SYNCHRONOUS GENERATORS 



347 



able to provide the required amount of air for the section in 

 question, the third being kept in reserve. Each fan inlet should 

 be equipped with a damper for controlling the air admission and 

 an automatic shut-off damper on the discharge so that when one 

 fan is shut down no leakage will occur through the fan from the 

 air chamber. The amount of air to each generator is regulated 

 by dampers in the ducts leading from the air chamber to the wheel 

 pits, and these dampers may be regulated from the generator floor. 

 The entrances to the wheel pits should be provided with air-tight 

 doors, and the pressure in the wheel pits should be kept approxi- 



SIDE ELEVATION 

 SECTION A-A 



FIG. 207. Plan and Sectional Elevation of Ventilating System for Large 

 Horizontal Slow-speed Generators. 



mately one inch of water, or just enough to insure a positive air 

 passage through the generator. 



The air ducts should be as straight as possible, and the per- 

 missible air velocity may vary from as low as 300 or 400 feet per 

 minute for small or very slow-speed machines to as much as 1000 

 or 1500 feet for higher speed machines. 



An approximate figure for the amount of air required is from 

 125 to 150 cubic feet per minute per kilowatt loss. Should, 

 however, the air in passing through the machine rise in tempera- 

 ture more than 15 to 20 C., it indicates that the air does not 



