PERFORMANCES OF COMPRESSORS AT THE ST. FARGEAU CENTRAL STATION._Scientific American Supplement, No. 803, May 23, 1891

--------------+-------+--------+------+-------+--------+--------+---------+

| R p | | E | | | | |

| l E m | by | c |through| through| Horse- | |

| t g n | sors. | e | Valves| per | and per| |

| o n t | | c |Revolu-| | | |

| n e e | | y | tion. | | | |

| s . | | . | | | | |

--------------+-------+--------+------+-------+--------+--------+---------+

_Compressor_ | 37 | 302 | .87 | 41.67 | 91,507| 261.3 | 90 |

Diameter of | 37 | 258 | .87 | 38.13 | 84,650| 276.1 | 90 |

23.62 in. | | | | | | | |

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 |

42.91 in.; | | | | | | | |

26.38 in.; | | | | | | | |

47.24 in. | | | | | | | |

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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.

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| | | |

| o t | e .| | | horse | horse

| n e | d | | | power.| power.

_________________|_s_.__|______|_________|________|_______|________

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.