64 AIR COMPRESSORS 



compressors, and the intermediate-reservoir capacity are so 

 proportioned as to divide the work of compression about 

 equally between the two compressors, thus obtaining the most 

 economical condition. To obtain the final pressure and free- 

 air capacity desired, it is important that the steam pressures 

 and approximate speeds be maintained as nearly as possible 

 to the values given in the accompanying table. This method 

 of compression is called the series- compound method. In this 

 method, the steam throttle of each compressor should be regu- 

 lated so as to cause the relative speeds to give the required 

 intermediate-reservoir pressure; otherwise the final delivery 

 pressure will not correspond to that given in the table. 



PISTONS AND RINGS FOR REBORED AIR- 

 COMPRESSOR CYLINDERS 



To facilitate repair work in railroad shops, reduce to a mini- 

 mum the number of pieces necessary to carry in stock, and 

 greatly simplify the ordering of repair parts, the Westinghouse 

 Air-Brake Company has adopted standard repair pistons and 

 rings for both air and steam cylinders as well as main-valve 

 bushings worn in service, when rebored to certain dimensions. 

 It recommends reboring steam and air cylinders in steps of 

 l"ff-in. and main-valve bushings in steps of aV-in. In no case, 

 however, should the maximum cylinder diameters specified in 

 the accompanying tables be exceeded. Also, for both new and 

 worn pistons the company advises that packing rings of the 

 standard thickness, viz, the dimension of a ring fitting between 

 the sides of a new piston groove, be employed. The adoption 

 of packing rings of standard thickness is a decision that has 

 been brought about by extended experience and experiments, 

 which demonstrated that the wear of the piston groove is 

 practically negligible except in case of defective material or 

 lack of maintenance. A true bearing is an indispensable con- 

 dition for satisfactory and efficient air-compressor operation, 

 hence, since a true bearing is rarely if ever obtained by filing 

 a piston ring to fit a groove, the practice is not recommended. 



In order to meet all possible conditions and methods of 

 fitting rings both to standard and to recut grooves, the 



