Variable Speed Pumping:Minimum Flow with Variable Speed

Pumping systems account for nearly 20% of the world’s energy used by electric motors and 25% to 50% of the total electrical energy usage in certain industrial facilities. Significant opportunities exist to reduce pumping system energy consumption through smart design,retrofitting, and operating practices. In particular, the many pumping applications with variable duty requirements offer great potential for savings.  The savings often go well beyond energy, and may include improved performance, improved reliability, and reduced life cycle costs.

Most existing systems requiring flow control make use of bypass lines, throttling valves, or pump speed adjustments.  The most efficient of these is pump speed control.  When a pump’s speed is reduced, less energy is imparted to the fluid and less energy needs to be throttled or bypassed. Speed can be controlled in a number of ways, with the most popular type of variable speed drive(VSD) being the variable frequency drive (VFD).

Motor Limitations

Most standard motors are capable of providing full torque output from 3 to 60 Hz; however, at lower speeds, where the integral motor cooling fans become less effective,supplemental cooling may be needed to operate at full torque output continuously. Therefore, VFD manufacturers recommend a minimum speed of 30 percent of their rated speed (18 Hz) for standard motors controlled by VFDs, to prevent motor over heating due to inadequate air flow. If lower speeds are required, then the motor manufacturer should be consulted for recommendations. Inverter duty motors can operate below 20 percent (12 Hz) of rated speed without problems in a variable load application, since they usually incorporate special cooling provisions and use a higher class insulation.

Pump Limitations

As mechanical devices, it appears that pumps are the most unforgiving component in the system. In high static head applications (such as large building or district energy heating and cooling pumps), a pump with a VFD can slow down such that it operates at or near shut-off head conditions. Operating in this region is very unstable for a number of reasons, including potentialtemperature rise of fluid, unstable operations or flow recirculation within the impeller.

Methods of Providing Minimum Flow

There are several proven methods for providing minimum flow for variable flow water systems:

(1) Locate a constant flow (continuous bypass) using orifice or balancing valve/constant flow control valve across the pump.
(2) Locate three-way control valve(s) within the piping network.
(3) Locate two-way valve across pump that is energized at low flow from signal from the VFD controller.

Of the above three options, only the last one is preferred because it is the most efficient since it does not bypass water from the supply to the return until low flow conditions are met. The third option also is the most complex and expensive to install, however, so there are trade-offs. The other two methods constantly dilute the return-water temperature with supply water, which detrimentally effects system efficiency and Delta T.

So instead of playing a game of limbo with your pumping network, guessing‘how low you can go,’ use the knowledge obtained from VFD and pump vendors to stay within 20 percent to 25 percent of your BEP flow –and stay ‘below the bar.’

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