1S47.] 



THE CIVIL ENGINEER AND ARCHITECT'S JOURNAL. 



47 



meter. Such boilers ought indeed to be named exploders, aud tlie legislative 

 restriction as to the amount of pressure to be used with them is, as far as it 

 goes, a salutary measure. Still better would the law stand if it began at the 

 other end, and limited the size of the vessels instead of the elasticity of the 

 (team within them ; for such an enactment would be free from the objection 

 (rf discouraging the use of high.pressure steam, now promising so much ad- 

 vantage to industry. We can scarcely hope, however, for the full realization 

 (rf our wishes in this respect, unless a bold and enlarged view is taken of the 

 system; for, as I shall hereafter show, the high-pressure engine cannot be 

 liiade to display its advantages with steam under about six atmospheres' 

 pressure. A compulsory enactment restricting the size of the generating 

 vessels would tend much towards promoting the use of steam of snch high 

 pressures, and, by producing a necessity for acquaintance with the working 

 of the engine, would undoubtedly further its real improvement." 



The comparison is not, however, here stated quite fairly. It is true, that 

 all things else remaining the same, the tension of the boiler increases with 

 its size; but then we set out by supposing the strength of the material in- 

 creased in like proportion. The author himself insists that it be presup- 

 posed, in limine, that the thickness of the metal be proportioned to the ten- 

 sion to be resisted ; and, as we have already said,'it is absurd to institute a 

 comparison on any other terms. In objecting therefore to the great size of 

 low-pressure boilers he should condemn — not their weakuess (which is sup- 

 posed to he provided against) — but the great weight of metal required to 

 make them sufficiently strong. 



The relation between the thickness of the metal and the dimensions aud 

 pressure of the boiler, may be easily determined in most cases ; and we in- 

 tend to lay before the reader, in a separate paper, the means of calculating, 

 with great facility, the proper thickness of a boiler of given form and size, 

 in order to sustain a given pressure. For the present, however, we may 

 observe, with Dr. Alban, that when the plates of a large boiler are increased 

 to a thickness proper to its dimensions, they may become so thick as to be 

 liable to crack from the sudden application of heat. This is a source of 

 danger altogether independent of those hitherto considered, — it must be 

 prevented either by making the plates of metal of superior temper and 

 tenacity ; or by gradual and careful heating ; or lastly, by reducing the 

 size of the boiler, and consequently, the thickness of its plates. Dr. Alban 

 mnst, however, recollect that, in respect to this danger of cracking, low- 

 pressure and high-pressure boilers are frequently on a par. He says — 



" It is indeed customary to give to boilers of great size a proportionate 

 thickness of metal, but this helps the case very little; for experience has 

 shown that thick plates, especially if of cast metal, are more liable to crack 

 by the action of the fire than thin ones ; inasmuch as the temperature of 

 tlieir two sides, exposed respectively to the fire without and the water within, 

 does not quickly assimilate ; whereby unequal expansion and contraction 

 raisues. It is moreover a difficult matter to determine what the proper 

 strength ought to be in proportion to the diameter and the pressure, and 

 there is a great dift'erence of opinion among those who have given their 

 attention to this point. It must also be noticed, that thick vessels tend more 

 to retard the transmission of heat to the water than thin ones, although this 

 fact seems often to have escaped the notice of engineers." 



But how extremely unphilosophical is it to urge this as an argument 

 against low-pressure boilers exclusively ! A boiler of large dimensions and 

 low pressure may require the same thickness of metal as a boiler ,of small 

 dimensions and high pressure. 



The second ohjection against high-pressure engines — the loss of heat — we 

 mnst, for the sake of brevity, dismiss with the following brief consideration, 

 which, in fact, embraces the sum of our author's arguments. By a well- 

 known property of steam, ascertained by Watt and many others, the sum of 

 the latent and sensible heats is constant at all pressures, and itherefore the 

 same fire will evaporate equal quantities of water in a given time, whatever 

 be the boiler pressure. Now, it may be demonstrated mathematically that 

 steam acts with most effect when used at a high pressure and worked ex- 

 pansively. Consequently there is, cceterU paribus, a greater economy of fuel 

 when the steam is generated at high pressure. 



Tlie third ohjection — the relinquishment of that force arising from con- 

 densation — is stated correctly by Dr. Alban, except in that he under-esti- 

 inates the amount of power obtained in practice by condensing the steam. 



