The demand for compressed air as a motive power is constantly
increasing in Paris; the company, according to its official reports,
is financially prosperous, and it seems difficult to understand how it
should continue as an actively going concern, unless it at all events
paid its way. The central station of St. Fargeau, originally started
on modest lines, for maintaining a uniform time by pneumatic pressure
throughout Paris, has grown rapidly to very large proportions, though
it has never been able to supply the demand made on it for power; and
at the present time a second and still larger station is being
constructed in another part of Paris. We confess that we do not
understand why such large sums of money should continue to be spent if
the enterprise is not commercially a sound one, nor how men of such
eminence in the scientific world as Professor Riedler should, without
hesitation, risk their reputation on the correctness of the system, if
it were the idle dream of an enthusiast, as many persons--chiefly
those interested in electric transmission--have declared it to be.
[Illustration: Fig. 1.--MAP OF PARIS WITH ST. FARGEAU STATION]
In describing the developments that have taken place during the last
two years, we shall confine ourselves entirely to the details of a
report recently made on the subject by Professor Riedler. As soon as
it became evident that a very largely increased installation was
necessary, it was determined that the new central station should be as
free as possible from the defects of the first one. These defects,
which were the natural results of the somewhat hasty development of an
experimental system, were of several kinds. In the first place, so
large a growth had not been contemplated, and the extensions were
made more or less piecemeal, instead of being on a regular plan; the
location of the central station itself was very unfavorable, both as
regards the facilities for obtaining coal and other supplies; the cost
of water was excessive, and the amount available, inadequate.
This evil was partly remedied by elaborate arrangements for cooling
the injection water so that it could be repeatedly used, a device
costly and ineffective, and resulting in extravagant working, to say
nothing of the high charges made by the Paris company for supplying
water. To these drawbacks had to be added others of an even more
serious character. The engines first laid down were not economical,
and the compressors employed gave but a very inferior result; with
each extension of the plant, the efficiency of both engines and
compressors was increased, the most satisfactory, we believe, having
been those supplied by the Societe Cockerill, and one of which was
exhibited at the Paris exhibition in 1889. Still it was clearly
recognized that much better results were possible, results which
Professor Riedler claims have been attained and which will be embodied
in the new installation now in progress.
This central station is located on the left bank of the Seine, close
to the fortifications, opposite Vincennes and not far from the
terminal stations of the Orleans and the Paris, Lyons, and
Mediterranean Railways; the plan, Fig. 1, shows the position. The
works are separated from the river by the quay, over which a bridge
will be constructed for the transfer of coal from the landing stages
belonging to the company, into the works; as will be readily seen from
the plan, it would be quite easy to run junction lines to the two
adjacent railways, but with all the advantages given by water
carriage, it was considered unnecessary to incur the expense. The
river also affords a constant and unlimited water supply, so that none
of the difficulties existing at St. Fargeau Station in imperfect
condensation and cooling will be met with.
The new installation, called the Central Station of the Quai de la
Gare, is laid out on a very large scale, the total generating energy
provided for being no less than 24,000 horse power; of this it is
intended that 8,000 horse power will be in operation this year, and an
extension of 10,000 horsepower in 1892; the power now in course of
completion comprises four engines of 2,000 horse power each. Four
batteries of boilers will provide steam for these engines. Figs. 2, 3,
and 4 show the first section of the installation now in progress; the
four groups of engines (three-cylinder condensing) are shown at 1, 2,
3, and 4; the four groups of boilers ranged behind them at F, F; the
feed water heaters belonging to each group at V V.
[Illustration: COMPRESSED AIR STATION ON THE QUA DE LA GARE, PARIS.
(FIG. 2,3,4)]
The end of the building abuts against the Seine, and the position of
the water conduits for inlet and discharge are indicated at C and A
respectively. The installation, when completed, will include very
extensive arrangements for transporting and storing coal, and the
interior of the boiler houses will be furnished with an overhead
system of rails and carriers for handling the coal automatically, as
far as possible. All the principal mains and steam pipes are made in
duplicate, not only for greater security, but in order that each set
of engines and boilers may be connected interchangeably without delay.
The Seine supplies an ample quantity of water, but not in a condition
either for feeding the boilers, for condensation, or for the air
compressors.
[Illustration: THE NEW COMPRESSED AIR STATION AT PARIS. (FIG. 5, 6)]
Special provisions have therefore to be made to filter the water
efficiently before it is used. For this purpose the water is led to a
group of four filters (see L, Fig. 4); from them it passes into the
tanks, JJ, and is pumped into the heaters. The filters can be rapidly
and automatically cleaned by reversing the flow of water through them.
Figs. 5 and 6 show the general form of the type of engine adopted, as
well as the engine house, some of the mains, etc. They are vertical
triple-expansion engines, and are being constructed by MM. Schneider
et Cie, of Creusot, with a guarantee of coal consumption not to exceed
1.54 lb. per horse power per hour, with a penalty of 2,000 francs for
every 100 grammes in excess of this limit. It is evident that with
this restricted fuel consumption, a large margin for economy will
exist at the new works, as compared with the St. Fargeau station,
where the best engines cannot show anything like this result, while
some of the earlier ones are distinctly extravagant, and the whole
installation is handicapped with imperfect means of condensation.
Moreover, according to Professor Riedler, the consumption of steam by
the new Schneider engines will be only 5.3 kilos. per horse power and
per hour as compared with some of the large engines requiring 9
kilos., and the Cockerill engines--using 8 kilos. per hour, not to
speak of the older motors that are very extravagant in the use of
steam. The St. Fargeau station is worked under a further disadvantage.
