Compressed air plant for mines; the production, transmission and use of compressed air, with special reference to mine service

Front matter

BgHSBHBSn&S

HBP

Class 7~JM f

Preface

The increasing use of compressed air makes the subject of interest to practitioners in nearly all branches of engineering. Besides its more important power applications, such as the operation of rock-drills, air brakes, riveting machines, and railroad switching and signalling systems, the uses of compressed air are numerous in many minor branches of mechanical engineering, in caisson work and the construction of subaqueous foundations, and in manufacturing industries, chemical works, etc., where it serves a multitude of purposes entirely distinct from that of the transmission of power.

A realization of the breadth of the field has suggested that a book may be acceptable, addressed especially to those who are engaged in mining, tunnelling, quarrying, and other work involving the excavation of rock, with its concomitant operations. While the literature bearing upon this branch of compressed-air service is by no means small, it is for the most part scattered through the technical periodicals and transactions of engineering societies, and therefore not readily accessible to those who are out of convenient reach of engineering libraries. I am aware that little that is new can be said on this subject, and in writing the book I have availed myself freely of existing sources of information.

In the first part, I have endeavored to present a view of current practice as to the construction and operation of compressors. Portions of the subject are dealt with at some length, such as: the types of compressor suitable for different kinds of service, heat losses occurring in air compression, and the various forms of valves, valve-motions, and governing and unloading mechanisms, that constitute prominent features of modern compressor practice.

iii j v PREFACE

A brief review is given of a few of the fundamental principles of air compression, but my intention has been to present only enough of the theory to make intelligible the formulas employed for the ordinary calculations of the power and capacity of compressed-air plant, together with the questions concerning temperature changes, as affecting the production and use of compressed air. Many details of the design of compressors and proportions of their parts have been omitted, since these fall properly within the province of the mechanical engineer. The second part is devoted to the applications to mine service of compressed-air transmission of power; including machine drills, pumps operated by compressed air, and mine haulage by compressed-air locomotives.

Many of the illustrations are reduced or adapted from working drawings kindly supplied by compressor builders. Others have been taken from catalogues of compressed-air machinery and from technical periodicals and books dealing with the different types. The origin of these has been stated in nearly every instance. My thanks are due to several of the technical journals, especially Compressed Air Magazine and Mines and Minerals, for many suggestions and in some cases for passages extracted either in substance or verbatim, from articles therein contained. For any important use or adaptation of published material, permission has been asked and obtained, and frequent references are given in foot-notes or in the body of the text. I also wish to acknowledge my indebtedness to Mr. Frank Richards's book on " Compressed Air," from which I have derived substantial assistance.

ROBERT PEELE.

School of Mines, Columbia University, New York, May, 1908.

Chapter II

Structure and Operation of Compressors : Straight-Line and Duplex. Compound Steam End; Stage Compressors; Direct- and Belt- Driven or Geared Compressors. Comparison of Types. Relation of Work Done in Air and Steam Cylinders. Proportions of Cylinders. Compressors Driven by Water Power, ..... 8

Chapter VII

Air Inlet Valves. Chief Requisites of . Poppet Inlet Valves: their Construction and Operation. "Skip-Valves" for the High- Pressure Cylinder of Stage Compressors. Ingersoll-Sergeant Piston-Inlet Valve. Johnson Valve. Humboldt Rubber Ring Valve. Leyner Flat Annular Valve. Arrangements for Admitting Inlet Air to the Compressor, 91

Chapter XVI

Conveyance of Compressed Air in Pipes. Loss of Power. Loss of Pressure or Head. Discharge Capacity of Piping. D'Arcy's Formula. Richards's Formula for Loss of Pressure. Comparison of Results of Current Formulas. Compressed-Air Piping. Effect of Bends in Pipe-Lines, 194

Chapter XVII

Compressed-Air Engines. General Considerations. Working at Full Pressure or with Partial or Complete Expansion. Ratios of Pressures and Temperatures due to Expansion in a Motor Cylinder. Corrections for Piston Clearance, etc. Nominal and Actual Cut-off. Work Done in a Motor Cylinder. Volume of Free Air Required. Cummings "Two-Pipe" System, . . . , 205

