AIR COMPRESSORS 55 



The second chart shows that at 90 Ib. constant air 

 pressure and 120 single strokes per min. the steam pressure 

 required is 98 Ib. The actual amount of free air delivered is 

 108 cu. ft. per min. The efficiency is 80%. Steam consump- 

 tion is 19 Ib. per 100 cu. ft. of free air and the displacement is 

 138 cu. ft. per min. 



OPERATING COMPRESSORS IN SERIES 



In industrial service, to deliver air pressures between 150 and 

 300 Ib., two compressors are often operated in series, as here 

 shown. The left-hand, or low-pressure compressor discharges 

 through a radiating coil into a long, narrow, intermediate reser- 

 voir, which has the greatest possible cooling surface. The 

 right-hand or high-pressure compressor receives its air supply 

 from this reservoir and discharges through a second radiating 

 coil into the air-storage reservoir. The governor is connected 

 to the steam-admission pipe to the high-pressure compressor 

 and is actuated by the pressure in the air-storage reservoir. 

 When the pressure in the reservoir reaches normal, the gov- 

 ernor stops the high-pressure compressor, and the low-pressure 

 compressor is in turn stopped by the intermediate-reservoir 

 pressure when that pressure becomes high enough to stop the 

 compressor. Each radiating coil should contain at least 25 ft. 

 of cooling pipe. The steam throttle valves of each compressor 

 should be regulated so as to cause the relative speeds to 

 give the required intermediate-reservoir pressure; otherwise the 

 final delivery pressure will not correspond to that in the accom- 

 panying table. 



All sizes of 8-in. compressors given in the accompanying 

 table refer to the new pattern compressor. 



OPERATING COMPRESSORS SERIES-COMPOUND 



When the question of economy in steam consumption is 

 important, two compressors of different sizes may be connected 

 in series, not to obtain a high -delivery air pressure, but to 

 increase the quantity of air compressed at the ordinary pres- 

 sure per unit of steam used. The low-pressure pump has an 

 air cylinder of large diameter and the pumps are connected as 

 before. The size of the air cylinders, the speeds of both 



