Compressed-air Table for Hoisting-engines.

(Ingersoll-Sergeant Drill Co.)

The following table gives an approximate idea of the volume of free air required for operating hoisting-engines, the air being delivered at 60 lbs. gauge-pressure. There are so many variable conditions to the operation of hoisting-engines in common use that accurate computations can only be offered when fixed data are given. In the table the engine is assumed to actually run but one-half of the time for hoisting, while the compressor, of course, runs continuously. If the engine runs less than one-half the time, as it usually does, the volume of air required will be proportionately less, and vice versa. The table is computed for maximum loads, which also in practice may vary widely. From the intermittent character of the work of a hoisting-engine the parts are able to resume their normal temperature between the hoists, and there is little probability of the annoyance of freezing up the exhaust-passages.

VOLUME OF FREE AIR REQUIRED FOR OPERATING HOISTING.
ENGINES, THE AIR COMPRESSED TO
60 POUNDS GAUGE
PRESSURE.

SINGLE-CYLINDER HOISTING-ENGINE.

Practical Results with Compressed Air.-Compressed-air System at the Chapin Mines, Iron Mountain, Mich.-These mines are three miles from the falls which supply the power. There are four turbines at the falls, one of 1000 horse-power and three of 900 horse-power each. The pressure is 60 pounds at 60° Fahr. Each turbine runs a pair of compressors. The pipe to the mines is 24 ins. diameter. The power is applied at the mines to Corliss engines, running pumps, hoists, etc., and direct to rock-drills.

A test made in 1888 gave 1430.27 H.P. at the compressors, and 390.17 H P. as the sum of the horse-power of the engines at the mines. Therefore, only 27% of the power generated was recovered at the mines. This includes the loss due to leakage and the loss of energy in heat, but not the friction in the engines or compressors. (F. A. Pocock, Trans. A. I. M. E., 1890.)

W. L. Saunders (Jour. F. I. 1892) says: "There is not a properly designed compressed-air installation in operation to-day that loses over 5% by transmission alone. The question is altogether one of the size of pipe; and if the pipe is large enough, the friction loss is a small item.

The loss of power in common practice, where compressed air is used to drive machinery in mines and tunnels, is about 70%. In the best practice, with the best air-compressors, and without reheating, the loss is about 60%. These losses may be reduced to a point as low as 20% by combining the best systems of reheating with the best air-compressors."

Gain due to Reheating.-Prof. Kennedy says compressed-air transmission system is now being carried on, on a large commercial scale, in such a fashion that a small motor four miles away from the central station can indicate in round numbers 10 horse-power, for 20 horse-power at the station itself, allowing for the value of the coke used in heating the air. The limit to successful reheating lies in the fact that air-engines cannot work to advantage at temperatures over 350°.

The efficiency of the common system of reheating is shown by the results obtained with the Popp system in Paris. Air is admitted to the reheater at about 83°, and passes to the engine at about 315°, thus being increased in volume about 42%. The air used in Paris is about 11 cubic feet of free air per minute per horse-power. The ordinary practice in America with cold air is from 15 to 25 cubic feet per minute per horse-power. When the Paris engines were worked without reheating the air consumption was increased to about 15 cubic feet per horse-power per minute. The amount of fuel consumed during reheating is trifling.

Efficiency of Compressed-air Engines.-The efficiency of an air-engine, that is, the percentage which the power given out by the air-engine bears to that required to compress the air in the compressor, depends on the loss by friction in the pipes, valves, etc., as well as in the engine itself. This question is treated at length in the catalogue of the Norwalk Iron Works Co., from which the following is condensed. As the friction increases the most economical pressure increases. In fact, for any given friction in a pipe, the pressure at the compressor must not be carried below a certain limit. The following table gives the lowest pressures which should be used at the compressor with varying amounts of friction in the pipe:

Friction, lbs.

2.9 5.8 8.8 11.7 14.7 17.6 20.5 23.5 26.4 29.4 Lbs. at Compressor... 20.5 29.4 38.2 47. 52.8 61.7 70.5 76.4 82.3 S8.2 Efficiency %. 70.9 61.5 60.6 57.9 55.7 54.0 52.5 51.3 50.2 49.2

An increase of pressure will decrease the bulk of air passing the pipe and its velocity. This will decrease the loss by friction, but we subject ourselves to a new loss, i.e. the diminishing efficiencies of increasing pressures. Yet as each cubic foot of air is at a higher pressure and therefore carries more power, we will not need as many cubic feet as before, for the same work. With so many sources of gain or loss, the question of selecting the proper pressure is not to be decided hastily.

