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CHAPTER XVII.

COMPOUND OR NON-COMPOUND ENGINES.

COMPOUND engines are more economical than singlecylinder engines, even when the latter are made condensing, mainly because of the higher pressures at which they can be worked without involving excessive strains, and by the more extended use they make of a given quantity of

steam.

Of course the same amount of use of a quantity of steam may be made in one cylinder of proper proportions, but the fall in temperature during the entire process is so great that the cylinder-walls cause serious condensation on the admission of the new steam. The practical economy of the use of the steam in two cylinders is considerable, and the additional advantage is gained of having two cranks on the turning shaft, by which more even working may be secured.

The second cylinder may, of course, be part of a separate engine altogether, provided its capacity and speed are exactly proportioned to receive, and use, the steam issuing from the high-pressure cylinder.

Two low-pressure cylinders may also be used, thus making a tri-cylinder engine, with which very easy turning may be performed. But this would cost nearly as much as a regular triple-expansion engine, which would afford better economy. The work for which a compound engine is inadvisable is that in which the major portion of its work is very variable and lays below its normal or regular power. Where this is the case, a single-cylinder condensing engine will frequently give better results. Where, however, the work is reasonably regular, and unless first cost stands in the

way, a compound engine should be adopted for all general purposes, and references further on will show that by the efforts of various manufacturers there are now on the market excellent compound engines from the smallest sizes upward, at commercial prices.

Chimneys for Compound Engines.-A chimney for the boiler of a compound engine does not obtain much assistance in draft from the blast of the exhaust, and may therefore need to be increased in height. Where, however, brick chimneys are built according to rules given in Chapter XXX., the steam jet or blast would make no economical difference to them, and the steam may as well in every case be utilized down to the lowest limit of pressure found to be practical, or be entirely condensed.

Amount of Water Required for a Steam-Engine. As previously pointed out, water is a prime necessity to all steamengines, and however economical a use is made of it, some waste is bound to occur. In many instances the cost of water is an important factor.

1. Water may be very dirty, and require the expense and employment of filtration machinery.

2. Its supply may fall to a minimum in summer months and require storage or great economy in use.

3. It may be derived from an expensive town supply.

4. It may need pumping machinery to lift it to a sufficient height for use.

Such matters need consideration when a type of engine is to be selected.

The average water consumption in steam-engines may be taken roughly as follows:

Non-condensing engines........

Where these engines are supplied

with a condenser Compound engines with a condenser...

each hour of work. about 40 lbs. per I.H.P. about 30 lbs. per I.H.P. each hour of work. about 20 to 22 lbs. per I.H.P. each hr. of work.

Triple compound engines

.......

about 15 to 18 lbs. per I.H.P. each hr. of work.

These results vary very considerably with various types of engines.

Thus the most excellent effects attainable on trials are very much better, and such records as the following have been made with very moderately sized engines:

A 20-H. P. non-condensing single- ( 22 lbs. per I.H.P. per cylinder engine by Paxman..... hour of work. A 20-H. P. non-condensing single- ( 23.9 lbs. per I.H.P. per cylinder engine by McLaren.... hour of work. A 20-H. P. double compound noncondensing engine by Paxman.. A 20-H. P. double compound noncondensing engine by McLaren.

17.8 lbs. per I.H.P.
per hour of work.
19.8 lbs.
per I.H.P.
per hour of work.

While large triple-compound condensing engines have used as little as 14 lbs. per I.H.P. per hour of work.

These high results are, however, only to be relied on with first-class machinery specially designed for the duty undertaken. It will be safer for the prospective user to reckon on the larger figures. A ready rule for good new engines is to divide the figure 200 by the square root of the boiler pressure :

200

√pressure

= lbs. of water per I. H. P.

per hour.

All, or nearly all, the above consumption may be, however, usefully re-converted into feed-water and used over again by means of condensation, with which we now proceed to deal.

Water for Condensation. The amount of water required for condensation is a totally different affair, and is more or less dependent on circumstances dealt with in the following chapter.

Impurities in Water. This is a matter of much importance, as nearly all waters contain some foreign substances which tend to produce scale or sediment in the boiler, and a great amount of future annoyance and expense may be saved sometimes by ascertaining beforehand the character of the water proposed to be employed for steam-raising. Carbonates and sulphates of lime and carbonate of magnesia are the most usual components of troublesome scaling, and one-sixteenth of an inch of such scale may mean the loss of as much as 13 per cent. of fuel, while one-fourth of an inch will cause a loss of 38 per cent. For very muddy water, especially if it hold lime in solution, filtration must be provided.

Some forms of feed-heaters will precipitate such substances, before they reach the boiler, to a great degree.

Lime salts are more soluble in cold than in hot water, and most of them are deposited before the water reaches 320° Fahr. In fact, nearly all the substances held in solution by water are parted with when that water is evaporated into steam, or when frozen into ice.

CHAPTER XVIII.

CONDENSATION.

THE wastefulness of turning exhaust steam loose into the atmosphere, to dissipate therein a part of the heat which it has cost money to impart to it, is apparent to the most uninformed observer. Its value as a means of increasing draught in chimneys cannot be set against this loss as a serious offset, though in special cases, such as the locomotive, it becomes so from force of circumstances.

But when it is found that by extracting the heat from it, it may not only be made use of again as feed-water, and thus avoid a corresponding amount of re-heating, but, in addition, that its effect on the action of the engine itself may be made a substantial addition to its power, the last possible particle of sense is cut out of the original wastefulness.

Condensation may be made use of in several ways, and these may be judiciously combined into a great working economy by the removal of pressure from the back of the piston, and by the use of part or all of the cooled steam as already heated feed-water for the boiler.

The amount of heat left in exhaust steam is ascertainable if its pressure be known, corresponding to the following:

TEMPERATURES OF EXHAUST STEAM.

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