Pump Optimization with Large Impellers Part 3: A Closer Look at the Benefits
/By Mark Bingham
Let’s take a closer look at the benefits of specifying a pump with an optimized impeller. As a reminder, an optimized impeller is the largest impeller that won’t overload the motor at any operating point on the pump curve for the given diameter. Using the same system conditions of 500 GPM and 65 feet that we used in Part 1 and Part 2, we’ll compare and summarize the results of a trimmed impeller selection vs. an optimized impeller selection with a variable frequency drive (VFD).
ESP Systemwize gives us an e-1510 3BD with an 9.5-inch impeller operating at 1620 rpm with a 15 horsepower non-overloading motor. The efficiency at this service condition is 84.7%. Note that a VFD allows the pump to operate at the exact required service conditions, while a pump with a trimmed impeller operates slightly above the specified requirement because impeller trimming is calibrated in 1/8-inch increments.
If, after installation, it turns out that the system head loss is only 55 feet instead of 65 feet, we can operate the optimized impeller pump at a reduced speed instead of throttling or re-trimming the impeller. (As we discussed in Part 2, ASHRAE 90.1 6.7.3.3.3 limits the use of throttling to consume extra pump head.) Running a selection at the new conditions prescribes a revised operating speed of 1521 rpm, which only reduces efficiency by 0.4%. This new operating condition can be implemented in a matter of minutes with an optimized impeller pump and VFD. Alternatively, we could throttle the pump or remove and trim the impeller. However, neither of these solutions is ideal, since throttling wastes excessive energy and removing and trimming the impeller is operationally disruptive and expensive.
Now, let’s consider a less likely scenario. What if the required head exceeds the specified pump head by 10 feet, resulting in a duty point of 500 gpm at 75 feet?
With an optimized impeller variable speed pump, we can run a Systemwize selection at the new conditions, allowing for motor overspeed should these new conditions require an operating speed above the nominal motor speed. (Note that overspeed was not required to meet the revised duty point.)
The table below compares the operating efficiencies of a traditionally selected pump with a trimmed impeller to those of an optimized impeller pump selection.
The effective efficiency resulting from throttling the excess head is calculated using the hydraulic horsepower equation rearranged as follows:
ⴄ=qh / 3960 bhp
where,
q= volumetric flow rate (gpm) - 500
h= head (feet) -55
bhp=hydraulic horsepower - 10.2
Solving at the operating condition yields an effective efficiency of 68.1%
It’s worth pointing out that we can actually get up to 100 feet at 500 GPM with an optimized (full diameter) impeller and VFD selection without operating in the motor service factor if we allow operation above 60hz (1770rpm). Remember, this extra capacity comes at no additional cost to the owner.
In summary, operating an optimized impeller pump at a speed that satisfies system flow requirements will result in the highest operating efficiency and the most flexibility over a wide range of heads at the desired flow.