The losses are, first, friction of the compressor. This will amount ordinarily to 15 or 20 per cent, and cannot probably be reduced below 10 per cent. Second, the loss occasioned by pumping the air of the engine-room, rather than the air drawn from a cooler place. This loss varies with the season and amounts from 3 to 10 per cent. This can all be saved. The third loss, or series of losses, arises in the compressing cylinder, viz., insufficient supply, difficult discharge, defective cooling arrangements, poor lubrication, etc. The fourth loss is found in the pipe. This loss varies with the situation, and is subject to somewhat complex influences. The fifth loss is chargeable to fall of temperature in the cylinder of the air-engine. Losses arising from leaks are often serious.

Effect of Temperature of Intake upon the Discharge of a Compressor.-Air should be drawn from outside the engine-room, and from as cool a place as possible. The gain amounts to one per cent for every five degrees that the air is taken in lower than the temperature of the engineroom. The inlet conduit should have an area at least 50% of the area of the air-piston, and should be made of wood, brick, or other non-conductor of heat.

Discharge of a compressor having an intake capacity of 1000 cubic feet per minute, and volumes of the discharge reduced to cubic feet at atmospheric pressure and at temperature of 62 degrees Fahrenheit: Temperature of Intake, F.. 0° 32° 62° 75° 80° 90° 100° 110° Relative volume discharged, cubic ft... 1135 1060 1000 975 966 949 932 916

Requirements of Rock-drills Driven by Compressed Air. (Norwalk Iron Works Co.)-The speed of the drill, the pressure of air, and the nature of the rock affect the consumption of power of drills. A three-inch drill using air at 30 lbs. pressure made 300 blows per minute and consumed the equivalent of 64 cubic feet of free air per minute. same drill, with air of 58 lbs. pressure, made 450 blows per minute and consumed 160 cubic feet of free air per minute. At Hell Gate different

The

machines doing the same work used from 80 to 150 cubic feet free air per minute.

An average consumption may be taken generally from 80 to 100 cubic feet per minute, according to the nature of the work.

The Popp Compressed-air System in Paris.-A most extensive system of distribution of power by means of compressed air is that of M. Popp, in Paris. One of the central stations is laid out for 24,000 horsepower. For a very complete description of the system, see Engineering, Feb. 15, June 7, 21, and 29, 1889, and March 13 and 20, April 10, and May 1, 1891. Also Proc. Inst. M. E., July, 1889. A condensed description will be found in Modern Mechanism, p. 12.

Utilization of Compressed Air in Small Motors.-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.

After a number of experiments M. Popp adopted a simple form of castiron stove lined with fire-clay, heated either by a gas jet or by a small coke fire. This apparatus answered the desired purpose until some better arrangement was perfected, and the type was accordingly adopted throughout the whole system. The economy resulting from the use of an improved form was very marked, as will be seen from the following table. EFFICIENCY OF AIR-HEATING STOVES.

The results given in this table were obtained from a large number of trials. From these trials it was found that more than 70% 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 0.2 lb. per horse-power per hour, but it is scarcely to be expected that in regular practice this quantity is not largely exceeded. The efficiency of fuel consumed in this way is at least six times greater than when utilized in a boiler and steam-engine.

According to Prof. Riedler, from 15% to 20% 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 480° F. an increased efficiency of 30% can be obtained.

A large number of motors in use among the subscribers to the Compressed Air Company of Paris are rotary engines developing 1 horse-power and less, and these in the early times of the industry were very extravagant in their consumption. Small rotary engines, working cold air without expansion, used as high as 2330 cu. ft. of air per brake horse-power per hour, and with heated air 1624 cu. ft. Working expansively, a 1 horsepower rotary engine used 1469 cu. ft. of cold air, or 960 cu. ft. of heated air, and a 2-horse-power rotary engine 1059 cu. ft. of cold air, or 847 cu. ft. of air, heated to about 50° C.

