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164

REPORT OF EXPERIMENTS ON THE EXPLOSIONS OF STEAM-BOILERS.

The experiments which follow the remark, "ten drops in six seconds," were all made by dropping several drops, not enough, however, to cool the surface down; measuring the time for the whole number, and dividing by that number. The point of maximum vaporisation is placed, with probability, at 1190, the times had certainly increased in rising to 444°; but in descending, the same certainty is to be found only on reaching 32140. This slightly rough but polished surface, as it may be considered, had its temperature of maximum vaporisation very certainly above that of the polished copper and of the smooth iron. The time of vaporisation of a drop at this temperature was less than one-sixth of the corresponding time for the polished copper, and less than that for the clean copper surface; agreeing more nearly with that for the smooth iron, which was much its inferior in lustre. A correct induction could only be had by varying the number of metals, and by frequent repetition of the results; but so far as these experiments go, they indicate that this repulsion does not depend alone upon the relative polish of the different metallic surfaces.

8. The conclusions which they fairly warrant are as follows:

1st. With the same metal, the temperature of maximum vaporisation of water is lower,, as the smoothness of the surface is greater, and the amount of vaporisation in a given time at this temperature is much diminished. In copper, the effect of polish and of oxida tion, the two extremes, is shown by a difference in the temperature of maximum vaporisation of 56°, that point being in the two cases, 2920 and 348°. Further, the ratio in the times of vaporisation at these two points

is as 12 to 1, or for the same drop of water, 3 seconds and of a second. In iron, the smooth surface gave, for the temperature of maximum vaporisation, 334, or 3370, the oxidated 3460, differing but little from the former; but when highly oxidated, gave 3810, or a difference of about 45°, the time of vaporisation not differing greatly in the two

cases.

2d. The temperatures of maximum vaporisation for copper and iron, in similar states of surface, differ between 30 and 40o, the iron having the higher point. The time of vaporisation at the maximum is less in the copper than in the iron, in the ratio, proba. bly, of 2 to 1, or nearly in the ratio of their conducting powers for heat, which are as 2 to 1.

3d. The temperature of maximum vaporisation for oxidated iron, or for highly oxidated copper, corresponds nearly to that at which steam has an elastic force of nine atmospheres. But the vapour was formed under atmospheric pressure only.

4th. A repulsion between the metal and water is perfect at from 20 to 40° above the point of maximum vaporisation, following more closely upon the temperature of maximum vaporisation in copper than in iron. At these temperatures the water does not wet the metal. The drops of water are put in rotary motion in variable directions, and sometimes remain at rest, slowly vaporising. When very small, they sometimes leap vertically from the surface of the metal. They seem to vaporise from the side next to the metal.

A general view of the facts just deduced is given numerically in the following table :

Table showing the Temperature of Maximum Vaporisation of Drops of Water in Copper and Iron

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REPORT OF EXPERIMENTS ON THE EXPLOSIONS OF STEAM-BOILERS.

begun to be developed. For we may assume that heat will tend to pass from the metal to the water the more rapidly as the temperature of the former exceeds that of the latter, which would tend to increase the vaporisation after the repulsive action had commerced.

The temperatures of maximum vaporisation are reached in practice in the highpressure steam-engines. The locomotives with flues of copper use steam of 60lbs. pressure upon the safety-valve, corresponding to nearly 306° Fah.; a temperature which is but 150 below that found for the maximum vaporisation by oxidated copper. The iron boilers of our high-pressure engines use steam of from ten to eleven atmospheres, or from 354° to 360° Fah., the higher temperature being about 20° below the temperature found for the maximum vaporisation of water by an oxidated surface of thick iron.

It is possible, and indeed probable, that pressure may modify these results, all of which were obtained under atmospheric pressure. Pressure, tending to counteract the effect of the repulsion between the heated metal and water, would probably raise the temperature of most rapid vaporisation. Vaporisation of considerable Quantities of Water.

