A modification of double-ended boiler is
that introduced by Mr. Alfred Holt. It has many decided advantages,
but is costly to make. The formation of the two ends into separate
fire-boxes leaves the bottom of the boiler free to adapt itself to the
variations of temperature to which it is exposed. The separation of the
furnaces from the combustion chamber, excepting through the opening
afforded by a connecting tube, is an advantage in the same direction,
and avoids almost entirely the racking strains due to irregular furnace
action. The weight of water carried is less, and that of the boiler
may also be made less; while the elliptical form of the two ends gives
greater steam space.
A type of boiler largely used in her Majesty's Navy, somewhat like a
locomotive boiler, is highly efficient in regard to weight and power
developed. Many examples have yielded one indicated horse-power in the
cylinders for every three square feet of heating surface, under natural
draught and with a very moderate height of funnel; and this with a
consumption of fuel not exceeding 21/2 lb. per indicated horse-power per
hour under a working pressure of 60 lb. With the aid of a steam jet in
the funnel, the heating surface per indicated horse-power has fallen
below 21/2 square feet.
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