The experiments made gave an evaporation of
7.61 lb. of water from 1 lb. of coal at 212 deg. Fahr., with 2 in. of
water pressure, and 6.41 lb. with 6 in. of pressure. These results are
low, but it is to be remembered that the heating surface is necessarily
small, in order to save weight, and the temperature of the funnel
consequently high, ranging from 1,073 deg. at the first pressure, and
1,444 deg. at the 6 in. With the ordinary proportions of locomotive
practice the efficiency can be made equal to the best marine boiler
when working under the water pressure usual in locomotives, say from
3 in. to 4 in., including funnel draught.
It has fallen to the lot of the writer to fit three vessels recently
with boilers worked under pressure in closed stokeholds. The results,
even under unfavorable conditions, were very satisfactory. The pressure
of air would be represented by 2 in. of water, and the indicated horse
power given out by the engines was 2,800, as against 1,875 when working
by natural draught, or exactly 50 per cent. gain in power developed.
Mr. Marshall then proceeded to refute the arguments which may be urged
against the use of the locomotive boiler at sea, and which we need not
reproduce. Coming to the engines, Mr. Marshall said that the total
working pressure of to-day may be accepted as 105 lb.
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