The explosibility of a steam generator may be measured by the relation of its total capacity to its vaporizing power. The old fashioned generators and some of the modern ones are so constructed as to contain from fifteen to twenty times more water than they are able to vaporize within one hour. Thus a great quantity of heat is obtained and a uniform pressure assured, but the steam-generating apparatus is costly, heavy, and cumbersome; it requires a long time before the necessary pressure is obtained, and the generator is only suitable for a stationary installation and where it can uninterruptedly work for a long period of time. Besides, the enormous quantity of hot water under pressure constitutes a constant danger, and the explosion of a steam generator with boiler tubes becomes a real disaster.
In order to satisfy the requirements which have newly arisen in connection with navigation, locomotion, small motors and apparatus which need for their working an intermittent supply of steam, it became necessary to modify the construction of steam boilers, to augment their heating surface, to diminish their residue of water, and to gradually construct so-called inexplosible apparatus, of which the Belleville boiler forms one of the most characteristic prototypes.
In trying to reduce the inexplosibility to the utmost, Messrs. Serpollet Brothers have succeeded in constructing a type of boilers which may be called absolutely inexplosible, and this result has been obtained by reducing the capacity of the boiler to practically nil, thus rendering the explosibility also nil, for under the circumstances the relation between capacity and vaporizing power becomes itself nil.
1. General view of boiler (experimental arrangement).
2. Cross section of boiler (natural size). The line A B indicates, at somewhat exaggerated scale, the cross section of the interior empty space of the boiler.
The method employed for this purpose by Messrs. Serpollet is an extremely simple one. A cylindrical steel tube of convenient diameter and sufficient thickness is rolled flat at a temperature below the white heat of the metal, and the last touch of the rollers is given to it when already cold. By this means a flat tube is obtained, the empty interior space of which looks in a cross section (Fig. 1, No. 2) like a black line not thicker than a hair, and measures from 0.1 to 0.3 millimeter. This tube is finally rolled up in the form of a spiral, or left straight, according to the use to be made of it, and put into an appropriate furnace (Fig. 1, No. 1). To either end of the tube a joint is attached, the one for the purpose of admitting the water, the other for admitting the steam.
When under these circumstances the tube has been heated to a high temperature in a convenient fire box, the water which has been pumped into it, by a feed pump fastened to one of its extremities, is instantly changed into steam and escapes at the other end at a pressure and in a state of dryness depending on the working conditions of the apparatus. The ingenious and really original and novel idea in this invention is this flattened tube, which constitutes an actual capillary boiler inside of which the water squeezed in between its walls cannot assume its spheroidal state, and the formation of drops becomes absolutely impossible. There exists no longer a residue of hot water, nor are water gauges, safety valves, or any other of those numerous accessories required which make all steam boilers so complicated and which augment considerably their cost.
It also becomes unnecessary to connect the joint from which the steam escapes by means of a valve with the motor for which the steam is to be used. If the supply of steam is to be stopped, this can be done by simply suppressing the supply of water, i.e., by emptying the boiler.
The regular working is assured by the quantity of heat contained in the heated iron tube, to which, for this purpose, an intentionally great thickness has been given, and it is this heat of the iron which replaces the heat furnished by the hot water in the steam generators with boiler tubes. From the above it will be easy to understand the general arrangement of the new steam generator, when connected with its motor. This motor works a small intermitting pump, which supplies the capillary boiler with water, according to the quantity consumed. The machine is started by means of a small special pump worked by hand.
Whenever the velocity of the motor tends to increase, a centrifugal regulator placed upon the motor reduces the action of the pump and, consequently, the supply of water to the tube, thus checking the velocity of the machine. If the velocity tends to slacken, the inverse process is employed. In order to stop the machine, it suffices to turn off the water furnished by the pump by means of a three-way cock, and to send the water back to the reservoir of supply. The boiler can be emptied in less than a second, and the motor stops in consequence of being deprived of motive power.
The whole is marvelously simple, and creates astonishment and admiration in the mind of even the most skeptical persons who see the apparatus.
The boiler of the one horse power type weighs 33 kilogrammes. It consists of an iron tube having a length of 2 meters and a height of 10.5 centimeters after it has been flattened; the total heating surface thus obtained being 48 square centimeters. The power of vaporization amounts to 20 kilogrammes of water per hour, while the quantity of coal consumed during the same period amounts to only 4 kilogrammes, which is comparatively little for a boiler of so small a power.
Fig. 2 shows the first model of a tricycle constructed by Messrs. Serpollet as an application of their boiler for locomotion. The writer has seen the working of this apparatus, and consequently is able to give some data. The total weight of the machine is 185 kilogrammes, or about 250 kilogrammes when mounted by a person. The boiler is placed behind the tricycle, the motor is under the seat, inside of which is the water reservoir and the supply of coal. In the motor employed in the present case the feed pump is a constant supply pump, but by means of a directing lever turning around its own axis and acting upon a three-way cock, the water can be divided into two streams, the one emptying into the feeding reservoir, the other into the boiler. By varying the position of the cock, the power of the machine can be modified and its velocity regulated. The machine can be brought to a stop within less than two meters by means of the combined action of a brake and the complete suppression of water in the boiler.
In order to start the machine, the water is sent into the tube by a little extra pump worked for a moment by the left hand of the cyclist when starting.
On July 25 some experiments were made before the Society of Civil Engineers with the tricycle above described, and on that occasion it traversed the Rue Girardon and the Rue de Norvino to Montmartre (streets in which the gradient rises to 15 centimeters per meter) with a velocity of three meters per second.
Fig. 3 represents the arrangement of the first stationary boiler of the new kind. The letters of reference will suffice to indicate the position of the principal parts of it, the forms of which may be varied according to the object for which the boiler is to be used.
Fig. 3. - COMPLETE VIEW OF A SERPOLLET BOILER.
1. Exterior view. 2. Cross section. 3. Horizontal section at the height of the tube.
Messrs. Serpollet are occupied at present with studying the special arrangements which will be needed for connecting their boiler with a quadricycle, a torpedo boat, a stove, a locomotive, or a stationary machine of 10 horse power, and with rectangular parts.
The inexplosibility of their boiler has been tested during an experiment made before the engineers of mines, on which occasion a manometer (steam gauge) graduated for a pressure of upward 200 kilogrammes per square centimeter was used, and the pressure raised far beyond the limits indicated. The result was that the hand of the manometer, being pressed against the walls of the box, became bent, and though the boiler was submitted to a pressure the degree of which it was impossible to measure, it was not changed in the slightest.
Incrustation of the boiler is not to be feared, for, in consequence of the great velocity with which the steam circulates through the tube, the solid matter dissolved in the water becomes pulverized and is forced out, mechanically assisting to lubricate and polish the parts of the motor.
The invention of Messrs. Serpollet is still too new to foretell all its possible applications, but their apparatus, in its present form, is exactly the steam generator which will be useful for producing a small motive force; while it will be necessary to wait until it has been ascertained, by further study, how the system can economically be used for high motive power.
The most natural and immediate application of the invention seems to be its use for the electric lighting of restaurants, in which case one of the instantaneous vaporization tubes might be connected with stoves which remain lighted all day, and which might thus besides supply the necessary motive force to work a small dynamo charging some accumulators. - E. Hospitalier, in La Nature.