RULES FOR STATIONARY ENGINEERS

If a gauge glass breaks turn off the water first and then the steam, to avoid scalding yourself.

Don't buy oil or waste simply because it is very cheap; it will cost more than a good article in the end.

When cutting rubber for gaskets, etc., have a dish of water handy, and keep wetting the knife blade; it makes the work much easier.

Don't forget that there is no economy in employing a poor fireman; he can, and probably will, waste more coal than would pay the wages of a firstclass man.

An ordinary steam engine having two cylinders connected at right angles on the same shaft consumes one-third more steam than a single-cylinder engine, while developing only the same amount of power.

A fusible plug ought to be renewed every 3 mo., by removing the old metal and refilling the case; and it should be scraped clean and bright on both ends every time that the boiler is washed out, to keep it in good working order.

When trying a gauge-cock, don't jerk it open suddenly, for if the water happens to be a trifle below the cock, the sudden relief from pressure at that point may cause it to lift and flow out, thus showing a wrong height. Whereas, if it is opened quietly, no lift will occur, and it will show whether there is water or steam at that level.

Always open steam stop-valves between boilers very gently, that they may heat and expand gradually; by suddenly turning on steam a stop-valve chest was burst, due to the expansive power of heat unequally applied. The same care must be exercised when shutting off stop-valves; explosions have been caused by shutting a communicating stop-valve too suddenly-due to the recoil. In order to obtain the driest possible steam from a boiler, there should be an internal perforated pipe (dry pipe, so called) fixed near the top of the boiler, and suitably connected to the steam pipe. The perforations in this pipe should be from one-quarter to one-half greater in area than that of the steam pipe. If a glass gauge tube is too long, wet a triangular file with turpentine, then holding the tube in the left hand, with the thumb and forefinger at the place where it is to be cut, saw it quickly and lightly two or three times with the edge of the file. Take the tube in both hands, both thumbs being on the side opposite the mark, and 1 in. or so apart, and then try to bend the glass, using the thumbs as fulcrums, and it will break at the mark, which has weakened the tube.

A stiff charge of coal all over a furnace will lower the temperature 200° or 300° in a very short time. After the coal is well ignited the temperature will rise about 500°, and as it burns will gradually drop about 200°, until the fireman puts in another charge, when the sudden fall again takes place. This sudden contraction and expansion frequently causes the bursting of a boiler, and it is for this reason that light and frequent charges of coal, or else firing only one-half of the furnace at a time, should be always insisted on. Be careful when using a wrench on hexagonal nuts that it fits snugly, or the edges of the nut will soon become rounded.

If a monkey-wrench is not placed on the nut properly, the strain will often bend or fracture the wrench.

The area of grate for a boiler should never be less than & sq. ft. per I. H. P. of the engine, and it is seldom advisable to increase this allowance beyond sq. ft. per I. H. P.

The area of tube surface for a boiler should not be less than 21 sq. ft. per I. H. P. of the engine.

The ratio of heating surface to grate area in a boiler should be 30 to 1 as a minimum, and may often be increased to 40 to 1, or even more, with advantage. Lap-welded pipe of the same rated size has always the same outside diameter, whether common, extra, or double extra, but the internal diameter is of course decreased with the increased thickness.

A good cement for steam and water joints is made by taking 10 parts, by weight, of white lead, 3 parts of black oxide of manganese, 1 part of litharge, and mixing them to the proper consistency with boiled linseed oil.

To harden a cutting tool, heat it in a coke fire to a blood-red heat and plunge it into a solution of salt and water (1 lb. of salt to 1 gal. of water), then polish the tool, heat it over gas, or otherwise, until a dark straw and purple mixed color shows on the polish, and cool it in the salt water.

Small articles can be plated with brass by dipping them in a solution of 9 gr. each of sulphate of copper and chloride of tin, in 1 pt. of water.

Don't be eternally tinkering about an engine, but let well enough alone. Don't forget that it is possible to drive a key with a copper hammer just as well as with a steel one, and that it doesn't leave any marks.

