X.

TABLE OF APPROXIMATE RATIOS FOR STEAM-GAS WORKING EXPANSIVELY IN A NON-CONDUCTING CYLINDER.

r Pm, ratio of whole gross work of steam on piston to gross work

P1

during admission.

P1

"P

ratio of gross work during admission to whole gross work.

XI.

TABLE OF APPROXIMATE RATIOS FOR PERFECT GASES WORKING EXPANSIVELY AT CONSTANT TEMPERATURE: ALSO FOR NEARLY

g Pm ratio of whole gross work of gas on piston to gross work

P.

during admission.

Pi

P

ratio of gross work during admission to whole gross work.

SECTION 7.-Of Binary Vapour Engines.

300. General Description of the Binary Vapour Engine.-This engine, the invention of M. Prospère-Vincent du Trembley, is driven by the combined action of two different fluids, a less and a more volatile, in two separate cylinders. The less volatile fluid is evaporated in a boiler, and drives the piston of its cylinder, in the usual way. On being discharged, it is passed vertically downwards through a set of small tubes, contained within a cylindrical vessel: the less volatile fluid, passing downwards through the tubes, is liquefied, and gives out its heat to the more volatile fluid, which ascends in the space surrounding the tubes, and reaches the top of the vessel in the state of vapour. This vapour drives the piston of a second cylinder, during the return stroke of which it is expelled into a second surface condenser, consisting also of a number of small vertical tubes; the vapour passes downwards through these tubes, which are surrounded by a copious stream of cold water; this abstracts heat from the vapour, and causes it to be condensed, and the liquid thus produced is pumped back into the evaporating vessel to perform its work over again.

The less volatile fluid is always water; for the more volatile, æther is usually employed.

Full details of the construction and mode of working of these engines are given in M. du Trembley's work, entitled, Manuel du Conducteur des Machines à Vapeurs combinées ou Machines Binaires (Lyons, 1850-51); and accounts of their performance are contained in a report by Mr. George Rennie, published in 1852; in a lithographed report by M. E. Gouin, on the experimental trip of the ship Brésil," in 1855; and in a paper by Mr. James W. Jamieson, read to the Institution of Civil Engineers in February, 1859.

301. Theory of the Steam-and-Æther Engine.—In fig. 115, let ABCEFA represent the diagram of the steam cylinder, and KLMPQK that of the æther cylinder.

OA be the absolute pressure of the steam at its admis

=

V1 A B, the volume of one lb. of it when admitted; r v1 = D C, the volume to which it expands;

Let H, denote the available heat expended, in foot-lbs. per lb. of steam; Ú = area A B C E F A, the energy exerted on the piston by one lb. of steam. Then the heat rejected by each lb. of steam, and given out through the tubes to the æther, is given by the equation

and several examples of the mode of computing this quantity of heat have been given in the preceding sections.

To find what volume will be filled with æther vapour by means of this heat, in the first place must be computed the expenditure of heat per cubic foot of ather vapour, produced at the pressure under which the æther is evaporated, which is supposed to be given and represented by p' OK, and is necessarily a pressure corresponding to a boiling point lower than the temperature at which the steam is condensed. That expenditure of heat is

where L': ==

=

d P is the latent heat of evaporation of one cubic d& foot of æther vapour under the given pressure, calculated by a formula of the kind given in Article 255, or by the aid of Table V.; J = 399·1 foot-lbs. per degree of Fahrenheit, is the specific heat of liquid æther; D' is the weight of one cubic foot of æther vapour, found by the formulæ of Article 256, or by the aid of Table V.; T is the temperature at which the æther is evaporated, and T" that at which it is condensed, and returned to the evaporating apparatus. Then the initial volume, represented by K L in the figure, of the æther evaporated per lb. of steam condensed, is found by means of the equation

H2

u' = KL = L' + J c D' (T′ – T")'

.(3.)

Let p❞ = ON denote the intended final pressure of the æther vapour, at the end of its expansion, and p" its mean back pressure, which appears to be about 5 lbs. on the square inch. Then from the data, p', p", p", T", by means of the formulæ of Articles 281 and 284, substituting only the constants which apply to æther for those which apply to steam, and using Table V. instead of Table IV., may be computed

The ratio of expansion r, and thence the final volume M N = r u of the æther evaporated per lb. of steam;

The energy exerted by that æther, represented by the KLM QK = U'.

The ratio

MN DC = r' u' ÷ r v1

..(4.)

is that of the volume of the æther cylinder to the volume of the steam cylinder. In practice, those cylinders are either of equal size, or the æther cylinder is somewhat the larger.

The heat per lb. of steam to be abstracted by the cold water which circulates in the æther condenser is given by the expression H1 – U – U ́........

..(5.)

The mean effective pressures in the steam cylinder and æthe cylinder respectively, are

The same amount of additional energy, which is obtained by the addition of the æther engine to the steam engine, might also be obtained by continuing the expansion of the steam sufficiently far, as represented by the line C H G, provided a sufficiently low back pressure could be insured; but this might require in some cases a cylinder so large as to be more costly than the binary engine.

302. Example of Results of Experiments.-The following quantities are means, computed from a long series of experimental results given in M. Gouin's report, already mentioned, on the performance of the steam and æther engines of the "Brésil:"

PRESSURES IN LBS. ON THE SQUARE INCIL.

Total mean effective pressure reduced to the area of one piston, the areas and strokes of the steam and æther pistons having been in this case the some,......

18.7

It thus appears that the proportions of the indicated power of the engine obtained in the steam and æther cylinders respectively, were as follows:-Steam, 11.6 18762; æther, 71 187 = •38.

=

The gain of power, however, by the addition of the æther engine, is not quite so great as this calculation shows; because, had the steam cylinder been used alone, the back pressure would have been in all probability about 3 lbs. on the square inch less; that is, about 46 instead of 76; so that the mean effective pressure in the