Chapter XVIII

Fieezing of Moisture Deposited from Compressed Air. Causes and Prevention of Freezing. Influence upon Freezing of High Pressures in Transmission. Deposition of Moisture by Reduction of Pressure. Protection of Air Piping, 220

Chapter XIX

Reheating Compressed Air. Appliances for, and Results of Reheating. Temperatures Employed and Consumption of Fuel. Construction and Operation of Reheaters. Use of Reheaters for Underground Work. Wet and Dry Reheating, . . . . . .225 viii CONTENTS

PAGE

Chapter XX

Compressed- Air Rock-Drills General Considerations as to Efficiency. Consumption of Air: Normal and at Altitudes above Sea-Level. Factors Affecting Air Consumption: Kind of Work, Character of the Rock, and Physical Condition of Drill. Examples from Practice. Proper Air Pressure for Machine Drills. Valve Motions, 240

Chapter XXI

Operation of Mine Pumps by Compressed Air. Disadvantages of Using Ordinary Steam Pumps. Simple, Direct-Acting Pumps. Cylinder Dimensions of Simple Pumps. Volume of Air for Non-Expansive Working. Horse-Power. Regulation of Initial Air Pressure. Prevention of Freezing of Moisture. Compressed-Air-Driven Compound Pumps : Discussion of Modes of Using the Air. Application of Reheating, 250

Chapter XXII

Pumping by the Direct Action of Compressed Air. Pneumatic- Displacement Pumps. Merrill, Latta-Martin, and Harris Displacement Pumps Pohle" Air-Lift Pump: Theory and Operation. Tests on Air-Lift Pumps. Application for Pumping Slimes in South -African Mills. Lansell's Air-Lift for Pumping in Mine Shafts, 265

Chapter XXIII

Compressed-Air Haulage for Mines. Compressed Air versus Electric Locomotives. Construction and Operation of Compressed-Air Locomotives. Modes of Dealing with Low Cylinder Temperature. Calculation for Pipe-Line and Charging Stations. Charging Apparatus. Calculation of Motive Power. Compressors for Charging Pneumatic Locomotives. Detailed Examples of Compressed-Air Haulage Plants, 282

ILLUSTRATIONS

PAGE

Figs, i and 2. — Laidlaw-Dunn-Gordon Straight-Line Compressor. Plan

and Elevation, s ... 10, 11

Fig. 3. — Ingersoll-Rand Straight -Line Compressor, Class A-i, ..... 13

Figs. 4 and 5. — Laidlaw-Dunn-Gordon Duplex Compressor. Plan and Elevation, , 14, 15

Fig. 6. — King-Riedler Compound Vertical Two-Stage Compressor, ... 16

Fig. 7. — Norwalk Compound Straight-Line, Two-Stage Compressor. Longitudinal Section, ...... 18

Fig. 8. — Norwalk Straight-Line, Two-Stage Compressor, with Simple Steam

Figs. 9 and 10. — Leyner Straight-Line, Two-Stage Compressor. Plan and

Elevation, 20

Fig. 11. — Sullivan Straight-Line, Two-Stage Compressor. Longitudinal Section, Inset page 22

Fig. 12. — Sullivan Duplex, Two-Stage Compressor. Longitudinal Section

through Low-Pressure Cylinder, 21

Fig. 13. — Leyner Duplex, Two-Stage Compressor, with Simple Steam Cylinders, 23

Figs. 14 and 15. — Riedler Cross-Compound Two-Stage Compressor; 15" and

24"X 36" Air Cylinders. Plan and Elevation, 24,25

Figs. 16 and 17. — Allis-Chalmers Cross-Compound Corliss, Two-Stage Compressor. Plan and Elevation, 26, 27

Figs. 18 and 19. — Laidlaw-Dunn-Gordon Duplex, Cross-Compound Compressor, with Two-Stage Air Cylinders. Perspective View and General Plan, Elevations and Sections, Inset and page 29

Fig. 20. — Combined Air and Steam Cards, 31

Fig. 21. — Duplex, i6"X3o" Risdon Compresser, Driven by 16 ft. Water-wheel, ^^

Figs. 22 and 23. — Water-Driven Risdon Duplex Compressor. Plan and Elevation, 34, 35

Fig. 24. — Ingersoll-Rand Water-Driven Compressor, 37

ix

ILLUSTRATIONS

PAGE

Figs. 25 and 26. — Rix Water-Driven Compressor at North Star Gold Mine,

Calif. Side and Front Elevations, 38, 39

Fig. 27. — Ingersoll-Sergeant Straight-Line, Belt -Driven Compressor, ... 41

Fig. 28. — Air Compression Temperature Diagram, 47

Fig. 29. — Air Indicator Card, 49

Figs. 30, 31 and 32. — Air Indicator Cards, Showing Effect of Cooling, Fig. t,^. — Humboldt Wet Compressor,

5 1

53 54

59 60

Fig. 34. — Hanarte Wet Compressor, . .

