Chapter 15. -PUMPS AND FORCED DRAFT BLOWERS 



38.105X 

 Figure 15-20.— Double-screw low-pitch pump. 



are in mesh. Everything in the chamber rotates 

 except the crescent, causing liquid to be trapped 

 in the gear spaces as they pass the crescent. The 

 trapped liquid is carried from the suction to the 

 discharge, where it is forced out of the pump by 

 the meshing of the gears. As liquid is carried 

 away from the intake side of the pump, the pres- 

 sure there is diminished, thus forcing other 

 liquid into the suction side of the pump. The di- 

 rection of flow in the internal gear pump can be 

 reversed by shifting the position of the crescent 

 180 degrees. 



CENTRIFUGAL PUMPS.-Centrifugal pumps 

 are widely used aboard ship for pumping water 

 and other nonviscous liquids. The centrifugal 

 pump utilizes the throwing force of a rapidly 

 revolving impeller. The liquid is pulled in at the 

 center or eye of the impeller and is discharged 

 at the outer rim of this impeller. By the time 

 the liquid reaches the outer rim of the impeller, 

 it has acquired a considerable velocity. The li- 

 quid is then slowed down by being led through a 

 volute or through a series of diffusing passages. 

 As the velocity of the liquid decreases, its pres- 

 sure increases— or, in other words, some of the 

 mechanical kinetic energy of the liquid is trans- 

 formed into mechanical potential energy. In the 

 terminology commonly used in discussion of 

 pumps, the velocity head of the liquid is partially 

 converted to pressure head . 



IDLER 

 ROTOR HOUSING 



LOWER THRUST 

 PLATE 



Figure 15-21. 



47.80 

 -Triple-screw high-pitch pump. 



Centrifugal pumps are not positive-displace- 

 ment pumps. When a centrifugal pump is operat- 

 ing at a constant speed, the amount of liquid 

 discharged (capacity) varies with the discharge 

 pressure according to the relationships inherent 

 in the particular pump design. The relationships 

 among capacity, total head (pressure), and power 

 are usually expressed by means of a character- 

 istics curve. ^ 



Capacity and discharge pressure can be 

 varied by changing the pump speed. However, 

 centrifugal pumps should be operated at or near 

 their rated capacity and discharge pressure 

 whenever possible. Impeller vane angles andthe 

 sizes of the pump waterways can be designed 

 for maximum efficiency at only one combination 

 of speed and discharge pressure; under other 

 conditions of operation, the impeller vane angles 



Characteristics curves are generally given in the 

 manufacturers' technical manuals or in the outline as- 

 sembly drawings of pumps. 



407 



