| As a user of pump myself I can understand the difficulties in choosing a pump that meets all our requirement and you bet I have tried many. That it must be efficient, draws minimum current, high flow rate, low noise, reliable. From catalogues alone one can never be sure, because a lot of data is purposely left out. Only a flow meter and ammeter could tell the performance of a pump. Adding to this confusion.
The name tag on the submersible indicates the power output of the motor which is same as the power input to the pump. No one can be sure of its power consumption, because the manufacturer would not tell you the motor efficiency in its catalog. You can take 10 different makes of submersible pump rated at 150 watt. The power consumption to each pump would be different so is the flowrate at the same H. We can only tell the power consumption by measuring the current drawn by each pump.
For a swimming pool type pump coupled to standard frame size motor. The name plate indicates the output power of the motor. The current rating of the motor always indicate no load current. If the name plate says power = 1/2 hp. Current 3 amperes. It means this motor is capable of delivering 1/2 hp to the pump. It draws a current of 3 amp when running free without any load. One can never tell how many amps it would draw when it is in operation. Only by measurement you can tell the power consumption. Therefore by comparing the name tag of submersible pump and standard frame size centrifugal pump one cannot tell which is more efficient until you measure its power consumption and flowrate at a fix hydraulic head.
If you have to guess the sweet spot of the pump, my opinion would be to target the operating hydraulic head to be at 1/3 away from the lowest recommended head. Let's say if the hydraulic head of system is 2 meters. Try to choose a pump that have a recommended range of 0.5 meter to 6 meter.
If we plot a power consumption graph with Y axis as Q (flowrate) and X axis as H (hydraulic head). The power consumption will increase quite linear at low Q and low H. Then the curve will incease exponentially upwards at a higher H.
This concept is not difficult to comprehend. If we have to deliver the same Q at a higher H. The pump will have less hydraulic efficiencies due to more eddies and frictional losses within the pump housing. Therefore requiring more power to deliver. For that reason always try to oversize the pipe, there would be more savings in the long run.
One can short cut the learning experience by measuring current consumption, hydraulic head and flowrate of friends pump. By comparing different makes a better picture will develop.
There is no fix and fast rule to say sub pump is more efficient than centrifugal
or vice versa. It depends whether the pump you selected operates at its optimum efficiency, which is only at one particular Q and H.
For centrifugal pump one can say with certainty, a closed impeller pump would be more efficient than a pump with semi open or open impeller and self priming type with open impeller would be the worst. |