gets hot, and, water being a bad conductor of heat, little cooling during compression takes place. As, however, a certain quantity of water is carried over into the delivery pipes at each stroke, the air is cooled before it gets to the receiver; but to be of any economical good, cooling should take place during compression. Indeed, it has been found that, to get good results, spray injection has to be introduced near to the outlet valve.

Compressors of this class resemble pumps, and must work at slow velocities. As a large body of water has to be set in motion and stopped at each stroke, considerable friction is caused, and the machine subjected to severe shocks. It must, therefore, be made very strong, and to produce the same quantity of air as a high-speed compressor must be considerably larger, and take more power to drive.

The actual position of affairs seems, therefore, to be that, by the assistance of a water piston and spray injection, a certain economy in compression is gained, while this advantage is neutralised by the extra power required to drive the machine. In addition, there is the difference between first cost and cost of maintenance in the two systems. It is impossible either to purchase large engines, or to keep them working, at the same cost as smaller ones.

The second division of wet compressors is that in which water is injected directly into the cylinder. This answers well in keeping down the temperature, provided that the water is in the form of fine spray, that it meets the piston during compression, and that it is in thorough. contact with the air. A further economy results from the fact that the power required to compress moist air is less than that required for dry air. The injected water also fills clearance spaces, and prevents loss from this cause. The absence of these in a compressor cylinder is a point of high importance. No spaces should exist between the piston and the cylinder cover at the termination of the stroke, because such spaces are filled with air at high pressure, and, on the retreat of the piston, this air expands and fills the cylinder, no free air entering until the pressure is reduced to that of the atmosphere.

To avoid clearance losses the pistons are often arranged to run dangerously close to the cylinder cover. Mr. Sturgeon has adopted a

A

sliding end, which is lifted slightly at the end of each stroke. preferable plan is to arrange by-pass grooves in the end of the cylinder (a a, Fig. 55) in such a manner that when the piston passes over them at the end of the stroke the high-pressure air escapes from the back to the front of the piston. As the pressure is thus suddenly taken off the steam piston must be properly cushioned or an injurious blow will result.

It is, however, believed that the cooling results obtained by the use of a spray of water are deceptive, as they take place principally after the air is completely compressed.

The objection to the injection system is the wear of the cylinder. and piston, caused by the fact that water is not only a bad lubricant itself, but its presence in the cylinder prevents oil, or grease, getting to the working parts, as it floats on the top of the water. The situation is bad when clean fresh water is used, but much worse when, as is often the case, it is necessary to employ acid water or water containing grit or sediment. Another objection is, that the compressed air produced contains a considerable amount of moisture, and that when used the exhaust ports of the motors become clogged up by the formation of ice. By a proper arrangement of reservoirs, or draining tanks, most of the moisture in the air can be removed before it is used in the motors.

The general type of modern air compressors consists of a pair of twin compound-condensing engines having the air cylinders arranged, tandem fashion, behind the steam cylinders. Compression should be in stages, and the air passed from the low-pressure cylinder through an intermediate cooling receiver to the high-pressure cylinder. Many high-class compressors are now fitted with mechanically-operated suction and delivery valves, but, speaking generally, this is a refinement which only pays in the case of large installations. With a single engine and air cylinder arranged in a straight line it is impossible to construct an economical machine, because the greatest work in the air cylinder has to be done at the end of the stroke. At the beginning of the stroke, when the steam has full pressure, the air cylinder contains air at atmospheric pressure, and offers no resistance, but at the end of the stroke, when the pressure in the steam cylinder would be low (if expansion were used), the resistance in the air cylinder is at its maximum. All sorts of arrangements have been designed to equalise the power and resistance, but have given way to the straight line pair type, with cranks set at right angles. Expansive working can then be used, as one steam cylinder is always exerting its maximum power at the moment when the air cylinder of the other engine is finishing its stroke.

This explains the seeming paradox, how steam, say, at 50 lbs., can compress air to 70 lbs., where both cylinders have the same diameter and stroke. When it is remembered that at the commencement of the stroke the pressure in the air cylinder is nothing, that for three-fourths of the stroke it is considerably below 50 lbs., and that only at the moment of discharge does it reach 70 lbs., the explanation is selfevident.

Various Valves on Air Compressors :

Walker's Valves.-The inlet valve is connected by a link (a, Fig. 56), and piston, b, with a controlling spindle, c, these reciprocating with the

movement of the valve. When the piston retreats suction opens the valve, which is prevented from going too far by the spring d, which becomes compressed. Immediately the piston starts to return the valve is closed by the spring, and prevented from being violently dashed on to its seat by the collar f, which moves with the spindle coming into contact with the india-rubber buffer k, carried on a fixed abutment, 1, suspended by two bars, m m', attached to the cylinder cover n. Messrs. Walker's experience has shown that it is also desirable to buffer the valve on its in-stroke, this being done by a second india-rubber washer, h, striking against another fixed abutment, i. It will be noticed from the drawing that the tension of the spring and the position of the stops can be varied, if desired, by a nut and lock-nut arrangement, e and f.