9. The results already presented, however interesting they may be in a practical or in a philosophical point of view, cannot be said to touch the question of the effect of the contact of water suddenly made with hot metal, in producing explosions. It is necessary to suppose so large a quantity of water, brought under the vaporising influence of the metal as, except where there is a violent repulsion by the heated metal, to reduce materially the temperature of the surface. To study the question in this point of view, we must ascertain, if possible, the law, according to which a variable quantity of water, thrown upon heated metal, is capable of reducing its temperature, so as to produce the maximum amount of vaporisation. That such a maximum may be found will be seen by considering the foregoing results. They show that an effective repulsion is developed between water and heated metal, increasing rapidly after a certain temperature, at which the vaporisation is a maximum. Now, water thrown upon a surface at its temperature of maximum vaporisation, would cool it down rapidly below this temperature. Again, if thrown upon it at a temperature when the repulsive effect was very strong, it would not be able to cool it down as low as the temperature of maximum vaporisation. Some where, then, between the points thus referred to, there will be an initial temperature, at which the vaporisation will be the greatest

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possible, or a given quantity of water will be vaporised in the least time. It is obviously not as easy to solve this problem as the preceding one, nor can so satisfactory results be expected, nor results so constantly reproducible; its practical importance required that its solution should be attempted; and the method adopted was as follows:-The same baths, viz. oil and tin, were used as in the foregoing series, to ascertain, generally, the effect of communicating heat through differ ent media. Different metals, copper and iron, with different thicknesses of each, and different states of surface, were subjected to trial. The quantity of water was gradually increased, from small quantities, scarcely capable of reducing the temperature of the surface when the repulsive tendency was fully. developed, to quantities as considerable as the bowls could contain. The study of each case was, of course, attended with much labour. In the greater quantities of water the temperature of the metal of the dishes was so much reduced as to affect that of the bath itself. Accordingly, a mean of the temperatures, observed at regular intervals, is taken as the temperature of the bath on which the water was thrown, and which, taking the entire mass into consideration, was supplying an amount of heat due to that temperature, to the parts adjacent to the bowls. The oil bath was stirred to produce, as nearly as practicable, a uniformity in temperature in the different parts.

Without knowing the temperature to which the parts of the heated metal, or of its bath, are reduced by the affusion of water, this kind of experiment supplies precisely the answer to the question in practice; at what temperature of a meial will water, thrown upon it in a limited quantity, be most rapidly turned into steam? Making due allowance for the different modes of communicating heat in the experiments and in practice.

Copper Bowl, No. VII.

10. The same bowl used in a former series of experiments was again applied, the surface being smooth. This bowl was a portion of a spheroid, approaching nearly, in its inner surface, to a spherical surface of 3.09 inches radius: the versed sine of the segment, or depth of the bowl, was 16 inches; and its chord, or the breadth of the bowl, 539 inches. The thickness of the metal was 07 of an inch.

The quantity of water first introduced was one-eighth of an ounce by weight (60 grains troy), the water being weighed in a small metallic dish, and thrown into the bowl placed in the bath. One experimenter ob- ` served the temperature of the bath, and gave notice to another of the instant of introducing the water; the other made a memorandum:

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REPORT OF EXPERIMENTS ON THE EXPLOSIONS OF STEAM-BOILERS.

of the temperature and time. The first observer gave notice of the instant at which the liquid began to boil, which was also entered upon the notes. The second then announced each minute, or half minute, as it passed, and the first gave the temperature of the bath at that time, stating also the circumstances taking place in the bowl when remarkable. The same observer also gave warning when the liquid was about to disappear, and a signal at the instant of its disappearance, which was marked by the second. The time between the introduction of the liquid into the bowl and its beginning to boil is deducted in each case in the following tables, so that they show the times necessary to vaporise the water, after it had been raised to the boiling point. At the high temperatures, the time required to raise the smaller quantities of water to ebullition, scarcely amounted to half a second. The times were noted, usually, by a pendulum beating seconds, sometimes by a quarter-second pendulum.

