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I AM frequently consulted on the question of the selection of a motive power suited to certain conditions, which conditions frequently vary so greatly that one cannot wonder at the perplexity they produce in the minds of those who are perhaps unacquainted with even the simpler technicalities connected with such matters. The facts, formulæ, and data resulting from experience with machinery, are scattered through books of all kinds, which are inaccessible to some, unknown to others, and, in any case, are so ill-arranged and often over-elaborated as to make them worse than useless to the uninformed.
While the settlement under very difficult conditions of the best motive power to be adopted, remains undoubtedly a matter in which the experience of an engineer is most properly applied, it has appeared to me that a compilation, or a condensation, of the facts that go to settle these questions in the hands of an expert, would prove of wide value not only to his class, but may be made sufficiently simple to be of practical use in those numerous cases where these questions have to be solved by those on the spot without technical aid.
I have aimed, therefore, in the following pages, at the double purpose of condensing and arranging these facts and figures for the easy reference of engineers, and for the ready comprehension of the non-technical; providing the former, not only with rules and formulæ compactly arranged, but with their results as far as possible worked out for them, and affording to the latter class sufficient direct information, without any but the simplest calculation, to enable them to come to a decision in any case in which the issue is not complicated, or at any rate to be in a position to present their requirements in an intelligent and practical form, either to an engineer for advice, or to a manufacturer for the purpose of estimating. An enormous amount of work, trouble, and anxiety is devoted by the members of my profession to advising and estimating upon requirements ignorantly stated, or in which essential points are ignored, both to the loss of their clients and themselves, and it is, perhaps, not going too far to say that the majority of small motive powers are decided upon, from this cause, in a somewhat hap-hazard manner.
It will be within the knowledge of most mechanical engineers that engines are frequently put to duties for which they are either unsuited, or to which a different system of motive power might have been applied, with economy both in working and in first cost.
Added to the above is a strong need for some practical work dealing with and finally disposing of the system of misdescription which has grown up around the powers of the steam-engine and boiler, and which, in the hands of the unscrupulous, is sometimes made use of to palm off upon the uninformed, machinery of less than proper dimensions. These misnomers and misunderstandings are kept alive by the absence of a direct and simple definition of the essential feature in common of engines and boilers ; and this I have in this book not merely formulated, but tabulated, so as to dispose of the last excuse for further use of the misleading term of a “Nominal horse-power." There should be no longer any excuse, with this tabulated information in hand, for any manufacturer to sell, or for any fairly informed buyer to purchase, engines or boilers defined by nominal powers.
It is really remarkable how many standard text-books and technical hand-books still cling to the use of this term, bolstering up its use by formulæ based upon it, formulæ, that is, which are dependent upon a figure, which as one instructive table in Section IV., Chapter XIX., exhibits, may vary twenty, thirty, even fifty per cent. in value, according to the ideas of liberality or economy of a designer or a manufacturer !
Surely an amazing admission of the personal error into any form of calculation.
An essential feature of this work is the presentation of the cost of the apparatus in each instance. So far as I know it is unique in this matter, which, however, is in nine cases out of ten the guiding or deciding consideration in any comparison. Realizing this it has been my object to present the cost prices all on as uniform a basis as possible, and the figures here given, while necessarily not presenting the lowest prices at which an apparatus might be obtained, are fair figures for a good class of machinery, derived largely from my own purchases and sales in and for many different markets. The essential point about them is, as I have said, that they are fairly on a uniform basis throughout, so that market fluctuations do not greatly affect their value for purposes of comparison one with another. For this reason, and for readier inter-translation, the value of the English pound sterling has been taken at $5.
As I have said, the information relative to the important subject here dealt with is nowhere to be found under one cover, but is scattered over a score or more of hand-books, guides, and even trade catalogues, and the collection of these into a form easy of reference, with the addition of all that my practical experience has brought to my knowledge, has been my task in this work.
In order to facilitate calculation, avoid cross-references, and present a complete view of all considerations of each subject, some repetition of formulæ and data has been resorted to in the different sections.
I have to acknowledge my indebtedness for information to a larger number of works of reference than I could find space to refer to in detail, having endeavoured to exhaust what has been written up to date on each part of my subject, but I desire to record the assistance I have derived from the work and information of the following in particu
MESSRS. THE BABCOCK & Wilcox COMPANY.
The first object of a Prime Motive power is to perform a given duty, under given conditions, in the best possible manner.
The secondary object, but, more often than not, that which must perforce take the first place, is that it shall cost less to purchase than any other.
First Cost. It is evident, then, that not only must the relative duties and economies of one motive power or the other be known, but to arrive at any definite conclusion their relative cost must also be available. For this reason prices of each are included in the succeeding sections. These prices will naturally vary in different localities, but being all on one basis, namely, that of higher class of English and American manufacturers, they afford a parallel of comparison throughout.
To these prices have to be added those of shafting, belts, pulleys, pipes, and sundries, but these apply almost equally to different systems of motive power, and do not, thus, greatly affect the comparison.
Cost of Freight.-In foreign countries, freight and the possibilities of transport often go to make a decision for or against the use of machinery. These important items of cost are arrived at as follows:
Shippers reckon a ton by weight to be equal to forty cubic feet of space occupied, which is called a “ton measurement.”
They usually reserve the right to assess freight on goods in whichever way is most profitable to them.