IT IS A PUMP IN WHICH A SCREW PROPELLER IS USED TO CREATE AN INCREASE IN PRESSURE BY CAUSING AN AXIAL ACCELERATION OF LIQUID WITHIN ITS BLADE. THE INCIDENTAL ROTATION IMPARTED TO LIQUID IS CONVERTED INTO STRAIGHT AXIAL MOVEMENT BY SUITABLY SHAPED OUTLET GUIDE VANES.
NOW BY LOOKING AT THE FIGURE WE CAN SAY THAT :
1. WITH DISCHARGE V/V SHUT :-
H(axial)=3*H(centrifugal)
P(axial)=2*P(centrifugal)
CLOSING OF DISCHARGE VALVE IN AXIAL PUMP WILL OVERLOAD ELECTRIC MOTOR AND CAUSE OPERATION OF PROTECTIVE DEVICE. IT ALSO CAUSES WATER HAMMER.
2. BEYOND NORMAL DUTY :-
P(axial) DECREASES WITH LOWER HEAD, DELIVERY INCREASES BY MODEST AMOUNT.
CENTRIGUGAL PUMP MOTOR CAN OVERLOAD AT LOW LOAD AND HIGH THROUGHPUT.
NOTE :- AXIAL PUMP RETAINS REASONABLE EFFICIENCY OVER WIDE HEAD RANGE THAN CENTRIFUGAL PUMP.
IMPORTANT POINTS :-
- AXIAL PUMP IS REVERSIBLE.
- AXIAL PUMP WILL IDLE AND OFFER LITTLE RESISTANCE WHEN FLOW IS INDUCED THROUGH IT BY EXTERNAL MEANS.
- UNDER LOW HEAD AND HIGH THROUGHPUT AN AXIAL FLOW PUMP WITH A HIGHER SPEED THAT EQUALLY MATCHED CENTRIFUGAL PUMP CAN BE USED. THE ELECTRIC MOTOR OF THE PUMP WILL BE OF SMALLER SIZE.