When a decided repulsion has commenced with these considerable quantities of water, the phenomena are of a very singular kind. The water assumes a rotary motion about an axis perpendicular, or nearly so, to the lowest point of the dish, and at the same time its figure changes, and, from being circular in its horizontal section, becomes of an irregular oval, which contracts and dilates alternately as the mass revolves; the transverse axis contracting until its place is occupied by the conjugate, and vice versa. The direction of this rotation is not at all uniform, and the mass sometimes becomes quiescent, Observed Time of

No. of Experiment.

Observed Temperature of Vaporisation.

Vaporisation.

and then assumes motion in an opposite direction. When this state of things first begins, vapour sometimes bubbles or bursts up through the liquid; but when fully established, it is most copiously given off from below. In fact, the appearance is that of a stratum of vapour, between the water and the bowl, which becomes at times visible when condensed at the edges.

This

If the results of the vaporisation of oneeighth of an ounce of water in bowl No. VII. be taken, and a curve be traced from them, of which the ordinates represent the differences between the times of evaporation and a constant quantity, and the abscissæ the differences between the temperatures and a constant quantity, a remarkable regularity will be found in the results, and an approach to a minimum in the time of vaporisation. affords good grounds for attempting to calcu late the temperature at which the maximum vaporisation, with this quantity of water, would have taken place; or the temperature above which the water introduced would not be able to cool the bowl as low as the temperature of maximum vaporisation for drops of water. The obvious approximation of the curve just referred to, (see plate 5, fig. 1*), to ` the ellipse, induced the trial of the equation of that curve to represent the observations. The following table shows the results of the comparison of calculation and observation, the transverse of the ellipse being assumed equal to 262o, and the conjugate to 200 seconds, and the co-ordinates of the centre being 5760 and 211.5 seconds.t

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We have not yet received from America the plates 5 and 6 referred to in the portion of the Report given this week.-ED. M.M.

That is, in the equation A2y2 + B2x2= A2, B2; A=262° and B =200 seconds. X=576°, and Y=211.5 seconds, are the co-ordinates of the centre. So that x=5769-the observed temperature, and y=211-5 seconds-the observed time of vaporisation.

REPORT OF EXPERIMENTS ON THE EXPLOSIONS OF STEAM-BOILERS. 167

cated, by the nature of the dotted line, to have been irregular, or the near coincidence of the calculated and observed numbers in the table, and the variable sign of the differences, justify us in assuming the true maximum of vaporisation at the temperature corresponding to the highest point of the ellipse, namely, to 576° Fah.

At about 576° Fah. then, a bowl of copper ⚫07 of an inch thick supplied with heat by a medium like oil, would be able so far to resist the cooling action of 60 grs. of water, as to produce the most rapid vaporisation; the quantity being sufficient to cover about onetenth of the surface exposed to heat.

Copper Bowl, No. IV.

11. This bowl was thinner than the last, its thickness being 05 of an inch. Its figure within approached nearly to a sphere of 3.1 inches radius, the chord of the segment being

Temperature of
Vaporisation.

5.25 inches, and the versed sine 145 inch; it deviated as little, therefore, from the figure of the last as could have been expected from the mode of forming it.

Nine observations were made of the vaporisation of one-eighth of an ounce of water in this bowl, placed in a bath of oil. Of these, seven are shown in the middle dotted line of fig. 1, plate 5, and agree very well with the ellipse traced in the full line; the two omitted were at temperatures lower than that of the lowest of the seven included in the figure. The following table shows the comparison of calculation and observation, assuming the major and minor axes of the ellipse to be respectively 251° and 214 seconds; and the co-ordinates of the centre 576° and 254 seconds. These values were not obtained rigidly, but they agreed better than numbers, greater and less, which were also tried.

No. of Observation.

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The temperature producing the greatest vaporisation with 60 grs. of water in a copper bowl 05 inch thick, would be nearly 576° Fah., or about the same temperature as with the greater thickness of 07 inches. The surfaces were nearly alike in the two cases, and both were clean but not polished.

