584 SECTIONAL TRANSACTIONS,— G. 



Installations are now in operation on land with pressures exceeding 1,250 lbs. per 

 square inch, and it has been estimated that, taking as the ' cycle limit ' initial condi- 

 tions of 1,250 lbs. per square inch and 900° F., with two reheating and eight feed-heating 

 stages, the consumption of about .37 lbs. of oil or .56 lbs. of coal per horse-power hour 

 may be expected. Although such low consumption figures are not yet commercially 

 practicable, the time may not be far distant when these high efficiencies are realised. 



Boilers of very large capacity with large furnace volumes are now constructed. 

 The tendency is to reduce the steam generating surface, increasing the rating of the 

 boilers but recovering more heat from the waste gases by other means, such as air 

 pre-heating, thus reducing the initial cost of the whole unit. Although the boiler 

 surface as a whole is reduced the surface exposed to direct radiation is not reduced, 

 and particularly is this the case where pulverised fuel is adopted, the addition of 

 water-cooled walls adding still further to the surface exposed to radiation. 



When burning pulverised fuel or oil there appears to be no limit to the temperature 

 to which the air may be heated. With mechanical grates, unless special heat-resisting 

 materials are used, the air temperature is, however, limited to about 400° F. if excessive 

 repairs are to be avoided. 



Rapid circulation is a necessity in high-pressure boilers, so that the steam bubbles 

 are swept away as soon as they are formed, and every encouragement should be given 

 to the water to circulate freely, so that tubes well inclined and as straight as possible 

 are preferable. It is essential, however, to ensure that pure feed water only is used 

 in high-pressure boilers, otherwise the formation of scale may occur and lead to 

 overheating. 



As boiler pressures increase it becomes more important to reduce riveted and 



bolted joints. Hollow-forged drums are now obtainable at a reasonable price both 



for the steam drum as well as the water collectors. These drums can be made with 



riveted ends or, for very high pressures, the drums are forged with closed-in ends, 



n which case rivets are entirely eliminated. 



In the design of superheaters for higher temperatures the creep of materials, which 

 has in recent years received considerable attention, has a very important bearing. 

 From ' creep ' consideration it would appear that steam temperatures of 750° to 800° F. 

 are approaching the dangerous Limit if ordinary carbon steel is used. Alloy steels 

 will stand higher temperatures and such materials will, no doubt, be more extensively 

 used as steam temperatures increase. 



The utilisation of the coal resources of this country is of such vital importance 

 that any development in the direction of improving the efficiency and economy of 

 steam-generating plants will be followed with much interest. While in electric power 

 stations coal is more or less universally adopted, in the mercantile marine the use of 

 oil in Diesel engines is a serious competitor. It is significant to note, however, that 

 the recent development in high-pressure boilers and steam turbines has resulted in 

 steam machinery in combination with coal being selected by certain shipowners Ln 

 preference to oil engines. 



Mr. W. J. Keabton. — An Investigation into the Throat Conditions during 

 the Adiahatic Flow of Mercury Vapour through Nozzles, within a 

 Unique Range of Initial Superheats. 



The paper deals chiefly with the adiabatic flow, through nozzles, of meroury vapour 

 in thermal equihbrium. An expression is deduced giving the ratio of throat pressure 

 to initial pressure for expansions in which initially superheated vapour becomes dry 

 and saturated before arriving at the throat of the nozzle. The actual analysis shows 

 that this ratio is sensibly constant. The limit of appUcation of this expression is 

 reached when the initial superheat is such that the vapour is just dry and saturated 

 at the throat. 



Considering expansions in which, owing to moderately high initial superheats, 

 the vapour may still be superheated after the nozzle throat is passed, it is shown that 

 there is a limit of application, for this type of expansion, corresponding to a certain 

 initial superheat, where once again the vapour at the throat is just dry and saturated. 

 Between the limits mentioned above there lies a range of initial superheats for which 

 none of the existing analytical methods can readily be applied. 



This range is investigated, and it is shown that the time-rate of mass flow per 

 unit area of nozzle section is not a continuous function of the pressure ratio of 

 expansion. Within the range the maximum rate of flow corresponds to the point 