Fig. 35. — Air Cylinder of Nordberg Compressor, .........

Fig. 36. — Air Cylinder, Class E, Laidlaw-Dunn-Gordon Co., ....

Fig. 37. — Air Card Showing Effect of Clearance, 63

Fig. 38. — Diagram of Effect of Clearance on Capacity of Dry Compressor, . 64

Fig. 39. — Section of Air Cylinder, Showing Method of Reducing Clearance, . 65

Fig. 40. — Section of Piston, Johnson Compressor, 65

Fig. 41. — Diagram of Norwalk Two-Stage Compressor, ....... 76

Fig. 42. — Horizontal Intercooler. Ingersoll-Rand Co., 81

Fig. 43. — Intercooler. Sullivan Machinery Co., .......... 82

Fig. 44. — Leyner System of Intercooling, . „ 85

Fig. 45. — Vertical Intercooler. Ingersoll-Rand Co., 87

Fig. 46. — Combined Air Card of Two-Stage Compressor, ....... 88

Fig. 47. — Norwalk Poppet Inlet Valve, 94

Fig. 48. — Laidlaw-Dunn-Gordon Poppet Inlet Valve, 95

Fig. 49. — Diagram of Effect of Valve-Spring Resistance on Volumetric Capacity of Compressors, 97

Fig. 50. — Air Card Showing Effect of Valve Resistance, 9§

Fig. 51. — "Skip-Valve." Norwalk Iron Works Co., 100

Fig. 52. — Cylinder of Piston-Inlet Compressor. Ingersoll-Rand Co., . . 101

Figs. 53 and 54. — Johnson Air Valves, 103

Fig. 55. — Humboldt Rubber Ring Valves, 104

Fig. 56. — Leyner Compressor. Part Section, Showing Flat Annular Air

Valves, 106

Fig. 57. — Leyner Annular Inlet Valve, 107

Fig. 58. — Laidlaw-Dunn-Gordon Poppet Discharge Valve, in

Fig. 59. — Norwalk Poppet Discharge Valve, 112

ILLUSTRATIONS xi

PAGE

Fig. 60. — "Express" Poppet Valve. Riedler Compressor, 113

Fig. 61. — Valve Motion of Low-Pressure Air Cylinder. Norwalk Compressor, 119

Fig. 62. — Section of Air Cylinder of Nordberg Compressor, 120

Fig. 63. — Section of Air Cylinder. Laidlaw-Dunn-Gordon Co., 121

Fig. 64. — " Cincinnati " Valve Gear. Laidlaw-Dunn-Gordon Compressor, . 122

Fig. 65. — Standard Air-Valve Motion. Allis-Chalmers Co., 124

Fig. 66. — Sullivan Air Cylinder, Showing Corliss Inlet Valves, 125

Fig. 67. — Riedler Air- Valve Motion, 127

Fig. 68. — Details of Riedler Inlet Valve, 128

Fig. 69. — Details of Riedler Discharge Valve, ........... 129

Fig. 70. — Cam-Controlled Inlet Valve, 130

Fig. 71. — Sturgeon Inlet Valve, 131

Fig. 72. — Ideal Air Card, 141

Fig. 73. — Diagram. Elements of Air Indicator Card, 141

Fig. 74. — Vertical Air Receiver. Norwalk Iron Works Co., 145

Fig. 75. — Horizontal Receiver-Aftercooler. Ingersoll-Rand Co., . . . 146

Fig. 76. — Clayton Governor and Pressure Regulator, 151

Fig. 79. — Clayton Pressure Regulator, 155

FlG. 80. — Rand Imperial Unloader. Sectional View, 157

FlG. 81. — Sullivan Governor and Unloader, 158

Fig. 82. — Ingersoll-Sergeant Regulator and Unloader, 160

Fig. 83. — Laidlaw-Dunn-Gordon Air Governor, 161

Fig. 84. — Air Cards Showing Results of Compression at Altitudes above

Sea Level, 165

FlG. 85. — Taylor Hydraulic Air Compressor, 185

FlG. 86. — Taylor Hydraulic Air Compressor. Detail of Head-piece, . . . 186

Figs. 87 and 88. — Hydraulic Air-Compressing Plant at Kootenay, B. C,

o . . . . . . . o . . . „ Inset and page 190

Fig. 89. — Expansion Curves of Steam and Air, „ . 209

Fig. 90. — Card Showing Work Done in Motor Cylinder, . . . . . . .213

Fig. 91. — Leyner Compressed-Air Reheater, . .... 231

Fig.

Fig.

Fig.

Fig.

Fig.

