Energy Saving Analysis of Variable Primary Flow System with Screw Chiller


 Ursula Dawson
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1 Energy Sving Anlysis of Vrible Primry Flow System with Screw Chiller YungChung Chng [1], PinCheng Chieng [2], JyunTing Lu [3], TienShun Chn [4], ChingLing Chen [5], ChengWen Lee [6] [1] Professor, Deprtment of AirConditioning nd Refrigertion, Ntionl Tipei University of Technology, Tiwn, ROC [2] Student, Deprtment of AirConditioning nd Refrigertion, Ntionl Tipei University of Technology, Tiwn, ROC [3] Student, Deprtment of AirConditioning nd Refrigertion, Ntionl Tipei University of Technology, Tiwn, ROC [4] Student, Deprtment of AirConditioning nd Refrigertion, Ntionl Tipei University of Technology, Tiwn, ROC [5] Student,Deprtment of AirConditioning nd Refrigertion, Ntionl Tipei University of Technology, Tiwn, ROC [6] Student, Deprtment of AirConditioning nd Refrigertion, Ntionl Tipei University of Technology, Tiwn, ROC [7] Student, Deprtment of AirConditioning nd Refrigertion, Ntionl Tipei University of Technology, Tiwn, ROC JyunTing Lu, Abstrct. This pper focuses on energysving opertion nd use of the vrible primry flow (VPF) of centrl irconditioning system chiller, by mens of regression nlysis to determine the reltionship eqution of chiller power consumption nd cooled wter flow rte, cooled wter supply temperture, cooling wter return temperture, nd prtil lod, nd the reltionship eqution of the cooled wter pump power consumption nd the cooled wter flow rte. Tke the reltionship equtions s bsis to identify the power consumption chnge curve of lods in different flow rtes, nd clculte the most energysving cooled wter flow rte of the chiller nd the cooled wter pump in vrious lods. Keywords: Chiller Vrible Primry Flow VPF 25
2 1 Introduction With the enhncement of the stndrd living of humn, the spce comfort requirements of residentil building environment lso will increse. In order to crete comfortble environment spce, often need the help of the centrl irconditioning system to mintin indoor comfort conditions; however, it is ccompnied by lrge mount of energy consumption. All such devices constituting the centrl irconditioning system s chiller, cooled wter pump, cooling wter pump, cooling wter tower nd blower re power consumption devices, their power consumption typiclly ccounting for most of totl building power consumption. The power consumption of the cooler unit ccounts for over hlf of totl centrl irconditioning system power consumption, so highefficiency cooler unit indeed cn effectively reduce the totl nnul power consumption of the system. For buildings of different forms nd uses, under the premises s the conditions to meet the comfortble environment requirements, there re vriety of different plnning nd choices in design of the centrl irconditioning system. The design of n irconditioning system with vrible primry flow (VPF) hs been widely dopted in Europe nd United Sttes [3], nd the use cses of which hve incresed yer by yer. The most direct influence of the VPF chnges of n centrl irconditioning system on power consumption is the reduced power consumption of the cooled wter pump; however, in terms of the chiller, it will chnge the evportion temperture, the cooled wter return/supply temperture, nd the evportion temperture difference, nd lso ffect the mount of het trnsfer in the evportor, nd influence the coefficient of performnce (COP) of the chiller. This study, therefore, ims t tckling the nlysis crried out to the influence of chnged VPF on vrious dt, to find the optiml opertion vlue of VPF. 26
3 2. Chilled wter system in n centrl irconditioning system 2.1 Chilled wter system As shown in Figure 1, the chilled wter system consists of chiller, chilled wter pump, chilled wter piping nd blower. The chilled wter pump mde the chilled wter circulte between the chiller nd the blower; the 7 chilled wter bsorbed the spce het in the irhndling unit, so tht the chilled wter temperture rose to 12, nd the 12 chilled wter ws returned to the chiller, cooling to 7 through the chiller evportor, nd then continued to the irhndling unit by the chilled wter pump to effect the cooling function in the irconditioning room. Figure 1. Concept of chilled wter ction 2.2. Primry Only System (POS) Actully, when the chilled wter system opertes, the lods re different depending on mbient temperture, personnel ccess, lighting nd other peripherls. The POS controls the chilled wter flow by threewy vlve; when the lod is low, the flow through the blower becomes smll, nd the excess flow through bypss pipe bypsses the blower; the flow rte through the blower nd the sum of bypssing flow re unchnged, so to mintin the constnt flow of entire chilled wter system. This system is the primry only system (POS), s shown in Figure 2. For this system, the chilled wter flow through the chiller is constnt, nd the chilled wter flow through the splitter of every irhndling unit is constnt too. The 27
