Steam:
Its Generation and Use

►i* HORSE-POWER OF BOILERS. Strictly speaking, there is no such thing as “ horse-power ” to a steam boiler ; it is a meas- ure applicable only to dynamic effect. But as boilers are necessary to drive steam-engines, the same measure applied to steam-engines has come to be universally applied to the boiler, and can- not well be discarded. In consequence, how- ever, of the different quantity of steam necessary to produce a horse-power, with different engines, there has been great need of an accepted stand- ard by which the amount of boiler required to provide steam for a commercial horse-power may be determined. This standard, as fixed by Watt, was one cubic foot of water evaporated per hour from 2120 for each horse-power. This was, at that time, the requirement of the best engine in use. At the present time, Prof. Thurston estimates, that the water required per hour, per horse-power, in good engines, is equal to the constant 200, divid- ed by the square root of the pressure, and that in the best engines this constant is as low as 150. This would give for good engines, working with 64 lbs. pressure, 25 lbs. water, and for the best engines working with 100 lbs., only 15 lbs. water per hourly horse-power. The extensive series of experiments, made under the direction of C. E. Emery, M. E., at the Novelty Works, in 1866-8, and published by Professor Trowbridge, show, that at ordinary pressures, and with good proportions, non-con- densing engines of from 20 to 300 H. P., required only from 25 to 30 lbs. water per hourly horse- power, in regular practice. The standard, therefore, adopted by the judges at the late Centennial Exhibition, of 30 lbs. water per hour, evaporated, at 70 lbs. pressure, from ioo°, for each horse-power, is a fair one for both boilers and engines, and has been favor- ably received by the Am. Soc. of Meeh. Engineers and by steam users, but as the same boiler may be made to do more or less work with less or great- er economy, it should be also required that the rating of a boiler be based on the amount of water it will evaporate at a high economical rate. For purposes of economy the amount of heating surface should never be less than one, and generally not more than two, square feet, for each 5,000 British thermal units to be absorbed per hour, though this depends somewhat on thoeharacter and location of . such surface. The range given above is believed to be sufficient to allow for the different conditions in practice, though a far greater range is frequently employed. As, for instance, in torpedo boats, where everything is sacrificed to lightness and power, the heating surface is sometimes made to absorb 12,000 to 15,000 B. T. U. per square foot per hour, while in some mills, where the pro- prietor and his advisers have gone on the princi- ple that ‘‘ too much is just enough, ’ ’ a square foot is only required to absorb 1,000 units or less per hour. Neither extreme is good economy. Square feet of heating surface is no criterion as between different styles of boilers — a square foot under some circumstances being many times as efficient as in others ; but when an average rate of evaporation per square foot for any given boiler has been fixed upon by experiment, there is no more convenient way of rating the power of others of the same style. The following table gives an approximate list of square feet of heat- ing surface per H. P. in different styles of boilers ; and various other data for comparison : Type of Boiler. Water-tube...... Tubular......... Flue............ Plain Cylinder .. Locomotive...... Vertical Tubular. Square feet of Heating Sur- face for One H. P. Coal per sq. ft. H.S.per hour. Relative Economy. Relative Rapidity of Steaming. xo to 12 •3 1.00 1.00 14 to 18 •25 •91 •50 8 to 12 •4 •79 •25 6 to 10 •5 .69 .20 12 to 16 •275 ■85 •55 15 to 20 •25 .80 .60 Isherwood. Prof. Trow- bridge. A horse-power in a steam-engine or other prime mover, is 550 lbs. raised 1 foot per second, or 33,000 lbs. i foot per minute. HORSE-POWER OF DIFFERENT NATIONS. Most nations have a standard for power simi- lar to, and generally derived from Watt’s “horse- power,” but owing to different standards of weights and measures, these are not identical, though the greatest differences amount to less than per cent. The following table gives the standard horse-power for each nation, in kilo- grammetres per second, and in foot-pounds per second, expressed in the foot and pound stand- ard in each country: TABLE OF STANDARD HORSE-POWER FOR DIFFERENT NATIONS. Country. •2 g ö tn m JO. fa France and I Baden... j Saxony...... Wortemberg ., Prussia..... Hanover..... England .. .. I Austria.... 75 75-045 75-24° 75-325 75-36i 76.041 76.119 500 500.30 501-36 502,17 502.41 506.94 507-46 529.68 530. 531.12 ss1-«)? 532.23 537-03 437-58 (/) £ v Wortem- berg Ft. pounds, per sec. 1 Prussian Ft, pounds, per sec. Hano varian Ft. pounds, per sec. English Ft. pounds, per sec. 52I-58 477-93 513-53 542-47 523-89 478.22 5’3-84 542-8o 525. 470.23 5I4-92 543-05 525.85 480. 5I5-75 544-82 526.10 480.23 516. 545'°S 530.84 484-56 520.65 550. 531-39 485.06 521.19 550-57 c/T o « fa 423.68 423-93 424-83 425-5I 425-72 429-56 430. h 59