Choice of centrifugal pump
The correct choice of the “motor + centrifugal pump” requests the analyze of the hydraulic circuit. If not, we may face one of situations :
- The under-sizing of the pump + motor.
Example: we need flow of 250 m³/h, but in spite taking biggest impeller and maximum power motor, the circuit has too big hydraulic resistance, so the pump is working at the middle of the curve and cannot deliver requested flow (line )Two situations are possible:
- We can accept lower flow in our application, but:
- Efficiency of the pump is lower.
- The pump is working “unbalanced”, so shorter life time for impeller.
- Efficiency of the motor is lower.
We cannot accept lower flow in our application, then:
- Or we replace the pump to bigger one
- Or we modify the piping ( decrease resistance )
- Or we install additional pump in series.
- The over-sizing of the pump + motor.
Example: we need flow of 250 m³/h, but taking biggest impeller and maximum power motor and the circuit has too small hydraulic resistance, so the pump is working at the end of the curve ( line )
- Efficiency of the pump is lower.
- The pump is working “unbalanced”, so shorter life time for bearing and seal.
- Cavitations problems.
Note:
We may throttle the flow to “move” the working point.
Example:
Centrifugal pump to circulate the water in closed circuit trough the chiller evaporator .The flow rate is imposed and the pump runs continuously.
Step1: Hydraulic circuit calculation
We introduce, as per or piping drawing, all elements with their hydraulic resistance (for the given flow and liquid), then, using specialized software, we make a table of the circuit.
Step2: Pump choice
After choosing the supplier of suitable pump ( type, quality of the pump for our application, service ability, etc. ) we verify the catalogue and locate our calculated working point on of the performances curves of the pre -selected pump. ( line  ). Then we choose the suitable impeller size and necessary drive.
Note:
- We verify right Rpm.
- We keep in mind that these curves are for water at 20 °C and can vary in function of manufacturing quality of given pump.
As result we choose for “our” pump impeller at ø 165 mm and the motor 22 kW |
Profits for “our” case :
- The pump will work in higher efficiency area
- We choose motor 22 kW that would work closer to nominal power, instead of 30 kW that would result from choice “biggest impeller – full power range motor” :
- Motor cost – lower
- Coupling drive-pump smaller.
- Electrical control, cables – smaller
- Power supply estimation – lower
- Motor efficiency - higher ( less reactive current for the network )
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