WATER TUBE BOILERS. 615 



question is the economy in fuel. The trials show that the Shann 

 boiler is as efficient in this respect as any of its size can be expected, 

 namely, 88 per cent, of Rankine's value for furnace efficiency. 



There are many other questions which come into prominence 

 under certain conditions, such as the value of having; a boiler which 

 will give a good efficiency under continuous steaming for a long 

 period of time. To ensure this condition the Shann boiler has 

 been so constructed that it can be readily cleaned while under full 

 steam. In this respect it is much superior to the existing water 

 tube boilers. 



The main problem of economy can be expressed roughly by the 

 equation. 



Comparative cost = P + 10 M + 20 C. 



Where P = Capital cost (first cost) of boiler. 



M = Annual cost of maintenance and depreciation on 



assumed life of boiler 

 C = Annual cost of fiiel, assuming interest at 5 per cent. 



One very important result of these trials has not yet been 

 mentioned, namely, the efficiency proved better in proportion as 

 the working pressure increased. The inference is that of all boilers 

 this is the best type for producing high pressure steam, and in 

 view of the extreme pressure of modern times, where it is 

 imperative to obtain the maximum amount of power with the 

 minimum expenditure, it is certain that the present low pressure 

 engines Avill soon be superseded ; in fact, in marine service they 

 have already been superseded by higher pressure of from 150lbs. 

 absolute. 



The writer understands that Mr. Shann has designed a larger 

 boiler with many improvements, among others a large heating 

 surface in proportion to grate area, and a mode of baffling the hot 

 gases between the furnace and the flue, both of which were proved 

 to be important matters by trials Nos. 2 and 3. He has carefully 

 avoided taking any interest in the design of these improvements 

 that he might have the more weight and liberty in expressing his 

 opinion on subsequent tests. 



How a boiler may be improved by baffle-plates is shown by the 

 following figures from Mr. Milton's paper, referred to above : — 



Lagrafel D'Allest Boiler with Baffle-plates. — Duration of trials, 

 6 hours ; weight of Cai-difF coal consumed, 1,008 K; evaporation, 

 10,760 litres : efficiency reduced to from and at 212'', 12'48. 



Lagrafel. — Duration of trials, 6 hours ; weight of Cardiff coal 

 consumed, 1,512 K; evaporation, 10,170 litres ; efficiency reduced 

 to from and at 212°, 7-78. 



