Chapter 6

In the earliest stages of the Popp system in Paris it was recognized that no good results could be obtained if the air were allowed to expand direct into the motor; not only did the formation of ice due to the expansion of the air rapidly accumulate and choke the exhaust, but the percentage of useful work obtained, compared with that put into the air at the central station, was so small as to render commercial results hopeless. The practice of heating the air before admitting it to the motor is quite old, but until a few years ago it never seems to have been properly carried out; in several mining installations where this motive power had been long used, more or less imperfect attempts had been made to heat the air; in one instance only, recorded by Professor Riedler, was an efficient means employed. In this case a spray of boiling water was injected into the cylinder and mixed with the air at each stroke, with the result that a very marked economy was obtained.

After a number of experiments, Mr. Popp arrived at the conclusion that the simplest mode of heating, if not the most efficient, was at all events the most suitable, as it was a matter of the first importance that subscribers should not be troubled with the charge of any apparatus involving complication or careful management; he therefore adopted a simple form of cast iron stove lined with fireclay, heated either by a gas jet or by a small coke fire. It was found that this apparatus, crude as it was, answered the desired purpose, until some better arrangement was perfected, and the type was accordingly adopted throughout the whole system. It was quite recognized that this method still left much to be desired, and the economy resulting from the use of an improved form was very marked.

From a large number of trials very carefully carried out by Professor Gutermuth, it was found that more than 70 per cent. of the total number of calories in the fuel employed was absorbed by the air and transformed into useful work. Whether gas or coal be employed as the fuel, the amount required is so small as to be scarcely worth consideration; according to the experiments carried out, it does not exceed 0.09 kilo. per horse power and per hour, but it is scarcely to be expected that in regular practice this quantity is not largely exceeded. Professor Weyrauch has also carefully investigated this part of the subject and fully confirms, if he, indeed, does not go beyond Professor Gutermuth. He claims that the efficiency of fuel consumed in this way is six times greater than when burnt under a boiler to generate steam. He goes so far as to assert that with a good method of heating the air, not only can all the losses due to the production and the transmission of the compressed air be made good, but also that it will actually contain more useful energy at the motor than was expended at the central station in compressing it.

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. ___________________________________________________________________ | R p | | | | e e | I | | Consumption of | v r | n | Temperature | air per horse | o | d h p| of air. | power and per | l m | i o o| | hour. | u i | c r w|__________________|________________ | t n | a s e| | | | Motor. | i u | t e r|Admission|Exhaust.|Nominal| Brake | o t | e .| | | horse | horse | n e | d | | | power.| power. _________________|_s_.__|______|_________|________|_______|________ | | | deg. C | deg. C | | Nominal 80 horse | 54.3 | 72.3 | 129 | 21 | 469 | 517 power single | 54.3 | 72.3 | 152 | 29 | 437 | 475 cylinder 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.

EXPERIMENTS ON LEAKAGE IN MAINS.

| | | | | | L P A | | | | | Air Pressure | Loss of | o e i | | | | | in Mains. | Pressure. | s r r | | | | |---------------|-------------| s | | | | | B | | | | C D | | |System of Mains | Length. | e T| | | | o e e | |N| Tried. | | g r| At | | | f n l | |u| | |A i o i| End |During| Per | t i | |m| | |t n f a| of |Trials|Hour. | A . v | |b| | | n l|Trials.| | | i e | |e| | | i s| | | | r o r | |r| | | n .| | | | f e | | | | | g | | | | 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 | ---------------------------------------------------------------------

These trials refer to the mains running from the St. 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.

It was found, during the first, second, and fourth tests, that considerable leakage occurred between the St. Fargeau central station and the Rue de Belleville. During the trials two and four, an uncertain amount of loss occurred from the consumption of air required to work the pneumatic clocks, and also motors in the circuit, that could not be stopped. The tests two and four include all losses in the service pipes, as well as the mains.

The production of compressed air at the central station is assumed at 30,000 cubic feet per hour (atmospheric pressure), and in all cases the loss in the mains is taken as a percentage of the total production.