The constantly increasing demand from subscribers taxes the resources
of the station to their fullest extent, so that practically there is
no reserve power.
In the new installation the work will be equally constant, but care
will be taken always to have a sufficient reserve. Electric lighting
will form a considerable part of the duty to be done from this
station, and in all cases it is intended to work with accumulators, so
that the resistance to be overcome by the engines, so far as this part
of the duty is concerned, will be well known and uniform. The
engineers of the Compressed Air Co., of Paris, have during the last
five years acquired an experience which could only be attained at a
high price and at the expense of a certain amount of failure; this
period, it is claimed, is now passed, and in the new installation it
is possible to put into practice all the valuable lessons learned at
St. Fargeau, to say nothing of the more favorable natural conditions
under which the extension is being started and the improvements in the
compression of the air made by Mr. Popp and Professor Riedler, and to
which we shall refer later.
Chiefly in consequence of the high value of the ground, vertical
engines were adopted at the new station; the proximity to the river
made the foundations somewhat costly, and the risk of occasional
floods rendered it desirable to set the level of the engine bedplates
20 inches above the floor of the building; the foundations of the
engines are continuous, but are quite independent of the building.
There are three compressing cylinders in each set of engines, one
being above each steam cylinder. Two of these are employed to compress
the air to about 30 lb. per square inch, after which it passes into a
receiver and is cooled; it is then admitted into the third or final
compressing cylinder and raised to the working pressure at which it
flows into the mains. In the illustrations, h, m, and b are the high,
intermediate, and low pressure cylinders of one set of engines; as
will be seen, each cylinder is on a separate frame connected by
girders; directly above the cylinders are the two low and the one high
pressure air cylinders, b¹, m¹, and h¹ respectively. The former
deliver the air compressed to the first stage into the receiver, T¹
(see Fig. 5), whence it passes into the third compression cylinder,
and thence by a main into the cylinders, R R, which are in direct
communication with the delivery mains; these mains terminate in the
subway, T. The water for condensation is brought into the engine house
by the channel, C, and the condenser pumps, a, draw direct from this
supply; the discharge main back to the river is shown at A. The
relative positions of the engine and boiler houses are indicated in
Figs. 2 to 5, where F shows the end of one group of boilers; the air
supply for the compressors is led from the central raised portion, S,
of the roof.
Professor Riedler's first experiments in improving the efficiency of
air compressors were made with one of the Cockerill compressors in use
at the St. Fargeau Station, and considerable difficulty attended this
work, because the machinery was necessarily kept almost in constant
operation. These compressors were designed by MM. Dubois and Francois,
of Seraing. Two of their leading features were the delivery of the
compressed air at as low a temperature as possible, and with a
relatively high piston speed of about 400 ft. a minute. The former
object is attained by the injection of a very fine water spray at each
end of the air cylinder, and its rapid removal with each stroke; the
free as well as the compressed air flows through the same passages,
one at each end of the cylinder; the inlet valves being placed at the
side of these passages, and the outlet or compressed air valves at the
top, the compressed air, entering a chamber above the cylinder, common
to both valves, and passing thence to the reservoir. The compressed
air valves, which are seven in. in diameter, are brought back sharply
to their seats at each stroke, by a small piston operated by
compressed air flowing through a by-pass from the chamber. The
illustrations published by us on page 686 of our forty-seventh volume
show the construction of these compressors. The engravings on page 683
of the same volume illustrate the compressors used in a somewhat older
part of the installation; they were made by M. Blanchod, of Vevey, and
a passing reference may be made to them. The air is admitted through
valves in the cylinder, and is forced out through spring-loaded
valves; water is admitted into the cylinder to cool the air.
Fig. 7 indicates the modification made by Professor Riedler in one of
the Cockerill compressors: a receiver, A, was placed under the two
compressing cylinders, B and C. The first stage is completed in the
large cylinder, B, the air being compressed to about 30 lb. per square
inch; from this it is discharged into the receiver, A, through the
pipe, B¹, where it meets with a spray injection that cools it to the
temperature of the water. The final stage is then effected in the
smaller cylinder, C, which, drawing the air from the receiver through
the pipe, C¹, compresses it to about 90 lb. and delivers it through
the pipe, d, to the mains. We hope shortly to publish drawings of this
compressor in its final form; in its elementary stage Professor
Riedler claims to have obtained some very remarkable results. He says
that the waste spaces in his modification were much smaller than in
the Cockerill compressor, while the efficiency of the apparatus was
largely increased. The actual engine duty per horse power and per hour
was raised, as a maximum, to 384 cubic feet of air at atmospheric
pressure, and compressed to 90 lb. per square inch, a marked increase
on the duty of the compressors in use at the St. Fargeau station. The
Cockerill compressors experimented on at the same time showed a
maximum duty of 306 cubic feet of air. A considerable advantage is
claimed in drawing clean and cool air from the outside of the
building, and beyond the main feature of carrying out the compression
in two stages, Mr. Riedler appears to have shown great skill in
introducing several minor alterations and improvements in the plant.
[Illustration: EFFICIENCY CURVES FOR THREE TYPES OF COMPRESSORS. (Fig.
8, 9, 10)]
Figs. 8, 9 and 10 are diagrams showing the comparative efficiency of
the three types of compressors at St. Fargeau--Fig. 10 being a diagram
of the Riedler compressor--and indicate the gain derived from the
intermediate cooling. The loss is shown to be only 12 per cent., as
compared with a loss of 43 per cent. in a large part of the plant, and
of 105 per cent. in the earlier compressors of the St. Gothard type.
The table given herewith contains a summary of trials made by
Professor Gutermuth, and are intended to show the comparative results
of an extended trial with three kinds of compressors at St. Fargeau.