SECT, in.] CONDENSATION OF STEAM. 145 



THE AREA OF CHIMNEYS FOR STEAM ENGINE BOILERS. 



274. Previous to giving particular rules for the area of chimneys, it may be 

 useful to remark, that a chimney may afterwards be convenient, if considerably 

 larger than is necessary for the use of the engine it is erected for, while the expense 

 bears a small ratio to the increase of size. Hence I would recommend that one 

 double the size of that given by the rule should be built, for the rules apply only to 

 one for the actual power of the engine. 



The height should not be less than about 50 feet, and should be higher if it be 

 desirable to avoid the nuisance of smoke in the immediate neighbourhood of the 

 chimney ; for though by increasing the height of the chimney there is no dimi- 

 nution of smoke, yet it is spread so as to fall over a large surface. 1 



275. RULE. The area of a chimney for a low pressure steam engine, when 

 above 10 horse power, should be 112 times the horse power of the engine, divided 

 by the square root of the height of the chimney. 



For less than 10 horse power, it should be 90 multiplied by the number opposite 

 the horse power in the first column of the table, (art. 221.) instead of 112. 



Example. Required the area of a chimney for an engine of 40 horse power, 

 the height of it being 70 feet. 



In this case 



40 x 112 4480 



- -g^- = 533-2 square inches. 



The square root of this is 23 inches, which will be the side of a square chimney. 

 Or multiply 533 by 1*27 and extract the square root for the diameter of a 

 circular one. 



But in either case I would advise to build a chimney of double the area, or 

 1066'4 area, that is, to make the side of the square 33 inches. 



In this rule it is supposed that the engine is done in the best manner, and 

 worked with the best coals, that is, one requiring only from 9 Ibs. to 11 Ibs. 



1 It is a curious circumstance, that when high pressure steam and smoke ascend in the same 

 chimney, the smoke becomes nearly invisible. It seems to have been first observed in Trevithick's 

 engine, when applied to a steam carriage in 1805 ; and was communicated to Nicholson's Journal 

 (vol. xii. p. 1.) by Mr. Gilbert, who offers no explanation, but states that the admission of the 

 steam into the chimney improved tha draught. Nicholson made an experiment which accounts 

 for the vapour becoming invisible, through the heat of the smoke preventing that degree of con- 

 densation which is essential to its being seen. (Journal, vol. xii. p. 47.) The disappearance of the 

 smoke is not accounted for ; but I think it seems to be deposited in consequence of the density 

 being diminished, by intermixture with steam, till it becomes incapable of suspending the particles 

 of sooty matter. 



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