One feature of a jacket of this kind must receive the most careful attention. It must be as near absolutely smooth as it can be made by good workmanship. The rivets must be countersunk on the outside and driven down absolutely flush so that a film of water will travel over their heads without any interruption, and in general everything must be done to remove even the slightest obstruction from the path of the water film, because this film travels down the surface with considerable velocity and even a minor obstruction will throw it off, leaving a dry spot below the obstruction. This dry spot becomes hotter than the rest of the jacket and causes contraction strains and in time cracks result, while if it be a large one the lining on the inside of the jacket will melt off and the jacket itself will then be exposed to the assault of the streams of iron and slag running down inside it, which will soon cut a hole through it. Such holes are almost impossible to stop without a long shutdown on the furnace to enable a patch to be put on, and as this can only be done with patch bolts it is never nearly so good as the original jacket.

This is one of the places where an ounce of prevention is worth a ton of cure, and the only proper course is to make it impossible for a dry spot to form on one of these jackets.

Fig. 167 shows a furnace reconstructed according to the plans of the writer with a jacket of this type in which absolute smoothness was obtained by welding it up with an oxy-acetylene burner, then chiselling off the roughness at the joints and grinding it down smooth, an operation which took little time and cost a relatively small amount, something like a couple of hundred dollars above the cost of boiler work.

This jacket was flanged out at top and bottom to avoid the necessity of riveting angles on it to make connections to the mantel plate above and to the trough at its base. By flanging the metal out bodily these riveted joints are avoided.

Water is sprayed onto the top of this jacket by a circular spray pipe divided into eight sections, each of these being connected to the water supply separately, and each of them also being in duplicate, only one being used at a time. If one becomes obstructed or fails the other can instantly be put on. This feature, as a matter of fact, is used only when it is occasionally necessary to clean out one of the sections, but it is a convenience at that time and contributes to safety at all times and is well worth the trifling expense.

Some care should be taken to supply the water to these sprays under a very moderate head and make the holes in the spray pipes fairly large. They are then much less likely to become obstructed by small threads of pump packing and like and can be made to deliver the water exactly at the top line of the jacket with a minimum of spattering, the water thrown on to the jacket flows down it in a smooth film and the spatter is caught by the annular funnel shown about half way down the jacket, which is not riveted to the latter but comes in contact with it through a serrated edge at the bottom of the funnel. This serrated edge gives sufficient flow at every point around the circumference, which would not be the case if the bottom edge of the funnel were straight as the contact would be close enough in that case to cut off all flow in places.

By having the funnel hung from above by independent hangers it is possible to adjust with small wedges the location of its bottom edge in relation to the jacket to regulate the flow and get it uniform around the furnace, which could not so readily be done if the two were rigidly connected. Moreover when the funnel is supported from the jacket this involves rivets in the jacket for the funnel brackets and below these rivets it is almost impossible to avoid having a dry spot as above described.

This jacket was so extremely smooth and worked so admirably in service that the intermediate funnel could probably have been dispensed with entirely by widening out the bottom trough a little more and perhaps putting some flanged extensions around its upper edge to catch spatter.

The trough as shown projects out beyond the base of the funnel, but should have been made three or four inches wider than it is, because some of the drops from the bottom edge of the funnel are deflected from the vertical line by capillary attraction and fall outside the line of the trough which causes disagreeable spattering down below.

When using a construction of this type it is not only possible but far preferable to use a very thin brick lining in it, because the brick being cooled only from the outside, no matter how thick originally will rapidly Wear down to that thickness at which as much heat is carried away from their bases as the currents of molten material and gas supply to their front surfaces. This is only a few inches and if three feet of brick were to be built inside such a bosh jacket in the course of a few months it would all be gone down to a few inches, exactly the same as if it had started at one foot or even nine inches in thickness. Consequently if the furnace were the correct shape to begin with it obviously could not be so with three feet of brick scoured off the bosh all around, therefore the thing to do is to put in only as much brick in the first place as the furnace may be expected to leave there after it has settled down to regular work.