4 POS mintins the whole system with the constnt circulting volume of chilled wter. In this system, it is no need to control the chilled wter pump, s long s renders the sme to operte; however, in lod shedding, the excess chilled wter will still circulte, resulting in wste of energy. Figure 2. Primry only system 2.3. Primry/secondry system Primry/secondry system is lso known s decoupling system, s shown in Figure 3. The primry pump is of constnt frequency to keep the chiller with constnt flow rte, nd the secondry pump is of vrible frequency control to coordinte with the size of the lod to regulte the flow rte, in order to chieve the energy sving effect. This system is minly to improve the energy wste of chilled wter in lod shedding cused by the secondry constnt flow rte. The bypss pipe is used to utomticlly djust the chilled wter volume, so tht when the lod chnges, the problem of different primry nd secondry chilled wter circulting volume cn be overcome. When the mximum lod of secondry flow comes out, the demnded chilled wter flow rte nd the primry circultion volume re sme, then the flow rte of the bypss pipe is close to zero; when the lod of secondry flow rte reduces, the secondry flow rte will be lower thn the primry flow rte, nd the excess primry flow rte will directly return to the chiller through the bypss pipe. For this design, when the lod is smll, reducing the secondry flow rte cn chieve the 28
5 energy sving purpose of the secondry pump; however, the primry flow rte is still constnt, resulting in the energy wste of the primry pump. Figure 3. Primry constnt/secondry vrible flow system 2.4. VPF system Generlly speking, the number of dys required with full lod opertion in yer will not be more thn 10 dys; most of the time, the opertion is under prtil lod mode. With the vrible primry flow design, pumps hve more time to operte in low wter volume nd low power consumption. Becuse the pump power consumption is directly proportionl to the third power of flow rte, so when the wter volume is lowered, there will be considerble lrge proportion of energy sving mount. The chilled wter pump sves energy, but t the sme time, the power consumption of the chiller chnges. Under overll considertion, should or should not drop the primry flow rte, nd how much of the drop cn be the most energyefficient? This is the focus to be discussed in this pper. First of the following is to instruct how to instll the VPF system Modified by the old system At present, most of the chilled wter system is primry/secondry system; s long s instlling frequency converter in the primry pump cn become VPF system, s 29
6 shown in Figure 4. However, proportionl vlve should be instlled in the bypss pipe, with the chnges of lod, to djust the flow rte of the bypss pipe [4]. Figure 4. Primry/secondry chilled wter circultion system New system The vrible primry flow system ws used to cncel the secondry wter pump, nd the primry constnt frequency pump ws replced with the high lift vrible frequency pump, s shown in Figure 5. And dedicted controller ws used to control the opertion nd frequency of the wter pump; the controller clculted the secondry lod by temperture difference nd flow rte, nd sent commnds to the wter pump inverter, to supply the chilled wter flow suitble for the lod. 30
7 Figure 5. Primry chilled wter circultion system 3. Prediction of power consumption 3.1 The chiller power consumption mode The prediction of the chiller power consumption ws tking the ctul mesured opertion dt by sttisticl softwre to obtin regression eqution coefficient, in order to estblish the chiller power consumption mode s follows: ~ 0 19 : Regression coefficient Pchiller: chiller power consumption (kw) Tchws: chilled wter supply temperture ( ) Tcwr: cooling wter return temperture ( ) LPM: chilled wter flow rte (Liter/minute) PLR: prtil lod rtio (%), the ctul refrigerting cpcity of the chiller divided by the rted refrigerting cpcity. And the ctul refrigerting cpcity of the chiller is: Q: refrigerting cpcity (kw) T: chilled wter return/supply temperture difference ( ) LPM: chilled wter flow rte (Liter/minute) Cp: Het cpcity t constnt pressure (Kcl/Kg. ) C: Constnt 3.2 The pump power consumption mode Through the lw of similrity, for pump, the power consumption is proportionl to the third power of speed, nd flow rte is proportionl to the speed, so we cn see the wter pump power consumption is proportionl to the third power of flow rte. The wter pump power consumption mode estblished by liner egression is s follows: b ~ b 0 3 : Regression coefficient Pchp: wter pump power consumption 31
