PROCEEDINGS OF THE POLYTECHNIC ASSOCIATION. 593 



P 

 the quantity of water discharged by it. la middle latitudes, with a mode- 

 rate temperature of the water used to condense the steam, the power 

 required to operate the air-pump is allowed to be ten per cent of the power 

 of the engine when attached to a jet condenser. 



Thus, it will be seen how much less power is necessary to operate the 

 air-pump when connected with a surface, than with a jet condenser. If 

 but one-half is saved, it is five per cent of the entire power of the engine. 



Estimate of the value of the gain referred to : 



Per cent. 



1st. As before stated, less power necessary to work the air pump 05 



2d. The heat lost in the hot water blown out to keep the boilers even 



partially clean 12 



3d. The heat lost by incrustation on the fire surfaces, in some cases 



more than twenty per cent 05 



Total 22 



4th. A higher pressure of steam may be used and a greater benefit ob- 

 tained by expansion. 



Where the losses named are prevented or saved, less coal and boilers 

 (and less water in them) will exert the same power, requiring fewer fire- 

 men and coal trimmers, thereby saving in the daily expenses of the vessel 

 for coal, wages, and food. 



Vessels fitted with two engines have two air-pumps. When a surface 

 condenser is attached, one is sufficient as an air-pump, the other to make 

 the water flow through the tubes. This pump, as before stated, works 

 against a pressure of but one to two pounds per square inch, instead of 

 against a pressure of thirteen or fourteen pounds per square inch when 

 used with a jet condenser. The other pump, although used as an air-pump, 

 has less than one-fourth part of the labor it would have if used with a jet 

 condenser, having only to discharge the water contained in the steam. 



By this arrangement there is no extra cost for pumps, but only for the 

 tubes and tube sheets for the condenser proper; and as less boiler will 

 exert the same power, the saving in cost will partly pay for the extra cost 

 of the condense!". 



In ordinary business transactions twenty per cent is considered a fair 

 profit, but the owner of a stearuship iising salt water in the boilers pays 

 for and carries in his vessel one-fifth or twenty per cent more -boilers (and 

 the additional water contained therein) than is necessary. 



He also pays for each day the vessel is steaming for twenty per cent 

 more coal than is necessary to exert the same power. 



This extra coal is not only wasted, but it requires additional firemen and 

 coal trimmers to take it on board and get rid of it, beside taking up the 

 freight room, and making the vessel draw more water, thus lessening her 

 speed. 



There are boilers using fresh water that have been in constant opera- 

 tion from fifteen to twenty years, requiring but little repairs during that time. 



The difference between the condensers constructed by him and those of 

 Mr. Hall is, that in his the water was passed through the small tubes; 

 whereas in Hall's the steam was passed through tubes, and the water was 

 thrown around the outside of the tubes. This plan required a much 



[Am. Inst.] M* 



