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CHAPTER XIV

THE TUBES AND FLUES

Q. What is a combustion chamber, and of what are its flues made?

A. In America, of iron and of Bessemer steel; in Europe, of these metals and also of copper and of brass. Q. When were copper and brass flues or tubes abandoned in America?

A. After coal was substituted for wood.

Q. What are the usual dimensions of locomotive flues or tubes?

A. Ten to 14 feet long, two inches in diameter.

Q. What is the best length of locomotive flues?

A. From sixty to eighty times the diameter-from 12.4 to 14 feet gives the best results.

Q. What is the disadvantage of great length?

A. Increased friction; less heating power at the front end than at the back; leakage owing to vibration.

Q. What is the minimum distance apart for tubes? A. Eleven-sixteenths inch in the clear; 3/4 inch is better.

Q. What is the disadvantage of having them close? A. They weaken flue sheets, retard circulation, and give sediment better lodging.

Q. What is the most difficult boiler trouble to control?

A. Leaky tubes.

Q. Will wide spacing cure leaks?

A. No.

Q. What are the influences affecting this leakage? A. Length of flue, quality of water and of coal, method of firing and of working the injectors or pumps,

weather, and severity of service; also the size of the nozzle.

Q. What is the influence of the nozzle on the flue or tube?

A. The smaller the nozzle the more trouble with them leaking.

Q. What causes tend to compel a reduction in the size of the nozzle?

A. Poor coal and severe service.*

Q. Why is this?

A. The smaller the nozzle the more severe the blast and the greater the blow-pipe action on the flue ends, making them hotter than the sheet in which they are expanded, so that when they cool down they become smaller than the holes in the sheet.

Q. In what part of the sheet are there usually the most leaky flues and tubes?

A. Usually below the boiler axis.

Q. What does this indicate?

A. That the short flames of highest temperature go into the lowest flues.

Q. To what design would this seem to point?

A. To one having a great depth of box below the flues, to keep them from the action of these hottest flames.

Q. Which are apt to have more trouble with leaking flues; through trains or heavy locals?

A. The latter, because as a rule with the severe exhaust in starting with the lever in the corner the ashpan opening is not able to take care of all the draft, taking it through the fire-box and heating it before it comes to the flues; therefore a certain amount of cold air comes through the fire-box door.

Q. How can this be avoided?

*See under "Exhaust Nozzles."

A. Only by a milder exhaust in pulling out from stations.

Q. Which ends of the flues give most trouble?

A. Those in the fire-box.

Q. Why not use tubes of a larger diameter?

A. Because it is best to divide the combustion-gases into small streams, each of which has it outer surface next a surface of metal, on the other side of which there

[graphic][graphic][subsumed][subsumed][subsumed][subsumed]

Fig. 78. Cross Section, Tube Sheets, K 4 s Engine, P. R. R.

is water to be heated. If the tubes were four inches in diameter, nearly all the heat of the central portion (say two inches in diameter) would be wasted, not having time to be delivered to the metal and through to the water on the other side.

Q. Why not have tubes only one inch in diameter, and give still more heating-surface?

A. Because there would be too great liability of clogging up, and also too much friction between the gases and the tube-surface.

Q. What is the disadvantage of excessive total crosssection of tubes?

A. Too slow draft, causing soot deposit.

Q. What is the disadvantage of too small total cross area of tubes?

A. Obstruction to the draft; besides which, the tubes are more liable to be clogged with cinders, and there is less space left when they are clogged.

Q. What is the disadvantage of too short tubes?

A. The combustion-gases get into the smoke-box before they have parted with enough of their heat, so the engine has both its capacity and its duty lessened.

Q. How are the tube-ends fixed steam-tight in the plates?

A. By expanding them.

Q. Are any additional means employed to render the tube-joints tight?

A. Usually there is a ferrule or thimble, either of copper, between the tube end and the edge of the hole, or of cast iron or steel, made tapering and driven in so as to force out the tube-end.

Q. What is the objection of the latter system of inside ferrules?

A. That it lessens the area of the tube orifices, and consequently diminishes the draft.

Q. What is the result of flue stoppage?

A. (1) There is less heating-surface, and (2) there is less draft to enable what heating-surface there is to be of use.

Q. How many tubes are there in a locomotive boiler? A. From 180 to 450.

Q. How much heating-surface is needed to evaporate six to eight tons of water per hour with the consumption of one ton of coal per hour?

A. From 1,000 to 1,500 square feet.

Q. Is there any other reason, besides the greater proportionate amount of heating-surface, for having small tubes?

A. They may be thinner to stand the same external pressure; this of course makes them cheaper, lessens the engine weight, and makes it raise steam rather more quickly.

Q. Why is the tube-plate thicker than the shell?

A. Largely by reason of its being greatly weakened by the large number of holes cut in it, and partly because it has to sustain half the weight and sag of the tubes.

Q. Are tubes best arranged in vertical or in horizontal rows?

A. Some think in vertical, as that gives the water better chance to ascend among them. Others contend

Fig. 79. Counterbored Tube Sheet.

that it is no advantage to have the water rise too fast; that it is better to have it delayed a little in its upward passage, so as to be longer in contact with the tubes. But it must be remembered that the bottoms of the tubes are not their hottest portion.

Q. What is an undoubted advantage of up and down as against crosswise rows of flues?

A. They are more easy to free from scale.

Q. How are the tubes made tight in the flue-sheet? A. By being expanded from within so as to bear hard and steam-tight against the reamed edges of the hole;

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