The efficiency of this type of rotary motors, with air heated to 50° C., may now be assumed at 43%. 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.

Tests of a small Riedinger rotary engine, used for driving sewing-machines and indicating about 0.1 H.P. showed an air-consumption of 1377 cu. ft. per

H.P. per hour when the initial pressure of the air was 86 lbs. per sq. in, and its temperature 54° F., and 988 cu. ft. when the air was heated to 338° F., its pressure being 72° lbs. With a one-half horse-power variable-expansion rotary engine the air-consumption was from 800 to 900 cu. ft. per H.P. per hour for initial pressures of 54 to 85 lbs. per sq. in. with the air heated from 336° to 388° F., and 1148 cu. ft. with cold air, 46° F., and an initial pressure of 72 lbs. The volumes of air were all taken at atmospheric pressure.

Trials made with an old single-cylinder 80-horse-power Farcot steam-en gine, indicating 72 horse-power, gave a consumption of air per brake horsepower as low as 465 cu. ft. per hour. The temperature of admission was 320° F., and of exhaust 95° F.

Prof. Elliott gives the following as typical results of efficiency for various systems of compressors and air-motors:

Simple compressor and simple motor, efficiency
Compound compressor and simple motor,

66

66

compound motor, efficiency.

Triple compressor and triple motor,

66

39.1%

44.9

50.7

55.3

The efficiency is the ratio of the indicated horse-power in the motor cylinders to the indicated horse-power in the steam-cylinders of the compressor. The pressure assumed is 6 atmospheres absolute, and the losses are equal to those found in Paris over a distance of 4 miles.

Summary of Efficiencies of Compressed-air Transmission at Paris, between the Contral Station at St. Fargeau and a 10-horse-power Motor Working with Pressure Reduced to 4% Atmospheres.

(The figures below correspond to mean results of two experiments cold and two heated.)

1 indicated horse-power at central station gives 0.845 indicated horse-power in compressors, and corresponds to the compression of 348 cubic feet of air per hour from atmospheric pressure to 6 atmospheres absolute. (The weight of this air is about 25 pounds.)

0.845 indicated horse-power in compressors delivers as much air as will do 0.52 indicated horse-power in adiabatic expansion after it has fallen in temperature to the normal temperature of the mains.

The fall of pressure in mains between central station and Paris (say 5 kilometres) reduces the possibility of work from 0.52 to 0.51 indicated horsepower.

The further fall of pressure through the reducing valve to 42 atmospheres (absolute) reduces the possibility of work from 0.51 to 0.50.

Incomplete expansion, wire-drawing, and other such causes reduce the actual indicated horse-power of the motor from 0.50 to 0.39.

By heating the air before it enters the motor to about 320° F., the actual indicated horse-power at the motor is, however, increased to 0.54. The ratio of gain by heating the air is, therefore, 0.54 0.39 = 1.38.

In this process additional heat is supplied by the combustion of about 0.39 pounds of coke per indicated horse-power per hour, and if this be taken into account, the real indicated efficiency of the whole process becomes 0.47 instead of 0.54.

Working with cold air the work spent in driving the motor itself reduces the available horse-power from 0.39 to 0.26.

Working with heated air the work spent in driving the motor itself reduces the available horse-power from 0.54 to 0.44.

A summary of the efficiencies is as follows:

Efficiency of transmission through mains 0.51+0.52 = 0.98:

Efficiency of reducing valve 0.50 0.51 0.98.

The combined efficiency of the mains and reducing valve between 5 and 41⁄2 atmospheres is thus 0.98 0.98 0.96. If the reduction had been to 4, 312, or 3 atmospheres, the corresponding efficiencies would have been 0.93, 0.89, and 0.85 respectively.

Indicated efficiency of motor 0.39 0.50 0.78.

Indicated efficiency of whole process with cold air 0.39. Apparent indi. cated efficiency of whole process with heated air 0.54.

Real indicated efficiency of whole process with heated air 0.47.

Mechanical efficiency of motor, cold, 0.67.

Mechanical efficiency of motor, hot, 0.81.

