CHAPTER XV.

GENERATORS.

By the term Generator is to be understood the electrical machine whereby mechanical energy is transformed into electrical energy and the prime mover in which the mechanical energy resides immediately before its conversion.

This definition, it will be seeu, is very comprehensive, and, on the one hand, includes dynamo machines of every conceivable variety, whether for producing continuous current or alternating of one or more phases, at low or high pressure; while, on the other, it comprises prime movers of all kinds, whether driven by steam, moving water, gas, or oil vapour.

It is obvious that it is out of the question to attempt to enter either into the theory or practical design of all or any of these machines in a single chapter of a book of this kind. Each item-dynamo machines, steam engines, turbines, gas engines-has a whole literature of its own, and to the standard works dealing with these matters the reader must be referred. It will only be attempted here to deal with a few aspects of the subject from a purely central station point of view.

A generator of electrical energy, as used in central station work, consists of two essential parts, viz.: (1) a prime mover of some kind to furnish the mechanical energy in a kinetic form; and (2) a dynamo machine, driven by the prime mover, and converting the mechanical energy into electrical energy in the particular form desired. There are other kinds of generators depending for their operation on chemical action, as primary batteries, or on the direct effect of heat, as thermo-electric apparatus; but up to the present all attempts to utilise these, on even a moderately large scale, have been entire failures, and, practically speaking, mass in motion is, so far, the link nearest to electrical power in the chain of conversion.

Mechanically, generators may be classified according as they are driven by steam, gas, or water, this being the order of relative importance in this country, though the first and last are practically interchanged in some parts of America and the Continent, while in the near future it is quite likely that the second may overtop the other two.

Electrically, generators naturally fall into the two main divisions of continuous current and alternating current machines.

Yet a third classification may be made according to the method of connecting the mechanical prime mover to the electrical transformer of energy, the classes being two in number, according as the driving is direct or through the intervention of gearing.

Probably the first classification is the most convenient, and the others may be described as subdivisions of one of its main headings, the remarks on them being taken as applicable to other headings, such modifications as they involve being pointed out.

Steam engines display an infinite variety in detail, and can be built to give a wide range of desired results, though a high degree of perfection in one direction may have to be attained by a corresponding sacrifice in another.

The chief essentials for engines for electrical work are the following:(1) Reliability and freedom from breakdown; (2) steady running; (3) high economy; (4) simplicity; (5) facility for repair; (6) ability to withstand sudden shocks and momentary or prolonged overload; (7) compactness, chiefly as regards floor space, but partly, also, as regards height; (8) small amount of attention required during running; (9) low first cost, so far as is compatible with the above conditions and small cost for repairs.

The reliability and freedom from breakdown of an engine will depend primarily upon its design, i.e., the suitability of the size and proportion of its various parts. This is best secured by employing makers of wide experience and first-class reputation. It is folly for a central station engineer to give a detailed specification of the sizes of the various parts. Let him detail the conditions under which the engine is to work, and the results that must be attained, and leave the design to those who have devoted their lives to this class of work and no other. By interfering and specifying certain leading dimensions, he may spoil the whole design, for symmetry and due proportion are of almost as much importance as the actual strength of an individual part. The Author has known an electrical engineer specify dimensions that to an experienced engine builder were wholly absurd, some parts being below and others above the strength usually allowed under ordinary practice, while parts actually transmitting identical stresses were of widely different sizes. The engineer will, however, of course, insist upon knowing the details of the design, and will exercise his judgment as to its

correctness.

A point second in importance only to the general design, in favour of freedom from breakdown, is simplicity. It is self-evident that a piece of mechanism with few parts is far less likely to give trouble than one with many complications. For this reason, it may often be advisable to sacrifice a certain amount of economy to secure the elimination of particular chances of failure.

Steady running is of importance in all cases, but for some work it is a primary essential, as in the parallel running of alternators, and especially in the operation of rotatory converters.

Steady running is of two kinds, viz.: (1) from revolution to revolution, and (2) during a revolution. The former is attained by good governing, the latter, which is the most important factor in alternating work, is secured first by even turning moment and then by the use of a fly wheel having a large amount of inertia.

The necessity for simplicity has already been touched on. It is secured by reducing the number of parts as much as possible, and avoiding complicated gears and automatic appliances.

Facility for repair is dependent chiefly on general design; it implies that any part requiring attention shall be accessible and removable with the minimum of disturbance of other parts.

The ability to withstand sudden shocks depends upon the strength of the elements of the mechanism, this being calculated, not on the normal working conditions, but on the most severe ones that it is apprehended can ever be imposed in case of emergency. The question of overload is chiefly one of size of cylinder; the necessity for meeting it has been shown in Chapter VIII. It is of little importance what the economy is at the overload rate, since the engine only works at this rate for a short time. The normal full load is that at which it should be designed to have maximum economy.

The compactness is important, because the value of the floor space is an important item in the standing charges, involving not only the cost of the land, but of the buildings also, the latter being affected as well by the height of the engine.