Keep on hand slips of thin sheet copper, brass, and tin, to use as liners, and if these are shaped properly, much time will be saved when they are needed. A few wooden skewer pins, such as butchers use, are very useful for many purposes in an engine room.

În running a line of steam pipe where there are certain rigid points, make arrangements for expansion on the line between those points.

Arrange the usual work of the engine and firerooms systematically, and adhere to it.

Don't forget that cleanliness is next to godliness.

Rubber cloth kept on hand for joints should be rolled up and laid away by itself, as any oil or grease coming in contact with it will cause it to soften and give out when put to use.

When using a jet condenser, let the engine make three or four revolutions before opening the injection valve, and then open it gradually, letting the engine make several more revolutions before it is opened to the full amount. Open the main stop-valve before the fires are started under the boilers.

When starting fires, don't forget to close the gauge-cocks and safety valve as soon as steam begins to form.

An old Turkish towel, cut in two lengthwise, is better than cotton waste for cleaning brass work.

Always connect the steam valves in such a manner that the valve closes against the constant steam pressure.

Turpentine well mixed with black varnish makes a good coating for iron smoke pipes.

Ordinary lubricating oils are not suitable for use in preventing rust.

It is possible to make a hole through glass by covering it with a thin coating of wax, warming the glass and spreading the wax on it; then scrape off the wax where the hole is wanted, drop a little fluoric acid on the spot with a wire. The acid will cut a hole through the glass, and it can be shaped with a copper wire covered with oil and rottenstone.

A mixture of 1 oz. of sulphate of copper, oz. of alum, teaspoonful of powdered salt, 1 gill of vinegar and 20 drops of nitric acid will make a hole in steel that is too hard to cut or file easily. Also, if applied to steel and washed off quickly, it will give the metal a beautiful frosted appearance.

COMPRESSED AIR

CLASSIFICATION AND CONSTRUCTION OF
COMPRESSORS

An air compressor consists essentially of a cylinder in which atmospheric air is compressed by a piston, the driving power being steam, water, oil, gas, or electricity. Steam-driven compressors in ordinary use may be classed as follows:

1. Straight-line type, in which a single horizontal air cylinder is set tandem with its steam cylinder, and provided with two flywheels; this pattern is generally adapted for compressors of small size.

2. Duplex type, in which there are two steam cylinders, each driving an air cylinder, and coupled at 90° to a crank-shaft carrying a flywheel.

3. Horizontal, cross-compound engines, each steam cylinder set tandem with an air cylinder, as in 2.

4. Vertical, simple, or compound engines, with the air cylinders set above the steam cylinders.

5. Compound or stage compressors, in which the air cylinders themselves are compounded; the compression is carried to a certain point in one cylinder and successively raised and finally completed to the desired pressure in the others. They may be either of the straight-line or duplex form, with simple or compound steam cylinders. The principle of compound, or two-stage,air compression is recognized as applicable for even the moderate pressures required in mining. Compressors of class 5 are frequently employed, as well as classes 1, 2, and 3.

Theory of Air Compression.-The useful effect or efficiency of a compressor is the ratio of the force stored in the compressed air to the work that has been expended in compressing it; this probably never reaches 80% and often falls below 60%.

Free Air is air at ordinary atmospheric pressure as taken into the compressor cylinder; as commonly used, this means air at sea-level pressure (14.7 lb. per sq. in.) at 60° F. The absolute pressure of air is measured from zero, and is equal to the assumed atmospheric pressure plus gauge pressure. Air-compression calculations depend on the two well-known laws:

1. Boyle's Law. The temperature being constant, the volume varies inversely as the pressure; or PV=P'V'a constant; in which V equals the volume of a given weight of air at the freezing point, and the pressure P; V' equals the volume of the same weight of air at the same temperature and under the pressure P'.

2 Gay-Lussac's Law.-The volume of a gas under constant pressure, when heated, expands, for each degree of rise in temperature, by a constant proportional part of the volume that it occupied at the freezing point; or, V'=V (1+at°), in which a equals for centigrade degrees, or for Fahrenheit degrees.

Theoretically, air may be compressed in two ways, as follows:

1. Isothermally, when the temperature is kept constant during compression, and in this case, the formula PV=P'V' is true.