Fig.

Fig.

Xll ILLUSTRATIONS

PAGE

Fig. 92. — Cast-iron Coils, Leyner Reheater, 232

-Sergeant Reheater, 233

. — Rand Reheater, 234

. — Sullivan Reheater, 235

-Merrill Pneumatic Pump, 266

. — Diagram of Pohle Air-Lift Pump, 271

-Diagram of Lansell's Air-Lift Pump for Mine Shafts, .... 280

-H. K. Porter Four-Wheel, Single-Tank Compressed-Air Mine Locomotive, 286

Fig. 100. — Small H. K. Porter Compressed-Air Locomotive, 287

Fig. ioi. — Baldwin Six-Wheel Compressed-Air Locomotive, 288

Fig. 102. — Baldwin Four-Wheel Compressed-Air Locomotive, 288

Figs. 103, 104 and 105. — Plan, Elevations and Sections of Baldwin Com-

pressed-Air Locomotive, 289, 290, 291

Fig. 106. — Compressed-Air Locomotive Charging-Station, 297

Fig. 107. — Norwalk Locomotive Charging Compressor, 301

Fig. 108. — Air-End of Ingersoll-Rand Three-Stage Locomotive Charger, . . 302

Figs. 109 and no. — Low- and High-Pressure Air-Ends of Ingersoll-Rand

Four-Stage Compressor, 303

Fig. hi. — Perspective View of Ingersoll-Rand Four-Stage Compressor, . . 304

Fig. 112. — E. A. Rix Compressed-Air Locomotive for Empire Mine, Grass

Valley, Cal., 307

Introduction

One of the most important applications of the transmission of power by compressed air is the driving of machine rock-drills; and to the necessity of providing for these drills a power medium suitable for use in mines and tunnels has been due, more than to any other cause, the development of the modern air compressor.

The time which has elapsed since the beginnings of this branch of engineering is short. The first percussion rock-drill, operating independently of gravity, was invented in 1849 by J- J- Couch, of Philadelphia. Though used only experimentally, it embodied the principal mechanical features of the modern machine drills, which have had such a striking influence in mining and tunnelling. Couch's machine, together with its immediate successors, such as the Fowle drill (1849-51) and the Cave (Paris, 1851), were steam-driven and therefore unsuitable for underground work. In 1852, the physicist Colladon proposed the use of compressed air for operating rock-drills, in connection with the driving of the Mont Cenis tunnel, in the western Alps. His idea was developed by Sommeiller and others between 1852 and i860, and in 1861-62 an air-compressor plant was first used successfully at that

l tunnel. It was driven by water power and furnished air for ventilation as well as for the drills.

The transmission of power by compressed air thus dates from about the middle of the last century. It is hardly necessary to say that the early air compressors were crude in both design and construction. Sommeiller's first plant, though of large size and effectual in fulfilling its purpose, had some resemblance in principle to the old hydraulic ram, possessing no moving parts except the valves. Piston compressors, driven by steam engines, such as the Dubois-Francois, and more or less similar fundamentally to some of the wet compressors still in use, soon made their appearance. Probably the first compressors built in the United States were those employed at the Hoosac tunnel, in western Massachusetts, in 1865-66. The Burleigh, Xorwalk, Clayton, and Rand compressors were among the earlier makes in this country.

