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Heating Water in Tanks by Steam.-In heating water by blowing steam into it through a perforated pipe placed at the bottom of a tank, the quantity of steam required is, approximately, 1 lb. for every 5 lbs. of water heated to 212° Fahr. The exact quantity may be calculated as follows:Suppose 200 gallons of water at a temperature of 52° Fahr. are to be heated to the boiling point, 212° Fahr., by steam of 60 lbs. per square inch pressure by the steam-gauge. Then to heat 1 lb. of water from 52° to 212° Fahr. requires 212° - 52° = 160 units of heat, and the total heat required is = 200 gallons x 10 lbs. 2,000 lbs. x 160 units 320,000 units. The total heat of steam of 60 lbs. + 15 lbs. 75 lbs. per square inch absolute pressure, is, from Table 1, page 16 1,207.2° Fahr.; and 1,207.2° - 212° 9952 heat units are available per lb. of water. Then 320,000 units 9952 units 321.5 lbs. of steam are required to heat the water. According to the above approximate rule, to heat this quantity of water, 2,000 lbs. of water ÷ 5 = 400 lbs. of steam are necessary, or about 25 per cent. more than the quantity actually required.

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Condensation of Steam in Pipes Cooled by Water. In some experiments with surface condensers, in which the steam was passed through the tubes, 500 units of heat by condensation were transmitted per square foot of tube-surface per hour per 1° Fahr. difference of temperaThe condensers were arranged in three groups of tubes, successively traversed by the condensing water. In another case, where the condenser was arranged in two groups, from 220 to 240 units were transmitted.

ture.

Mr. B. G. Nichol experimented with an ordinary surface-condenser brass tube,inch external diameter and 18-wire gauge in thickness, encased in a 3 inch diameter iron pipe. Steam of 32 lbs. total pressure per square inch occupied the inter-space, and cold water at 58° Fahr. flowed through the brass tube. Three experiments were made with the tube in a vertical position, and also in a horizontal position, the results of which were as follows:Vertical position. Horizontal position.

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Velocity of water through the tube in feet per minute :—

81

278 390

78

6

307 415 feet. Steam condensed per square foot of surface per hour for 1° Fahr difference of temperature:

*335 *436 457

*480 *603 699 lb.

Heat absorbed by the water per square foot per hour for 1° Fahr. difference of temperature:

346 449

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The rate of condensation was greater in the horizontal position than in the vertical position. Also, the efficiency of the condensing surface was increased by an increase of velocity of the water through the tube, nearly in the ratio of the fourth root of the velocity for vertical tubes; and nearly as the 45 root for horizontal tubes.

Comparative Rate of Emission of Heat from Pipes.-The rate of emission of heat from pipes varies considerably. For equal total difference of temperature the rate of emission of heat from steam-pipes placed in water, is from 150 to 250 times as great as the rate when they are placed in air, according as the pipes are in a vertical or horizontal position.

The rate of emission of heat from water-tubes placed in water is about 20 times as great as the rate when they are placed in air. In one experiment it was proved to be 25 times. When the water-tube was moved through the air at a speed of 59 feet per second, it was cooled in one-twelfth of the time occupied in still air. In water moved at a speed of 3 feet per second, the water in the tube was cooled in one-half the time.

Refrigerating Machinery. For the cooling of brine and other liquids by the alternate compression and expansion of air, Mr. D. Thomson gives the following formulæ, in which the machine is supposed to be perfect :— P T ; C = X 772 T-i

Р

=

772 C x

In which P the power required to do the cooling work C, in foot-pounds.

C

T

t

=

=

=

=

the cooling work done, in thermal units.

the absolute maximum temperature in degrees Fahr. of the air in the hot or compression-end of the machine.

the absolute minimum temperature in degrees Fahr. of the air in the cold or expansion-end of the machine.

These formulæ indicate that the most economical results, as regards consumption of power, are obtained when the machine is worked within a small range of temperature, as in breweries, where the temperature of the water has frequently to be lowered only 10° Fahr.

