OPINIONS FROM OTHERS.
Page 24
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The Editor invites correspondence on all subjects connected with ,the use of commercial motors, Letters should be on one side of the paper only and typewritten by preference, The right of abbreviation is reserved, and no responsibility for views expressed is accepted.
Steam Vehicle Developments.
. The Editor, THE COMMERCIAL MOTOR.
[1,725] Sir,—I note from the letter of Mr. T. A. Jones in your issue of the 16th ultimo that in referring to the eminent professors—Osborne Reynolds and Perry—he does not desire to imply any expression of opinion by these ancient authorities concerning the efficient operation of thimble tubes for steam generation.
Mr. Jones's observation that the difference between a horizontal and a vertical tube is only a matter of degree, savours more of a lawyer's than of an engineer's argument. Actually, as everyone knows, the difference between vertical and horizontal is a matter of 919 degrees, or, in other words, the one position is 'as far removed from the other as it is possible to be. On the same line of legal argument, it might be stated that the difference between black and white is merely a matter of degree—that both of these may be correctly described as an extreme shade of grey—these extremes being united by an infinite variety of shades of the same colour.
Comparing horizontal and vertical heating surfaces, engineers find that the former has double the efficiency of the latter—so much for the question of degrees of difference.
I take the liberty of reproducing Mr. Jones's sketch showing how circulation is promoted in a syphon tube,' by the application of heat to one of the legs, but I must point out that Mr. Jones has only told half the story.
Mr. Jones's sketch (reproduced here in Fig. 1), showing the upward circulation in the leg that is heated, is quite correct, but he does not tell us what would happen if the heating is transferred to the other leg after the current has started. The mere theorist who nourishes himself on scientific formulas would, doubtless, tell us that the direction of circulation is reversed by heating the other leg. The practical engineer knows that it does nothing of the kind, and that once the current has been started by heating one leg, it will continue merrily in the same direction, no matter which leg is heated (see Fig. 2).
I can claim to be familiar with most scientific formulas as applied to engineering, but I know of no formula that would predict such a phenomenon. Possibly Mr. Jones may know of, or can devise, a. suitable formula to meet the case.
The formula introduced by Mr. Jones in his last letter is the usual formula for predicting the velocity of a body falling freely from a given altitude—or the velocity of efflux of a liquid issuing under a given head, but to attempt to apply this formula to boiler circulation is very misleading. I can claim to have as great a respect for scientific formulas as any engineer, but I hold that it is a disc42 service to science to misapply a formula, and I maintain that science should never supersede, but should ever be the handmaid of practice.
I should like to see how Mr. Jones would design a boiler purely upon scientific formulae and I do not
• hesitate to say that it would be a freak of no practical
In the first place, science teaches that the strongest and lightest vessel to resist internal pressure should have a spherical form, as, for example, the soap bubble—but practical considerations upset this entirely, because lightness and strength (important though they be) are not the only conditions to be fulfilled by the ideal boiler. Some of the other conditions are:—
Facility for the transfer of heat from the products of combustion to the water.
Reducing the temperature of the products of combustion to the lowest degree before rejection.
Facility for cleaning—both internally and externally. Freedom from leakage at all pressures, especially when forcing the highest rate of steaming. The maximum evaporative power and efficiency, in relation to the weight of boiler, and cost of maintenance.
These are a few of the conditions to be satisfied, and as some of them conflict, a satisfactory result can only be achieved by compromise, as all engineers of experience are well aware.
Mr.. Jones states "there is a vast difference between ebullition and circulation," and that " a horizontal thimble tube has very little value as a circulator of water." He admits that there may be " ebullition" in the thimble tube, but not " circulation." Let us examine this contention.
The word ebullition is defined in the standard dictionary As "the action of boiling." I maintain that this action is the fundamental function of a steam generator. I maintain also that ebullition is impossible unless two conditions are fulfilled :—
1. Water must be present.
2. Heat must be applied to the water.
The result of applying heat is to disperse the water as steam, and the action of ebullition cannot go on unless fresh water comes to replace that which is evaporated. In other words, for the maintenance of .eliullition there must be circulation, and in recognizing the existence of ebullition in the thimble tube Mr. Jones unwittingly admits, at the same time, the existence of circulation. If circulation, stopped, ebullition would soon cease and, immediately afterwards, the material of the boiler would be destroyed. by the fierce heat impinging upon it, which was not being carried away by the steam generated.
That thimble tubes do not burn out, even under the fiercest firing, is sufficient proof of effective circulation, although I do not pretend to give a formula for it.
Mr. Jones's attitude towards thimble tubes is quite untenable and savours more of " salesman's logic" than anything else of which I can think.
I do not suggest that Mr. Jones is either a salesman, a lawyer, or an engineer—or a combination of all three, but I am perplexed to understand the mentality of a man who persists in making erroneous assertions concerning an engineering device which be is unable to substantiate, either by ancient authorities, scientific formulas, or observed facts, and of which said device he has obviously no practical experience whatseever.—Yours faithfully,
Tnoliaa CLART.7.60N;
M.anst. Civil Engineers.