The outlet valve (Fig. 57) is balanced by making a portion of the spindle passing through the stuffing-box hollow, the outer end passing into a small cylinder, a, into which air is admitted at the same pressure as in the receiver. The valve is prevented from opening too rapidly by the spring b, and is buffered on its in-stroke and out-stroke by stops c c', arranged to engage the india-rubber blocks e e' (carried by a fixed cross bar f), just prior to the termination of the valve's travel.

The india-rubber blocks are annular in form, but somewhat of a T-shape in cross section, the faces of the annulus being widened out to leave a projecting flange at the inner and outer periphery. With this shape it has been found that the life of the blocks is considerably increased.

Sturgeon's Valve.-The feature of Sturgeon's air compressor consists of a stuffing-box inlet valve, which is opened by the piston-rod at the commencement of its stroke, this doing away with the necessity of forming a vacuum in order to cause the valve to open. A complete cylinder, full of air at atmospheric pressure, is taken in at each stroke, and immediately the piston starts to return the valve shuts. In

Fig. 58, i is the inlet valve attached to the stuffing-box of the pistonrod. By means of the nuts a a sufficient grip can be obtained to ensure the valve opening on the forward and backward strokes of the

PISTON

a intet

ROD

Fig. 58.

piston. The stops b b, screwed to the valve, limit its travel in one direction, while its flange portion performs the same office in the other. The piston is recessed to fit over the valve at the termination of each stroke, and reduce clearance to a minimum. The outlet valves o o are usually eight in number, and can be taken out separately for repairs, or removed by unscrewing. A spiral spring, c, in each one serves to bring it back sharply on to its seat. The arrows show the direction of the air both from the inlet and delivery valves.

Ingersoll-Sergeant Valve. This consists of two annular valves (a, Fig. 59) placed in a hollow piston of a double-acting air cylinder, free air being admitted through a hollow tail-rod attached to the piston. The valves do not require the aid of springs or other connections, but are opened and closed at the proper moment by their own inertia. The arrows show the direction of the intake and

delivery; the outlet valves are shown at b. To reduce clearance small recesses, c, are turned in the cylinder covers, into which the inlet valve fits at the termination of the stroke. As there are no inlet valves in the cylinder covers water-jackets, d d, are provided at each end, as well as around the sides e e. The air passes into the receiver through f. A perspective view of the valve is shown in Fig. 60. Additional advantages are derived from the absence of valves in the cylinder covers, and the admission of air through the hollow pistonCompressed Air Production (Wm. L. Saunders, New York, 1891), 20.

rod. Not only can the cylinder be water-jacketed all round, but what is of far more importance, air can be drawn from outside the engineroom through the piston-rod, resulting in a considerable higher compression efficiency, as on a cold winter's morning the difference in temperature between the inside and outside of the engine-room may amount to 70° F.

Riedler Valve. -Both the inlet and outlet valves of the Riedler compressors are closed in the same manner as the pump valves of this design (for description see chapter on Pumping). The valves are perfectly free to open, but are closed at the proper moment by a cam controlled from the wrist-plate driving the steam admission valves, at a time when the wrist-plate is moving at its highest velocity, while the air piston is nearly at a state of rest. As each inlet valve is closed at the same time as the opposite outlet valve, and as the theoretical velocity of the piston movement at the time of closure is zero, there should be no wear nor shock on the valve face and seats. As no springs nor loose pieces are employed in the gear, the speed of operation is dependent only on the practical speed at which the steam engine may be worked.

West and Jenkin's Valve.*-The inlet valve is of the mushroom type, provided with two springs on the valve stem. The lower one tends to open the valve, while the upper one, which is much stronger, tends, when free, to keep it on its seat. By an arrangement of levers and cams similar to Fig. 62 the latter spring is compressed, or let go, at the desired moment. When the piston is nearly at the end of the compression stroke, the cam compresses the stronger upper spring and relieves the valve of the downward pressure. If it were not for the pressure of air in the cylinder, the weaker opening spring below would then immediately open the inlet valve, but this cannot happen until all the compressed air in the cylinder has been exhausted through the delivery valve and the piston commences to make its suction stroke.

The outlet valves are also controlled by a cam, but are arranged with springs, and are free to open of their own accord in case the pressure in the receiver falls below the normal.

This is a point of great importance in all mechanically-actuated valves, because, unless some such device is in operation when the air is being used from the receiver faster than the compressors can supply it at standard pressure, the compressor presses air up to the maximum, and then delivers it into the mains where it expands down to the pressure existing there.

Means to prevent "Dancing" of Valves. In the ordinary form of valves to which a spring is connected vibratory motion is set up, because the air tries to pull the valve open and the spring to shut it, and first one and then the other prevails.

The dancing of the valve in Walker's air compressor is reduced by causing a certain amount of friction to be set up between the spindle (c, Fig. 56) and one or more of its bearings. To accomplish this, where the spindle passes through the cross-bar (p, Fig. 56), the bearing is split longitudinally, so that the bore of the bush can be slightly contracted by means of a screwed spindle (a, Fig. 61), having a handwheel, b, and lock-nut, c, connected to the top half of the step. To provide a greater frictional surface, the spindle is made of larger dia*Fed. Inst., vii., 239.