CHARACTERISTICS OF THE IDEAL BOILER

A

STEAM boiler is an apparatus for abstracting the heat from hot gases and transferring it to water so as to form steam of a desired pressure. The hot gases are supplied by the combustion of fuel in a furnace. Theoretically, the furnace is not part of the boiler, but in practice it is generally convenient to have them so intimately associated that they are parts of the same structure.

The essential part of the boiler is the heating surface, composed of metal sheets in plates or tubes, with the hot gases on one side and the water and steam on the other. There must, of course, be an exit for the waste gases and it is convenient and almost universal to have a reservoir for steam and water.

The efficiency of the boiler really consists of two factors, the efficiency of the furnace and that of the heating surface. The former depends upon a proper supply of air and its intermixture with the burning fuel so as to produce perfect combustion, so that the fuel shall all have been gasified before the transmission of heat to the water begins. This involves a commodious chamber in which the gases will have room for mixture and preferably with non-conducting sides. This last is not possible as an entirety in usual practice.

The efficiency of the heating surface depends chiefly upon the proper circulation of the water and the hot gases with reference to each other, and obviously the more intimately and thoroughly they interpenetrate each other, the greater the absorption of heat and the higher the efficiency. A little thought will show that this interpenetration will come about most thoroughly when one of the fluids passes through a series of tubes.

Although apparatus like modern water-tube boilers existed nearly two thousand years ago (as shown by the excavations at Pompeii), they seem to have been entirely forgotten and the evolution of the steam boiler started from the humble pot or kettle. When powers were small, pressures low and weights relatively unimportant, the serious and inherent defects of this class of boiler and its evolutionary improvements were not clearly seen, but, with high pressures and forced combustion, they were brought out and the advantages of the water-tube boiler commended it. As bearing on efficiency, one of these is that, nearly all the heating surface is in tubes, and they can be so disposed with reference to the path of the gases as to give the most complete interpenetration of the two fluids. It would be anticipated that this would result in the highest efficiency, and careful tests show this to be the fact.

Accessibility for cleaning the interior and exterior surfaces of the tubes is most important for efficiency and with it facility for removing and replacing a tube without disturbing others.

The boiler must be safe against disastrous explosion. The watertube boiler from its small parts, which have an excess of strength, and from the small amount of contained water, possesses this feature in a high degree.

The boiler should be able to withstand sudden and rapid changes of temperature. This means that its parts must have great freedom of expansion. The entire lack of this feature is one of the great drawbacks to the old-type shell boiler.

The boiler should be rugged or robust, that is, the scantlings of its various parts should be adequate to enable it to withstand, uninjured, the kind of treatment a boiler is sure to receive. Certain classes of water-tube boilers, designed for extreme lightness, can not possess this feature, and, as they were first in the marine field, many engineers have supposed that this fragility is an essential of water-tube boilers. This is not only not true, but some water-tube boilers, notwithstanding their much less weight, are actually more robust than the old-type shell boilers.

ADMIRAL MELVILLE'S LIST OF ESSENTIAL CHARACTERISTICS OF THE IDEAL WATER-TUBE BOILER

T

HE late Admiral George W. Melville was Engineer-in-Chief of the United States Navy from 1887 to 1903, and was recognized throughout the world as one of the foremost leaders of the engineering profession, and one of the most lucid writers on engineering topics. His last published article appeared in the Engineering Magazine (New York) for January, 1912, and was entitled, "The Development of the Marine Boiler in the Last Quarter Century." After reciting his study of water-tube boilers and early experience with them he says:

"From my study of the subject, I had reached the conclusion that the thoroughly satisfactory water-tube boiler should possess, among others, the following characteristics:

66

'Reasonable lightness, with scantlings sufficient to promise reasonable

longevity;

"An adequate amount of water, so that failure of the feed supply or any inattention thereto would not immediately cause trouble;

"Accessibility for cleaning and repairs on both water and fire sides;

66

'Straight tubes, with no screw joints in the fire, but the simple expanded

joints so well tested out for years;

"No cast metal, either iron or steel, subjected to pressure;

"Ability to raise steam quickly;

"High economy of evaporation;

"Economy of space;

"Interchangeability of parts, and, as far as possible, the use of regular commercial sizes, so that repair material could be procured anywhere;

"The ability to stand severe forcing without injury;

"The ability to stand abuse—that is, to be of rugged construction and not so delicate as to require skilled mechanics to run it;

"Safety against disastrous explosion, meaning that only the part of the boiler which gave way would be damaged."

He then tells of making several installations of Babcock & Wilcox boilers and of the "Alert" design, and says of this, "To my mind it fulfilled almost perfectly my list of characteristics and gave a water-tube boiler about as good as we are likely to get." He adds later, “Believing that I had found the boiler best adapted to use on large war vessels, and confirmed in this view by their performance in service, I continued to install the Babcock & Wilcox boiler as long as I remained Engineer-in-Chief, and my successors have done the same."

