Pump Optimization with Large Impellers Part 2: Changing Pump Speed with a Variable Frequency Drive
/By Mark Bingham
A retired engineer I know once said, "There is no glory in an undersized pump." He was right, of course. There's less obvious fallout for specifying a pump with too much "oomph" than one with not enough. The point is pumps are often oversized and even occasionally undersized, and both scenarios have consequences. The question is, what do we do about it?
In our last blog, we discussed the pump affinity laws, which define the mathematical relationship between flow, RPM, and impeller diameter. If we have too much or too little flow, we can correct this by altering the impeller diameter or speed. But how do we determine which is the best method?
Let’s explore this question with an example of a system designed for 500 gpm at 65 feet of head.
If we select a constant speed pump for these conditions, ESP Systemwize gives us an e-1510 3BD pump with an 8.875” impeller operating at 1770 rpm with a 15hp non-overloading motor (Figure 1). The efficiency at this condition is 82.8%. The operating point is slightly above the specified requirement at 505 gpm and 66.3 ft. because impellers are trimmed in 1/8” increments.
Figure 1. esp systemwize Constant Speed pump selection for a system designed for 500 gpm at 65 feet of head.
Ooops! We Overestimated the Head Loss
What happens if the head loss turns out to be only 55 feet? The easiest solution is to consume the extra head by throttling the pump. However, that comes at an operating cost penalty to the owner. We can estimate the throttling losses by using the following equation for hydraulic horsepower:
bhp = (qh) / (3960η) where:
q = volumetric flow rate (gpm)
h = head (feet)
η = pump efficiency
Using this equation, we calculate that the throttling losses required to achieve 500 gpm at 55 feet of head with this unaltered pump will cost us an extra 1.52 horsepower. Assuming this pump operates continuously, the annual operating cost penalty, based on the 2023 US average cost of 12.79 cents per kWh and a motor efficiency of 92%, amounts to about $1,380.
This throttling loss is certainly undesirable and, in many circumstances, not allowed under ASHRAE 90.1-2022, which states:
If we wish to eliminate this cost, which will accrue over the operating life of the pump, we can trim the impeller or operate the pump at a reduced speed.
For the impeller trimming option, ESP Systemwize indicates a required impeller diameter of 8.375", which reduces our efficiency to 81.3% (Figure 2). The pump efficiency is reduced by 1.5% due to the increased impeller trim. Note that this duty point would allow for a ten-horsepower non-overloading motor.
Figure 2. esp systemwize Constant Speed pump selection with impeller Trimmed to adjust for 55 feet of head
If we were to install a variable frequency drive and reduce the pump speed to meet the 55-foot head requirement, we would operate the pump at 1650 rpm, resulting in an efficiency of 82.3% versus the 81.3 efficiency that would result from trimming the impeller (Figure 3).
Figure 3. Adding a variable speed drive results in an efficiency ( 82.3%) versus trimming the impeller (81.3%)
Ooops! We Underestimated the Head Loss
Let's consider the less likely scenario of the required head exceeding the specified head by 10 feet, resulting in a duty point of 500 GPM at 75 feet.
We could replace the constant speed pump with a larger diameter (9.25") impeller, increasing efficiency to 83.6% (Figure 4). However, replacing the impeller is costly and may be disruptive.
figure 4: If a pump is undersized for an system, we could replace the impeller with a larger one, which would increase efficiency, but may be costly and disruptive.
We could instead apply a variable speed drive and operate the 8.875” impeller at approximately 1835 rpm, resulting in an efficiency of 82.6%.
Figure 5. alternatively, we can leave the impeller as is and add a VFD which will still result in a small efficiency gain.
The examples presented here illustrate the benefits of changing pump speed with a variable frequency drive, as long as the increase in speed doesn’t overload the motor. These benefits are even more significant when we select a pump with an optimized (larger) impeller.
The next blog will revisit the scenarios above with an optimized pump selection.
