Your questions answered: Optimizing Variable Flow Pumping Control Modes for Energy Savings

Kevin Anderson, senior technical training specialist at Grundfos Pumps Corp. in Olathe, Kan. tackled unanswered questions from the Jan. 19, 2016 webcast on optimizing variable flow pumping control modes for energy savings.


Kevin Anderson, senior technical training specialist, Grundfos Pumps Corp., Olathe, Kan. Courtesy: Grundfos Pumps Corp.Kevin Anderson, senior technical training specialist at Grundfos Pumps Corp. in Olathe, Kan. tackled unanswered questions from the Jan. 19, 2016 webcast on optimizing variable flow pumping control modes for energy savings.

Q: What are the pros and cons of an electrically commutated motor (ECM)?

Kevin Anderson: Pros: an ECM is very efficient, especially across a wider motor load, and higher torque. Con: the purchase price is higher than that of a conventional motor.

Q: If PMGs have higher efficiency, why is it not as widely used as the induction motor (PMGs are not in common use in U.S.).

Anderson: Purchase price.

Q: Everything you've mentioned so far refers to ECMs. How would this be different with an induction motor?

Anderson: From a variable speed pump perspective, the main difference between an ECM controlled motor and an induction motor is efficiency. Both can be controlled the same way, but the power savings will be much higher with the ECM motor because the electrical efficiency is higher, especially at partial load—where it counts—on a variable speed pump.

Q: Somehow, I missed how the efficiencies for the zones were calculated based on the 81% motor efficiency.

Anderson: The efficiencies for the pumps were based on where the pump was operating on its efficiency curve. The efficiency for the motor was based on the efficiency of an average wet rotor motor.

Q: Why are people still using induction motors?

Anderson: The most common reason is the initial cost of an induction motor is cheaper.

Q: I question your savings. At constant speed, pump efficiency at a given point on the pump curve is unchanged. If the original motor is 81%, then at constant speed, maximum savings would be 1/0.81 = 1.23 or 23%.

Anderson: The reason for the large energy savings is due to the lower power usage overall of a permanent magnet motor, not just the motor efficiencies themselves. Also, the permanent magnet pump has a very similar flow and head profile, but has a higher hydraulic efficiency. The permanent magnet motor used for this example has a maximum load of 610 W (0.61/1.29 = 0.47). This means only 47% of the total energy of the induction motor is used by the permanent magnet motor for a total savings of 53%.

Q: What is the job of the primary pumps compared to the secondary pumps? What is the use of the green line (bypass pipe) in your diagram?

Anderson: To achieve proper variable volume pumping, we must have two-way valves, primary and secondary pumps, and a common pipe. If we leave three-way valves in the system, we will still have constant flow. The green line is the common pipe. A common pipe hydraulically "decouples" the primary part of the system (the boilers or chillers) from the secondary portion (distribution and loads) of the system. A common pipe allows independent operation of the primary and secondary pumps. Because the primary pump is only responsible for flow through the boiler and a small amount of piping, the pressure drop is relatively small, so the pump horsepower is usually small. The secondary pumps have to do more work. They have to meet maximum demand and have much more piping head loss to overcome. Fortunately, demand varies, so when demand is less, flow is less and friction loss is less—saving operating costs.

Q: It would have been really nice if you would have shown what the flow reduction was relative to, or maybe put a scale on the graphs.

Anderson: I'm sorry. I should have explained more thoroughly. The changing flow was due to zone valves opening and closing in a hydronic heating loop.

Q: By proportional differential pressure control, do you mean the pressure transmitter is out in the system as opposed to across the pump?

Anderson: The proportional differential pressure can be accomplished using differential pressure sensors either out in the system or across the pump. However, across the system is the most accurate way to design the system. A remote mounted sensor can result in optimized energy savings because "actual" head loss is accounted for versus a calculated or estimated head loss.

Q: How does the reduction in power cost compare with capital cost investment i.e. internal rate of return?

Anderson: Depending on the energy costs and the load profile for the application, we have seen as low as 18 months or even less payback on many of these applications.

Q: From your technical experience, which control mode do you think is better and why?

Anderson: I have to give the standard answer of "that depends." If it is a closed loop heating system as described in this webcast, the auto-adaptive control is best more than 85% of the time. However, high head losses at low flow, as in a coil with a high head loss close to the pump discharge, then constant differential pressure or a medium-to-high proportional differential pressure may be required.

Q: Why not try to design your example system using only primary pumps all of the time? Will that ultimately save even more money?

Anderson: Often, using single-loop systems is much less efficient for pump or boiler energy use. In a single-loop system, the pump must be sized to overcome the full head and flow of the entire system. This can cause a very large pump to run near shutoff a majority of the time depending on the load profile. In most cases, it is more energy efficient to operate smaller pumps at lower flows and heads, then adding additional pumps as needed to achieve higher flows and heads. We have a prerecorded webcast titled "Efficient Pump Selection" on our training website that gets into the efficient staging of pumps based on load profile.

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