A large number of motors in use among the subscribers to the Compressed Air Company, of Paris, are rotary engines developing one horse power and less, and these in the early times of the industry were extravagant in their consumption, to a very high degree. To some extent this condition of things has been improved, chiefly by the addition of better regulating valves to control the air admission.
As altered, the two horse power rotary motors, when employed as cold air engines, a method often desired in special industries, consume 1,059 cubic feet per hour and per indicated horse power; with a moderate degree of heating, say to 50 deg. Cent., this consumption falls to 847 cubic feet. The efficiency of this type of rotary motors with air heated to 50 deg. may now be assumed at 43 per cent., not a very economical result, it is true, and one that may be largely improved, yet it is evident that with such an efficiency the use of small motors in many industries becomes possible, while in cases where it is necessary to have a constant supply of cold air, economy ceases to be a matter of the first importance.
Some useful results were obtained with compressed air used in crank engines; it is to be regretted that with this, also, apologies have to be made for the imperfect design and construction; they were old steam engines, some of those of two horse power losing from 25 to 30 per cent. by their own friction; some of the others tried, however, were far better, a newer type losing only from 8 to 10 per cent., while the 80 horse power referred to below showed an efficiency of 91 per cent. From these trials Prof. Riedler deduces—assuming 85 per cent. efficiency—a consumption of 611, 752, and 720 cubic feet per brake horse power. It is very evident from the foregoing that the Compressed Air Company, of Paris, will never do itself justice until as much thought and care has been devoted to the economical use of the motive power as has been expended in the means of producing it, and Professor Riedler's recent investigations should be especially useful in this respect. The question has indeed attracted the attention of more than one manufacturer, and reference is made to a particular type of small rotary motors which are being constructed by MM. Riedinger & Co., and which is stated have given very excellent results. These engines were specially used for working sewing machines and developed on the brake an efficiency of 34.07 and 51.63 foot pounds per second. Trials were made with a half horse power variable expansion Riedinger engine.
TRIALS OF A SMALL ROTARY RIEDINGER ENGINE.
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| |
Number of trials. | I. | II.
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| |
Initial air pressure. lb. per square inch | 86 | 71.8
" temperature. deg. Cent. | +12 | +170
Ft. pounds per second measured on the brake. | 51.63 | 34.07
Revolutions per minute. | 384 | 300
Consumption of air for one horse power per | |
hour. | 1,377 | 988
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TRIALS OF A 0.5 HORSE POWER RIEDINGER ROTARY ENGINE.
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| | | |
Number of trials. | I. | II. | III. | IV.
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| | | |
Initial pressure of air. lb. per sq. in. | 54 | 69.7 | 85 | 71.8
" temperature of air. deg. Cent. | 170 | 180 | 198 | 8
Final " " " | 25 | 20 | ... | 25
Revolutions per minute. | 335 | 350 | 310 | 243
Foot pounds per second measured on | | | |
brake. | 271 | 477 | 376 | 316
Consumption of air per horse power | | | |
and per hour. | 883 | 791 | 900 |1,148
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TRIAL OF AN 80 HORSE POWER (NOMINAL) FARCOT STEAM ENGINE.
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| | | |
| Re- | In- | | Consumption of
| vo- | di- | Temperature | air per horse
| lu- | ca- | of air. | power and per
|tions | ted | | hour.
| | |_____________|________________
| per |horse | | | |
Motor. | | |Admis-| Ex- |Nominal| Brake
| min- |power.| sion.|haust.| horse | horse
| ute. | | | | power.| power.
_________________|______|______|______|______|_______|________
| | |deg. C|deg. C| |
Nominal 80 horse | 54.3 | 72.3 | 129 | 21 | 469 | 517
power single cy- | 54.3 | 72.3 | 152 | 29 | 437 | 475
linder Farcot | 54.0 | 72.3 | 160 | 35 | 424 | 465
engine. | 40 | 65.0 | 170 | 49 | 438 | 477
_________________|______|______|______|______|_______|______________
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.