The amount of attention required by an engine while running depends chiefly upon the governing arrangements and on the system of lubrication adopted.

Finally, low first cost depends upon the simplicity of the engine, and very largely on the speed; while the amount of repairs required depends on the excellence of the design and the quality of the material and workmanship employed in construction.

With these few preliminary remarks, we may now consider briefly the various types of engine available.

Engines are either single, compound, triple, or quadruple expansion, according as the steam is expanded in one, two, three, or four stages respectively. The necessity for the series of expansions arises from the fact that only a certain ratio of expansion in a single cylinder is economical, and since the efficiency of the machine as a heat engine depends on the difference in the temperatures between which it works, and the initial temperature on the pressure of the steam, it follows

that for high economy a high steam pressure and therefore a number of cylinders must be used.

The difference in economy between single and compound engines is so great that practically the former are never employed; the difference between compound and triple does not, under the most favourable conditions, exceed 10 per cent., while between triple and quadruple the difference is, comparatively speaking, trifling.

In comparing the merits of the various kinds, it must be borne in mind that the saving effected by increasing the number of expansions is on the steam consumption, and on nothing else; while against this must be set the increased capital cost of the engine, the additional complication and liability to breakdown in consequence of the larger number of parts, and the increased friction, which last item alone, it is affirmed by some authorities, is sufficient to counterbalance the gain in economy due to diminished steam consumption.

Now, what is the precise advantage of decreasing the steam consumption? In the first place, less fuel has to be consumed to produce the lessened quantity of steam. This involves a saving not only on the actual cost of the coal, but also on the cost of handling it, firing the boilers, and disposing of the ashes. Next, the capacity of the boilers need not be so great, and a saving is made on the capital cost of the boiler plant, including such accessories as feed pumps and mechanical stokers. Lastly, smaller steam pipes, costing less to provide and wasting less heat by radiation, are required.

The most important saving is obviously on the cost of fuel. In an ordinary central station this amounts to only about 30 per cent. of the total cost of production at the utmost, and in many cases it is far less; hence, the saving of 10 per cent. in the fuel is made on 30 per cent. of the cost only, or, in other words, it represents but a saving of 3 per cent. on the total costs.

In order, however, to effect this saving, which is made, be it remembered, on the fuel consumption per indicated horsepower, the engine must run at or near full load, for the rate of saving diminishes as the load falls off. In a central station, it is well-nigh impossible to secure this, for, with the changing load experienced, it is inevitable that the load on individual engines shall fluctuate. This will happen no matter how carefully the sizes may be chosen, for, as the load grows, the engines at work, which at a given time may have been fully loaded, gradually become overloaded, and when another is put in to relieve them, they all become underloaded. Again, in certain conditions of the weather, it is necessary to run more plant than is actually required, in order to be ready for a sudden rush of demand.

Lastly, as has already been pointed out, the saving is made on the

consumption of fuel per indicated horsepower; when the economy reckoned on the brake horsepower, or the electrical output is considered, the saving may completely vanish.

This matter is one that has engaged the close and anxious attention of the Author, especially in connection with the design of a station of 50,000 H.P., and the conclusion at which he has arrived is that, under ordinary central station conditions, quadruple expansion engines would be quite unsuitable, while the fuel consumption of compound engines per unit of electrical energy generated would not appreciably exceed that of triple expansion engines, and the balance is amply turned in favour of the former by their smaller cost and greater simplicity and freedom from risk of breakdown.

The analogy of marine practice is frequently urged in favour of triple and even quadruple expansion engines for central station work, but the analogy does not hold, for the conditions are wholly different. In marine work, the load being a steady one, the maximum saving of coal can be ensured, and the fuel consumption is the ruling consideration, for, not only is the saving of its cost of great importance, but there is the fact that storage room for the coal has to be found on board the vessel, and every ton saved means so much less dead weight to carry, and so much more space for remunerative cargo.

The advantages of condensing and superheating are referred to elsewhere in this book, and need not be further enlarged on here. By the use of the latter process, the steam consumption of compound engines can be so materially improved as to remove the last doubt about their use.

To sum up the matter, the Author's opinion is that for central station work a compound condensing engine, using superheated steam at a boiler pressure of 160 lbs. per square inch, is the most suitable that can be employed.

The next point to determine is whether the engine shall run at a high speed or at a low one. This point has already been touched upon in Chapter VIII.; and it is one that has been very much debated, but it cannot be said that any definite conclusion can be safely drawn. Practice in this country up to quite lately has been in favour of high speeds, while American and Continental practice has been unmistakably on the side of slow speed. With the introduction of larger sizes of engine, the number of revolutions per minute of the high-speed engine necessarily falls, so that for very large powers they are hardly high speed more than in name, and, even in English practice, there is a tendency to revert to slow speeds.

The term slow speed is usually restricted to engines making not more than 100 revolutions per minute or thereabouts. Up to this speed, Corliss valve gear works well, but it cannot safely be pushed much higher.