Pump efficiency equation and power

Pump efficiency equation and power

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Power is consumed by a pump, fan or compressor in order to move and increase the pressure of a fluid. The power requirement of the pump depends on a number of factors including the pump and motor efficiency, the differential pressure and the fluid density, viscosity and flow rate. This article provides relationships to determine the required pump power.

Pump efficiency equation and power

Industrial solutions

How pump efficiency can be preserved by changing impeller speed rather than reducing it diameter will also be examined. Then I will compare the value of peak efficiency versus the breadth of efficiency over a range of flow. The discussion will end with the importance, or sometimes unimportance, of efficiency as it relates to a particular application or process.

Pump efficiency equation and power

A Simplified Method of Determining the Efficiency of a Motor-Driven Centrifugal Pump

To sidestep these issues, a different equation can be used to determine centrifugal pump efficiency without having to know the liquid’s specific gravity and pump’s output shaft horsepower. This equation allows you to determine a centrifugal pump’s efficiency by only knowing these values: the pump’s discharge pressure (P2) and suction pressure (P1), flow rate (Q), electric motor efficiency (effm), and motor power in kilowatts (kW ).

See also

Head = 160 feet. We measured it at the discharge side of the pump and corrected it for the fact that the gage was two feet above the pump center line. Look at the following diagram where we have calculated the discharge head from the formula shown on the right hand side of the illustration. If there were any positive head on the suction side of the pump that head would have to be subtracted. A negative suction head would be added to the discharge head.

Main principles of pumps selection

Capacity (delivery, flow rate) – volume of medium pumped by a pump per unit of time. It is denoted by letter Q and has dimension in m3-h, l-s, etc. Flow rate quantity includes only factual volume of displaced fluid ignoring return leakages. The theoretical and factual flow rate ratio is expressed by volumetric efficiency quantity:

Pump efficiency equation and power

For a pump that has been already purchased or has been ordered for manufacturing, the efficiency can be determined using the pump performance curves provided by pump manufacturer. Here the problem statement has specified pump efficiency to be 70%.

Pump efficiency equation and power


Previous Motors@Work Blogs have focused mainly on motor energy efficiency.  Albeit there are numerous benefits to be gained from a motor efficiency program, a more holistic approach ultimately provides the greatest potential benefits.


Efficiency is how much energy you get out of an appliance, compared to how much energy you put into running it. A 97% efficient furnace gives you back 97% of the energy you put into it in the form of heat for your home, the other 3% is lost up the flue. Since electric baseboard has no flue, 100% of the energy that goes in, stays in your home in the form of heat. The higher the efficiency of your system, the lower your heating bills.

When we talk about the size of an air conditioning system (whether by tons of cooling, BTU-h, or kW), we are specifying the cooling capacity (power) of the system. The actual electrical power used to operate such a system is less. As described in this document, the electrical power used is one half to one third (or less) of the cooling power.