THE CORNISH BOILER.] 



APPLIED MECHANICS. 



849 



in the front portion of the tube ; and the products o 

 combustion pass along the tube, spread at the end ] 



Fig. 151. 



into the two side flues F F 

 descend at G G to the 

 bottom flue B, and pas; 

 thence to the chimney. The 

 quantity of flue-surface in a 

 boiler of this kind exceed, 

 that in the simple cylindrica 

 boiler by nearly the intern a 

 surface of the tube. But a 

 the hot products of com 

 bustion pass chiefly along 

 the upper side of the tube 



TKAN0VKRSE SECTION. 



and leave its lower side generally covered with a coat of 

 non-conducting soot and ashes, we cannot safely reckon 

 much more than half the surface of the tube as effective 

 heating surface ; that is, 1J time the tube's diameter 

 multiplied by ita length. 



In order, then, to estimate the power of a Cornish 

 boiler, we should calculate the external surface as before, 

 and add to it the product of li time the diameter of the 

 tube by the length for the total effective surface ; allow- 

 ing 10 square feet for every horse-power. 



Or, more simply : To the external diameter add the 

 diameter of the tube (in feet), multiply by the length, 

 add one-half to the product, and divide by 10 for the 

 horse-power of the bofler. 



Example. A. Cornish boiler, 5 feet diameter, and 12 

 feet long, has a flue-tube 3 feet diameter : required its 

 power. 



Diameter of boiler 6 feet. 



Add diameter of tube . 3 ,, 



Multiply by length . . 

 Add one-half of % = 



i: 



96 square feet. 

 48 



Divide by ...... 10) 144 



Horse-power of boiler .... linearly. 



The converse operation for finding the dimensions of 

 the boiler when the power is given, would be : From 

 10 times the power subtract its Jrd part ; and the re- 

 mainder gives the product of the length by the sum of 

 the external and internal diameters. 



Exftm/ik. Thus, for a boiler 14J horse-power 

 Since 10 X 14J- ....... 145 



Subtract ^rd of 145 ...... 48 



VOL. I. 



97 nearly. 



Thus, 97 is the product of the length by the sum of the 

 external and internal diameters. 



In this case, again, we must consult the circumstances 

 of position for determining the suitable length and 

 diameters. The diameter of the tube should not greatly 

 exceed half that of the boiler, because there should be 

 an ample covering of water over all the heated surfaces. 

 And again, it should not be less than 1 or 2 feet, because 

 it must admit sufficient area of fire-grate without exces- 

 sive length. The fire-grate in the case given, reckoning 

 Jths of a square foot per horse-power, should be about 

 11 square feet ; and as a length of fire exceeding 5 or 6 

 feet would become inconvenient, we must take it at least 

 2 feet in breadth that is to say, the diameter of the 

 tube must be 2 feet. The diameter of the boiler might 

 then be 4 feet, and the length would be for these 



97 

 diameters 2.4.4 = 16 ^ ee *- Were we to take the diameters 



as 3 feet for the tube and 5 feet for the boiler, the length 



would be 



97 

 3 + 5 



= 12 feet. 



Fig. 153. 



We believe it will be found practically advantageous 

 to make the length a little more than three times the 

 diameter of the boiler, the diameter of the tube being 

 rather more than half that of the boiler. For 14 horse- 

 power, according to this proportion, we should have 

 Diameter of boiler .... 4 feet 6 inches 

 Diameter of tube .... 2 6 ,, 

 Length of boiler . . . . 14 ,, 

 In cases where the dimensions of the boiler are con- 

 siderable, two or more 

 tubes are introduced, as in 

 Fig. 152. The tubes are 

 always placed as low as 

 possible, allowing 4 to 6 

 inches between them and 

 the outer casing, in order 

 that their upper and hottest 

 surfaces maybe well covered 

 with water, without inter- 

 fering inconveniently with 

 the steam space above. 



MARINE AND LOCO- 

 MOTIVE BOILERS. 

 For marine steam-boilers, where brick-work setting would 

 10 inconvenient, it is usual to arrange the whole heating- 

 surface within the boiler, by means of flues pervading it 

 D all directions (Fig 1 53). The water is thus divided 

 into sheets about 6 inches thick ; heated on one or both 

 aides by the products of combustion as they pass along 



Tig. 153. 



he flues. In all such boilers a heating-surface of at 

 east 10 square feet per horse-power, and fire-grate from 

 to Jths of a square foot per horse-power, should be 

 rovided. Nor should the flues be too small in sectional 

 rea, nor too much broken up or prolonged, lest the 

 raught or rapidity of movement of air, and consequently 

 of combustion, be interfered with. Of late years, tubular 

 boilers have been very extensively adopted, both for 

 marine and for land engines. These were first employed 

 principally in locomotive engines, as they afforded a 

 means of securing very large flue-surface within a limited 



5q 



