Maxims and Instructions for the Boiler Room / Useful to Engineers, Firemen & Mechanics; Relating to Steam Generators, Pumps, Appliances, Steam Heating, Practical Plumbing, etc.

by N. Hawkins

WATER TUBE STEAM BOILERS.

Water Tube Boiler.—Fig. 26.

A popular form of steam boiler in use in the United States and Europe is what is called the water tube boiler. This term is applied to a class of boiler in which the water is contained in a series of tubes, of comparatively small diameter, which communicate with each other and with a common steam-chamber. The flames and hot gases circulate between the tubes and are usually guided by partitions so as to act equally on all portions of the tubes. There are many varieties of this type of boiler of which the cut illustrates one: in this each tube is secured at either end into a square cast-iron head, and each of these heads has two openings, one communicating with the tube below and the other with the tube above; the communication is effected by means of hollow cast-iron caps shown at the end of the tubes; the caps have openings in them corresponding with the openings in the tube heads to which they are bolted.

In the best forms of the water tube boilers, it is suspended entirely independent of the brick work from wrought iron girders resting on iron columns. This avoids any straining of the boiler from unequal expansion between it and its enclosing walls and permits the brick work to be repaired or removed, if necessary, without in any way disturbing the boiler. This design is shown in [Fig. 26].

The distinguishing difference, which marks the water tube boiler from others, consists in the fact that in the former the small tubes are filled with water instead of the products of combustions; hence the comparison, frequently made, between water-tube and fire tube boilers—the difference has been expressed in another way, “Water-tube vs. shell boilers,” but the principle of steam production in both systems remains the same; the heat from the combustible is transferred to the water through the medium of iron plates and in both, the furnaces, steam appliances, application of the draught, etc., is substantially the same. In another important point do the systems agree, i.e., in the average number of pounds of water evaporated per lb. of combustible; it is in the thoroughness of construction and skillfulness of adaptation to surroundings that produce the best results. Water tube or sectional boilers, have been made since the days of James Watt, in 1766, in many different forms and under various names. Owing, however, to the imperfection of manufacture the system, as compared to shell boilers, has been a failure until very recently; various patterns of water-tube boilers are now in most favorable and satisfactory use. The advantages claimed for this form of steam generator are as follows:

1. Safety from disastrous explosions, arising from the division of the contents into small portions, and especially from details of construction which make it tolerably certain that the rupture will be local instead of a general violent explosion which liberates at once large masses of steam and water.

2. The small diameter of the tubes of which they are composed render them much stronger than ordinary boilers.

3. They can be cheaply built and easily repaired, as duplicate pieces can be kept on hand. The various parts of a boiler can be transported without great expense, trouble or delay; the form and proportions of a boiler can be suited to any available space; and, again, the power can be increased by simply adding more rows of tubes and increasing the grate area.

4. Their evaporative efficiency can be made equal to that of other boilers, and, in fact, for equal proportions of heating and grate surfaces, it is often a trifle higher.

5. Thin heating surface in the furnace, avoiding the thick plates necessarily used in ordinary boilers which not only hinder the transmission of heat to the water, but admit of overheating.

6. Joints removed from the fire. The use of lap welded water tubes with their joints removed from the fire also avoid the unequal expansion of riveted joints consequent upon their double thickness.

7. Quick steaming.

8. Accessibility for cleaning.

9. Ease of handling and erecting.

10. Economy and speediness of repairs.

The known disadvantages of these boilers are

1. They generally occupy more space and require more masonry than ordinary boilers.

2. On account of the small quantity of water which they contain, sudden fluctuations of pressure are caused by any irregularities in supplying the feed-water or in handling the fires, and the rapid and at times violent generation of steam causes it to accumulate in the contracted water-chambers, and leads to priming, with a consequent loss of water, and to overheated tubes.

3. The horizontal or inclined water tubes which mainly compose these boilers, do not afford a ready outlet for the steam generated in them. The steam bubbles cannot follow their natural tendency and rise directly, but are generally obliged by friction to traverse the tube slowly, and at times the accumulation of steam at the heated surfaces causes the tubes to be split or burned.

4. The use of water which forms deposits of solid matter still further increases the liability to overheating of the tubes. It has been claimed by some inventors that the rapid circulation of the water through the tubes would prevent any deposit of scale or sediment in them, but experience has proved this to be a grave error. Others have said that the expansion of the tube would detach the scale as fast as it was deposited and prevent any dangerous accumulation, but this also has been proved an error. Again, the use of cast iron about these boilers has frequently been a constant source of trouble from cracks, etc.