when the pressures of the steam-engine valve gears may be become equal.

The inlet valves open used for compressors, and designed by just after the piston commences its the same methods, observing only that stroke, when the expansion of the com the compressor is in every way a repressed air remaining in the cylinder be versed steam engine, so that its discharge hind the piston has lowered the pressure port and valves are duplicates of the

inlet details of the engine, while the en light springs, causing less resistance to gine exhaust and the compressor inlet the air passing through. valves are also similarly related.

Where the expense of full mechanical Varying initial (or boiler) pressure is action is warranted on account of the compensated, except in throttling en superior efficiency obtainable, poppet or gines, by varying time of inlet-valve clos rotary valves may be arranged to open ure or "cut-off ;” and varying discharge by means of springs or air dashpots. The pressure in a compressor calls for vari- opening device is released either through ation in time of opening of the discharge the rising pressure in the cylinder easing valves. In both cases, the means of vari the valve on its seat, and reducing the ation constitute the chief problem for friction until the valve, when balanced, the designer.

slips freely open; or through the same Mechanically-moved inlet valves of pressure acting on a piston attached to a compressors act always at the same

pusher, the operation of which results points, opening a trifle after the piston either in starting the valve in spite of starts on the intake stroke, and closing friction or in lifting a catch and thereby exactly at the end of the same stroke; freeing the spring or dashpot mechanism but the discharge valves must open at of the compressor. the instant the piston has compressed the air in front of it to a pressure equaling that above the valves in the discharge


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pipe, and must close always at the same

Compound Compressors instant, at the end of the stroke. As While low-compression pressures are the compressor may be working against accompanied by only moderate heating a pressure greater or less than that regu of the air during compression, the prolarly carried, the discharge valves must duction of high pressures is attended be so controlled as to open at whatever with excessive heat and considerable inpoint is required by the pressure then crease in the volume of the compressed being carried.

The requirements are air. As the air leaving the cylinder soon sometimes met by putting automatic lift resumes the normal temperature, and valves above, or even directly upon, the decreases in volume accordingly, the exmechanical discharge valves, giving the tra work done in compressing the incombination the effect of valves auto creased volume is wasted. Compressing matic as regards their opening, but posi- cylinders in operation are always cooled tively closed by the mechanism at the by water or otherwise; but it is imposproper instant. Such lift-valves, being sible, even by spraying water into the shut off entirely at the proper closing cylinder, to keep the air from rising coninstant, seat themselves without noise or siderably in temperature. For high presshock, and may therefore have very sures, resort is therefore had to com

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pound compression, the air being passed stage compressor may be desirable for successively through larger low-pressure pressures that would call for only single to smaller high-pressure cylinders, be cylinders at sea-level. Thus, at many tween which are located inter-coolers of the mines in the Rocky Mountain whose function is to restore the air to its region, the atmospheric pressure is as original temperature before it enters on low as eleven pounds per square inch, the next stage. The volume of the air so that 90 pounds' air pressure by gauge is thus kept as small as possible; and the requires a compression ratio of nine to successive stages of compression result one, which is considerably beyond that in producing the required pressure, with proper for a single-stage compressor. In a minimum of loss from heating dur- general, high-level compressors should ing the process. Two-stage machines

be specially proportioned for their work. are preferred to single-stage where air

Methods of Driving must be compressed to one-sixth or a smaller fraction of its volume at atmos Like pumps and other machinery, compheric pressure (measuring pressures pressors are direct-connected to engines from absolute vacuum); and three or or are driven through gears or belts from more stages are required in compressing separate sources of power. The reciproto one-sixth or less. Cylinder diameters cating piston compressor requires a varyare selected which will provide for about ing effort to balance the cylinder presthe same aniount of work being done in sure, since, during the stroke, the piston each cylinder.

moves against an increasing air pressure, Most compressor problems deal with and finally against the full discharge air taken directly from the atmosphere at pressure, in pushing out the contents of its sea-level pressure; but, as at moderate the cylinder. Direct-connected compreselevation there is a marked decrease of sors are either "straight-line" (tandem). the atmosplieric pressure, compressors

having steam and air pistons on the same for high locations must deal with air at piston rod, or they are connected to pressures below fifteen pounds absolute. cranks set at an angle on a common Under such conditions the volume of air shaft. The first method reduces floor taken into the compressor at each stroke space and cost, but requires very heavy weighs less, and therefore less air is de- fly wheels, and makes the machine liable livered by the compressor, while there is to stop on a center if run much below full a corresponding decrease in the power to speed and capacity. run the machine. The ratio of compres

It is evident that steam used expansion, and the rise in temperature, are pro- sively supplies against the piston a force portionately increased, so that a two decreasing toward the end of the stroke,

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