2.

Adiabatically, when the temperature is allowed to rise without check during the compression.

As the pressure rises faster than the volume diminishes, the equation P' PV=P'V' no longer holds, and P specific heat of air at constant pressure is .2375, and at constant volume .1689, and n=.2375÷.1689=1.406.

= (√)”, in which n equals 1.406. The

In practice, compression is neither isothermal nor adiabatic, but intermediate between the two. The values of n for different conditions in practice as determined from a 2,000-H. P. stage compressor at Quai de la Gare, Paris, are as follows: For purely adiabatic compression, with no cooling arrangements, n=1.406; in ordinary single-cylinder dry compressors, provided with a water-jacket, n is roughly 1.3; while in the best wet compressors (with spray injection), n becomes 1.2 to 1.25. In the poorest forms of compressor, the value n=1.4 is closely approached. For large, well-designed compressors with compound air cylinders, the exponent n may be as small as 1.15.

Construction of Compressors.-Compressors are usually built with a short stroke, as this is conducive to economy in compression as well as the attainment of a proper rotative speed. In ordinary single-stage compressors, the usual ratio of length of stroke to diameter of steam cylinders is 1 to 1 or 11 to 1. In some makes, such as the Rand, the ratio is considerably greater, varying from 1 to 13 to 1, as in several large plants built for the Calumet & Hecla Mining Co. Many compressors have length and diameter of steam cylinders equal. The relative diameters of the air and steam cylinders depend on the steam pressure carried, and the air pressure to be produced. In mining operations, there is usually but little variation in these conditions. For rockdrill work, the air pressure is generally from 60 to 80 lb.

In using water-power, a compressor is driven most conveniently by a bucket impact wheel, such as the Pelton or Knight. The waterwheel is generally mounted directly on the crank-shaft, without the use of gearing. As the power developed is uniform throughout the revolution of the wheel, the compressor should be of duplex form, in order to equalize the resistance so far as possible. The rim of the wheel is made extra heavy, to supply the place of a flywheel. When direct-connected, the wheel is of relatively large diameter, as its speed of rotation must of necessity be slow. With small high-speed wheels, the compressor cylinders may be operated through belting or gearing. In most cases, however, the waterwheel may be large enough to render gearing unnecessary. Impact wheels may be employed with quite small heads of water, by introducing multiple nozzles. To prevent the water from splashing over the compressor, the wheel is enclosed in a tight iron or wooden casing. The force of the water is regulated usually by an ordinary gate valve. If the head is great, it may be necessary to introduce means for deflecting the nozzle, so that, when the compressor is to be stopped suddenly, danger of rupturing the water main will be avoided.

Rating of Compressors.-Compressors are rated as follows: (1) In terms of the horsepower developed by the steam end of the compressor, as shown by

indicator cards taken when running at full speed and when the usual volume of air is being consumed; (2) compressors for mines are often rated roughly as furnishing sufficient air to operate a certain number of rock drills; a 3-in. drill requires a volume of air at 60 lb. pressure, equal to 100 or 110 cu. ft. of free atmospheric air per minute; (3) in terms of cubic feet of free air compressed per minute to a given pressure.

As the actual capacity of a compressor depends on the density of the intake air, it will be reduced when working at an altitude above sea level, because of the diminished density of the atmosphere. The accompanying table gives the percentages of output at different elevations.

EFFICIENCIES OF AIR COMPRESSORS AT DIFFERENT ALTITUDES

EXAMPLE.-Calculate the volume of air furnished by an 18"X24" compressor working at an elevation of 5,000 ft. above sea level, making 95 rev. per min., and having a piston speed of 380 ft. per min.

254.3
144

X380: =668.8

SOLUTION. 92X3.14-254.3 sq. in. = piston area. cu. ft.=volume displaced per minute by the piston; deducting 10% for loss gives 602 cu. ft. At sea level at 80 lb. gauge pressure, this equals X602-95 cu. ft. At an elevation of 5,000 ft., the output of a compressor would be 95X84%-79.8 cu. ft. per min.

