According to Professor Riedler, from 15 to 20 per cent. above the power at the central station can be obtained by means at the disposal of the power users, and it has been shown by experiment that by heating the air to 250 deg. Cent. an increased efficiency of 30 per cent. can be obtained. Better results than those heretofore obtained may, therefore, be confidently expected with a more perfect and economical application of the fuel in heating the air, and a better means of regulation in admitting it to the motors. In his report Professor Riedler indicates a method by the use of which he considers considerable advantages may be secured. This is the heating the air in two stages instead of at one operation, and passing it through two motors, to the first of which the air is admitted heated only to a moderate extent; the exhaust from this motor then passes into a second heater and thence into the second motor. A series of experiments with this arrangement were recently carried out.
The consumption of air per brake horse power was reduced from 812 cubic feet per hour, a favorable duty in the single motor, to 720, and in the best result to 646 cubic feet with the two motors and double heaters. It should be added that these trials were carried out with steam engines but ill adapted for the purpose. It is to be regretted that the experiments of Professor Riedler could not have been conducted with more perfect appliances, but it must be borne in mind that the utilization of compressed air, especially as regards the motors, is still in a very imperfect stage, and that a great deal remains to be done before the maximum power available at the motor can be obtained. Investigations in this direction for a considerable time to come must be directed, therefore, toward improving the design and construction of the motors and the treatment of the air at the point of delivery into the engine.
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. ______________________________________________________________ | | Number of trials. | I. | II. ______________________________________________|_______|_______ | | 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 ______________________________________________|_______|_______ TRIALS OF A 0.5 HORSE POWER RIEDINGER ROTARY ENGINE. _____________________________________________________________________ | | | | Number of trials. | I. | II. | III. | IV. __________________________________________|______|______|______|_____ | | | | 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 __________________________________________|______|______|______|_____ TRIAL OF AN 80 HORSE POWER (NOMINAL) FARCOT STEAM ENGINE. ______________________________________________________________ | | | | | Re- | In- | | Consumption of | vo- | di- | Temperature | air per horse | lu- | ca- | of air. | power and per |tions | ted | | hour. | | |_____________|________________ | per |horse | | | |