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Optimizing unitary pumping solutions

Unitary pumping solutions integrate variable speed drives (VSDs) to allow for pumps to self-optimize.

Stephen Putnam, Grundfos Pumps Corp., Downers Grove, Ill.
02/20/2017

Learning Objectives


Pump manufacturers can offer engineers and building owners the ability to control equipment through onboard logic. These capabilities are being underused, however.

The market standard for a pump is to be driven by a third-party motor whose performance is dictated by a third-party variable speed drive (VSD) that was programmed by a third-party contractor. As capable as all of those involved in this process may be, this is not the best design to run pumping systems as efficiently as possible. As in the boiler and chiller market, it is the pump manufacturers know how best to use and control their products to achieve maximum utility while minimizing energy consumption. Pump manufacturers who design and produce their products are best equipped to control their pumps using the unitary pumping solution. 

What is a unitary pumping solution?

Electrically commutated motors (ECMs) originated in fans, as the technology was limited by low torque ratings in the fractional horsepower segment. But it did not take long for the ECM technology to spread into pumping systems, and this was the key that opened the door to incorporate integrated VSDs and ultimately "self-optimizing" logic onboard a pump.

ECM is driven by a permanently magnetized rotor, and the rotational force comes from alternating the polarity of the stator in the motor. There is an inherent risk of rotor lock in this design; the solution, engineers found, is to incorporate speed controls in the motor to protect against this threat. The solution proved to be valuable: With integrated speed controls, the energy-saving benefits VSDs offered to larger horsepower pumps become available in smaller horsepower pumps.

Once the market had integrated speed controls, a self-optimizing ability of a unitary pumping solution was the logical next step. Self-optimizing ability allows the pump to "rightsize" itself for the specific application, into which it is installed without end-user interaction.

All unitary pumping solutions have onboard controlling ability that allows for the pump to self-optimize its performance in which pump speed is dictated by an integrated VSD that is driven by an ECM. With the proper anatomy and performance capabilities, energy-saving rates of 80% to 90% can be achieved as compared with the market's standard definition of a pump (wet-end and induction motor with no self-optimizing ability). For engineers, contractors, and end users looking for low-hanging fruit for energy savings, this is the most ripe. 

What is self-optimizing?

Before diving into depth on the economic argument for a unitary pumping solution, a basic engineering overview of pump curves and system curves is required.

Pump curve: The market-standard pump operates at a constant speed along its fixed pump curve. The impeller diameter inside of the volute is a fixed size, and the rotational speed at which the motor-and, consequently, the pump-operate is constant. Note that as flow (gallons per minute) increases, total dynamic head (TDH, commonly referred to as "head"), will decrease.

TDH = friction losses + elevation in system + discharge head

The reverse is true as well; if flow decreases, the head will increase. The only variable in the system is the flow, but the pump will only operate along its curve.

Figure 1: No matter the control scheme (fixed-speed or variable), a pump will always operate at the intersection of the pump curve and system curve. Constant-speed pumps dominate the market today, and the introduction of integrated variable speed drives (System curve: Every piping system has a system curve that denotes how much resistance is embedded (due to pipe friction, valves, fittings, elevation, and delivery pressure requirements) at any given flow. The system curve is static so long as there are no dynamic changes in the system that would change the system characteristics (pipe length, valves, or fittings added/taken away, etc.). Open systems (hot-water recirculation) have a fixed system curve as there is a fixed amount of piping, valves, and fittings. Closed systems (HVAC), have dynamic system curves. These dynamic systems are the result of heating zones opening and closing based on occupant demands. When all zones in an HVAC system are open, there is more length of pipe through which the product must be pumped as compared to one zone open, where there is less length of pipe through which the product must be pumped. Even though the pump is servicing the same system, the system curve changes depending on the number of zones that are demanding heat. Duty point:

The pump will always operate at the point of intersection of the pump curve and system curve. The duty point can migrate left and right along the pump curve.

Proportional pressure curve: Note that the pump curve and the system curves are inverses of one another. As flow may increase or decrease, the system curve adjusts itself along the pump curve at the new duty point. The performance is constrained by the pump and the system into which it is installed. The opportunity for self-optimizing ability comes in answering the question, "What does the end user need to do the job?"

Proportional pressure operation answers this question by attempting to mirror the system curve in that specific system. It will run at the necessary flow while eliminating the excess head produced by the fixed-speed pump, saving energy in the process (see Figure 1).

Self-optimization: The onboard algorithms will continually monitor the system, finding and operating on the most appropriate proportional pressure curve into the specific system into which it is installed. The end result is a pumping system that will continually rightsize itself down to the lowest head production while still doing the job, saving a significant amount of energy in the process. [subhead]

What are the benefits?

The greatest benefits of unitary pumping solutions depend on who you ask within the system design and operation:

Engineer: Cast one net, catch all of the fish. The ability to specify a minimal number of pumps for all of the applications found in a facility. Moreover, unitary pumping solutions provide the value of energy optimization to the end user.

End user: One energy-saving opportunity that is often overlooked is that unitary pumping solution systems will continually consume minimal energy while maintaining comfort. Additionally, there is a minimal amount of maintenance (if any) for these systems.

Contractor: The contractor does not have to size or select a pump with a great deal of specificity. Instead, a single pump can be installed in a vast array of applications. Among the largest opportunities for a unitary pumping solution is in hydronic heating applications. The application is more popular across the northern U.S., residentially, and is very common across the U.S. commercially. Often, there are circulating pumps employed in these systems, and the unitary solution has yet to penetrate the market in a significant way.

As an example, compare a market-standard fixed-speed circulator pump versus a unitary pumping solution. Both pumps run 5,000 hours/year and the end user has an energy cost of 9.5 cents/kWh. Most applications will have a variable load profile, with the most universally accepted sample load profile being the Blauer Engel load profile

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