APPLIED MECHANICS. 



[STEAM BOILERS. 



ill -adapted for the purpose of a boiler where a large 

 amount of heating surface is important. Next to the 

 spherical in point of strength, and superior to it in re- 

 pect of superficial area, ui the cylindrical form, with 

 spherical or rounded ends (Fig. 14'J) ; and acoord- 

 . this form is very generally adopted for steam- 

 boiler*. 



In onler to render available as much as possible of 

 the surface for receding the heat, the heated products of 

 combustion are not permitted to escape directly from the 

 fir* into the chimney, but are carried round the boiler 

 by flues generally in the manner indicated in Fig. 160. 



That them should be some relation between those quan- 

 tities will be evident from the following considerations. 

 If the quantity of fuel consumed be very great while 

 the flue-surface is small, the products of comhii 

 will not have sufficient opportunity for parting with 

 their heat, and will, therefore, carry up the chimney, 

 and waste, a large amount of beating power, which, by 

 better arrangements, might be made to tell upon the 

 water. If, on the other hand, the fire-grate be too 

 small, while the fluo-Hurface is largo, the products of 

 combustion will have parted with their heat before 

 rc.-u'hiiig the chimney a large portion of the flue-surface 

 will thus be rendered useless, the draught (caused by the 

 ascent of heated air in the chimney) will be sluggish, 

 and the combustion slow. We believe that, practically, 

 it will be found advantageous to adopt the following 

 rules as to size of fire-grate and quantity of flue-surface 

 in a boiler, such as we have described the cylindrical, 

 egg-ended boiler. 



To the diameter of the boiler, multiplied by its length, 

 add one- half : the result may be taken as the Hue-surface ; 

 and this product (in square feet) should be 10 times the 

 horse-power. Thus in a boiler 4 feet inches diameter, 

 and 16 feet long : 



Since 15 feet X 4i feet 

 Add one-half . 



07 i square feet 

 33J 



The boiler is placed on two banks of brick-work A A, 

 between which are fixed the fire-bars B, so that the flame 

 may play on the bottom of the boiler. The products of 

 c-i,iii)iution, heated to a high temperature, pass along 

 i !,!. r tin- bottom of the boiler, upwards at the far end 

 ('. thence alone the side flues DID formed by brick-work, 

 till they finally proceed by any convenient flue or 

 channel E to the chimney. By this arrangement a large 

 portion of the heat contained in the products of com- 

 mution is absorbed during their passage along the bottom 

 and tide surface* of the boiler, and is given to the water 

 contained in it ; while the brick-work, being a very im- 

 perfect conductor of heat, permit* very little to escape 

 ineffectively. The power of a boiler arranged in this 

 manner depends upon the extent of fire-grate, or quan- 

 tity of combustible matt* consumed in a given time, 

 and the superficial area^f boiler on which the product* 

 of combustion play in their course towards the chimney. 



The flue-surface may be taken at 101 J- 



equivalent to 10 horse-power. Again, for every 

 horse-power there should be jths of a square foot 

 of fire-grate. For 10 horse-power there should 

 therefore be 7i square feet of fire-grate, a surface 

 that might be made up by taking the length of 

 the fire, 3 feet 9 inches, and the width 2 feet, 

 since 3 feet 9 inches X 2 feet = 7$ square feet. 



The converse rule for finding the dimensions of a 

 boiler suitable to a given power is the following : 

 From 10 times the horse-power subtract its Ird 

 part, and the result will be the product of the dia- 

 meter by the length. Thus, to make a boiler of 10 

 horse-power : 



Since 10 X 10 = 100 



Subtract Jrd = 33J 



The product of diameter X length = . 66 

 We are at liberty to take any convenient diame- 

 ter and length that might make up this product 

 within proper limits. Thus, making the 



ft. ft, in. 



Diam. 3, the length must be 223, product 6CJ sq. ft 

 4, 168, CGJ 

 5, 134, 66J 



6, 112, 67 

 7, 90, 66J 



Any of these dimensions may be chosen accord- 

 ing to the particular circumstances of the case. 

 Were we to take a diameter smaller than 3 feet, 

 " with a greater length than 22 feet, or a diameter 

 greater than 7 feet, with a lei 'han 9 feet 



6 inches, we should find practical difficulties in fixing 

 and working, and lose useful effect from the extreme 

 lengthening or shortening of the flues. 



CORNISH BOILER. When it is desired to obtain 

 greater heating surface within a smaller space, it is found 

 very advantageous to construct the boiler of cylindrical 

 form, with a cylindrical flue or tube passing through the 

 water ; so that not only may the exterior surface exposed 

 to the flues receive heat from the products of conilmx- 

 tion, but also the interior surface of the tube. When 

 this tube is made of sufficient size to admit the fire with- 

 in it, as in Fig. 151, the boiler is called a Cornish boiler, 

 from the circumstance of its being first extensively 

 applied with excellent effect in Cornwall. The arrange- 

 ment of flues for a Cornish boiler is generally similar to 

 that represented in the figure. The boiler rests on two 

 banks of brick-work A A, with a space for the bottom 

 flue B, left between them. The fire-grate is fixed at C, 



