Steam, Its Generation and Use

by Babcock & Wilcox Company

A positive and efficient circulation assures that all portions of the pressure parts will be at approximately the same temperature and in this way strains resulting from unequal temperatures are obviated.

If a shell or fire-tubular boiler explodes, the apparatus as a whole is destroyed. In the case of water-tube boilers, the drums are ordinarily so located that they are protected from intense heat and any rupture is usually in the case of a tube. Tube failures, resulting from blisters or burning, are not serious in their nature. Where a tube ruptures because of a flaw in the metal, the result may be more severe, but there cannot be the disastrous explosion such as would occur in the case of the explosion of a shell boiler.

To quote Dr. Thurston, relative to the greater safety of the water-tube boiler: “The stored available energy is usually less than that of any of the other stationary boilers and not very far from the amount stored, pound for pound, in the plain tubular boiler. It is evident that their admitted safety from destructive explosion does not come from this relation, however, but from the division of the contents into small portions and especially from those details of construction which make it tolerably certain that any rupture shall be local. A violent explosion can only come from the general disruption of a boiler and the liberation at once of large masses of steam and water.”

Economy—The requirement probably next in importance to safety in a steam boiler is economy in the use of fuel. To fulfill such a requirement, the three items, of [Pg 63] proper grate for the class of fuel to be burned, a combustion chamber permitting complete combustion of gases before their escape to the stack, and the heating surface of such a character and arrangement that the maximum amount of available heat may be extracted, must be co-ordinated.

Fire-tube boilers from the nature of their design do not permit the variety of combinations of grate surface, heating surface, and combustion space possible in practically any water-tube boiler.

In securing the best results in fuel economy, the draft area in a boiler is an important consideration. In fire-tube boilers this area is limited to the cross sectional area of the fire tubes, a condition further aggravated in a horizontal boiler by the tendency of the hot gases to pass through the upper rows of tubes instead of through all of the tubes alike. In water-tube boilers the draft area is that of the space outside of the tubes and is hence much greater than the cross sectional area of the tubes.

Capacity—Due to the generally more efficient circulation found in water-tube than in fire-tube boilers, rates of evaporation are possible with water-tube boilers that cannot be approached where fire-tube boilers are employed.

Quick Steaming—Another important result of the better circulation ordinarily found in water-tube boilers is in their ability to raise steam rapidly in starting and to meet the sudden demands that may be thrown on them.

In a properly designed water-tube boiler steam may be raised from a cold boiler to 200 pounds pressure in less than one-half hour.

For the sake of comparison with the figure above, it may be stated that in the U. S. Government Service the shortest time allowed for getting up steam in Scotch marine boilers is 6 hours and the time ordinarily allowed is 12 hours. In large double-ended Scotch boilers, such as are generally used in Trans-Atlantic service, the fires are usually started 24 hours before the time set for getting under way. This length of time is necessary for such boilers in order to eliminate as far as possible excessive strains resulting from the sudden application of heat to the surfaces.