METHODS TO CYLINDRICAL RETURN TUBULAR BOILERS. 167 



The designs referred to were limited to boilers with separate combustion cham- 

 bers for forced draft, and boilers with common combustion chambers for natural 

 draft, all with three or four furnaces, but did not cover two-furnace boilers. 



In the accompanying diagrams, boilers with separate combustion chambers for 

 natural draft have also been included, and the diagrams for boilers with common 

 combustion chambers have been extended to include two-furnace boilers. 



While some two-furnace boilers are fitted with separate combustion chambers, 

 these boilers generally are small, as in the case of small vessels for coastwise or 

 harbor service or of donkey boilers, and it would seem that a common combustion 

 chamber type would be equally acceptable in such cases and would be considerably 

 cheaper to construct. 



FURNACES. 



The diameter of furnace that can be used conveniently in a boiler of a given 

 size is a question that may be open to considerable discussion. It is, however, one 

 of the first and most important features of design, since a furnace too large will 

 result in either unduly crowding the tubes and reducing the steam space avail- 

 able or having an insufficient amount of heating surface and area through tubes, 

 whereas a furnace too small in diameter will result in poor combustion and insuffi- 

 cient grate surface. 



In determining upon a standard design, therefore, the diameter of the furnace 

 should be a prime factor, and Plate 57 gives furnace diameters which not only are in 

 accord with average practice but which increase systematically in proportion to the 

 diameter of the boiler. 



Attention is again drawn to the fact that there seems to be considerable dif- 

 ference of opinion on this subject, some builders using smaller, others larger fur- 

 naces, but the dimensions given will ensure a well-balanced design when all the 

 other features have been taken into account, such as the ratios between heating 

 and grate surface and between area through tubes and grate surface. 



It should be remembered that in Plate 57 the diameters of furnaces given are 

 the extreme outside diameters and not the efifective diameters. 



TUBES. 



While there are many dififei-ent sizes of tubes used the most common practice 

 seems to be to use 3-inch O. D. tubes for natural and 2 ^/^ -inch O. D. tubes for ■ 

 forced draft. 



It is evident that by the use of smaller tubes the heating surface in a boiler can 

 be increased, but there seems to be no really sound reason why in a series of stand- 

 ard designs of boilers the above diameters could not be adhered to. 



Plate 58 gives the number of tubes for each size of boiler. 



At this point it is well to note that the number of tubes which can be placed in 

 boilers of a standard type and with standard characteristics will not give quite as 

 regular a curve as plotted on the above diagrams. 



