Why does pump efficiency increase with flow rate?

In a typical pump curve, the X-axis indicates flow rate (usually in GPM) and the Y-axis corresponds to head (given in feet or meters). This is exactly the case in the example above, taken from the Price Pump website for their centrifugal family of pumps.

No matter what system you are considering, when you say efficiency you mean the amount of energy put into a system compared to the amount of work done by the system. When it comes to pumps, the work done by the system and put into the system is usually measured in horsepower (HP) or kilowatts (kW). The efficiency of a pump can be calculated as:

To calculate the work output by a particular pump, flow rate (often referred to as capacity) is multiplied by head, the density of the fluid being moved, the fluid’s specific gravity, and the appropriate conversion factor for the desired units (such as horse power in the example above). To calculate the energy put into a system, the simplest way is to refer to the pump curve provided by the manufacturer.

In some cases, an efficiency curve may already be provided. In the example above efficiency is observed to increase with flow rate up to around the 100GPM point. After this point efficiency begins to decrease. Note that when the flow rate is zero, the efficiency is also zero. This makes sense intuitively because if there is no flow, no work is being done i.e. the output in the efficiency equation is zero. Recall that flow rate is in the numerator of our equation as well. Flow rate must be a number greater than zero to achieve a positive efficiency.

The most interesting and perhaps least obvious part of the efficiency line above is that it begins to decrease with flow rate, approximately after the 100GPM mark. Notice that as flow rate increases beyond 100GPM, power into the system also increases. This input power is the denominator in the efficiency equation. Shortly after the 100GPM mark head also begins to decrease. The numerator in the efficiency equation is the product of head and flow rate primarily. Though flow rate continues to increase, the rate that output power increases is stunted by the loss in head.

When material constants are separated from the efficiency equation, a simple relationship between the three variables flow, head and input power becomes apparent. Peak efficiency occurs in a pump when the ratio of head and flow compared to the power into the system is maximized.

Looking at all of this information, you might think, “I’m more confused now than when I started!”  That’s fine, feel free to reach out to one of us here at G&D and we can help you find the perfect chiller for your needs based on our 26+ years of experience!