SCREW COMPRESSOR. 



63 



rod C of this piston performs the function of a ram working 

 through a stuffing box in the smaller ram cylinder D. If 

 the ram or piston rod is, say, 3 in. in diameter, the respec- 

 tive areas of the piston and ram are in the proportion of 

 100 to 1 ; and, neglecting friction, the water pressures will 

 be in the inverse ratio. Thus, if water at 50 Ib. per sq. in. 

 pressure be supplied at E to the under surface of the 

 piston, it will force water from the outlet pipe of the 

 upper cylinder at a pressure of 



50 x 100 = 5,000 Ib. per sq. in. 



The supply of water to the straining cylinder of the 

 machine can either be directly controlled by a valve in the 

 supply pipe F, or it can be regulated by controlling the 

 supply of low-pressure water to A through E. The latter 

 is the better way. 



30. The "Quiet" or Screw Compressor. Here the 

 water is supplied at high pressure by a ram being forced 

 into a ram cylinder by means of screws and gearing worked 

 by belts from the line shafting. Its mode of action is shown 

 in Fig. 24. A countershafc on the compressor is rotated 

 by means of a belt from an overhead line shaft. This 



u 



FIG. 24. Quiet or Screw Compressor. 



countershaft drives, through intermediate gearing, two spur 

 wheels, acting as nuts upon the pair of screws, which 

 are in this way drawn in the direction of their length along 

 the two sides of the ram cylinder. The ram itself is 

 carried by a crosshead, which is attached to the ends 

 of the screws, so that a movement of the screws means 

 also a movement of the ram. As the ram enters the 

 cylinder it drives out a supply of water at the required 

 pressure. 



When working the machine, the movement of the ram 

 in the straining cylinder is brought about by the movement 

 of a hand lever, which shifts the belt on the compressor 