8 4. Experimentl Explortion nd Anlysis In this experiment, n office building ws tken for the experimentl re, nd the ctul dt collected by monitoring system ws nlyzed. Tble 1 shows the specifictions of this system. The smpling time for this experiment is 12 dys. Smpling dt nd opertion rnge of the vrious prmeters of the chiller re shown in Tbles 2 nd 3. Tble 1 Specifictions of experimentl irconditioning system Item Specifictions evportor type compressor type Refrigernt type chiller refrigerting cpcity chiller rted power rted chilled wter flow rte Horse power of chilled wter pump Flooded type Spirl type (dul pressure) R RT 174 kw 2200 LPM 20 HP Dte Tble 2 Smpling dtes nd climte dt of the experiment Sun. to temperture rnge Humidity rnge Mon. % 2012/5/17 Thu. 23.4~ ~ /5/18 Fri. 26.9~ ~ /5/21 Mon. 25.4~ ~ /6/26 Tue. 27.2~ ~
9 2012/6/27 Wed. 28.8~ ~ /6/28 Thu. 30.1~ ~ /7/12 Thu. 31.0~ ~ /7/13 Fri. 31.2~ ~ /7/16 Mon. 27.5~ ~ /7/17 Tue. 29.2~ ~ /7/19 Thu. 30.9~ ~ /7/20 Fri. 31.8~ ~96.7 Tble 3 Smpling dt rnge of the chiller Item Rnge PLR(%) 23~102% T chws ( ) 5~20 T cwr ( ) 23~40 Chilled wter flow rte (LPM) 1000~2360 LPM The independent vribles in this experiment re prtil lod rtio, chilled wter supply temperture, chilled wter return temperture, chilled wter flow rte, nd the regression eqution coefficients obtined by the sttisticl softwre s shown in Tble 4. The regression results show tht the 2 R vlue is , nd the verge error rte is 2.67%, indicting the ccurcy is quite high which is vilble for prediction. Figure 6 shows the comprison of the ctul chiller power consumption nd simultive power consumption. 33
10 Tble 4 regression coefficients The regression coefficients of the chiller power consumption mode Vlue Vribles constnt PLR T chws T cwr LPM PLR T chws 2 T cwr E06 2 LPM PLR Tchws PLR Tcwr PLR LPM Tchws Tcwr T chws LPM T cwr LPM PLR T chws Tcwr PLR T LPM chws PLR T LPM cwr 34
11 T T LPM PLR T T LPM 19 chws chws cwr cwr Simultive chiller Power consumption (kw) Rel chiller Power consumption (kw) Figure 6 Comprison of ctul chiller power consumption nd simultive power consumption The power consumption of the wter pump under the condition of constnt system pressure drop reltes to wter volume only; the obtined regression eqution coefficients re shown in Tble 5. From similrity theorem tht power consumption is proportionl to the third power of wter volume, but the obtined third power coefficient b3 is 0, so the modeling by the second power is s follows: P chp b 2 0 b1 LPM b2lpm b ~ b 0 2 : Regression coefficient P chp : wter pump power consumption (kw) (1) Tble 5. The regression coefficients of the chiller wter pump power consumption regression coefficients Vlue Vribles b constnt 0 35
12 b LPM b E06 2 LPM 2 The regression nlysis shows tht the R 2 vlue is nd the verge error rte is 4.46%; Figure 7 shows the comprison of ctul power consumption of chilled wter pump nd simultive power consumption. From the figure, it shows the ccurcy of the chilled wter pump modeling is quite high nd vilble for prediction. Figure 7. Comprison of ctul power consumption of chilled wter pump nd simultive power consumption 5. Metnlysis of the chiller nd the chilled wter pump power consumption There is only one independent vrible in the regression eqution of the chilled wter pump power consumption, which is the chilled wter flow rte. power consumption curves of 1000~2400LPM. Figure 8 shows the 36
13 Figure 8. The chilled wter pump power consumption There re four independent vribles of the regression eqution of the chiller power consumption, including prtil lod rtio, chilled wter supply temperture, cooling wter return temperture, nd chilled wter flow rte. This pper took the constnt chilled wter supply temperture 7, nd cooling wter return temperture 25 for nlysis. First, the prtil lod rte ws fixed s 100%, s shown in Tble 6. The chnges of the wter pump power consumption, the chiller power consumption, nd totl power consumption when the chilled wter flow rte vried between 1000 to 2400LPM were illustrted s shown in Figure 9. The blue curve below refers to the chiller power consumption, nd the purple curve bove refers to totl power consumption. Found out the chilled wter flow of the lowest power consumption mong totl power consumption s the optiml flow rte of totl power consumption. Using the tringlepoint s mrk, totl power consumption curve ws smiling curve, nd the optimum flow rte of the lowest power consumption mong totl power consumption ws t the lowest point of 1800LPM smiling curve. The flow rte strted to decrement from 2200LPM nd decremented 100LPM every time until 1000LPM. The seprtion distnce of two curves refers to the wter pump power consumption; it cn be found tht the smller the flow rte, the smller the seprtion distnce, refers the smller the wter pump power consumption. Tble 6 The power consumption nlysis of the chilled wter flow chnges when the chiller lod is of 100% 37