These formulæ are applicable to all cooling machines, whether they operate by means of air, ether, ammonia, or any other fluid.

In the ammonia machine, or other machine working on the same principle, in which no mechanical power is applied, the value of P, it is understood, is the heat theoretically required, at the rate of 1 heat-unit for 772 footpounds of power; and the first formula given above becomes :

(Ammonia) Heat required to do the work C=(T − 1) ÷ t.

The ammonia machine has, theoretically, a great economical superiority, as heat is so much less expensive than its equivalent of mechanical power. The nature of the vapour employed influences the size of the machine, as will be seen from the following table :

Table 57.-RELATIVE CAPACITY OF CYLINDER FOR DIFFERENt Vapours.

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CUTTING METALS.

The most advantageous speed in lineal feet per minute, for planing, shaping, slotting, and turning metals, is, for copper 120 feet, brass 50 feet, wrought-iron 20 feet, cast-iron 18 feet, steel 12 feet. By dividing these numbers by the circumference in feet of the work to be turned, the number of revolutions of the lathe-spindle is obtained. For boring work in a lathe, the speed is limited by the overhanging of the tool, to from 6 to 10 feet per minute; for screwing bolts and tapping nuts the surface-speed is from 4 to 8 feet per minute. The speed of cutters for wheel-cutting and milling machines should not exceed 18 feet per minute at the largest cutting diameter.

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The speed for turning chilled rolls is from 3 to 4 feet per minute. The speed for cutting screws is equal to two-thirds of the speed for turning metals. Table 59.-SPEED IN REVOLUTIONS PER MINUTE OF PLAIN DRILLS AND TWIST DRILLS FOR DRILLING VARIOUS METALS.

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The rate of the feed of a drill should be in proportion to the hardness of the metal. In drilling wrought iron, the number of revolutions of the drilj per inch of feed may generally be 300 for drills of, and inch diameter; 200 for a drill of inch diameter; 150 for drills of,, and inch diameter, and 100 for drills of from inch to 4 inches diameter.

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The feed or advance of tool suitable for the speeds given in the table of cutting speeds for lathe work, is given in the following table for roughing cuts. The finishing cut should be as light as possible, with a broad advance or feed of cut.

Table 60.-FEED OR ADVANCE OF CUT FOR ROUGHING CUTS IN

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As each revolution of the lathe moves the tool forward the portion of an inch given in this table, a 3-inch shaft making

would be turned with a rough cut at the rate of

in length per minute.

25 revolutions per minute
25 revolutions
16 advance

= 1 inch

The feed or advance of the tool of a planing machine should be 14 or 12 cuts per inch for roughing cuts, and the finishing cuts should be done with a broad tool having an advance for each cut of from to inch.

Speed of circular saws for cutting metal, for brass 350 lineal feet per minute, for cast-iron 190 feet per minute, for wrought-iron 150 lineal feet per minute.

The speed per square foot of surface at which metals can be cut, depends greatly upon the efficiency and rigidity of the machine tools, as well as upon the softness and quality of the metal; some iron is very scaly and dirty, and soon blunts the tool. In the following table is given the time required to finish work, including one roughing cut and one finishing cut, the average of a great quantity of work done by ordinary good tools: the finishing cut being light with a broad advance.

Lathe Centres.-The usual angle for lathe centres is 60°; but for heavy work a more durable angle is 75°. For heavy work the centre should have a small hole bored up its centre, and another hole drilled at right angles to meet it, by which means the bearing surfaces can be properly oiled without stopping the lathe.

Cutting Angle of Lathe Tools.-The cutting angle best adapted for turning tools for soft wood is 30°, for hard wood 40°, for wrought-iron and steel 60°, for cast-iron 70°, for brass 80°, for very hard metals 842, for gun metal 85°, for hard brass and hard gun metal 90°, and for chilled rolls 90° The angle of clearance of these tools is 3°.

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