THE

WORKS MANAGER'S HAND-BOOK.

SECTION I.

STATIONARY AND LOCOMOTIVE STEAM ENGINES, GAS ENGINES, &c.

WORK AND HORSE-POWER.

A Unit of Work is equivalent to one pound avoirdupois raised vertically one foot. The units of work done in raising a given weight to a given height, are found by multiplying the height in feet by the weight in pounds. The units of work done in raising a weight up an inclined plane, are equal to the work that would be done in raising the weight vertically through the height of the plane.

The Modulus of a Machine is the fraction which expresses the relation of the work done to that of the work applied, or the percentage of the power absorbed which a machine will give out in useful work.

Horse-power. A strong horse can travel 2

miles per hour and work 8 hours a day, doing the equivalent of pulling a load of 150 lbs. weight up out of a shaft by means of a rope. 2 miles an hour is 220 feet per minute, and at that speed the load of 150 lbs. is raised vertically the same distance, that is equal to 300 lbs. raised 110 feet high, or 3,000 lbs. raised 11 feet high, or 33,000 lbs. raised one foot high per minute. The unit of power is the mechanical force necessary to lift 33,000 lbs. one foot high in one minute; but, in dealing with steam engines, two terms are used, viz., nominal horse power, and actual horse-power.

Nominal Horse-power is a commercial term used by makers of engines to denote only the size of an engine without regard to the actual power it will exert.

Nominal Horse-power of Non-Condensing Engines.-The rule of ordinary practice is to make the sectional area of the cylinder equal to from 9 to 10 square inches for each nominal horse-power. The nominal horse-power of non-condensing engines may be found by the following rule, which accords with the best modern practice. Rule: Multiply the square of the diameter of the cylinder in inches by 7, and divide the result by 80.

Nominal Horse-power of Condensing Engines.-Rule: Multiply the square of the diameter of the cylinder in inches by 7, and divide the product by 200.

Actual Horse-power of an Engine.-To find the actual horse-power. Rule: Multiply the area of the cylinder in square inches by the average effective pressure in lbs. per square inch, minus 3 lbs. per square inch for friction; and by the speed of the piston in feet per minute. The product will be the number of foot-pounds per minute which the engine will raise. Next, divide the product by 33,000, and the quotient will be the actual horse-power.

Power and Weight of Men and Animals.-In working a crane handle, a man can apply a force of 60 lbs. in an emergency with difficulty, or a force of 30 lbs. for a short time with difficulty, or a force of 20 lbs. for a short time easily, or a force of 15 lbs. in continuous work at a velocity of

220 feet per minute; hence the power of a man is 15 × 220=3,300 foot pounds per minute, or one-tenth of a horse-power. A soldier on march travels about 30 inches per step, and occupies a front of 21 inches in the rank; the average weight of men is 150 lbs. each; five men can stand in a space of one square yard; the weight of ordinary crowds of people is 80 lbs. per square foot; the absolute force of a man in pulling horizontally or pushing with his hands is 110 lbs., his lifting power with both hands is 280 lbs., and the greatest load he can support on his shoulders is 336 lbs. A horse will exert a pulling force of 120 lbs. at the rate of 2 miles an hour during 10 hours. A pony or mule will exert a pulling force of 60 lbs. at the rate of 2 miles an hour during 10 hours. An ass will exert a pulling force of 30 lbs. at the rate of 2 miles an hour during 10 hours. These animals will each carry a load on its back equal to one-fourth its own weight, at the rate of 24 miles an hour during 10 hours. A horse will draw a load of one ton at the rate of 2 miles an hour during 10 hours. A pony or mule will draw a load of 12 cwt. at the rate of 2 miles an hour during 10 hours. An ass will draw a load of 7 cwt. at the rate of 2 miles an hour during 10 hours. These forces are for a straight pull; when animals work by pulling while walking in a circle, their pulling force is only about 60 per cent. of their force for a straight pull; the diameter of the circular path should not be less than 25 feet, and the velocity should not exceed 2 miles an hour. The average weight of a cart-horse is 13 cwt.; a cob, 7 cwt.; a mule, 6 cwt.

Resistance of Carts and Waggons to Traction on Level Roads and Rails.—The resistance to traction in proportion to the whole weight is on fields; on gravel and on broken-stone roads in bad condition; on dry hard turf; on good macadamized roads; on underground tramways with 8-inch diameter wheels; on wood pavement; London pavement; on street tramways with grooved rails; underground tramways with 12-inch wheels on round top rails; asphalte pavement; on granite tramway; on railways.

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The force required to drag a weight on a level firm wood floor without rollers is the whole weight, and with the weight placed on rollers 3 inches diameter, it is of the whole weight.

CONDENSATION IN STEAM CYLINDERS.

Condensation. It is found in practice that nearly all steam engines use half as much more steam than is theoretically required, and this loss is mostly caused by condensation of the steam in the cylinder. When steam enters a cold cylinder, it is rapidly condensed during the operation of warming the cylinder and piston, and raising their heat up to the same. temperature as the steam, because the piston will not move until both it and the surrounding surfaces are heated to a temperature approaching more or less that of the steam. Re-evaporation takes place during the

whole time of exhaust, because the steam, when exhausting after expansion, being lower in pressure and temperature, cools the cylinder and steam passages, and absorbs the heat. The heat thus abstracted must be restored to the metal by the entering steam, a portion of which must be condensed to restore the heat thus lost, because, as already stated, until the metal is considerably raised in temperature, the heat in the entering steam will be expended in heating the surfaces, instead of moving the piston. Condensation also goes on in the cylinder, due to the performance of work during expansion in driving the piston. The steam falls in temperature owing to its change in volume during expansion, and the temperature of the interior surfaces of the cylinder also falls during expansion, nearly with that of the steam, parting with heat to re-evaporate the water formed. Therefore, at the commencement of each stroke, a portion of the entering steam must be condensed to restore the heat lost by condensation and the cooling of the cylinder by re-evaporation during the previous expansion, as well as the heat abstracted by the steam during exhaust.

The extent to which cylinder condensation takes place depends upon the extent of the cooling surfaces opposed, and also upon the quantity of water mixed with the steam and carried with it from the boiler; but part of the water formed from the condensed steam is re-converted into steam during expansion, and the heat necessary for its re-evaporation is supplied from three sources. First, from the heat stored in the metal which was abstracted from the entering steam. Secondly, from the sensible heat given up by the steam as it falls in pressure and temperature during expansion. Thirdly, from the latent heat given up by the steam during condensation. So that the action of condensation and re-evaporation is continually going on in the cylinder. Condensation varies as the size of cylinder, for as the diameter is increased, the condensing surfaces increase directly as the diameter; but the area and consequently the volume of steam increase as the square of the diameter; the condensing surfaces of the piston and cylinder-ends increase as the square of the diameter; but the volume of steam cut off at a given proportion of the stroke increases directly as the length of stroke, so that the loss from condensation diminishes as the diameter of cylinder and the length of stroke are increased. Condensation also varies with the rate of expansion; the weight of steam condensed increases rapidly with each increase in the ratio of expansion.

CYLINDERS.

Cylinder Condensation causes a great loss of both steam and fuel, and forms an obstacle to working expansively; in fact, unless the cylinder is protected in some way, so as to keep up the temperature of the steam during expansion to its initial pressure, little or no gain will be derived from working expansively. If steam could only be maintained at a suitably high temperature during expansion, without condensation, then the reduction of