What is efficiency?
When we talk about the efficiency of most types of machines, we are usually referring to how well it can convert one form of energy to another. If there is one unit of energy supplied to the machine and its output, in the same units of measure, is one half unit, the efficiency of it is 50%.
What exactly is specific speed and how does it affect the pumps curve?
Specific speed was first applied to centrifugal pumps on the 1800’s and was then a modified version of one developed for water turbines. You will find that most pump designers tend to see specific speed as the most important contributor to centrifugal pump design, allowing the use of existing design and the test data in order to design similar higher and lower flow pumps as the specific speed of a pump is independent of the size.
What are the individual efficiencies that affect operation?
- Volumetric efficiency
- Hydraulic efficiency
- Combined efficiency
- Mechanical efficiency
How is efficiency preserved?
One of the most important parts of the volute is the cutwater, or tongue. The cutwaters purpose is to maintain the flow into the throat while still minimizing recirculation back into the case. The optimum clearance between the cutwater and the impeller periphery is the smallest distance that doesn’t give rise to pressure pulsations during the vane tip passing. A properly designed pump will generally have a full sized impeller which meets these clearance criteria. When your impeller is getting trimmed, the distance increases and then allows more fluid to recirculate back into the case. As the recirculation increases, the hydraulic efficiency will decrease.
How does the curve shape affect the efficiency?
The typical performance curve is basically flat at low values of specific speed and it will become steeper as the Ns increases. The pump efficiency is lowest at low values of Ns and it will increase as the Ns increases. It reaches its max in the mid-to-high 2000 range and it will also begin to decrease above 3000.
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