38 AIR COMPRESSORS 



With 200 Ib. steam pressure, full throttle, and working against 

 a main-reservoir pressure of 130 Ib., the piston cycles will be 

 about 65 per min.; when working against 70 Ib. main-reservoir 

 pressure the piston cycles will be about 81 per min. With 

 200 Ib. steam pressure and pumping main-reservoir pressure 

 from 30 to 70 Ib., the piston makes about 82 f cycles per min. 

 Thus, the possibility of racing is eliminated. 



10i-IN. f CROSS-COMPOUND, AIR COMPRESSORS 



The lOs-in., cross-compound, air compressor, shown in 

 Fig. 5, was designed for industrial service and not for train 

 air-brake service. It was designed for 100 Ib. steam pressure 

 working against 80 Ib. air pressure. Under these conditions, 

 its normal speed is 131 single strokes per min. and its displace- 

 ment is 150 cu. ft. per min. It will operate satisfactorily on 

 steam pressures ranging from 80 to 160 Ib., the air pressures 

 ranging from 40 to over 150 Ib. depending on the steam pres- 

 sure. The rated boiler power for operating the compressor is 

 25 H. P. The construction of the 10^-in. compressor is similar 

 to that of the Sf-m., and the operation of the two is the same. 

 The lift of the air valves is: Intermediate, i-in.; suction, 

 &-in.; discharge, &-in. 



MAINTENANCE OF SIMPLE AIR COM- 

 PRESSORS 



CAUSES OF PUMP FAILURES 



Speed of Compressor. The heating of the air cylinder is a 

 feature of air compression that cannot be prevented. As an 

 example of the normal heating resulting from extreme duty, a 

 9^-in. compressor in good order that for 1 hr. maintained an 

 average speed of 174 single strokes or exhausts per min., work- 

 ing constantly against 100 Ib. of air pressure, was discharging 

 the air at a temperature of 408 F. Higher speed or greater 

 air pressure would have increased the heating, while slower 

 speed, shorter time of test, or lower air pressure would have 

 decreased it. 



Speaking generally, the speed should not exceed 140 ex- 

 hausts per min. and such a speed should not be continuously 



