Steam, Its Generation and Use | Page 7

Plants a Total of 9280 Horse Power of Babcock & Wilcox Boilers]

REQUIREMENTS OF STEAM BOILERS
Since the first appearance in "Steam" of the following "Requirements of a Perfect Steam Boiler", the list has been copied many times either word for word or clothed in different language and applied to some specific type of boiler design or construction. In most cases, although full compliance with one or more of the requirements was structurally impossible, the reader was left to infer that the boiler under consideration possessed all the desirable features. It is noteworthy that this list of requirements, as prepared by George H. Babcock and Stephen Wilcox, in 1875, represents the best practice of to-day. Moreover, coupled with the boiler itself, which is used in the largest and most important steam generating plants throughout the world, the list forms a fitting monument to the foresight and genius of the inventors.

REQUIREMENTS OF A PERFECT STEAM BOILER
1st. Proper workmanship and simple construction, using materials which experience has shown to be the best, thus avoiding the necessity of early repairs.
2nd. A mud drum to receive all impurities deposited from the water, and so placed as to be removed from the action of the fire.
3rd. A steam and water capacity sufficient to prevent any fluctuation in steam pressure or water level.
4th. A water surface for the disengagement of the steam from the water, of sufficient extent to prevent foaming.
5th. A constant and thorough circulation of water throughout the boiler, so as to maintain all parts at the same temperature.
6th. The water space divided into sections so arranged that, should any section fail, no general explosion can occur and the destructive effects will be confined to the escape of the contents. Large and free passages between the different sections to equalize the water line and pressure in all.
7th. A great excess of strength over any legitimate strain, the boiler being so constructed as to be free from strains due to unequal expansion, and, if possible, to avoid joints exposed to the direct action of the fire.
8th. A combustion chamber so arranged that the combustion of the gases started in the furnace may be completed before the gases escape to the chimney.
9th. The heating surface as nearly as possible at right angles to the currents of heated gases, so as to break up the currents and extract the entire available heat from the gases.
10th. All parts readily accessible for cleaning and repairs. This is a point of the greatest importance as regards safety and economy.
11th. Proportioned for the work to be done, and capable of working to its full rated capacity with the highest economy.
12th. Equipped with the very best gauges, safety valves and other fixtures.
The exhaustive study made of each one of these requirements is shown by the following extract from a lecture delivered by Mr. Geo. H. Babcock at Cornell University in 1890 upon the subject:

THE CIRCULATION OF WATER IN STEAM BOILERS
You have all noticed a kettle of water boiling over the fire, the fluid rising somewhat tumultuously around the edges of the vessel, and tumbling toward the center, where it descends. Similar currents are in action while the water is simply being heated, but they are not perceptible unless there are floating particles in the liquid. These currents are caused by the joint action of the added temperature and two or more qualities which the water possesses.
1st. Water, in common with most other substances, expands when heated; a statement, however, strictly true only when referred to a temperature above 39 degrees F. or 4 degrees C., but as in the making of steam we rarely have to do with temperatures so low as that, we may, for our present purposes, ignore that exception.
2nd. Water is practically a non-conductor of heat, though not entirely so. If ice-cold water was kept boiling at the surface the heat would not penetrate sufficiently to begin melting ice at a depth of 3 inches in less than about two hours. As, therefore, the heated water cannot impart its heat to its neighboring particles, it remains expanded and rises by its levity, while colder portions come to be heated in turn, thus setting up currents in the fluid.
Now, when all the water has been heated to the boiling point corresponding to the pressure to which it is subjected, each added unit of heat converts a portion, about 7 grains in weight, into vapor, greatly increasing its volume; and the mingled steam and water rises more rapidly still, producing ebullition such as we have noticed in the kettle. So long as the quantity of heat added to the contents of the kettle continues practically constant, the conditions remain similar to those we noticed at first, a tumultuous lifting of the water around the edges, flowing toward the center and thence downward; if,