APPLIED MECHANIC& 



[SDPEB-HKATr-D STEAM. 



pressure aide of t ! . Mid where, by evolving va- 



poor on the vacuum *:.! it produces back or retarding 

 p feature. It paaaes with the steam into the condenser, 

 and there abaorbs uselessly a Urge portion of tin- 1 

 power, whether of in. ; of cold surface, and 



therefore, for a given power of engine, necessitates a 

 proportionally greater cooling power, a proportionally 

 larger air-pump, mil ; illy increased waste of 



useful effect in "iknr; it. To avoid these evils it has 

 long been UMI.I! t .am-pipea and cylinder 



with thick felt and wood ca.-r .:. <>r non conducting ma- 

 terials of a similar I. in many < 

 cylinders have been wade double, havm.; a jacket or 

 apace all round them tilled with steam from the boiler, 

 which serves to keep tlm inner cylinder hut, and to pre- 

 vent loss of pressure and condensation during the actual 

 expansion of the steam within it. Notwithstanding 

 these precautions, however, a great loss of effect is still 

 incurred, amounting probably to 20 or 25 per cent, under 

 the most favourable conditions, and, under average con- 

 ditions, to a great deal more. Engineers accordingly 

 begin to adopt the super-heating process, which is found 

 to present an almost complete remedy against these 

 louses. It may simply be described as a mode of sur- 

 charging the steam with heat, and raising its temperature 

 considerably above what may be called its natural tem- 

 perature, or the temperature due to its pressure. In 

 order fully to understand this, it should be remembered 

 that the temperatures given in the Table are those of the 

 ateam when it is in contact with water. For instance, 



lie pressure is equivalent to -1 atmospheres, the 

 team in contact with watt T, and the water, have the 

 temperature of 290' ; if the temperature were reduced, 



11 of the steam would be condensed, and the re- 



t would present a proportionally diminished pres- 

 sure. 'Were the temperature increased, a fresh portion 

 of water would be converted into vapour, which, 

 minimi,' with the rest, would proportionally increase 

 the pressure. But if the steam were removed into a 

 Teasel separate from the water, while it would still be 

 true that a i ; temperature would cause a con- 



densation and diminution of pressure, it would no longer 

 be true that an accession of temperature would cause an 

 increase of pressure from the generation of fr. ,-h steam, 

 because there would be no water from which to generate 

 it In that case, the steam would probably follow the 

 law of expansion, which is common to all gaseous bodies, 

 increasing in bulk by a certain fraction of its volume, 



Hied to its elevation of temperature ; or, if its 

 volume were confined, increasing in pressure in the same 

 But steam, so elevated in temperature, 

 would )>u in a condition to sustain a certain loss of heat 



any part of it becoming condensed. It would, 

 in fact, have a relay of heat stored within it, ami 

 be exposed in pipes or cylinders to cooling surfaces 

 without losing any appreciable percentage of its pressure, 

 or depositing water of condensation. The ] 

 modes by which this super-heating process is effected 

 are various. Their general principle, however, is simply 

 to expose the steam, in its passage from the boiler towards 

 the cylinder, to the heat of the boiler flues ; and, in order 

 that this heat may be rendered effective as rapidly and 

 completelyas possible, the steam-pipe is subdivided i 

 numerous small pipes or passages, exposing a very ex- 

 ! tended surface to the hot products of combustion which 

 play upon it Almost all the super-heating apparatus 

 thus bear a close resemblance to the surface-condensers, 

 though intended for an operation of an opposite cha- 

 ' surface condenser the steam i sub- 

 divided into numerous tubes or channels exposed to cold 

 water for the purpose of cooling and condensing it, so 

 in the super-heater, it islikewi < suMivided into channels 

 exposed to hot flame, smoke, and gases, for the purpose 

 of surcharging it with heat 



It is said, that by the use of super-heating apparatus, 

 an economy of fuel baa been cti. '!..!, amounting, in some 

 eaaea, to 20 and 25 per cent Probably a portion of thU 

 economy may be duo to the circumstance that super. 

 heating apparatuses are generally placed in the last part 



of the boiler flue leading; to the chimney, where 

 utilise a quantity of heat which, under oni 



tancea, passes up the chimney, and is totally wasted. 



