CLARKSON'S HEDGEHOG BOILER.
Page 13
Page 14
If you've noticed an error in this article please click here to report it so we can fix it.
A New Form of Coke-fired Construction With Blind Water Tubes.
AFEW WEEKS AGO, when we were considering the possible and probable effects of the impending new regulations foreshadowed by the Commissioner of Police for the Metropolis with regard to the licensing of future models of motorbuses for use in and round London, we drew attention to the fact that these stipulations would not• only affect the petrol-driven bus, but that the steam public-service vehicle would inevitably come under official review as well. That being so, it follows that the successful steam-driven chassis, developed and operated by that skilful and persevering engineer, Mr. Thomas Clarkson, will call for the attention of its designer with a view to a reduction, if possible, of the total weight of the chassis, or, at any rate to the redistribution of its weight to more effective rate,
The Boiler Under the Bonnet.
So far as the Commissioner's new requirements are concerned, there is only one steam chassis at present running to which they will apply, the Clarkson being unique, so far as public-passenger-service motor vehicles are concerned. The Clarkson, it will be recalled, has this boiler situated under a capacious bonnet in front of the machine, the engine and transmission gear being compactly disposed beneatfh and to the rear of the driver's seat. Necessarily, this arrangement throws considerable weight on the front wheels, but, so far, the present distribution has not adversely affected the suitability of the Clarkson sys. tern for such special employment. It is obvious, however, that, in the coming review of design, the boiler will be the tomponent which will cause as much anxiety as any other part of the chassis in any at tempt at redistribution of weight. The present Clarkson motorbus chassis have special vertical tubular boilers which are oil fired. Mr. Clarkson has also, with great ingenuity, perfected a method of automatic coke firing, but this method is at present not employed on chassis used for passenger-carrying vehicles in the Metropolis. Atihis stage of development, therefore, we, are particularly interested to note the publication of the complae,specification of yet another form of Clarkson boiler, embodying principles which are quite novel, so far as motor chassis construction is concerned.
The New Hedgehog Design. ,
We publish on this page a sectional elevation drawing of this new boiler, which is, for those who can read a mechanical drawing, almost i self-explanatory. In place of the usual vertical or horizontal fire tubes, a number of blind tapered water tubes of short length are arranged in such a manner as to protrude into the fire space. The boiler in question is similar, in one sense, to certain water tube patterns, but, so far as the main tubes are concerned, there is no through circulation Of the kind with which we are familiar. The protruding tubes, which are the outstanding feature of the design, are in section something like the old-fashioned candle extinguisher, or, perhaps even more, they may be likened to elongated thimbles. The boiler has elsewhere been dubbed the hedgehog, presumably from the lucus a non lucendo principle that its spikes do not stick outwards !
So far as the shell of the boiler is concerned, this consists of a vertical cylinder with anoth% of smaller diameter inside it, leaving an annular space between the two walls which contains the water. The "thimbles" are arranged staggered, and, as shown in the illustration on this page, protruding into the interior of the central fire space, thus meeting the lull up-draught of the heat from the fire bars below.
A central tube is arranged to facilitate firing with coke or coal. This also prevents the lodging of solid fuel on the tubes. Advantage is taken of this central construction to build round it a full single row set of coils, through which the steam from the top of the annular water space circulates and is superheated on its way to the main steam valve.
Circulation of a Complex Nature.
It will be seen that the principal novelty is the employment or the blind tubes, with a view to rapid steam raising. Suitable baffles are placed above the water level to prevent priming, and also, if necessary, in the fire space over the tubes, to cogill.eract any tendency towards unequal heating of the tubes. Many possibly' will doubt the effectiveness of circulation in and out of the blind tubes, and it is quite obvious that, while, no doubt, rapid temperature raising takes place through their media, the character of the convection currents induced within them must be of an extremely complicated and agitated nature. It should suffice, if rapid and steady generation is secured, even if exact information. as to the manner . in which the water rushes into the tubes and is expelled out of them in the form of steam bubbles is not thoroughly available.
Some of our readers may recall that at a time when there was a great clamour for the further development and adoption of the water-tube boiler, particularly for marine purposes, Yarrows' conducted very exhaustive tests with a view to determining the nature of the circulatory paths induced in high-capacity water-tube boilers which they were building for torpedo boats a number of years back. If we recall correctly, the result of these tests was to establish
the fact, at any rate for the time being, that the circulation would not be satisfactory, in the sense desired, unleSs very rample tubes were provided to facilitate the downward motion of the water moving to take the place of that which had rushed upwards as it was heated. We believe that, based on very exact experiments as were those conclusions, the necessity for these big downward tubes has for a long while past been abandoned. This evolution of design is cited with a view to showing how complex is the problem of circulation in ,ateam boilers, and, in particular, in the water-tube patterns. There is little doubt that the "thimble tubes of this latest Clarkson design will prove very fruitful in problems for the student.
A Lightweight Form of Construction.
We have not yet had the advantage of learning from Mr. Clarkson exactly the extent to which success has attended the introduction of this interesting model, nor do we know if, -as a soli,d-fuel-fired it will prove to be of unusually light weight compared with its steam-raising capacity ; but there appears to be every possibility• of such a result, in which event Mr. Olarkson's prolonged and successful efforts to produce a light-weight steam-driven chassis of relatively high-speed capacity appear likely to develop even further.
Taking into account the present severe restrictions on the use of liquid fuel of all kinds, and the probability that such restriction as will not be removed for many months ahead at least, it is quite probable that, manufacturing facilities permitting, the use of coke and coal-fired steam lorries will be extended on a very considerable scale. In the National interests particularly, Mr. Clarkson should be afforded very special facilities for further experiment at the present time. M.A.C.