Bowl, No. I.

12. Was thinner than either of the foregoing; its thickness being only ⚫025 of an inch. The figure was nearly the same as the

Temperature of
Vaporisation.

foregoing, and the quantity of water used and nature of the bath were the same.

Of eight observations made and recorded in the following table, five only appear to belong to the same curve; this is seen in the lowest curve, plate 5, fig. 1, in which the dotted line represents the curve of observation. These five may be represented by a circle determined from observations 3, 4, and 8, which give for the radius 262°. The coordinates of the centre are 604° and 309 seconds.

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REPORT OF EXPERIMENTS ON THE EXPLOSIONS OF STEAM-BOILERS.

of the temperature and time. The first observer gave notice of the instant at which the liquid began to boil, which was also entered upon the notes. The second then announced each minute, or half minute, as it passed, and the first gave the temperature of the bath at that time, stating also the circumstances taking place in the bowl when remarkable. The same observer also gave warning when the liquid was about to disappear, and a signal at the instant of its disappearance, which was marked by the second. The time between the introduction of the liquid into the bowl and its beginning to boil is deducted in each case in the following tables, so that they show the times necessary to vaporise the water, after it had been raised to the boiling point. At the high temperatures, the time required to raise the smaller quantities of water to ebullition, scarcely amounted to half a second. The times were noted, usually, by a pendulum beating seconds, sometimes by a quarter-second pendulum.

When a decided repulsion has commenced with these considerable quantities of water, the phenomena are of a very singular kind. The water assumes a rotary motion about an axis perpendicular, or nearly so, to the lowest point of the dish, and at the same time its figure changes, and, from being circular in its horizontal section, becomes of an irregular oval, which contracts and dilates alternately as the mass revolves; the transverse axis contracting until its place is occupied by the conjugate, and vice versa. The direction of this rotation is not at all uniform, and the mass sometimes becomes quiescent, Observed Time of

No. of

Experiment.

Observeu Temperature of Vaporisation.

Vaporisation.

and then assumes motion in an opposite direction. When this state of things first begins, vapour sometimes bubbles or bursts up through the liquid; but when fully established, it is most copiously given off from below. In fact, the appearance is that of a stratum of vapour, between the water and the bowl, which becomes at times visible when condensed at the edges.

If the results of the vaporisation of oneeighth of an ounce of water in bowl No. VII. be taken, and a curve be traced from them, of which the ordinates represent the differences between the times of evaporation and a constant quantity, and the abscissæ the differences between the temperatures and a constant quantity, a remarkable regularity will be found in the results, and an approach to a minimum in the time of vaporisation. This affords good grounds for attempting to calculate the temperature at which the maximum vaporisation, with this quantity of water, would have taken place; or the temperature above which the water introduced would not be able to cool the bowl as low as the temperature of maximum vaporisation for drops of water. The obvious approximation of the curve just referred to, (see plate 5, fig. 1*), to ` the ellipse, induced the trial of the equation of that curve to represent the observations. The following table shows the results of the comparison of calculation and observation, the transverse of the ellipse being assumed equal to 262o, and the conjugate to 200 seconds, and the co-ordinates of the centre being 5760 and 211.5 seconds.†

Ordinates from Observation.

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A similar comparison which addresses itself, even more directly to the eye, is given in fig. 1, plate 5, in which the upper dotted line is that traced from the observations, and

the full line is the ellipse which has been assumed.

The general coincidence of these lines, varying only when the observations are indi

We have not yet received from America the plates 5 and 6 referred to in the portion of the Report given this week.-ED. M.M.

That is, in the equation A2y2 + B2x2= A2, B2; A = 262° and B = 200 seconds. X=576°, and Y=211.5 seconds, are the co-ordinates of the centre. So that x = 576o-the observed temperature, and y➡211.5 seconds-the observed time of vaporisation.

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