\\l-l-o\V KU._ the. 



purposes to which steam-power is applied are Die 

 following : 



I. Pumpin!> water for the drainage of mines and 

 docks, or at water-works for the supply of towns. 



II. Driving machinery for raising ore from mines, for 

 moving heavy weights, or for agricultural and manufac- 

 turing purposes. 



III. For locomotion on railways. 



IV. l-'or propeilinur Mram-vessels. 



\Ve ill ' iss the modes in which steam-power 



is generally made available for these diff.-rent uses, and 

 the forms of steam-engines most advantageously applied 

 in each case. 



I. PCMI-IXO. In pumping water, until of late years, 

 single-acting engines were almost universally applied. 

 We may most readily account for this, not on the ground 

 of any advantage derived from the use of single-acting 

 engines, but from the circumstance that the earliest form 

 of engine that was rendered practically available was that 

 of Newcomen, which was single-acting, and suited only 

 for working pumps ; that Watt's improvements were first 

 applied to engines of this kind, and that his engines 

 were introduced at mines to supersede the labour of men 

 and horses formerly applied to pumping ; that these 

 engines were of first-rate quality, effective and durable, 

 and naturally impressed the miners with a preference for 

 their form and arrangement ; and that engineers in 

 mining districts, applied themselves rather to the per- 

 fecting of forms already in use, than to the introduction 

 of new arrangements. So strong, indeed, has been the 

 preference for single-acting engines, especially those of 

 the kind employed in Cornwall, when applied to pump- 

 ing, that few double-acting engines have ever been em- 

 ployed for this purpose until very recently. We believe, 

 however, that the results have shown decided advantages 

 in favour of the double-acting engines, and that ere long 

 they will supersede the more cumbrous and less advanced 

 form of those that are single-acting. 



The pumps employed in mines are of the lifting or 

 forcing kind, having a stroke seldom exceeding 8 or 10 

 feet. When the mine is deep, the water is raised by 

 stages, each 150 to 200 feet in height. The 1 

 pumps discharge into a reservoir about that height above 

 the bottom of the mine ; the second set of pumps draw 

 from that reservoir, and discharge into the next higher ; 

 and so on, until the water is finally delivered at such a 

 level that it may be permitted to flow off by natural 

 drainage. A apparatus is generally required at mines 

 for stamping and crushing the ore, it is not unusual to 

 deliver the water at such a height above the general 

 level of the ground, as may permit its use for driving a 

 water-wheel, from which motion is given to the ore- 

 crushing machinery. But as power is also required for 

 raising the ore, it is generally more advantageous to 

 employ a double-acting engine, besides the main pumping 

 engine, for this purpose, as well as for the preparation 

 of the ore. The power required for raising water depends 

 upon the quantity raised in a given time, and the h- 

 to which it is raised. If we suppose that it is necessary 

 to raise 100 cubic feet of water per minute 150 fathoms, 

 or 900 feet, since each cubic foot of water weighs li'JA Ibs., 

 the. total to be raised is 100 X "'.H = U-JMI Ibs. per 

 minute. This load, raised through 900 feet, is equivalent 

 to !,_'.% x 900 = 6625000 Ibs. raised 1 foot, and would 



"(' '"I Ul t 



require ~ - 170 horse-power, without allowance 



for friction and excess of power, to give the velocity of 

 movement to the column of water. To estimate this 

 quantity, we should add at least 10 percent., or ,' ,.th, 

 making the power 187-h"rv. which would be tin 

 power, after making all allowances for tho friction of 

 the engine, and tho various losses occasioned by the 

 cooling or leakage of steam, and the working of the air- 

 pump and feed-pump. 



