ASHRAE ST-16-015-2016 Improving Variable-Speed Pumping Control to Maximize Savings.pdf

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1、 2016 ASHRAE 141ABSTRACTAccording to some estimates, pumps account for between10% and 20% of world electricity consumption (EERE 2001;Grundfos 2011). Unfortunately, about two-thirds of all pumpsuse up to 60% too much energy (Grundfos 2011), primarilybecause of inefficient flow control. Varying pump

2、speed usinga variable-frequency drive (VFD) on the pump motor is one ofthe most efficient methods of flow control. As a consequence,about one-fifth of all U.S. utilities incentivize VFDs (NCSU2014), and many of these drives control pumping systems.However, field studies and research show that few va

3、ri-able-flowsystemsareoptimallycontrolled,andthefractionofactual to ideal savings is frequently as low as 40% (Kissock2012;Maetal.2015;L.Song,AssistantProfessor,Departmentof Mechanical Engineering, University of Oklahoma, m., July, 2013). Utility incentive programs that rely onidealenergysavingcalcu

4、lationscouldoverestimatesavingsby30% (Maxwell 2005).Previous work has shown the importance of changingmotor efficiency, VFD efficiency, and pump efficiency onsavings(BernierandBourret1999;Maxwell2005).Thisworkconsiders the difference between actual and ideal savingscausedbyexcessbypassflow,positions

5、andsetpointsofcontrolsensors, and control algorithms. This paper examines theinfluenceofthesefactorsonenergysavingsusingsimulations,experimentaldata,andfieldmeasurements.Ingeneral,energysavingsareincreasedwhenbypassisminimizedoreliminated,pressure sensors for control are located near the most remote

6、end use, and the pressure control setpoint is minimized.INTRODUCTIONAccordingtosomeestimates,pumpsaccountforbetween10% and 20% of world electricity consumption (EERE 2001;Grundfos 2011). In industrial applications, pumps frequentlyaccount for 25% of plant energy use (EERE 2001). Unfortu-nately, abou

7、t two-thirds of all pumps use up to 60% too muchenergy (Grundfos 2011). The primary reasons are 1) althoughpumps are designed for peak flow, most pumping systemsseldom require peak flow and 2) the energy efficiency of flowcontrol methods varies significantly. Before variable-frequency drives (VFDs)

8、were commonly used, bypass andthrottling were common, but inefficient, methods of varyingflow to a specific end use. Today, the most energy efficientmethodofvaryingflowisbyvaryingpumpspeedwithaVFD.Previous work has shown that excluding the effects of chang-ing motor efficiency, VFD efficiency, pump

9、efficiency, andstatic head requirements results in overestimating savings(Bernier and Bourret 1999; Maxwell 2005).The quantity of energy saved in variable-flow systems ishighly dependent on other factors in addition to motor, pump,andVFDefficiencies.Fieldstudiesandresearchshowthatfewvariable-flow sy

10、stems are optimally controlled and that thefraction of actual to maximum savings can be as low as 40%in poorly controlled flow systems (L. Song, Assistant Profes-sor, Department of Mechanical Engineering, University ofOklahoma, pers. comm., July, 2013; Kissock 2012; Ma et al.2015). This work conside

11、rs the difference between actual andideal savings caused by excess bypass flow, positions andsetpointsofcontrolsensors,andcontrolalgorithms.Thepaperbeginsbydefining“ideal”flowcontrolasthemostenergyeffi-cient type of flow control and compares pump power fromreducing flow by throttling to the ideal ca

12、se. Because someImproving Variable-Speed Pumping Controlto Maximize SavingsAlexandra Brogan Vijay Gopalakrishnan Kathleen SturtevantStudent Member ASHRAE Student Member ASHRAEZachary Valigosky Kelly Kissock, PhD, PEAssociate Member ASHRAE Member ASHRAEAlexandra Brogan is an energy engineer at Plug S

13、mart, Columbus, OH. Vijay Gopalakrishnan is a project engineer at Energy and ResourceSolutions, North Andover, MA. Kathleen Sturtevant and Zachary Valigosky are graduate students in the Renewable and Clean Energyprogram and Kelly Kissock is a professor and chair of the Mechanical and Aerospace Engin

14、eering Department and Director of the Renewableand Clean Energy program at the University of Dayton, Dayton, OH.ST-16-015Published in ASHRAE Transactions, Volume 122, Part 2 142 ASHRAE Transactionsminimumflowisrequiredinmostpumpingsystems,theeffectof excess bypass flow on pumping energy use is consi

15、dered.The control variable for most variable-flow pumping systemsis pressure; hence, the effect of the locations and setpointvalues of pressure sensors on pump power is considered.Finally,acasestudyispresentedthatdemonstrateshowpump-ing energy can be reduced through application of these prin-ciples.

