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.
PERFORMANCES OF COMPRESSORS AT THE ST. FARGEAU CENTRAL STATION.
--------------+-------+--------+------+-------+--------+--------+----------+
| | | | | | | |
|Revolu-| Horse- | |Amount |Quantity| Cubic | |
Compressors. | tions | Power |Effic-|of Air | of Air |Feet of |Final Air |
|of Eng-|Absorbed|iency.|Passing| Passing|Air per |Pressure. |
|ine per| by | |through| through| Horse- | |
|Minute.|Compres-| | Inlet | Valves | Power | |
| | sors. | | Valves| per | and per| |
| | | | each | Hour. | Hour. | |
| | | |Revolu-| | | |
| | | | tion. | | | |
--------------+-------+--------+------+-------+--------+--------+----------+
| | | | cubic | cubic | |lb. per |
1. | | | | feet | feet | |sq. in. |
Sturgeon | | | | | | | |
Compressor | 37 | 302 | .87 | 41.67 | 91,507| 261.3 | 90 |
Diameter of | 37 | 258 | .87 | 38.13 | 84,650| 276.1 | 90 |
cylinder, | | | | | | | |
23.62 in. | | | | | | | |
and 21.66 in.;| | | | | | | |
stroke, | | | | | | | |
48.63 in. | | | | | | | |
| | | | | | | |
2. | | | | | | | |
Cockerill | 40 | 337 | .83 | 46.61 | 111,864| 281.83 | 90 |
Compressor. | 45 | 353 | .83 | 46.61 | 125,844| 302.66 | 90 |
Diameter of | 40 | 342 | .88 | 49.43 | 118,632| 296.65 | 90 |
cylinder, | 46 | 377 | .85 | 48.02 | 132,534| 298.77 | 90 |
25.98 in.; | 38.67 | 324 | .89 | 50.14 | 116,434| 306.19 | 90 |
stroke, | 38.5 | 337 | .89 | 50.14 | 115,818| 294.18 | 90 |
47.24 in. | 38.6 | 329 | .91 | 50.84 | 117,740| 305.13 | 90 |
| | | | | | | |
| | | | | | | |
3. | | | | | | | |
Riedler | 52 | 615 | .985 | 77.34 | 241,300| 353.50 | 90 |
Compressor. | 60 | 709 | .985 | 76.98 | 277,128| 353.50 | 90 |
Diameter of | 38 | 422 | .985 | 77.34 | 176,330| 376.12 | 90 |
low-pressure | 39 | 424 | .985 | 77.34 | 181,030| 384.60 | 90 |
cylinder, | | | | | | | |
42.91 in.; | | | | | | | |
diameter of | | | | | | | |
high-pressure | | | | | | | |
cylinder, | | | | | | | |
26.38 in.; | | | | | | | |
stroke, | | | | | | | |
47.24 in. | | | | | | | |
--------------+-------+--------+------+-------+--------+--------+----------+
The results thus obtained were so satisfactory that the designs were prepared for the great compressors to be operated at the new central station on the Quai de la Gare by the 2,000 horse power engines.
The transmission of the compressed air through the mains is unavoidably attended with a certain percentage of loss, which, of course, increases with the length of the transmission, the presence of leakage at the joints, etc. Professor Riedler has devoted considerable time to the investigation of this source of waste, and we shall presently refer to the results he has recorded; in the first place, however, we propose to consider what he has to say on the subject of utilizing the air at the points of delivery, and the means employed for obtaining a relatively high efficiency of the motor.
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.