But the Mont Cenis tunnel, about eight miles long and completed in 1 87 1, the first connecting link through the Alps between the railway systems of France and Italy, was undoubtedly the field where were fought out on a large scale the initial problems of compressed-air production and use; and to Sommeiller is due the honor of having laid the foundations of new practice, by which that great work was brought to a successful completion. From 1857 to 1 861 the tunnel headings had been progressing slowly and in the face of great difficulties. Drilling was done by hand labor and blasting by black powder, the average advance for this period, in each of the two headings, being only about one and a half feet per day. At this rate, even granting that the work could have been finished at all by the means employed, over forty years would have been required to connect the headings and years more to complete the enlargement to full section. With machine drills, the speed of advance in each heading rose to four and three-quarters feet per twenty-four hours and later, when dynamite was introduced, to a little over six feet; this average being maintained for a period of six years.

Machine drills did not make their way into mining to any extent for some years after their successful application to tunnel driving.

INTRODUCTION 3

It is difficult now to name the mining district in this country where they were first used, but their most important trial was probably in the Calumet and Hecla copper mine, Michigan. After strong and concerted opposition from the miners, the Rand drill was introduced there in 1878, and the value of machine drilling for hard ground was soon demonstrated by decreased costs of drifting and stoping and higher speeds of advance.

Compressed air has now a wide application in various branches of mechanical engineering and the arts and manufactures. In this book it is intended to deal only with its production and uses in connection with mining and tunnelling operations. Its two rivals in these fields of work are steam and electricity, regarding which a few general considerations may here be mentioned.

As compared with steam, compressed-air transmission of power is especially valuable and convenient for three reasons: first, its loss in transmission through pipes is relatively small; second, the troublesome question of the disposal of exhaust steam underground is avoided ; third, the exhausted air is of some assistance in ventilating the working places of the mine. In large mines, where steam may be carried thousands of feet, down shafts and through lateral workings, for operating pumping engines, etc., the disadvantages attending its use become very apparent; the amount of condensation is serious, even when the piping is provided with good non-conducting covering, and the working efficiency falls to an abnormally small figure. Furthermore, aside from the heat produced by the use of steam, it is rarely feasible to employ efficient condensers for underground engines other than pumps, on account of the difficulty of obtaining the necessary condensing water and the additional space required. If the exhaust be discharged into the mine workings, even though they are large and well ventilated and the volume of the exhaust steam comparatively small, the temperature and quantity of moisture in the air would be considerably increased. Deterioration of the timbering is thereby hastened, the roof and walls of the workings are softened and slacked off, especially in collieries, and the mine atmosphere is rendered oppressive and unwholesome. The presence of hot steam pipes in confined workings, or in the narrow compartments of shafts, is also objectionable.

Although the loss from condensation in long steam lines may be diminished by covering the pipe with efficient non-conducting material, still, even with the best covering, the effective pressure at a distant underground engine is greatly reduced, and very uneconomical working is the result. On conveying steam a distance of several thousand feet the pressure may be reduced to half the boiler pressure, or even less. For example, in the case of a pump, or other engine, situated 2,000 feet from the boiler and using 200 cubic feet of steam per minute at a boiler pressure of 75 pounds, with a four-inch mineral-wool-covered pipe, the effective pressure at the engine would be only about 58 pounds; or, with a poor covering, like some of the asbestos lagging often used, it might easily be as low as 35 pounds. For compressed-air transmission, on the other hand, the reduction of pressure for the same volume of air, size of pipe, and initial pressure, would be 9.3 pounds, giving a terminal pressure of 65.7 pounds. However, as the speed of flow in pipes for economical transmission is greater for steam than for air, a comparison based solely on piping of the same diameter cannot justly be made. In the above example, if the diameter of the pipe were smaller the gain in reduced radiation would outweigh the increased f rictional loss, and the net loss would be diminished. Since the frictional loss varies inversely, and the loss from radiation directly, with the diameter, the size of the steam pipe can be so proportioned as to produce a minimum loss under the given conditions. With compressed air the case is different, since the question of radiation is eliminated. If the diameter of the pipe be increased to 5 inches the loss of pressure, or head required to overcome friction, is reduced to 2.8 pounds and increasing the distance to one mile it would be only 7.4 pounds. Furthermore, the increased cost of the larger air pipe would be offset by the expense of the non-conducting covering necessary for steam transmission.

