These motors, it may be assumed, represent the best practice that has been obtained up to the present time in the construction of compressed air motors; with the smallest of them, indicating about one-tenth of a horse power, the consumption of air, when admitted cold, was 1377 cubic feet and 988 cubic feet when the air was heated before admission. The half horse power engine consumed 1148 cubic feet of cold air, and of heated air 791 cubic feet per horse power and per hour. It should be mentioned that these, the most valuable and suggestive of all the trials carried out by Professor Riedler, were conducted with the greatest care, two distinct modes of measuring the air supplied being followed on two occasions for each test; it may therefore be considered that the results given are absolutely correct. The trials were made with an old single cylinder Farcot engine, nominally of 80 horse power, but indicating over 72.3. With this engine the consumption of air varied from 465 to 517 cubic feet, the larger consumption being due to the lower temperature (129 deg. Cent.) to which the air was raised before admission; in the most economical result the temperature was 160 deg.
Cent. The volumes of air referred to are, of course, in all cases taken at atmospheric pressure.
Among the important losses that have to be reckoned with in every system of distributing motive power from a central station - whether by steam or by electricity, water, or compressed air - losses must occur in the mains by which the power generated is transferred from the point of production to that of consumption. In the case we are now considering very careful tests were conducted in 1889 by Professor Kennedy, to whose report we have already referred. Since that time important changes have been made by the Compressed Air Company, at Paris, in the details of distribution, and on this account the later investigations of Professor Riedler on the losses due to this cause are of special interest.
Before its admission into the mains a certain loss occurs at the St. Fargeau station, in the large reservoirs to which the air is delivered from the compressors. This question of preliminary storage was one that received considerable attention when the designs of the new station on the Quai de la Gare were being considered. It was intended to construct very large receivers in the basement of the station, and the foundations for these were even commenced. It was decided, however, that for the 10,000 horse power which is to form the first section of the new station, and for which the complete system of mains has already been laid down, storage reservoirs would be unnecessary, and a saving both in first cost and subsequent loss of air would be effected. The length of mains of 19.69 in. diameter is so considerable that they will contain at all times a sufficient reserve of air to prevent any irregularities in pressure at the motors.
With reference to these mains it may be mentioned that, unlike the 11.81 in. conductors of the St. Fargeau system, of which 17 kilometers are laid in the Paris subways, the new mains are entirely laid in the streets, it having been found impossible to make room for these large pipes in the subways already crowded with telegraph and telephone wires, water mains, etc.
Professor Riedler investigated the two causes of loss in the mains - leakage and resistance. It was superficially evident that the mains of the old system were so well laid, and the joints so well designed, that the loss from leakage was never a serious one. In order, however, to ascertain the amount accurately, a series of careful experiments were carried out by Professor Gutermuth with the 11.81 in. mains of the St. Fargeau system.
These trials refer to the mains running from the St.
EXPERIMENTS ON LEAKAGE IN MAINS. --------------------------------------------------------------------- | | | | | | L P A | | | | | Air Pressure | Loss of | o e i | | | | | in Mains. | Pressure. | s r r | | | | |---------------|-------------| s | | | | | | | | | C D | | |System of Mains | Length. | | | | | o e e | |N| Tried. | | At | At | | | f n l | |u| | |Begin- | End |During| Per | t i | |m| | |ning of| of |Trials|Hour. | A . v | |b| | |Trials.|Trials.| | | i e | |e| | | | | | | r o r | |r| | | | | | | f e | | | | | | | | | d | --+-----------------+---------+-------+-------+------+------+-------| | | | yards. | atm. | atm. | | | | |1|Southern reseau | | | | | | | | | to Place de la | | | | | | | | | Concorde. | 9,980 | 6.5 | 6.0 | 0.5 | 1.5 | 3 | |2| Total reseau | 18,500 | 6.9 | 5.9 | 1.0 | 1.5 | 6.3 | |3|To Place de | | | | | | | | | la Concorde | 9,980 | 7.0 | 6.43 | 0.57 | 0.75 | 2.16 | |4|Total reseau | 18,500 | 6.7 | 5.28 | 0.88 | 1.32 | 5.5 | |5|Northern reseau | | | | | | | | | to Rue de Belle-| | | | | | | | | ville. | 1,530 | 6.0 | 5.0 | 1.0 | 0.6 | 2.3 | |6|To the Rue des | | | | | | | | | Pyrenees. | 600 | 6.1 | 3.7 | 2.4 | 0.56 | 2.2 | ---------------------------------------------------------------------
Fargeau station to the Place de la Concorde, a length of 9.142 kilometers; to the whole system of mains, 16.5 kilometers; to the northern mains running from St. Fargeau to the Rue de Belleville, 1.4 kilometers; and from St. Fargeau to the Rue des Pyrenees, 6.5 kilometers. It will be seen from the figures given in the table that the actual loss is small, and it is stated that this is due chiefly to the elastic joint employed throughout the system, excepting in the Rue de Belleville, where rigid couplings are used, and continual trouble is experienced from loss by leakage. In all cases the losses given are the maximum, which only occur under the most unfavorable conditions.