1«40.] 



THE Cn^L ENGINEER AND ARCHITECTS JOURNAL. 



89 



EXPLANATION OF SOME TECHNICAL TERMS USED IN 

 STEAM ENGINE CALCULATIONS, 



WITH REMARKS ON THE CORNISH QUESTION. 



Sir — The full and satisfactory account you liave given in your 

 February number, of the new engine at the East London Water-works, 

 must not only be highly interesting to those of your readers who are 

 attached to pursuits connected with the steam-engine, but also to 

 those who value truth for its own sake, inasmucli as it will very soon 

 settle the long-disputed Cornish question, besides being of the greatest 

 practical importance to the proprietor of mines and other targe works 

 in all parts of the kingdom. 



It now appears that in my comparison of the Cornish and Lancashire 

 systems in your number for January, I had, as indeed I wished to do, 

 rather over than under-rated the power of the engine above referred 

 to, and when I have all the data for going into the commercial part of 

 the question — the comparative expense — I am afraid it will be found 

 that the advantage of the Cornish system has been somewhat more 

 largely overrated by others, especially with reference to the propriety 

 of adopting that system in cotton factories. At any rate, when the 

 proper corrections are made in my table of comparative duty, from 

 the statement you have furnished, I think no one will be found to con- 

 tend thjit/our, five, and even six times more work (as has been often 

 asserted) is performed by the steam-engines in Cornwall than in the 

 north of England for the same quantity of fuel of like quality. Indeed, 

 the excessive degree of perfection hitherto claimed for the Cornish 

 engine is much to be regretted, even if true, as it carries a certain 

 degree of aVjsurdity on the face of it, that has not a little indisposed 

 engineers on both sides of the question to a fair and dispassionate 

 inquiry. With a view to expedite the settlement of the most im- 

 portant parts of the question, and prevent that divergence from the 

 main point at issue which is liable to occur with tlie best-intentioned 

 disp\itants, I have made the following attempt to define certain tech- 

 nical terms which prevail in this district, and it will be of use, per- 

 haps, to some engineers both in and o\it of Cornwall. I am also 

 induced to submit these definitions to the approval of your readers, 

 because I observe, in Mr. Enys' remarks in your last number, a few 

 slight misconceptions of my meaning, which, together with perhaps 

 a want of strict accuracy of application in some of the expressions 

 used by me, have led that gentleman to underrate the comparative 

 duty of the Lancashire engine ; there are also errors in his statement 

 that go to the disparagement of the Cornish system, which I am sure 

 must be quite obvious to that gentleman, as well as the rest of your 

 readers, on the slightest reconsideration of the subject — I more par- 

 ticularly allude to the concluding portion of Mr. Enys' communication. 

 No guess work allowances are at all requisite either for "vacuum 

 imperfections" or engine friction and resistance in my estimate of the 

 Lancashire engine, as the load on the piston of 10 His. per circular 

 inch was not the calculated, but the obserred, steam pressure taken by 

 the indicator, as I distinctly stated, and it of course includes the fric- 

 tion of the engine, shafting, &c. The average steam pressure acting 

 on the piston of the pumping-engine, was, on the other hand, not ob- 

 served, but calculated to be 10-l)5 lbs. per circular inch, which would 

 be the difference of pressure between one side of the piston and the 

 other, due to the given load on the other end of the beam, including 

 of course a small allowance for the friction of tlie engine itself, as was 

 required to render it equivalent to the indicator pressure ; but no 

 allowance was required in this case, any more than in the othf r, fur 

 "vacuum imperfections." I purposely chose this method of avoiding 

 the risk of making erroneous deductions from what I think is properly 

 termed the "gross horse power," so that a more just comparison of 

 the two systems might be obtained. Possibly some allowance may be 

 required for pit-work friction, but as Mr. Enys seems to think that 

 nearly equivalent to deficient water delivery, the omission cannot 

 make much difference. 



From the corrected data now given by Mr. Wicksteed, it appears 

 that the load in the shaft, 66,443 Bs. must be reduced for the leverage 

 of the beam in the proportion of 10 ft. 3 in. to i1 ft., or to r)8,39S tbs. 

 and this sum, jilus an allowance for friction, is the gross load in the 

 cylinder, instead of 68,160 His., which I had before assumed from the 

 data then furnished to nie. The jiroper substitutions corresponding 

 to this correction being made in my tatile of " Comparative Duty," 

 it will be seen that the latter will be materially altered in favour of 

 the Lancashire system. 