Thus, compressed air may be conveyed long distances with but small loss of pressure, and is readily distributed for application to a variety of underground uses, for which steam is not practicable.

INTRODUCTION

Compressed air is always ready to do its work, and, aside from leakage of transmission pipes, which is in large measure preventable, suffers no loss nor diminution of power when not in actual use. For performing work intermittently, at a distance from its source, it is therefore particularly valuable, because the air pressure is maintained nearly constant during intervals of work, without further expenditure of power. With steam transmission, on the contrary, power is continually dissipated by radiation, whether in use or net, and a steam engine, when stopped for any length of time, loses much of its normal working temperature and becomes a receptacle for water of condensation.

Though in mining compressed air is employed mainly for operating machine drills, other applications are found in the driving of underground hoists and pumps in confined workings. Mechanical coal cutters, for mining bituminous coal, are sometimes operated by compressed air, and the employment of compressed-air locomotives in mines and extensive tunnelling operations furnishes an example of its capacity for storing power, in contradistinction to its function as a power transmitter. The introduction of compressed-air drills has facilitated the rapid driving of long railroad and mining tunnels, which otherwise would have been greatly delayed or completed only with extreme difficulty. Had compressed-air power, together with the high explosives, not been available, it may well be doubted whether the great tunnels through the Alps and elsewhere, and the numerous long mine tunnels driven in recent years in this country, would have been at all practicable.

Without attempting to review in detail the comparative merits of electricity and compressed air, it may be pointed out that the application of electricity for transmitting power in mines has increased enormously in importance during the past twenty-five years. The peculiar requirements of mine service have been in nearly all cases successfully met by modifications and adaptations of standard forms of electric apparatus. Both means of power transmission possess characteristics which adapt them particularly for underground work. But, although by virtue of its numerous successful applications, electricity has become a rival of compressed air in most branches of mine work, their spheres of usefulness are not identical and the field is broad enough for both. It is often stated that the first cost of an electric plant is lower than that of an equivalent compressed-air plant. A broad generalization, however, does not cover the case. There is actually but little difference between the costs of the power plants themselves, the advantage being generally with the compressor. Considering the question of the transmission of a given power, the cost of the electric conductor line for short distances is much less than that of compressed-air pipe; but the cost of the electric line increases as the square of the distance, while the cost of the pipe line increases only as the first power of the distance. Hence, a point is soon reached where compressed-air transmission becomes the cheaper. It is in the greater efficiency of generation that electric power has the advantage.

In one direction only has electricity failed hitherto to meet evenrequirement. While compressed-air drills, though far from being economical considered simply as machines, nevertheless admirably fulfil their purpose, no perfectly satisfactory electric rock-drill has yet been produced. However, this problem has for years been receiving much attention from electricians, both in this country and abroad, and there is reason to anticipate a successful solution in the near future. The Temple " electric-air " drill, brought out some four years ago, and already well tested under a variety of conditions, mav be referred to here as a remarkably efficient and ingenious machine, but it is not an electric drill in the ordinary meaning of the term. It is rather a combination of a compressed-air drill, operated by a small, electric-driven compressor which is mounted on a truck close to the drill itself. As there is no exhaust, the same air being used over and over, one of the incidental advantages of the ordinary air drill is missing, viz., that of assisting somewhat in ventilating the mine workings, in those places where ventilation is most needed. Keeping this in mind, together with such minor uses of compressed air as the cleaning of drill holes preparatory to charging, and driving out the smoke of blasting from working places, it seems doubtful whether, for underground mining, electric drills of any kind can be expected to supersede entirely those oper-

INTRODUCTION 7

ated by compressed air. Given the necessity for a compressed-air plant for the rock-drills, as is the case in most metal mines, it may often be more advantageous to provide the additional compressor capacity required for driving underground pumps, hoists, and other machines as well, than to erect a separate and distinct plant for generating electricity.

