THE STEAM ENGINE 333 



373. Condensing Engines. Non-condensing or high- 

 pressure engines are less economical than condensing or low- 

 pressure engines, because they use much more steam. When 

 the waste steam is let out of the cylinder, the air rushes in and 

 takes its place. This air presses hard against the piston so 

 that it takes power to drive it down. 



After the steam is condensed in the condensing engine 

 there is a vacuum, or an empty space, on one side of the pis- 

 ton, so that but little fresh steam is necessary to drive it. 

 Thus the object of condensing is to do away with the back 

 pressure on the piston and thereby increase the mean effec- 

 tive pressure. There is a gain of 20 to 33 H % m economy, 

 depending on the size and type of engine. In small engines 

 the saving is not enough to be considered. 



Where fresh water is scarce, it is of great importance to 

 the marine engineer to condense the steam by leading it into 

 a condenser when it has finished moving the piston. In this 

 process the steam as it leaves the cylinder enters a condenser 

 and passes over a number of copper tubes, through which sea 

 water is circulated by means of a pump. The steam is thus 

 condensed into water and a vacuum is created. Since this 

 water is warm, it is pumped into a hot-water well, whence a 

 pipe leads it to a pump, which in turn carries it back to a 

 boiler. 



374. Installation of Pipes. In installing pipes and metal 

 fittings of all kinds it is absolutely necessary to make proper 

 provision for expansion. (See Chapter IX, " Heat and Ex- 

 pansion.") When steam is turned on the temperature is 

 raised and the pipes expand. Pieces of curved pipe called 

 bends are usually used to take up the expansion and prevent 

 the joints from leaking. When steam is suddenly admitted 



