Centrifugal Pumps. 99 



Pumps with single fans can therefore be run at a smaller 

 number of revolutions — a great advantage when used for 

 drainage purposes. The water passes to the opening through 

 a trumpet-shaped mouth continued downwards a short dis- 

 tance to prevent the pump drawing air when the water is 

 pumped to the lowest level. The gradually decreasing size 

 of the opening at the entrance produces a corresponding in- 

 crease in the velocity, which is again decreased on leaving 

 the fan by guide blades, the apertures between the guide 

 blades being smaller near the fan than above. 



The use of guide blades is found to increase very materially 

 the discharging power of the pump. In experiments made 

 by Mr. Parsons, a pump, which without the guide blades dis- 

 charged 1500 gallons a minute^ increased this quantity to 

 50CK) gallons a minute when the guides were added, the 

 pressure of steam remaining the same. 



This form of pump is fixed in an iron case or brick well, the 

 outlet from which is at the lowest level to which the water 

 in the outlet channel is ever likely to fall No delivery or 

 suction pipes are required. The opening in the well is either 

 protected by a self-acting valve to prevent back flow when 

 the pump is not working, or doors are placed at the end of 

 the channel leading into the main outfall drain. The pump 

 is hung by its spindle to a girder across the well at the top 

 by a gun-metal bearing that is quite accessible, the spindle 

 being stayed by horizontal guides in the well. No footstep 

 is required. The bearings of the different parts have conical 

 seats, and the fan can at any time be taken out for repairs 

 and replaced without emptying the water from the pump 

 well. This arrangement secures the machine from the wear 

 and tear due to its exposure to the grit and dirt contained 

 in the water, and facilitates repairs when required. It is 

 necessary to arrange the well or case so that it can at any 

 time be pumped dry if required — a pi'ecaution, however, that 

 i^ seldom wanted. No valves are required, the pump being 



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