These experiments proved conclusively that the size of the steam drums in the cross drum design has no appreciable effect in determining the amount of liberating surface, and that sufficient liberating surface is provided in the circulating tubes alone. If further proof of the ability of this design of boiler to deliver dry steam is required, such proof is perhaps best seen in the continued use of the Babcock & Wilcox marine boiler, in which the cross drum is used exclusively, and with which rates of evaporation are obtained far in excess of those secured in ordinary practice.
Quick Steaming--The advantages of water-tube boilers as a class over fire-tube boilers in ability to raise steam quickly have been indicated.
Due to the constant and thorough circulation resulting from the sectional nature of the Babcock & Wilcox boiler, steam may be raised more rapidly than in practically any other water-tube design.
In starting up a cold Babcock & Wilcox boiler with either coal or oil fuel, where a proper furnace arrangement is supplied, steam may be raised to a pressure of 200 pounds in less than half an hour. With a Babcock & Wilcox boiler in a test where forced draft was available, steam was raised from an initial temperature of the boiler and its contained water of 72 degrees to a pressure of 200 pounds, in 12½ minutes after lighting the fire. The boiler also responds quickly in starting from banked fires, especially where forced draft is available.
In Babcock & Wilcox boilers the water is divided into many small streams which circulate without undue frictional resistance in thin envelopes passing through the hottest part of the furnace, the steam being carried rapidly to the disengaging surface. There is no part of the boiler exposed to the heat of the fire that is not in contact with water internally, and as a result there is no danger of overheating on starting up quickly nor can leaks occur from unequal expansion such as might be the case where an attempt is made to raise steam rapidly in boilers using water leg construction.
Storage Capacity for Steam and Water--Where sufficient steam and water capacity are not provided in a boiler, its action will be irregular, the steam pressure varying over wide limits and the water level being subject to frequent and rapid fluctuation.