SUCTION AIE VESSELS 619 



the law (pressure X volume = constant), for if H = height of top of 

 air vessel above centre line of pump : 



li a {H v h v } = constant = K 



... hv = H v - *j- (7) 



"a 



01 fe = H^k. < 8) 



The mean height h v may be adjusted by admitting more or less air 

 into the air vessel, and this adjustment should be made until h v is as low 

 as is consistent with no air being drawn over into the suction chamber as 

 the level varies. 



With a single-acting pump, the total cyclical variation in volume in 

 the air chamber is about *56 of the piston displacement per stroke. 



Since the fluctuation in level diminishes with an increase in cross- 

 sectional area, an increase in a. v has the further advantage that it permits 

 the mean working value of h v to be reduced. 



Again, substituting in (6) from (8), it appears that while the fluctuation 

 in a s corresponding to a given variation in h w diminishes as H v increases 

 yet this effect is small compared with that of an increase in a v . In effect 

 then, the area of the vessel is of much greater importance than its length, 



i 



and for a given volume, the ratio T - rr should be as large as possible. 



EXAMPLE. 



A = 1*0 square foot. l s = 30 feet. 



a s = *25 square foot. h s = 9'5 feet. 



a v = rOO square foot. H v = 4 feet. 



Length of stroke = TO foot. No. of revolutions = 100 per inin. 

 When the pump is standing let h v = 6 inches. We then have h a = 

 (34 10) = 24 feet absolute head, 



so that 24 {4 \ j- = constant = K 



.'. K = 84. 



Assuming the total fluctuation of volume in the air vessel to be equal 

 to '56 times the delivery per stroke, this gives a total fluctuation equal to 

 56 cubic feet, and therefore a fluctuation in level of '56 feet. 



The acceleration in the supply pipe corresponding to any value of the 

 piston acceleration may now be obtained from equation (6). 



- 9-5 -^V + h, (-gig- - l) | 



4 ~ fe t> \6A A / ) 



32-2 34 



-25 h 30 



