Objections to the duplex construction are: The strains are indirect,
angular and intermittent. It is necessary therefore to largely increase
the strength of parts; to add a crank shaft of large diameter with
enormous bearings, and to build expensive and very secure foundations.
Should the foundations settle at any point, excessive strains will be
brought upon the bearings, resulting in friction and liability to
breakage. A steam engine meets with a resistance on its crank shaft that
is uniform throughout the stroke; while an air compressor is subject to
a heavy maximum strain at the end of the stroke, hence the importance of
direct straight line connection between power and resistance.
[Illustration: FIG. 8.]
The friction loss on a duplex compressor seldom gets lower than 15 per
cent., while straight line compressors show as low a loss as 5 per cent.
Fig. 8 illustrates the Rand Duplex Air Compressor, a machine largely
used in America, especially in the Lake Superior iron mines. Fig. 9
illustrates a Duplex Compound Condensing Corliss Air Compressor built by
the Ingersoll-Sergeant Drill Company.
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