" Partly through imperfect condensation, partly through the working of 

 the air aud cold water pumps, and from other causes of the same descrip- 

 tion, the useful efi'ect of low-pressure engines is reduced from about 17 11). 

 per square inch absolute pressure upon the piston, to about seven, as made 

 available in power obtained ; so that the use of condensation only in reality 

 offers a gain of from 4| to 5 lb. per square inch, or one-third of the atmo- 

 spheric pressure The objection loses in weight as we use steam of 



higher pressure, and at seven or eight atmospheres is scarcely to be con- 

 sidered, because the surface of the piston becomes proportionately less as the 



elasticity is increased, and therefore the loss of the vacuum is less to be 

 felt ; while the advantages of the system are increased by such increase of 

 elasticity. When the pressure used is too low, for example, only two or 

 three atmospheres, as is most common, the loss maybe imporiant, and the 

 advantages of the high-pressure system are not sufficiently developed to 

 cover it. For instance, an engine of 10-horse power at two atmospheres' 

 pressure, will require about twice as much steam as a condensing one of the 

 same power : it must be of about the same dimensions, and by the want of 

 a vacuum must be supplied with sleam of a double elasticity to produce the 

 same effect. Here, therefore, a power of ten horses will be sacrificed by the 

 want of the vacuum ; that is, as much as the whole power of the engine. 

 But if a pressure of eight or ten atmospheres be used, and the principle of 

 expansion apphed, the proportionate loss, by the sacrifice of the vacuum, 

 will be scarcely equal to 2-horse power nut of ten, — a loss of very trifling 

 weight when comjiared with the advantages possessed by such an engine 

 over a low-pressure one. Yet more in favour of the high-pressure engine 

 would the comparison be if we could substitute steam of sixteen atmospheres 

 for that of eight; but unfortunately, through practical difficulties in the 

 working of the machinery, our hmits of available elasticity are at present 

 too confined." 



We may here observe, that elsewhere Dr. Alban recommends that the 

 steam should always be generated at a pressure of eight or ten atmospheres, 

 or 120 lb. to 150 lb. to the square inch. He speaks of employing what we 

 should consider excessive pressures, with great composure. " Once," says 

 he, " I worked an engine, for the sake of experiment, to a pressure of 1000 lb. 

 on the square inch, and it was found that under this tremendous pressure, 

 the engine itself remained perfectly firm and steam-tight" 1 He tells us 

 also, that in the ordinary working of his steam engines " the steam makes 

 its exit from the cylinder with a pressure of about three atmospheres," or 

 45 lb. Now, in English railway locomotives, stieam is often admitted into 

 the cylinder at the pressure with which Dr. Alban suffers it to escape. So 

 that if it were possible for him to send his steam here when he had done 

 with it, we might use it in working our locomotives. It seems scarcely pos- 

 sible that there can be any economy where steam is suffered to escape at 

 this high pressure : for supposing it to be admitted to the cylinder at eight 

 atmospheres, and to maintain uniform pressure throughout the stroke, the 

 efi"ective pressure is (8 — 3, or) 5 atmospheres: consequently, fths of the 

 power is wasted. The reason assigned for expelhng the steam at a high 

 pressure exhibits some very odd philosophy : 



" The steam leaving the cylinder would at the end of each stroke retain too 

 little excess of pressure above the atmosphere, and therefore would blow 

 out with too small a velocity, and leave behind an increased resistance to 

 the piston. For example, steam of three atmospheres, expanded to three 

 times its volume, would scarcely balance the atmosphere, and would thus 

 have no tendency to blow out ; while steam of two atmospheres similarly 

 expanded, would sink so much under the atmospheric pressure, as to cause 

 a very injurious counter-resistance to the piston from the entering air." 



The phrase, " leave behind an increased resistance to this piston," is to us 

 perfectly unintelligible. If the escaping steam be of high elasticity, it will 

 doubtless have a great tendency to rush out by the eduction-port — but the 

 same elasticity also acts to retard the motion of the piston. Of course, it 

 is desirable that the steam should be got rid of with facility ; aud, in order 

 that the piston may drive it out with as little resistance as possible, the 

 escape-pipes should be made of ample size. But a " tendency to blow out" 

 in the steam itself is of no advantage : for it has no power of itself to 

 " blow out," after so much of it has escaped as to make the pressure of the 

 remainder equal to that of the atmosphere. This remainder is expelled by 

 the piston. Consequently, the piston will always have to drive out steam of 

 not less pressure than the atmosphere — if it be much greater than the atmo- 

 sphere, an unnecessary resistance is created. \<e come, then, to the conclu- 

 sion that, in all engines in which the steam issues directly from the cylinder 

 into the air, the issuing steam ought to be as nearly as possible of atmo- 

 spheric pressure : in other words, if the steam be admitted from the boiler 

 at a high pressure, its maximum effect is obtained by working it expansively 

 and reducing it to about 15 lb. pressure, before the eduction-port is opened. 



The fifth objection — the wear and tear of metal from rapidity of motion — 

 our author tries to overcome by theoretical arguments : but the facts are 

 too strong for him. That the rubbing parts of the best locomotive engines 

 suffer a great wear and tear, due to the rapidity of motion, is an incontest- 

 able truth, which no arguments such as the following can get rid of: — 



" It is inconceivable how the apparatus for transmitting the motion of the 

 piston of a high-pressure engine to the machinery can be more subject to 

 destruction, iii regard to the durability of its joints, than in a low-pressure. 

 If the power of each be the same, the machinery must have in each case 

 equal strength : the stress to which it is subject is the same (or rather is 

 less in the high-pressure engine, on account of the diminished prejudicial 