16、IDEAL FLOW CONTROLTo consider the effects of excess bypass flow, positionsand setpoints of pressure sensors, and control algorithms onpumping system energy use, it is useful to define the maxi-mum savings that can be expected from reducing flow.Figure 1 shows two operating points of a pumping system

17、.Point 1 represents the pump operating at full flow. Point 2Visthe operating point if pump speed is slowed by an optimallycontrolled VFD. Point 2Vlies on a system curve in which thepressure head dh approaches zero as volume flow rateapproaches zero and pump head varies with the square ofvolume flow

18、rate. This ideal case represents the minimumpumping power that can be expected when flow is reducedfrom to . The reduction in pump power is also definedby the pump affinity law shown in Equation 1, where W isfluid work and is volume flow rate at respective operatingpoints:(1)Fewactualpumpingsystemsa

19、chievethepowerreductiondefined by the pump affinity law because of throttling, mini-mumflowconstraints,staticheadrequirementsduetochangesinelevation,velocityandpressurebetweentheinletandoutletof the piping system, and control losses. In the followingsections,thesedeviationsfromtheidealcaseareinvesti

20、gated.THROTTLED FLOW CONTROLOne of the most common methods of varying flow is tothrottle flow by partially closing a valve in the piping system.In this section, we compare pump power from reducing flowby throttling to pump power from reducing flow by slowingthe pump with a VFD. The University of Day

21、ton HydraulicsLab (UDHL) is equipped with two pumps, a VFD, a parallelpiping network with four branches, pressure sensors, andflowmeters. In Figure 2, Point 1 is the operating point with thepump at full flow . Point 2Tis the operating point whenflowwasthrottledtovolumeflowrate .Point2Vistheoper-atin

22、gpointwhenpumpspeedwasslowedbyaVFDtovolumeflow rate . Pump power is proportional to the product ofhead and flow, which is represented by the rectangular areadefined by each operating point. The data showed that whenflow was controlled by throttling at Point 2T, the pump ran at1800 rpm (30 Hz) and co

23、nsumed 8.5 kW. When flow wascontrolled by the VFD at Point 2V, the pump ran at 1180 rpm(19.7 Hz) and consumed 3.25 kW; pump power decreased by62%. Clearly, reducing flow with a VFD is more energy effi-cient than throttling flow.In pumping systems, the power transmitted to the fluidWfluidis given by

24、Equation 2, where dh is the pressure headacross the pump and is the volume flow rate:(I-P) (2)(SI) (2)Figure 1 The ideal system curve approaches zero head atzero flow.VV1V2VW2VW1V2V1-3=Figure 2 Measured energy penalty due to throttled flow.V1V2V2VWfluidkWdh ft H2O V gpm3960 gpmftH2Ohp- 0.746 kW/hp=W

25、fluidkWdh m H2O V L/s76.2 L/smH2Ohp- 0.746 kW/hp=Published in ASHRAE Transactions, Volume 122, Part 2 ASHRAE Transactions 143At Point 2V, the fluid work was:However, the electrical power to the pump motor isalways greater than fluid work because of efficiency losses inthe motor, pump, and VFD. Using

26、 the measured power drawand calculated fluid work, the combined efficiency, combined,is given by Equation 3:(3)At Point 2V, the combined efficiency was:The combined efficiencies at Points 2Tand 2Vwere 32%and 34%, respectively. These results indicate that about 70%of pump power was lost due to ineffi

27、ciencies in the motor,pump, and VFD.BYPASS FLOW CONTROLMinimum Flow RequirementsPumping systems require some minimum flow due toconstraints such as minimum VFD speed, minimum flowthrough the pump, or minimum flow through equipment suchas a chiller evaporator. Hence, a bypass is needed to provide apa

28、th for minimum flow when end uses reduce flow below thisminimum. In the ideal case, no bypass flow is permitted whenend uses require more than minimum flow. Flow in excess ofthe minimum required flow increases pumping power andwastes energy.Excess Bypass FlowFlow in excess of the minimum required fl

29、ow alwayswastes pumping energy. Three common piping systems thatresult in excess bypass flow are shown in Figure 3. The first isa three-way valve at one or more end uses (Figure 3A). Three-way valves always allow some bypass flow, thus wastingpumping energy. Another piping system that allows excessb

30、ypass flow uses a manually controlled bypass valve(Figure 3B). Manually controlled valves are always at leastpartially open and hence allow bypass flow even when nobypass is required. The recommended commissioning prac-tice in a system with a manually controlled bypass valve is tocloseallend-useload

31、sandthrottletoallowminimumrequiredflow (Kelley, J., Energy Engineer, Plug Smart, LLC, E, m.,February2015).However,whenend-usevalvesclose,flow through the bypass valve increases due to rebalancing offlow, allowing more than minimum flow through the bypass.(Rebalancing of flow is shown experimentally

32、by the differ-ence in flows and in Figure 5.) In industrialsystems, manually controlled bypass valves are oftenneglected after installation and allow unmonitored excessbypassflow.Athirdcommonpipingsystemthatallowsexcessbypass flow uses an automatic flow limiter on the bypass pipe(Figure 3C). Automat