The losses due to resistance in the mains were also examined with great care, over independent sections, as well as through the complete _réseau_. During the early part of these trials, an unusual and excessive loss was recorded, the cause of which could not be at first ascertained. At intervals along these mains are placed a number of water reservoirs which receive the water injected into the mains; in addition to these the direct flow of the air is interrupted by numerous siphons, the stop valves to branches, etc. Investigation showed that the presence of these reservoirs created considerable resistance on account of an increased and subsequently reduced section. The exact loss from this cause was, therefore, carefully measured, as well as the losses existing in the mains not so interrupted. The results show that the loss by expansion at one reservoir, when the speed of the air flow was 23 ft. per second, was equal to 0.15 atmosphere; with a speed of 29 ft. 6 in. per second, it amounted to 0.2 atmosphere.

Therefore, the presence of five such reservoirs would cause a loss in pressure equal to one atmosphere. This very undesirable arrangement is not repeated in the new system, the sumphs being connected in such a way as not to modify the section of the tube, nor consequently the pressure of the air. The presence of the siphons and stop valves did not seem to affect the pressure to any measurable extent. The following table contains a list of the more important mains tested, and it may be mentioned that the resistance, due to the reservoirs, was at first partially included. The trials were carried out while the mains were not being drawn upon by subscribers.

| | Section of Mains Tested. | Length. |No. of | |Tests. | | ---------------------------------------------------+------------+------ | yards. | From the central station to the end of reseau and | | back to central station by return circuit | 18,100 | 7 From the central station to the Rue Fontaine au |\ 14,600 |/ 3 Roi |/ 9,900 |\ 4 From the central station to the Rue de la | | Charonne | 9,490 | 5 From the Rue de la Charonne to Fontaine au | | Roi | 4,770 | 3 From the central station to the Avenue de la | | Republique | 1,860 | 8 Various trials on different lengths of mains |770 to 8,000| 11 -----------------------------------------------------------------------

Over the whole system of 16.5 kilometers, which was also tested when no air was being taken off, there were four reservoirs of considerable size, and which offered a large resistance with a corresponding loss of pressure; on the line there were also 23 siphons and 42 stop valves.

These trials were repeated several times to secure accuracy, and the speed of the air was brought to 49 ft. a second. The results obtained in one of these trials may be taken as an example. The main between the Rue St. Fargeau and the Fontaine au Roi, on which there are no collecting reservoirs, but three siphons and eight stop valves, gave, with an average speed of 21 ft. 3 in., a loss in pressure of 0.05 atmosphere for each kilometer of main.

From these experiments it would appear that, assuming a speed of 21 ft. per second, a loss in pressure of one atmosphere would correspond to a distance of 20 kilometers; that is to say, a central station could extend its mains on all sides with a radius of 20 kilometers, and the motors at the ends of the lines would receive the air at a pressure 15 lb. less than at the central station. Professor Riedler states that as an actually measured result, the velocity of the air through the mains of the St. Fargeau system is 19 ft. 8 in. per second, and that the loss in pressure per kilometer is 0.07 atmosphere. From this it follows that including the resistances due to the four reservoirs, and other obstructions actually existing, an allowance of one atmosphere loss on a 14 kilometer radius is ample. By increasing the initial pressure of the air, much better results can be obtained, and future attention in practice should be devoted to this point. The amount of work required to compress air does not increase in the same ratio as the pressure, and for this reason considerable economy can be effected at the first stage, and the loss in the mains will be reduced.

Passing to another point of the same subject, Professor Riedler considers the best dimensions that should be given to the mains. Resistance decreases with an increase in the diameter of these and in direct ratio to their diameter; for this reason--still assuming a pressure corresponding to a velocity of 20 ft. per second--with a fall of one atmosphere, a length of 40 kilometers could be succesfully worked.

The mains of the new _réseau_ for the Quai de la Gare station are 19.69 in. in diameter; they are built up of steel plates riveted, and this Professor Riedler considers to have been a serious error on account of the extra resistance offered by the large number of rivet heads.