FOR AUXILIARY PURPOSES ON BOARD SHIP. 161 



EFFICIENCY. 



As mentioned earlier in the paper regarding the poor unit economy of such 

 apparatus as the Arkansas circulating pump, it must be borne in mind that when 

 operating auxiliaries non-condensing and utilizing the exhaust steam either in 

 the feed-water heater or in the low-pressure main turbine, a big sacrifice can be 

 made in the auxiliary efficiency without materially affecting the over-all fuel con- 

 sumption of the whole ship. When a small turbine operates with an efficiency of 

 30 or 40 per cent you are only extracting this said 30 or 40 per cent from the 

 steam. The consequence is, the remaining heat — namely, 70 and 60 per cent re- 

 spectively — is not destroyed but can be utilized to good advantage elsewhere. 

 Curves (Plate 84) show the heat extracted per pound of steam by an auxiliary 

 turbine, and also show that a very big difference in steam consumption per horse- 

 power is permissible without materially affecting the efficiency of the whole cycle. 



On curve, Plate 85, is plotted the quantity of auxiliary steam used at various 

 speeds in percentage of the total. In the curve the 100 per cent speed represents 

 the maximum speed on trial trips. 



It was interesting to note that, after plotting the curve, the percentage of aux- 

 iliary steam could be closely approximated by the following formula: — 



^ , 100 — S 

 1300 -|- 



where A = percentage of auxiliary steam to total steam and 5" = percentage of 

 maximum speed of the boat. 



The following calculations will show the very small effect actually resulting 

 in a marked improvement in the steam consumption of a piece of auxiliary appa- 

 ratus. These calculations are not meant to be accepted as absolute figures, but 

 will tend to serve the purpose intended, namely, to show that the unit of economy 

 of the auxiliary apparatus is not of such vital importance as would be at first 

 imagined. 



It has been necessary at the outset to make certain assumptions, and the type 

 of apparatus used in the one instance is a direct-connected low-speed turbine circu- 

 lating pump, the second case being a gear-driven unit. It has also been assumed 

 that all the steam exhausting from the turbine can be returned to work in the feed- 

 water heater. This, of course, is subject to criticism, but, as mentioned above, the 

 figures are not intended to be absolute but merely an indication of the difference 

 that is to be expected between a high-efficiency and a low-efficiency unit. 



One very strong point that this question brings out is the fact that it is not 

 in the turbine itself where we must look for efficiency, but in the driven end of the 

 apparatus. Within reasonable limits the efficiency of the turbine matters little in 

 the over-all fuel cost, as the heat rejected can be turned to useful work, but in 

 the driven member anything below 100 per cent efficiency must be considered as a 

 dead loss. 