14 PLR % T chws ( ) T cwr ( ) Chill ed wter flow rte (LPM) Chiller power consumption (kw) COP Pump Totl power power consumption consumption (kw) (kw) 100% % % % % % % % % % % % % % %
15 Figure 9 The power consumption chnges when the chilled wter flow rte chnges (the chiller lod is of 100%) Anlyzed the stte of prtil lod rtio t 90%, 80%, 70%, 60%, 50%, 40%, 30%, nd 25%, nd compiled the power consumption messge of vrious lods t rted flow nd the optimum flow rte, s shown in Tble 7. It cn be seen from Tble 7, under the lod from 25% to 100%, there ws lwys energy sving effect of reducing chilled wter flow rte, with the energy sving volume between 1.4kW to 9.6kW, nd the energy sving rte ws from 0.9% to 3.6%. When the lod ws 100%, the energy sving rte ws 1.4kW (0.9%) only; the smller the lod, the smller the optimum flow rte, nd the greter the energy sving volume, nd the greter the energy sving rte s well. The energy sving rte ws obtined from the energy svings divided by totl power consumption of rted flow t the sid lod, nd the energy svings were up to 23.6%. If lwys tkes the totl power consumption 154.3kW of rted flow t 100% lods s bsis, the energy sving rte is of 0.9% to 6.2%. Tble 7. The power sving nlysis 39
16 Optiml flow (LPM) Interntionl Journl of Electronics Communictions nd Electricl Engineering PLR Rted flow rte P chiller P totl Flow (LPM) Opimum flow rte sving % P chiller P pump P totl kw % 100% % % 90% % % 80% % % 70% % % 60% % % 50% % % 40% % % 30% % % 25% % % optiml flow of chiller power optiml flow of totl power % 30% 40% 50% 60% 70% 80% 90% 100% 40 Prtil lod rte (%)
17 Figure 10. Optiml flow rte trcking Integrted the optimum flow rte of the lowest power consumption t different lods into Figure 10; when the lod ws greter thn 60%, the chiller s optimum flow rte ws the rted flow rte, nd when the lod ws less thn 60%, the chiller s optimum flow rte grdully reduces. Figure 10 shows the chiller s rted flow is not the optimum flow rte, the lower the lod, the lower the optimum flow rte. 6. Conclusions The control of the chiller ws bsed on the supply temperture of chilled wter. During the experiment, when the chilled wter flow rte chnged, the chiller cnnot knew the flow rte chnges, but only through the chnge of flow rte leding to chnges of temperture tht the chiller will rect. So, when the flow rte chnges, the control of the chilled wter temperture will be reltively unstble; to chnge the speed of the chilled wter flow rte must consider the chiller s rection speed to void too much devotion of temperture from the set point to ffect the system stbility. VPF system is used to sve energy by low flow rte. If the chilled wter return/supply temperture difference is gret, nd the return wter temperture is higher thn the sme of primry only system, the chiller will misjudge low lod into high lod in terms of stgebckwter control, resulting in too low return temperture, nd in severe cse, my cuse the chilled wter freezing. It is recommended tht to 41
18 use VPF for energy sving, using the freestge supply wter control of the chiller is better. Too low chilled wter flow rte will ffect the het exchnge efficiency of the chilled wter in the evportor. For the minimum flow rte restrictions, plese refer to 2~3ft/s, the minimum requirement of vrible primry flow system in flow velocity of the chiller evportor mentioned by MQcy Interntionl [2]. The users cnnot know the internl design of the evportor, nd so no wy of knowing the flow velocity of cooper pipe of the evportor. The best is to sk chiller mnufcturer provide the flow rte nd flow velocity tble s the reference for vrible flow rte opertion. 7. References [1] Trne Engineers Newsletter Vol.28, No.3, 1999 [2] McQuy Interntionl, chiller plnt Design Appliction Guide AG , 2002 [3] Ke, MingTsun, Cheng, HsiYi, Chiu, PinFeng, Energysving Anlysis nd Review of Vrible Primry Flow Chilled Wter Cses, Journl of Refrigertion & AirConditioning Technology, July [4] Liu, ChungChe, Liu, ChiHung, The Energysving Appliction of Vrible Screw Chillers in Vrible Primry Flow System, Journl of Refrigertion & AirConditioning Technology, October
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