32 AIR COMPRESSORS 



admission to, and exhaust from, both the high- and low-pressure 

 steam cylinders. The low-pressure steam and high-pressure 

 air pistons are connected by a solid piston rod, having no 

 mechanical connection with the valve gear, being simply 

 floating pistons. 



The valve gear and its operation are essentially the same as 

 those of the 9^-and 11-in. pumps. The reversing valve performs 

 the same duties and is operated by the re versing- valve rod in 

 the same manner as that of the 9 5- and 11-in. pumps. 



The main slide valve is provided with the usual exhaust 

 cavity and four elongated steam ports in its face. The two 

 outer and one of the intermediate ports communicate with 

 two cored passages extending longitudinally in the valve and 

 serve to make the proper connection between the high- and 

 low-pressure cylinders during the expansion of steam from one 

 to the other. The remaining port controls the admission of 

 steam to the high-pressure cylinder. The cavity governs the 

 exhaust from the low-pressure cylinder to the atmosphere. 



The valve seat has five ports. Of these the two back ones 

 lead to the bottom and top ends, respectively, of the high- 

 pressure cylinder; the first and third ports to top and bottom 

 ends of the low-pressure cylinder and the second port to the 

 exhaust. 



The steam cylinders are compounded. Steam from the 

 boiler is admitted into the high-pressure cylinder. After doing 

 its work, it is delivered to the low-pressure cylinder where it 

 is expanded again and is then exhausted to the atmosphere. 

 The air cylinders, also, are compounded. Free air is taken 

 out the larger air cylinder and by compression is forced into 

 the smaller cylinder where it is compressed to main-reservoir 

 pressure and forced into the reservoir. 



The compressor is designed for 200 Ib. steam pressure, 

 working against 140 Ib. air pressure. Its normal speed under 

 those conditions is 131 single strokes per min., and its displace- 

 ment is 150 cu. ft. per min. The weight of the pump is 1 ,500 Ib. ; 

 the lift of all air valves is A -in. 



The piece number of the 8$-in., cross-compound air com- 

 pressor is 23,736; the number of the various parts are given in 

 the accompanying list. 