Because of the view usually taken of the lack of economy in the operation of compressed-air drills, it has been customary in the past to consider compressed air in general as a form of power respecting which the questions of convenience and expediency are more weighty than the attainment of a high degree of efficiency. In recent years, however, as the principles of air compression have become better understood, a substantial improvement has taken place, not only in the design of the compressors themselves, but also in the installation of pipe lines and in the operation of the machines using the compressed air. The consequences of overloading a compressor, and thereby driving it beyond its proper speed, are now comprehended by every intelligent master mechanic as being wholly different from those produced by overloading a steam engine. The results of leaks in air pipes, and of using air mains of too small a diameter, are also understood and avoided. Better practice prevails in the field, and in the production, transmission, and use of compressed air a much higher total efficiency is now realized than was formerly thought possible.

Chapter II

An air compressor consists essentially of a cylinder in which atmospheric air is compressed by a piston, the power for driving which may be derived from a steam engine, water-wheel, or electric motor. The air cylinder is almost invariably double-acting, and as such is provided with inlet and discharge or deliver}' valves in each cylinder head. On the forward stroke the air is compressed by the advancing piston, while the decrease in pressure, or, as it is commonly termed, the tendency to form a vacuum, behind the piston causes the inlet valves to open under atmospheric pressure, thus allowing the outside air to flow into the cylinder. At each stroke a certain volume of compressed air is forced from the cylinder through the discharge valves, into a pipe leading to a large reservoir or receiver, whence the air enters the transmission pipe or main.

Before considering the operation and various appurtenances of the air and steam cylinders, it will be well to examine the general mechanical structure of the compressor and the modes of applying the power. Probably no single classification of air compressors can be made sufficiently comprehensive to present intelligibly all of their salient features. In attempting a classification three widely different bases of comparison suggest themselves. First, several clear distinctions result from a consideration of the general structural characteristics of air compressors regarded purely as engines; second, they may be classed according to the mode of dealing with the heat necessarily produced during compression of the air; and third, the numerous and varying types of valves and valve-motions devised in modern practice for controlling the distribution of the air in the compressing cylinders constitute a basis for comparison which, though not so simple as the others, is in some respects quite as useful and important.

The first classification only will be given here, the others being taken up respectively in Chapters IV to VI and VII to IX. Airbrake and gas compressors, vacuum pumps and other special forms of air-compressing machinery are not included, as this book will not deal with compressors other than those which are applicable to mine service.

Under the first classification and taking the steam-driven compressor as the type form, four subdivisions may be named :

1. " Straight-line' * Compressors. In these, which are made by all builders, the steam and air cylinders are set tandem on a common piston-rod. They are always provided with a pair of fly-wheels, one on each end of the crank-shaft, which are driven by outside connecting-rods from a cross-head between the steam and air cylinders. Their structural form is thus simple and compact to a marked degree. Figs. 1 and 2 illustrate a Laidlaw-Dunn-Gordon straight-line compressor, with Meyer valve gear, a section of the steam cylinder being shown in the plan and of the air cylinder in the elevation. Fig. 3 is a perspective view of an Ingersoll-Rand straight -line compressor.

2. Duplex Compressors, (a) Two engines are placed side by side, each being complete in itself and consisting of tandem steam and air cylinders, with their cranks set at 90 degrees on a common fly-wheel shaft. Each side of the duplex is in effect a straight-line compressor. They are almost invariably horizontal, and the steam cylinders are always nearest the crank-shaft. Figs. 4 and 5 show a type of the duplex compressor, (b) One air and one steam cylinder, side by side, with a common crank-shaft, may in one sense be classed as duplex, though its operation is entirely different from (a) in the disadvantageous distribution of the load. While obsolete in America, this form is occasionally adopted by some European builders for special purposes. It is not well balanced, occupies at least three times the floor space of a straight-line compressor of the same capacity, and requires more expensive foundations.

IO

3. Compressors with Compound Steam Ends.

(a) Duplex, horizontal, cross-compound; a simple or single-stage air cylinder being set tandem to each steam cylinder. This form is now rarely used. The considerations leading to the compounding of the steam end make it desirable to adopt stage compression for the air end. (b) Vertical compound; the air cylinders being placed respectively above the high- and low-pressure steam cylinders. This also is a somewhat unusual design. It is advantageous in saving floor space, though this consideration is rarely of consequence at mines. As an example, the King-Riedler compressor may be cited (Fig. 6).* Some very large plants of this type, up to a capacity of 8,oco cu. ft. of free air per minute, have been built for South African mines. Vertical compressors have the disadvantage of being complicated and difficult to maintain and repair.