For the purpose already stated, and also in order that a clear under- 

 standing of the meaning intended to be conveyed in future, when com- 

 paring the power or economy of steam engines, it seems necessary 

 that some teclmical terms commonly used by engineers and others 

 should be strictly defined. The following are dehnitions of such as 



are used in reference to the power of the factory or cotton mill engine ; 

 and I trust that some of our Cornish friends will favo\u- us with a simi- 

 lar elucidation of the equivalent terms that obtain in Cornwall, such 

 as " duty, efficiency, &c." 



The "nominal power" is what an engine is called by its maker, 

 and Mr. Watt's standard, it is w'ell known, was that due to an eli'ective 

 pressure of steam in the cylinder of 6 His. per circular inch, and a 

 speed of "220 ft. a minute for each horse power. The " gnms power" 

 is the total power exerted by the steam in the cylinder, including that 

 required to work the engine itself, or to overcome what are called the 

 friction ami resistances of the engine, and is ecjuivalent to the whole 

 force of the steam acting on the piston against a vacuum more or less 

 perfect; or, in other wiu'ds, it is the force resulting from the average 

 difference of pressure between one side of the piston and the other ; 

 this average is that obtained by the indicator, and it is in general 

 sufficiently correct for all practical purposes. The indicator pressure, 

 it will be observed requires no correction or allowance for what are 

 called vacuum imperfections, such allow'ance only being rei|uired 

 when, for want of iuilicalur experiinents, the steam pressure in the 

 cylinder can only be estimated from that in the boiler. The "cffe-clive 

 power" is the total power exerted l)y the engine, or delivered at the 

 crank shaft, after overcoming its own friction. This friction, of course, 

 not only incluiles the friction, properly so called, of the piston, pump 

 buckets, stulfing boxes, &:c., as well as all the bearing parts of the 

 engine, but it also includes the resistances due to the water lifted by 

 the engine pumps, and is a quantity that varies in dill'ercnt engines 

 according to the dilVerent degrees of excellence in their workmanshij), 

 situation, and other circumstances. In general it is found to be equal 

 to from one to two pounds per circular inch on the area of the piston 

 in the best mo<lern engines, but in a much less ratio in large engines 

 than small ones. When an engine can be unconnected with the sliaft- 

 ing, its own friction can be readily ascertained by the indicator; this, 

 however, would only be what M. Pambour properly calls the "unloaded 

 friction," fur, of course, the friction of nearly all the bearing parts of 

 the engine uuist increase with the load in some ratio corresponding to 

 the goodness of workmanship. This loaded friction is variously esti- 

 mated by different engineers, at from one fifth to three tenths of thi^_ 

 gross load ; and Tredgold estimates it at about -23^^ of tlu^ whole of 

 the force of the steam in the boiler, or with the resistance to the steam 

 in the passages, tlie loss of power by cooling, &c., included, he calls it 

 •368 of that force (see Tredgohl, new edition, page i;)6). Although 

 the unloaded fricti»ii of t\\e engine, when the speed of the latter 

 admits of being easily regulated, is capable of correct ascertainment, 

 as I have already stated, yet it is rarely so obtained in factory engines 

 separately from the friction of the shafting; but when so obtained and 

 deducted' from the gross power, the result gives a certain amount 

 larger than the real effective power, by so much as the loaded exceeds 

 the unloadetl friction of the engine. This result has been proposed 

 to Ijc termed the "effective indicated power." 



The "net effective pontr," or available power of an engine, is usually 

 understood to be the power delivered at the machine pulleys, or that 

 which is eifective or available in turning the machinery, exclusive of 

 that required to turn the shafting, the straps, and the loose pulleys- 

 The friction of the shafting, when ascertained by the indicator, (the 

 machine straps being thrown on the loose pulleys) is of course the un- 

 loaded friction, and as in the case of the unloadeil engine friction 

 when deducted from the effective power, leaves a result for net 

 effective power somewhat greater than the truth; this result, however, 

 has been proposed to be denominated the "net effective indicated 

 power." This last is what is meant when the number of hoises 

 power required to turn any given portion of uiachinery is said to be 

 ascertained by the indicator. It is ahvavs understooil to include so 

 much of the friction of the engine and shafting as is due to the in- 

 creased load, ami is comraonlv, and I think properly, termed the 

 " indicated horse power of the metchinery:' It is also sometimes called 

 " available " power, but evidently without due consideration, that term 

 being only strictly applicable wli'eu used to signify the net effective 

 power, and which may be ascertained in many cases independent ol 

 indicator experiments. 



Should it meet with your approbation, I shall be glad to furnish you 

 with practical illustrations of the above remai>ksby indicatcn-, diagrams, 

 and calculations taken from engines now at work in this county, pre- 

 vious to going farther into the consideration of the question of thR 

 economy of the Cornish system. 



I am. Sir, 

 Manchester, Your obedient servant, 



Fti. nth, 1840 R. Armstrong. 



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