Treatise on the steam engine by Renwick James 1790?-1863

Author:Renwick, James, 1790?-1863
Language: eng
Format: epub, pdf
Tags: Steam-engines
Publisher: New-York, Carvill & Co.
Published: 1839-03-25T05:00:00+00:00

we have assumed in Chap. VI. In addition then, to the estimate in horse powers, which has now become of no other use than a mode of describing the size of an engine, in contracts between the maker and purchaser, it has become customary to compare the work of engines with each other, by a mode of estimate which is called their Duty. The mode in which the duty of a steam engine is estimated is in the numbers of pounds which can be raised 1 foot high by the combustion of a single bushel of coals. We have seen that this quantity of coal is capable oi evaporating 12 cubic feet of water, and therefore of keeping an engine of twelve horsepower in action ior an hour. It ought, therefore, according to the estimate we have just made, raise to a height of 1 foot

24000 X60 X 12=17,2800001bs. or upwards of seventeen millions of pounds. Watt and Boul-ton constructed an engine, whose duty reached as high as 19 millions; and it was said that their own engine at Soho, did work equivalent to a duty of 21,6000001bs.; but, on an examination, in legal form, of all the engines they had put up in Cornwall, two years before the expiration of their patent, it was found that the average duty was no more than 17 millions, or in strict conformity with our estimate. Many of these engines acted expansively ; and one performed a duty of 27 millions, in spite of which the average fell to the limit we have stated. The expiration of Watts' patent left engineers free to make such improvments as experience or science might suggest. The expansive action of steam was the improvement which was principally relied upon; and, in order to obtain from it the greatest practicable advantage, for the old boilers of Watt, such as are figured on PI. I. were gradually substituted cylindric boilers capable of bearing steam of great tension. In this way the force of the steam has been gradually raised from little more than a single atmosphere to 10, and an intelligent Cornish engineer states that he has seen it raised as high as 20 or 30 atmospheres. In this way the average duty has been regularly on the increase, being in 1833, 19| millions ; in 1814, 20^- millions ; in 1815. the same ; in 1816, nearly 23 millions ; in 1817, 26^- millions ; in 1818, 25^- millions; in 1819, 26| millions; in 1820, 28f

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