357 



Diesel engines. In Germany alone, the output during 1912 was estimated to be 450,000 

 tons, the average price 453. per ton. 3,000 new coke-ovens were installed in 1912, and for 

 1915 in Germany alone the estimated output of creosote oil is 900,000 tons. These figures, 

 large as they seem, are small in comparison with the total amount of coal now used for 

 steamship propulsion. The evolution of a successful internal-combustion engine using coal 

 power as fuel may be another great step in the forward progress of power production. 



(ARCHIBALD SHARP.) 



STEAM BOILERS 1 



There has been very little change in 1909-13 in the design or construction of the vari- 

 ous types of steam boilers used. Modifications have been due either to the use of 

 somewhat higher steam pressures or to the more general application of superheating. 



The increase of steam pressures is more noticeable in marine practice than in other 

 cases. At the present time many marine boilers are being made to work at pressures of 

 from 215 Ibs. 10,225 Ibs. per square inch, supplying steam to quadruple expansion en- 

 gines. Where triple expansion engines are used the most common pressure is 1 80 Ibs. 

 per square inch. The strength necessary for the extra pressure is generally obtained 

 by using thicker plates for the shells and more closely pitched stays for the combustion 

 chambers. In numerous boilers for large ocean liners, where the question of weight 

 to be carried assumes an exceptional importance, steel plates having a tensile strength 

 of from. 3 5 to 38 tons per sq. in. have been used for the cylindrical shells, wherqas that 

 of ordinary boiler shell plates is from 28 to 32 tons per sq. in. 



An improvement has been introduced by one firm of steel plate makers, who now roll 

 long plates having two different thicknesses, the two end portions being thinner than the 

 part near the middle of the length. The variation of thicknesses made gradual over a 

 space of about six inches. The thicker portion can be arranged in position and in di- 

 mensions with precision. Such plates can be made in widths up to, but not exceeding 

 seven feet. They are used for the wrapper plates of the combustion chambers of marine 

 boilers, the thin parts being employed for the sides of the chambers, which are supported 

 by numerous screwed stays, whilst the thick central portions form the bottoms of the 

 chambers which are unstayed. 



The use of superheated steam has become more frequent in marine and stationary 

 work, but more especially so in locomotive boilers. In practically all cases the super- 

 heaters are formed of pairs of headers made of cast steel, connected together by numerous 

 steel tubes which form the superheating surfaces. One header of each pair is connected 

 by a steam pipe with the stop valve on the boiler, and when in use the saturated steam 

 from the boiler enters this header, then traverses the small superheating tubes and passes 

 into the other, whence it is conducted through suitable steam pipes to the engine. 



The steam pipe arrangements are sometimes made so that all the steam must pass 

 through the superheaters; in other designs regulating valves are provided which permit 

 of any desired proportion of the steam passing through the superheater, the remainder 

 being mixed with the superheated steam before passing to the engine. This plan per- 

 mits of the temperature of the steam entering the cylinders being regulated through a 

 certain range so that the temperature which experience shows to be most advantageous 

 can be used. About 600 F. is generally considered to give the best result. 



In locomotives the regulation of the temperature of the steam is effected by more or 

 less throttling the passage of the heated furnace gases in their passage across the super- 

 heating tubes. It is customary to use only iron or steel for the superheated steam 

 pipes to the exclusion of copper. It is also necessary to fit a safety valve to the super- 

 heater if the arrangement permits of its being shut off from the boiler at any time. 



In the E. B. article, figures n and 12, superheaters are shown of the types used and 

 in the positions adopted with stationary boilers of the Babcock and Wilcox and of the 

 Stirling types. In some marine boilers the superheaters are fitted in the uptakes, where 

 they are exposed to the heat of the products of combustion after the gases have left the 

 ordinary boiler tubes. In such cases as the temperatures to which the superheating 

 surfaces are exposed are not very high only a low degree of superheat is possible. The 



1 See E. B. iv, 141 el seq. 