We will now go over to the Admiral's list in detail and give data

showing how thoroughly the Babcock & Wilcox boiler fulfills the require

ments.

REASONABLE LIGHTNESS, WITH SCANTLINGS SUFFICIENT TO PROMISE

REASONABLE LONGEVITY

Table I. shows the weight of the Babcock & Wilcox boiler as compared with other water-tube boilers, and it will be seen that for the same diameter and thickness of tubes it is as light as any. It is very important to note that, as built for naval vessels and mail steamers, its weight, with water, for 250 lbs. pressure is about 25 lbs. per square foot of heating surface, as against 60 lbs. for naval vessels and 75 lbs. for mail steamers using cylindrical or Scotch boilers and carrying 175 lbs. pressure. (In the case of large boilers, arranged for oil-firing, the weight is below 20 lbs. per square foot of heating surface.)

At the same time, the tubes are actually thicker than in Scotch boilers, while the headers are as thick as the furnace and combustion chambers. The resistance to corrosion and corresponding promise of longevity are in proportion to the thickness.

AN ADEQUATE AMOUNT OF WATER, SO THAT FAILURE OF THE FEED SUPPLY OR ANY INATTENTION THERETO WOULD NOT IMMEDIATELY CAUSE TROUBLE

There is no useless water in the boiler, every particle being in active circulation. Consequently, although the weight of water per square foot of heating surface is only from four to five pounds, the amount is sufficient, when carried at the normal level (the horizontal diameter of the drum), to permit the operation of the boiler under moderate combustion (15 to 20 lbs. of coal per square foot of grate) for a period of twenty to thirty minutes without feed before the water falls low enough to cause danger. This should not encourage carelessness about the feed, but shows that the same degree of care that is adequate for Scotch boilers will answer for Babcock & Wilcox.

ACCESSIBILITY FOR CLEANING AND REPAIRS ON BOTH WATER AND

FIRE SIDES

A reference to the detailed cuts and description of the boiler, already given, will show that the Babcock & Wilcox boiler secures this condition perfectly. It may be added that no other boiler does or can.

STRAIGHT TUBES, WITH NO SCREW JOINTS IN THE FIRE, BUT THE SIMPLE EXPANDED JOINTS SO WELL TESTED OUT FOR YEARS

This is one of the special characteristics of the Babcock & Wilcox

boiler. In conjunction with the expanded joints and the headers, with hand holes opposite groups of tubes, it enables any tube to be removed and a new one installed without disturbing any other.

NO CAST METAL, EITHER IRON OR STEEL, SUBJECTED TO PRESSURE

The tubes are of seamless steel; the drums, of the highest-grade boiler plate. The headers are made from the best quality of open-hearth flange plate in a series of presses, where the process is flanging and not drawing. The headers are the backbone of the boiler and it may be asserted with confidence that no part of any kind of steam apparatus is made of better material or with more care. Owing to process of manufacture, all the original good quality of the plate is retained. It may be remarked that tests to destruction have shown the headers to have a factor of safety of more than ten when carrying 300 lbs. pressure.

ABILITY TO RAISE STEAM QUICKLY

Repeated tests have shown that, with coal fires, steam can be raised from cold water (100 degrees temperature) to a pressure of 200 lbs. in about fifteen minutes. It is to be noted that this is not only a merit as making the boiler ready for quick use at any time, but it is the strongest testimony to its ability to stand rapid changes of temperature and its remarkable elasticity and freedom to expand and contract. It would be impossible to raise steam in any such period in a shell boiler, and such a boiler would be quickly ruined if steam were frequently raised in even a few hours. The safety with which steam can be quickly raised in the Babcock & Wilcox boiler gives it foremost rank as a Naval boiler and accounts for the great number that are used in Naval vessels and fireboats.

HIGH ECONOMY OF EVAPORATION

This is a point of great importance which has not been as fully appreciated as it should be. The efficiency of the Babcock & Wilcox boiler averages about ten per cent. greater than that of well designed Scotch boilers or 75% against 65%, with coal burned at moderate rates. The efficiency with oil fuel goes as high as 80%.

ECONOMY OF SPACE

Here again the Babcock & Wilcox boiler takes first place. Table I. shows a comparison with other water-tube boilers. With respect to Scotch boilers, the article of Admiral Melville, already referred to, takes the case of the great steamers "Mauretania" and "Luistania," whose space for Scotch boilers is 340 feet long by 61 feet wide, and shows that, disregarding their superior economy, Babcock & Wilcox boilers burning the same amount