WATER TUBE BOILERS. 



613 



boiler fail to attain the high efficiency which its inventor justly 

 anticipates, the accompanying trials show clearly that as it at 

 present exists it is a distinct step in advance of anything which 

 has yet been effected. In general design it follows the Heine 

 boiler, only that the tubes are placed at a considerable inclination 

 in the fire, and the front header is divided by the division plate. 

 The essential difference is not so much in this plate as in the fact 

 that the circulation is in a complete cycle through the tubes and 

 the two end headers or water jackets. In this connection it is 

 well to point out that in most other water tube boilers the steam 

 drum contains a large quantity of water which shares the circu- 

 lation, through the tubes being connected with both front and 

 back headers; in consequence there mvist be a considerable 

 impeding of the current through the tubes, as will' be readil)^ 

 understood by considering the very small head to cause flow 

 through these pipes from the variations in density of the water in 

 different parts of the water drum. 



In the Shann boiler the circulation is quite beyond all pre- 

 cedent, and this can be readily understood since the only 

 impediment to the free flowing of the water under the influence 

 of the steam which rapidly forms in every part of the tubes is the 

 vena contracta at the entrance to the tubes. The arrangement of 

 the tubes is such that the hottest part of the fire receives the down 

 draught of the water, which returns through the upper tubes after 

 it has delivered its steam at the division plate ; this will be clearly 

 seen by the accompanying diagram (Plate XVIII.). 



APPENDIX No. 'i. 



Table nhniviiig Dimensions of Boilers to accompany Record of their Performances. 



* Grate area was varied in trial No. 4. 

 t A large quantity of cold air was admitted above the tire in all these trials. This circum- 

 stance must have injured the results considerably, but owing to the nature of the fuel it 

 -seemed unavoidable. 



