50 District Energy

at or near shut-off head conditions. Operat-ing in this region is very unstable for anumber of reasons, including potentialtemperature rise of fluid, unstable opera-tions or flow recirculation within the impeller.

Fluid Temperature RiseAt low flow conditions, the inefficiency

of the pump (the difference between thebrake horsepower consumed and the waterhorsepower developed) transmits heat tothe fluid creating a potential for overheatingthe water. Overheating the water can causebearing failure as well as excessive radialthrust loads and shaft deflection, since theheat is conducted to all components. There-fore, the Hydraulic Institute recommendslimiting the temperature rise of the fluidflowing through the pump to 15 F. This isespecially true for pumping higher-temper-ature water systems. Equation 1 quantifiesthis flow rate:

Equation 1: Q = P / (2.95 x Cp x S)whereQ = minimum flow rate for a 15 F

temperature riseP = Input power at minimum flow rate

(HP). [Assume that shut-off HP isequivalent to minimum flow rate HP]

2.95 = constant [(HP-lb-min-F)/(Btu-gal)]Cp = specific heat of fluid [(Btu/(lb -F)]

[1 for water]S = specific gravity of fluid [1 for water]

Unstable OperationIf the pump selected does not have a

flat curve characteristic, then there couldpossibly be multiple operating points forreduced flows at the same pump head. Thisnot only creates hunting by the control sys-tem but also translates to unstable opera-tion resulting in excessive shaft deflectionand vibration due to unbalanced radialthrust and rotating element instability.

Recirculation Within Pump CasingLow flow conditions also may create

reduced pressure in the pump vortex. Whenthe pressure drops below the liquids vaporpressure, cavitation will occur, damagingthe pump impeller and therefore its per-formance. These scenarios become moreevident as suction pressures increase beyondthe net positive suction head required(NPSHR) by the pump.

with turning down the VFD to zero. Sowhats the big deal then? As VFD and pumpvendors will tell you, the limitation is notwith the VFD, but rather, the culprits arethe motor and pump.

Motor LimitationsMost standard motors are capable of

providing full torque output from 3 to 60 Hz;however, at lower speeds, where the integralmotor cooling fans become less effective,supplemental cooling may be needed tooperate at full torque output continuously.Therefore, VFD manufacturers recommenda minimum speed of 30 percent of theirrated speed (18 Hz) for standard motorscontrolled by VFDs, to prevent motor over-heating due to inadequate air flow. If lowerspeeds are required, then the motor man-ufacturer should be consulted for recom-mendations. Inverter duty motors canoperate below 20 percent (12 Hz) of ratedspeed without problems in a variable loadapplication, since they usually incorporatespecial cooling provisions and use a higher-class insulation.

Pump LimitationsAs mechanical devices, it appears that

pumps are the most unforgiving componentin the system. In high-static head applications(such as large building or district energyheating and cooling pumps), a pump witha VFD can slow down such that it operates

Editors Note: Inside Insights is a column

designed to address ongoing issues of interest

to building owners, managers and operating

engineers who use district energy services.

Today, with the higher cost of elec-tricity, an increasing number of heat-ing, ventilation and air-conditioningsystems are using variable speed-drivenequipment to save energy and optimizethe system performance. For years engineersand designers have provided a minimumflow to protect the pumping system. Onemay ask, Protect the system from what?and How low can you go? To answerthese questions, we have to analyze all thecomponents pump, motor and variablefrequency drive (VFD).

Since pumps, motors and VFDs arenot 100 percent efficient, these inefficienciesare usually radiated as heat to the surround-ings. Excessive heat will lead to componentdamage and premature failure. So, just likemy beer, keeping it cool is the secret tosuccess! Hence, VFDs and motors haveintegral cooling fans to remove this heat.In the case of the motors, the fan is attacheddirectly to the drive shaft that forces airover the windings to cool them. VFDs alsohave internal fans that take room air andcirculate through the cabinet. The fans alsoadd to the drive inefficiencies. As long asthe room air is clean and tempered (below104 degrees F), there should be no issue

Variable SpeedPumping: How lowcan you go?Steve Tredinnick, PE, Infrastructure Project Engineer/Manager, Affiliated Engineers Inc.

InsideInsights

Third Quarter 2006 51

Volume Pumping, Pumps and SystemsMagazine, March 1999; Ed H. Edwards,HBE Engineering, Ensure Minimum Flowfor Centrifugal Pumps, Pumps andSystems, March 2003.

Steve Tredinnick, PE, is aproject engineer/managerfor Affiliated Engineers inMadison, Wis., with morethan 20 years experiencerelated to building HVACsystems. The past 10 years ofhis work have been focusedon district energy systems. Tredinnick is agraduate of Pennsylvania State University witha degree in architectural engineering. He is amember of IDEA and ASHRAE and is currentlychair of ASHRAE TC 6.2 District Energy.Tredinnick may be reached at stredinnick@aeieng.com.

Pump Minimum Flow CriteriaIt is interesting to note that there are

no published industry standards that estab-lish precise limits for minimum flow inpumps, but ANSI/HI 9.6.3-1997 Centrifugaland Vertical Pumps Allowable OperatingRegion presents all of the factors involvedand provides recommendations for thepreferred operating region. As a rule ofthumb, the Hydraulic Institute and pumpmanufacturers typically recommend a min-imum flow rate of the 20 percent bestefficiency point (BEP) flow rate, which cor-responds to the VFD manufacturers sug-gested minimum as well. However, somepump manufacturers recommend 25 percentof BEP flow rate, so the designer mustconfirm this value.

Figure 1 illustrates the BEP as the pointon a pump's performance curve that corre-sponds to the highest efficiency at a givenflow rate and pump head. Pump operatingconditions selected within the identified BEParea ensures that the impeller is subjectedto minimum radial forces promoting smoothoperation with low vibration and noise.Figure 1 also indicates the NPSHR curve agood graphic indicator of the suggestedpump operating range. The designer shouldnot select a pump to operate to the left orright of this curve without consulting themanufacturer.

Methods of ProvidingMinimum Flow

There are several proven methods forproviding minimum flow for variable flowwater systems: (1) Locate a constant flow (continuousbypass) using orifice or balancing valve/constant flow control valve across the pump.(2) Locate three-way control valve(s) withinthe piping network.

(3) Locate two-way valve across pump thatis energized at low flow from signal fromthe VFD controller.

Of the above three options, only thelast one is preferred because it is the mostefficient since it does not bypass waterfrom the supply to the return until lowflow conditions are met. The third optionalso is the most complex and expensive toinstall, however, so there are trade-offs.The other two methods constantly dilutethe return-water temperature with supplywater, which detrimentally effects systemefficiency and Delta T.

So instead of playing a game of limbowith your pumping network, guessinghow low you can go, use the knowledgeobtained from VFD and pump vendors tostay within 20 percent to 25 percent ofyour BEP flow and stay below the bar.

The author acknowledges the follow-ing reference sources: Kenneth R. Luther,ITT Fluid Handling, Applying Variable

Column also available at www.districtenergy.org/de_magazine.htm

OperationalEnvelope

500 1000 1500 2000 2500 3000 3500 4000 4500 5000gpm

175

150

125

100

75

50

25

ft

NPSH Curve

76% 78%80% Design Point (to the left ofcapacity range midpoint)

Approx. BEP

System Curve

15

9

Figure 1. Best Efficiency Point and Optimum Pump Selection. Figure illustrates a typical pump curvewith the optimum selection area indicated with best efficiency points.

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