I have already described the earlier and cruder methods of cleaning water-tube boilers. It is not too much to say that these methods were so ineffective and the difficulty of keeping the tubes of these boilers in condition was so great that it seriously retarded the introduction of this type of boiler.

When 'scale becomes too heavily incrusted upon the tubes of a horizontal tubular boiler to be removed in any other way, the tubes can be cut off and removed bodily from the boiler, and the scale can then be cleaned off of them without much trouble or expense. The tubes can then be welded out to their original length and replaced in the boiler. This is not a matter of prohibitive expense. But when a water tube becomes heavily incrusted with scale it cannot be cleaned even in this way because the scale is as inaccessible when the tube is out of the boiler and when it is in place. This is so serious a consideration that the writer on one occasion needing additional boiler capacity installed a horizontal tubular boiler rather than contend with the difficulties of cleaning the water-tube type.

In more recent years these conditions have been greatly improved for the water-tube boiler in two ways.

First, by the very general introduction at large plants which do not possess a natural supply of good boiler water, of purifying plants which neutralize any injurious acid which the feed water may contain (this in coal-mining regions is sometimes a very large quantity), and then, by the addition of suitable chemicals, precipitate out a large part of the scale-forming material which the water contains, and throw that which is left into a form which makes sludge that can be blown out instead of forming scale. Without such treatment this material settles on the tubes and other heating surfaces in a deposit almost as hard as iron, and almost as non-conducting as asbestos.

Second, by the introduction of power-driven boiler cleaners of various types. Generally the power is transmitted to the cleaner by water under pressure or by compressed air; the fluid under pressure in either case drives a motor small enough to go inside the tube and pass through it freely. This motor carries at its forward end steel cutters of various forms, generally loosely pivoted to the shaft of the motor and thrown out to the wall of the tube by centrifugal force.

The motor is run at a very high speed and the rapid whirling of these cutters removes the scale which is carried out of the tube by the "exhaust" of the fluid which drives the motor.

Much ingenuity has been employed both in making and in selling devices of this kind, and some of them are extremely meritorious. The motors in general are of two types, turbine and sliding-vane positive-pressure motors. It would be thought that this would be the ideal field for the turbine, but I have obtained the best results with the sliding-vane positive-pressure motor.

As between the two fluids, water and air, the advantages, in my judgment, are so strongly in favor of compressed air as to put it beyond comparison. This is for two or three reasons. First, the men working with the apparatus are at close quarters with it in a confined space, and no matter how carefully made and operated water-driven apparatus may be, under such circumstances it is impossible for the operator to avoid becoming wet and very dirty from the sludge produced by the grinding up of the scale. This makes the task an excessively disagreeable one for the operator, and materially reduces his speed in accomplishing it. Secondly, the exhaust water from the motor cannot be prevented from reaching the brickwork of the setting to a greater or less extent, and in the Babcock & Wilcox boiler it is almost impossible to prevent its running down into the underground flue with a bad effect upon both the draught and the flue.

With air none of these disadvantages exist. The air is perfectly clean, and its expansion even tends to cool to a small extent the air in the drums which the operator has to breathe, so actually ameliorating the conditions under which he has to work, while, of course, it has no effect whatever on the setting. Moreover, the amount of power needed may be transmitted through a much smaller hose with compressed air than with water, and therefore the hose which connects the motor with the supply of power, and by which it is pushed through the tubes, is correspondingly smaller, lighter and easier for the operator to handle.

By the use of the best types of these cleaners boiler tubes can be kept clean at the fraction of the expense and with a fraction of the time lost from service that are required when hand-cleaning is used.

Moreover, it is very much to be doubted whether hand-cleaning ever cleans the tubes so thoroughly as do power cleaners, no matter how conscientiously the former may be done.

I have seen the power cleaner applied to boilers supplied with the best natural feed water which had received the most thorough care, were thought to be absolutely clean, and were, as far as could be seen, but at certain points the cleaner found and removed scale a quarter inch or more in thickness, which had been entirely invisible from the ends, although these were straight tubes.

Owing to the short length of these cleaners they may be used on curved tubes as well as straight ones and are equally effective therein.

This goes far to remove the disadvantage under which the Stirling and other curved-tube boilers formerly labored and to put them on a parity from an operating point of view with the straight-tube types, but, of course, it does not remove their disadvantage in the matter of removing and replacing tubes.

The power boiler cleaner is like many other useful tools in that it must be used with care and discretion.

In order to cut the scale the cutters must be good tool steel, almost as hard as glass and very sharp. In order to be effective they must run at a speed of many hundred revolutions per minute. It is obvious that if allowed to run in contact with the bare tube in one place they would soon cut it away enough to weaken it seriously if not to cut it through entirely, and the operator must be carefully instructed and advised to prevent this.

These two developments of the past decade and a half - the feed-water purifying plant which keeps out of the boilers much of the scale-forming material, and the power-drive tube cleaner which removes that which gets in - have reduced to a fraction of their former amounts the time, labor, worry, and expense of boiler maintenance, increased their average efficiency and decreased the percentage of spares necessary. They have thus gone far to solve the problems of boiler operation at blast-furnaces.