15 80+15

Cooling. Compressor cylinders may be cooled by injecting water into the cylinder, in which case they are known as wet compressors; or by jacketing the cylinder in water, when they are known as dry compressors.

TRANSMISSION OF AIR IN PIPES

The actual discharge capacity of piping is not proportional to the crosssectional area alone, that is, to the square of the diameter. Although the periphery is directly proportional to the diameter, the interior surface resistance is much greater in a small pipe than in a large one, because, as the pipe becomes smaller, the ratio of perimeter to area increases.

To pass a given volume of compressed air, a 1-in. pipe of given length requires over three times as much head as a 2-in. pipe of the same length. The character of the pipe, also, and the condition of its inner surface, have much to do with the friction developed by the flow of air. Besides imperfections in the surface of the metal, the irregularities incident on coupling together the lengths of pipe must increase friction. There are so few reliable

data that the influences by which the values of some of the factors may be modified are not fully understood; and, owing to these uncertain conditions, the results obtained from formulas are only approximately correct.

Among the formulas in common use, perhaps the most satisfactory is that of D'Arcy. As adopted for compressed-air transmission, it takes the form:

D=c√ √ d3 (P1 — p2)

wil

in which D = volume of compressed air discharged at final pressure in cubic feet per minute;

c = coefficient varying with diameter of pipe, as determined by experiment;

d = diameter of pipe in inches (actual diameters of 14- and 1-in. pipe are 1.38 in. and 1.61 in., respectively; nominal diameters of all other sizes may be taken for calculations);

= length of pipe, in feet;

Pi initial gauge pressure, in pounds per square inch;
P2=final gauge pressure, in pounds per square inch;

w-density of air, or its weight at initial pressure p1, in pounds per

cubic foot.

The values of the coefficients c for sizes of piping up to 12 in. are:

1 in...

2 in.

3 in..

4 in..

..45.3
52.6

5 in..

6 in.

59.0

9 in..

61.0

.61.2

61.8

62.0

Some apparent discrepancies exist for sizes larger than 9 in., but they cause no very material differences in the results.

Another formula, published by Mr. Frank Richards, is as follows:

in which H = head or difference of pressure required to overcome friction and

maintain flow of air;

V = volume of compressed air delivered, in cubic feet per minute;
L-length of pipe, in feet;

D-diameter of pipe, in inches;

a=coefficient, depending on size of pipe.

Values of a for nominal diameters of wrought-iron pipe:

The values of a for 11 and 1-in. pipe are not consistent with those for other sizes, for the reason already stated. When using this formula with its constants, the calculated losses of pressure are found to be smaller, and, conversely, the volumes of air discharged are larger, under the same conditions than those obtained from D'Arcy's formula.

It must be remembered that, within certain limits, the loss of head or pressure increases with the square of the velocity. To obtain the best results, it has been found that the velocity of flow in the main air pipes should not exceed 20 or 25 ft. per sec. When the initial velocity much exceeds 50 ft. per sec., the percentage loss becomes very large; and, conversely, by using piping large enough to keep down the velocity, the friction loss may be almost eliminated. For example, at the Hoosac tunnel, in transmitting 875 cu. ft. per min. of free air at an initial pressure of 60 lb., through an 8-in. pipe, 7,150 ft. long, the average loss including leakage was only 2 lb. A volume of 500 cu. ft. per min. of free air, at 75 lb., can be transmitted through 1,000 ft. of 3-in. pipe with a loss of 4.1 lb., while if a 5-in. pipe is used the loss will be reduced to .24 lb. The velocity of flow in the latter case is only 10 ft. per sec.

When driving the Jeddo mining tunnel, at Ebervale, Pa., two 31-in. drills were used in each heading, with a 6-in. main, the maximum transmission distance being 10,800 ft. This pipe was so large in proportion to the volume of air required for the drills (230 cu. ft. per min. of free air) that the loss was reduced to an extremely small quantity. A calculation shows a loss of .002 lb., and the gauges at each end of the main were found to record practically the same pressure.

A due regard for economy in installation, however, must limit the use of very large piping, the cost of which should be considered in relation to the