Most of the compressed air in Paris is used for driving motors, but the work done by these is of the most varied kind. A list of motors driven from St. Fargeau station shows 225 installations, nearly all motors working at from horse-power to 50 horse-power, and the great majority of them more than two miles away from the station. The new station at Quai de la Gare is much larger than the one at St. Fargeau. Experiments on the Riedler air-compressors at Paris, made in December, 1891, to determine the ratio between the indicated work done by the air-pistons and the indicated work in the steam-cylinders, showed a ratio of 0.8997. The compressors are driven by four triple-expansion Corliss engines of 2000 horse-power each.

Shops Operated by Compressed Air.-The Iron Age, March 2, 1893, describes the shops of the Wuerpei Switch and Signal Co., East St. Louis, the machine tools of which are operated by compressed air, each of the larger tools having its own air engine, and the smaller tools being belted from shafting driven by an air engine. Power is supplied by a compound compressor rated at 55 horse-power. The air engines are of the Kriebel make, rated from 2 to 8 horse-power.

Pneumatic Postal Transmission.-A paper by A. Falkenau, Eng'rs Club of Philadelphia, April 1894, entitled the "First United States Pneumatic Postal System," gives a description of the system used in London and Paris, and that recently introduced in Philadelphia between the main post-office and a substation. In London the tubes are 24 and 3 inch lead pipes laid in cast-iron pipes for protection. The carriers used in 24-inch tubes are but 14 inches diameter, the remaining space being taken up by packing: Carriers are despatched singly. First, vacuum alone was used; later, vacuum and compressed air. The tubes used in the Continental cities in Europe are wrought iron, the Paris tubes being 2% inches diameter. There the carriers are despatched in trains of six to ten, propelled by a piston. In Philadelphia the size of tube adopted is 6% inches, the tubes being of cast iron bored to size. The lengths of the outgoing and return tubes are 2928 feet each. The pressure at the main station is 7 lbs., at the substation 4 lbs., and at the end of the return pipe atmospheric pressure. The compressor has two air-cylinders 18 x 24 in. Each carrier holds about 200 letters, but 100 to 150 are taken as an average. Eight carriers may be despatched in a minute, giving a delivery of 48,000 to 72,000 letters per hour. The time required in transmission is about 57 seconds.

Pneumatic postal transmission tubes were laid in 1898 by the Batcheller Pneumatic Tube Co. between the general post-offices in New York and Brooklyn, crossing the East River on the bridge. The tubes are cast iron, 12-ft. lengths, bored to 8% in. diameter. The joints are bells, calked with lead and yarn. There are two tubes, one operating in each direction. Both lines are operated by air-pressure above the atmospheric pressure. One tube is operated by an air-compressor in the New York office and the other by one located in the Brooklyn office.

The carriers are 24 in. long, in the form of a cylinder 7 in. in diameter, and are made of steel, with fibrous bearing-rings which fit the tube. Each carrier will contain about 600 ordinary letters, and they are despatched at intervals of 10 seconds in each direction, the time of transit between the two offices being 3% minutes, the carriers travelling at a speed of from 30 to 35 miles per hour.

The air-compressors were built by the Rand Drill Co. and the IngersollSergeant Drill Co. The Rand Drill Co. compressor is of the duplex type and has two steam-cylinders 10 x 20 in. and two air-cylinders 24 × 20 ̊in., delivering 1570 cu. ft. of free air per minute, at 75 revolutions, the power being about 50 H.P. Corliss valve-gear is on the steam cylinders and the Rand mechanical valve-gear on the air-cylinders.

The Ingersoll-Sergeant Drill Co. furnished two duplex Corliss air-compressors, with mechanically moved valves on air-cylinders. The steamcylinders are 14 X 18 in. and the air-cylinders 264 X 18 in. They are designed for 80 to 90 revs. per min. and to compress to 20 lbs. per sq. in.

Another double line of pneumatic tubes has been laid between the main office and Postal Station H, Lexington Ave. and 44th St., in New York City. This line is about 31⁄2 miles in length. There are three intermediate stations: Third Ave. and 8th St., Madison Square, and Third Ave. and 28th St. The carriers can be so adjusted when they are put into the tube that they will traverse the line and be discharged automatically from the tube at the station for which they are intended. The tubes are of the same size as those of the Brooklyn line and are operated in a similar manner. The initial aircompression is about 12 to 15 lbs. On the Brooklyn line it is about 7 lbs.