33、ic flow limiters are better than manuallycontrolled valves because they prevent rebalancing of flow.However, they still allow a fixed amount of bypass flow whenno bypass is required, resulting in wasted pumping energy.Ideal Bypass Flow ControlIdeal bypass flow control is achieved with a dedicated,ac

34、tuated two-way bypass valve controlled to eliminatebypass flow when end uses require more than the minimumflowconstraintandtoallowminimumbypassflowwhenenduses require less than the minimum flow constraint (Averyand Richel 2009). Examples of ideal chilled-water systembypass flow control are shown in

35、Figure 4: the first is aprimary-only pumping system and the second is a primary-secondary pumping system. In the primary-only system(Figure 4A), the minimum flow constraint is flow throughthe chiller evaporator. The bypass valve opens based on aflowmeter on the return header (Taylor 2012) or the pre

36、ssureWfluidkW71.6 ft H2O 81 gpm3960 gpmftH2Ohp- 0.746 kW/hp=1.09 kW=WfluidkW21.8 m H2O 5.1 L/s76.2 L/smH2Ohp- 0.746 kW/hp=1.09 kW=combinedWfluidkWP kW-=combined1.09 kW3.25 kW- 100% 34%=Figure 3 (A) Three-way bypass valve at one end use, (B) manually controlled bypass valve, and (C) automatic flow li

37、miterbypass valve.V2TOV2VOPublished in ASHRAE Transactions, Volume 122, Part 2 144 ASHRAE Transactionsdifference through the chiller evaporator (Fauber, J., SeniorMechanical Engineer, Heapy Engineering, E, pers. comm.,May 2015) when end uses require less than minimum flow.The bypass valve is closed

38、when the end uses require morethan minimum flow.In the primary-secondary system (Figure 4B), a constant-speed primary pump provides minimum flow to the chiller,which eliminates the need for a bypass valve by maintainingminimum flow through the chiller evaporator (Taylor 2002).The secondary pump prov

39、ides flow to end uses.Primary-only pumping systems always use less energyand have a lower first cost than primary-secondary systems(Taylor 2002). However, primary-only systems are compli-cated to commission and difficult for facilities withoutconstant personnel support to maintain them (Taylor 2002)

40、.Primary-secondary pumping systems provide minimumflow with smaller, primary pumps while allowing larger,secondary pumps to vary flow with a VFD, as shown inFigure 4B.Savings from Eliminating Excess Bypass FlowThe effect of excess bypass flow on energy savings invariable-flow pumping systems was exp

41、erimentallymeasured in the UDHL. In this experiment, flow through“the process” end use was 81 gpm (5.1 L/s) with the manu-ally controlled bypass valve closed. Next, flow through “theprocess”endusewasmaintainedat81gpm(5.1L/s)withthebypass valve about 50% open. Figure 5 shows that with thebypassvalvec

42、losed,theVFDranat1180rpm(19.7Hz),totalflow was 81 gpm (5.1 L/s), and the operating point is 2VC.With the bypass valve 50% open, the VFD ran at 1212 rpm(20.2 Hz), total flow was 134 gpm (8.4 L/s), and the operat-ing point shifted to 2VO. Because flow through the processend use was maintained at 81 gp

43、m (5.1 L/s), 53 gpm (3.3 L/s)traveled through the bypass loop. In this case, pump powerincreased from 3.25 to 4.12 kW, or about 27%. When themanually controlled bypass valve was fully open, pumppower increased by 54%. This indicates that if the effect ofexcessbypassflowisneglectedinsavingscalculatio

44、ns,thosecalculations will significantly overestimate savings. Theseresults also suggest that savings can be significantlyincreased by minimizing excess bypass.PUMP SPEED CONTROLAutomatically controlled VFDs modulate pump speedbased on data from a control variable; the most commoncontrol variable is

45、pressure. The setpoint pressure determinesthe y-intercept of the system curve; hence, a high pressuresetpoint increases pump power at all flows. This concept isdemonstrated in Figure 6. The rectangle defined by Point 2V80represents pump power when the pressure setpoint value is80 ft H2O (24.4 m H2O)

46、. The rectangle defined by Point 2V40represents pump power when the pressure setpoint value is40 ft H2O (12.2 m H2O). The difference in the size of the rect-anglesrepresentstheadditionalenergyassociatedwithsettingthePat80ftH2O(24.4mH2O)comparedtosettingitat40 ftH2O (12.2 m H2O).Figure 4 (A) In a pri

47、mary-only system, the bypass can be controlled by a flowmeter or a differential pressure sensor;(B) a primary-secondary system eliminates the need for a bypass valve.Figure 5 Measured energy penalty associated with bypass50% open.Published in ASHRAE Transactions, Volume 122, Part 2 ASHRAE Transactions 145The setpoint pressure must be large enough to push fluidthrough the end uses and hence depends on the location of thesensor in relation to the pump and end uses. Figure 7A showsa closed-loop chilled-wat