Imágenes de páginas

chants is to alter the nominal powers so as to supply a smaller engine for a given power, establishing a higher rate of speed to make up the difference.

Thus, one list may be found to give for a compound engine of 20 nominal horse-power cylinders 10" and 18" X 21" stroke at go revolutions and 80 lbs., while another gives only 9" and 14" X 16" at 135 revolutions and 100 lbs., both thus offering to the user 60 indicated horse-powerneedless to point out how great a difference in cost exists between the two.

It will thus be seen how unreliable a factor is the mere statement of a nominal horse-power to the purchaser of an engine.

While the preceding table may be found of use in comparing the first costs of various steam-engines from pricelists, the work to be done by them should be accepted only upon calculation of the indicated horse-power due to size, speed, and mean steam pressure.

These are, in the succeeding chapters, tabulated for all ordinary commercial sizes and considered in the following order :

A. Vertical patterns.
B. Horizonal patterns.
C. The portable and semi-portable engine.
D. Special types of high-speed engines.



The vertical engine, or, as it should more properly be called, the “inverted engine,” is arranged with its cylinder on a standard, or frame-work, over the crank-shaft, in which position it occupies little floor space, and is very handy of access if well designed. For very many purposes of small motive power it is the most handy and useful engine, and it is quite the cheapest form of first motive power as regards first cost. The chief fault of the vertical engine is, that it is not so free from vibration as the horizontal pattern, but if its construction be of a solid character this objection disappears.

In many cheap patterns the glands are very difficult of access and the lubricating arrangements are ineffective. These points should be looked to in purchasing.

A distinct advantage of the vertical engine is the even wear of the cylinder, there being no tendency to wear oval, as in the horizontal, due to the weight of the piston.

The dead weight of the working parts is also freely supported on the crank-pin, and these two features should make a vertical engine under exactly similar conditions more economical than a horizontal pattern. In practice, however, the difference is inappreciable, and the adoption of one pattern or other is to be decided by convenience.

The small vertical engine may be made portable at very moderate expense. Its wastefulness of fuel is due to the poor design of the vertical boilers commonly supplied with

it for the sake of economy in first cost, and in space occupied.

As a set-off to this an arrangement should always be insisted upon whereby the feed-water is heated to some extent.

This can be conveniently accomplished by having the bed-plate on which either engine or boiler, or sometimes both together, stand, made hollow and arranged to act as a tank or hot well, the contents being warmed by part of the exhaust steam from the engine. The whole of the exhaust cannot be condensed, as in these little boilers the bulk of the steam is usually needed to be turned up the chimney to create a draught for the fire.

The commercial vertical engine is not to be recommended as a motor when exceeding a cylinder of 10 inches diameter, although manufactured by some firms up to 15 inches bore.

Some firms still continue to manufacture vertical engines attached to the shell of vertical boilers. This is not an advantageous method, throwing strains on to the shell which it is not suited to bear, and the only possible economic advantage that can be seen about the arrangement, besides cheapness, is that the vibrations of the engine are communicated to the water and thus aid the separation of the steam bubbles.

For very small powers, however, the arrangement would be open to no serious objection, provided the engine is mounted on a complete plate without a riveted or welded joint between the working parts either in line or across it.

Some capital small compound vertical engines are now being put on the market of as small powers as 2 and 3 nominal horse-power.

The prices of these, complete with boiler and waterheater base-tank, are £65, or $325, and £83, or $415, respectively.

Dynamo-Driving.—A very commendable use of the vertical engine, which is rapidly extending at present, is for the purpose of driving dynamos, and such fast-running machines as centrifugal pumps and fans, either directly attached to its crank-shaft or by one belt.

The following is a list of suitable proportions for the engines destined for this special work, which is now of such growing importance and wide-spread use as to demand special consideration among applications of power.



[graphic][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][ocr errors][subsumed][subsumed][subsumed][subsumed][ocr errors][subsumed][subsumed][subsumed][subsumed][ocr errors][ocr errors][subsumed][subsumed][subsumed][ocr errors][ocr errors][subsumed][subsumed][subsumed][ocr errors][subsumed][subsumed][subsumed][subsumed][subsumed][ocr errors][ocr errors][subsumed][subsumed][subsumed][subsumed][ocr errors][subsumed][subsumed][ocr errors][subsumed][subsumed]

For small powers, a single-cylinder engine may be employed, standing upon an extension of the dynamo bed-plate.

Engines specially designed for this purpose are made by most of the large agricultural engineers and also by many of the high class engine-builders of the United States and England, and for confined spaces, especially on board ship, these machines leave little to be desired.

In this direct driving it must be clearly borne in mind that a very high rate of speed is necessary for the engine, as otherwise the size of the dynamo has to be increased and the first cost runs up excessively. Therefore the ordinary commercial vertical engine will not do for this work, as greater rigidity, shorter stroke, and better special lubricating arrangements are a necessity for the duty.

The high speed at which these direct coupled engines are run renders them uneconomical as regards their consumption of steam, and it would not be safe to assume that they would absorb less than about 40 lbs. per indicated horse-power per hour.

When, therefore, this is taken into consideration, together with the extra dimensions of a dynamo required to give the electrical output at what is to the latter a slow speed, it will be evident that direct coupled engines are not to be considered advisable unless space is of primary importance.

Vertical engines are well adapted to the driving of dynamos by a belt, connecting a fly-wheel or belt-pulley to the dynamo pulley. For such an arrangement, a high speed of rotation is still necessary for the engine, but a little longer proportion of stroke is permissible and advantageous, while a much smaller dynamo can be utilized, giving an equal output at a much higher rate of speed, and the whole arrangement thus becomes cheaper in first cost.

The two machines can be very compactly arranged, as will be seen by the two succeeding lists of combination plants.

« AnteriorContinuar »