Scientific American Supplement, No. 360, November 25, 1882

by Various

This last is of itself a most important point, and demands careful consideration.

For example, I was at a mill a short time ago when the governor belt broke. The result was a stoppage of the whole mill. Had the motive power of this mill been subdivided into a number of small engines only one department would have been stopped. During the stoppage in this case the windows of the mill were a sea of heads of men and women (the operatives), and considerable excitement was caused by the violent blowing off of steam from the safety-valves, due to the stoppage of the steam supply to the engine; and this excitement continued until the cause of the stoppage was understood. Had the power in this mill been subdivided the stoppage of one of a number of engines would scarcely have been noticed, and the blowing off of surplus steam would not have occurred.

In building a mill the first item to be considered is the interest on the first cost of the engine, boilers, etc. This item can be subdivided with advantage into the amounts of interest on the respective costs of,

First. The engine or engines;

Second The boiler or boilers;

Third. The engine and boiler house.

In the same connection the form of engine to be used must be considered. In some few cases--as, for instance, where engines have to be placed in confined situations--the form is practically fixed by the space available, it being perhaps possible only to erect a vertical or a horizontal engine, as the case may be. These, however, are exceptional instances, and in most cases--at all events where large powers are required--the engineer may have a free choice in the matter. Under these circumstances the best form, in the vast majority of cases where machinery must be driven, is undoubtedly the horizontal engine, and the worst the beam engine. When properly constructed, the horizontal engine is more durable than the beam engine, while, its first cost being less, it can be driven at a higher speed, and it involves a much smaller outlay for engine house and foundations than the latter. In many respects the horizontal engine is undoubtedly closely approached in advantages by the best forms of vertical engines; but on the whole we consider that where machinery is to be driven the balance of advantages is decidedly in favor of the former class, and particularly so in the case of large powers.

The next point to be decided is, whether a condensing or non-condensing engine should be employed. In settling this question not only the respective first costs of the two classes of engines must be taken into consideration, but also the cost of water and fuel. Excepting, perhaps, in cases of very small powers, and in those instances where the exhaust steam from a non-condensing engine can be turned to good account for heating or drying purpose, it may safely be asserted that in all instances where a sufficient supply of condensing water is available at a moderate cost, the extra economy of a well-constructed condensing engine will fully warrant the additional outlay involved in its purchase. In these days of high steam pressures, a well constructed non-condensing engine can, no doubt, be made to approximate closely to the economy of a condensing engine, but in such a case the extra cost of the stronger boiler required will go far to balance the additional cost of the condensing engine.

Having decided on the form, the next question is, what "class" of engine shall it be; and by the term class I mean the relative excellence of the engine as a power-producing machine. An automatic engine costs more than a plain slide-valve engine, but it will depend upon the cost of fuel at the location where the engine is to be placed, and the number of hours per day it is kept running, to decide which class of machine can be adopted with the greatest economy to the proprietor. The cost of lubricating materials, fuel, repairs, and percentage of cost to be put aside for depreciation, will be less in case of the high-class than in the low-class engine, while the former will also require less boiler power.

Against these advantages are to be set the greater first cost of the automatic engine, and the consequent annual charge due to capital sunk. These several items should all be fairly estimated when an engine is to be bought, and the kind chosen accordingly. Let us take the item of fuel, for instance, and let us suppose this fuel to cost four dollars per ton at the place where the engine is run. Suppose the engine to be capable of developing one hundred horse-power, and that it consumes five pounds of coal per hour per horse-power, and runs ten hours per day: this would necessitate the supply of two and one-half tons per day at a cost of ten dollars per day. To be really economical, therefore, any improvement which would effect a saving of one pound of coal per hour per horse-power must not cost a greater sum per horse-power than that on which the cost of the difference of the coal saved (one pound of coal per hour per horse-power, which would be 1,000 pounds per day) for, say, three hundred days, three hundred thousand (300,000) pounds, or one hundred and fifty tons (or six hundred dollars), would pay a fair interest.