SUPERHEATED STEAM IN MARINE PRACTICE. 183 



45. Piston Rings (cylinder packing) should be of strong, hard cast iron. 

 There are several designs of piston packing rings on the market which are re- 

 ported as giving good satisfaction on marine engines using 200° of superheat. 

 These designs are also applicable for valve rings. 



46. Rod Packings, for high-pressure piston and valves, as now arranged are 

 durable and satisfactory. Trouble was hitherto experienced in obtaining the 

 proper designs of rod-packing, but this is no longer the case. It is the author's 

 understanding that there is a growing tendency towards the use of soft cast iron 

 instead of white metal in these rod packings, and it is claimed to meet all the re- 

 quirements. If such is the case our ideas of this detail may have to be revised. 



47. Cylinder ratios to be suitable for use with highly superheated steam re- 

 quire consideration, on account of the steam characteristics. The published dimen- 

 sions of English and German superheated steamships show that a lower cylinder 

 ratio is used. This is generally accomplished by increasing the diameter of the 

 high-pressure cylinder, the intermediate and the low-pressure diameters, on a triple 

 job, remaining as before. In quadruple engines, the high and first intermediate- 

 pressure cylinders are increased in diameter, while no change is made in the sec- 

 ond intermediate and low-pressure cylinders. In converting saturated engines to 

 the use of superheated steam, an increase in the high-pressure cylinder diameter is 

 not always advisable, and many times not possible. In such cases, if the same indi- 

 cated horse-power is to be obtained, it is necessary to increase the cut-ofif in the 

 high-pressure cylinder, thus giving an increased volume of steam at cut-ofif, and, 

 in efifect, decreasing the cylinder ratio. If the volume at cut-ofif is not increased the 

 indicated horse-power will be slightly decreased, assuming, of course, that the 

 boiler pressure is not changed. To explain the necessity for these modifications, 

 it will only be necessary to point out that the expansion curve of superheated steam 

 falls a little more rapidly than saturated steam, and, to make up for this, the volume 

 of steam at cut-ofif, with say 200° of superheat, should be about 10 per cent greater 

 than the volume with saturated steam in order to get equal power. This does 

 not mean, of course, that the same weight of steam is used per stroke after super- 

 heating. It merely means that a 10 per cent greater volume of steam, which weighs 

 about 30 per cent less per unit of volume than saturated steam, is used. These figures 

 will indicate that there is still a considerable margin for steam economy, although 

 a greater volume of steam per stroke is taken. 



ENGINE OPERATION WITH SUPERHEATED STEAM. 



48. The operation of engines using superheated steam is but very little dif- 

 ferent from saturated steam operation. The two chief points under this topic are 

 as follows : — 



1st. Starting up. 

 2d. Lubrication. 



