III. HIGH-PRESSURE COMPRESSORS.
B. High-duty Apparatus.-
2. Compressors cooled by Water-Envelope.-In the majority of compressors the refrigeration is accomplished by Jacketing the cylinder and causing a circulation of water in the annular space between. Failure of this means is mostly due to the fact that the air remains dry, and in this condition the compression causes a development of heat which increases rapidly with the pressures This heat is incompletely absorbed by the water because of the high velocity with which the air traverses the cylinder, and the consequence is that piston-packing and valves speedily deteriorate. These machines are most advantageously employed for pressures between 3 and 4 atmospheres.
One of the most successful compressors of this, class is that constructed by Mr. Sturgeon, in England, the disposition of which is shown in Fig. 148. The air-cylinder is attached to one side of a hollow bed or receiver, and is worked by a steam-engine attached to the other side through a crank-shaft carrying a fly-wheel at each end. To these fly-wheels the crank-pins are attached at right angles to each other, so that the piston of the steam-engine may be at the middle of its stroke and the best point of its power, when the piston of the air-engine is approaching the end of its stroke, where it meets the greatest resistance from the compression of the air.
The valve-boxes of the air-cylinder (Fig. 149) serve as covers,
and are bolted to the receiver. The inlet-valves are at the centre
'of the boxes. The construction is such that, as the piston begins
to recede, the rod carries the valve with it until its progress
is checked by a stop, it then being full open, and the rod, continuing
its movement throug the valve, holds the latter open until the
end of the stroke. On the commencement of the return-stroke the
valve is at once closed in the same manner. The delivery-valves
consist of a number of small valves distributed over the surface
of the cylinder-cover or valve-box, and affording a large area
of outlet opposite the direction of movement of the piston. The
following data relate to one of these compressors exhibited at
the Manchester (England) Exhibition of 1874: Absolute air-pressure,
3 atmospheres; volume of air furnished at this pressure, 35 cubic
feet. Motor: Diameter of piston, 11.7 inches; stroke, same; diameter
of fly-wbeels, 3 feet 10 inches. Compressor (double-acting):
Diameter of piston, 10.4 inches; stroke, 11.7 inches; useful volume
of cylinder, 1,037 cubic inches; revolutions per minute (average),
145-these have been carried as high as 440; theoretic volume delivered
per minute, 174 cubic feet.