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About 1/240th of the capacity of the cylinder answers very well for the capacity of the feed pump in the case of low pressure engines, supposing the cylinder to be double acting, and the pump single acting; but it is better to exceed this size. Q. Is this rule for the size of the feed pump applicable to the case of high pressure engines?
Q. You stated that the capacity of the feed pump was 1/240th of the capacity of the cylinder in the case of condensing engines, the engine being double acting and the pump single acting, and that in high pressure engines the capacity of the pump should be greater in proportion to the pressure of the steam.
In low pressure engines the pressure in the boiler may be taken at 5 lbs. above the atmospheric pressure, or 20 lbs. altogether; and as high pressure steam is merely low pressure steam compressed into a smaller compass, the size of the feed pump in relation to the size of the cylinder must obviously vary in the direct proportion of the pressure; and if it be 1/240th of the capacity of the cylinder when the total pressure of the steam is 20 lbs., it must be 1/120th of the capacity of the cylinder when the pressure is 40 lbs. per square inch, or 25 lbs. per square inch above the atmospheric pressure.
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