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    Auditac tg8 how manufacturers could help auditors Auditac tg8 how manufacturers could help auditors Document Transcript

    • Technical guides for owner/manager of an air conditioning system: volume 8 How manufacturers could help the unfortunate energy auditor
    • Team France (Project coordinator) Armines - Mines de Paris Austria Slovenia Austrian Energy Agency University of Ljubljana Belgium UK Université de Liège Association of Building Engineers Italy BRE Politecnico di Torino (Building Research Establishment Ltd) Portugal University of Porto Welsh School of Architecture Eurovent-CertificationAuthors of this volumeCleide Aparecida Silva (Université de Liège)Cristian Cuevas (Université de Liège)Jules Hannay (Université de Liège)Jean Lebrun (Université de Liège)Vladut Teodorese (Université de Liège) The sole responsibility for the content of this publication lies with the authors. It does notrepresent the opinion of the European Communities. The European Commission is notresponsible for any use that may be made of the information contained therein. 2
    • IntroductionAs already demonstrated in other chapters of the present report, the energy audit ofa whole HVAC system is not a funny game. On site measurements are unavoidableand cost a lot of time and money. Obviously, in most installations, the highest energyconsumers are the fans. The chillers are then coming in second position.The nominal performances of these components are usually well identified inlaboratory and in manufacturers catalogues. But their average performances aremore questionable in actual conditions of use.From other part, providing that these components are correctly identified andmodelled, they may become valuable measuring instruments.The object of this chapter is to show how the components manufacturers could helpthe auditor by allowing him to make a better use of the information they already have.Using a fan as air flow meter The principleAirflow rates measurements are difficult in existing distribution networks: long enoughstraight lines are seldom available or accessible and velocity profiles are usually notuniform enough. A large series of measuring points is required and the final accuracyis often disappointing. A much better solution consists in using the fan as an air flowmeter.This can be done in two ways: 1) By taking profit of the well known characteristics of the fan and using easy measurements, as supply and exhaust static pressures, rotation speed and/or electrical power; 2) Even better, by measuring just one reference pressure drop at fan supply.Both procedures are illustrated and validated hereafter. Fan characteristics identified on the basis of manufacturer dataA fan is currently modelled with the help of similarity variables: flow, pressure andpower factors. These factors can be correlated to each other by polynomialexpressions.The main output of a fan model can be the airflow rate, expressed in “specific” value(in kg/s of dry air), as usually in air conditioning. Other outputs can be the differentfactors, exhaust air speed, total pressure difference, isentropic power and (isentropic)temperature increase across the fan (these two last outputs can be used as checkinginformation).The fan is supposed to be characterised by the diameter of its impeller (scalevariable), its (fictitious) exhaust area and the coefficients of the polynomialcorrelations.Supply air conditions (temperature, pressure and moisture content), rotation speedand static pressure difference are taken as input variables. 3
    • This gives the information flow diagram of Figure 1. Figure 1: Example of fan polynomial modelThe first equations of this model are built on the basis of the definitions of two (flowand pressure) similarity factors:Two dynamic pressures are considered: one at the exhaust and the other one at theperiphery of the impeller: 4
    • The other non-dimensional variables considered are the isentropic effectiveness andthe power factor:The three factors are inter-correlated through polynomial laws such as:This model is from long time well validated and easy to tune…Polynomials are fittedto manufacturer’s performance data.Total and static pressures have to be carefully distinguished at fan exhaust:manufacturers present fan performance in terms of total pressure rise, whereas themeasurements are usually made in terms of static pressures.Fan characteristics are generally provided by the manufacturers as “data sheets”,showing the operating curves of the fan (relationships among the different variablesof the system).A typical example of data sheet is presented in Figure 2.This information might be better used if the manufacturers were giving:The experimental points actually available;The correlation equations actually used to generate the curves.Still today, the fan simulation model built by the manufacturer is only “offered” to“important” customers as manufacturers of air handling units. The equations are thenembedded inside black box selection and simulation software. 5
    • Figure 2 Fan characteristics as presented by the manufacturerSome points can be selected in the diagram of Figure 2, in order to identify thepolynomial laws already presented. An example of selection is made on Figure 3; itcorresponds to three different rotation speeds. Figure 3 Reference points selected for parameter identificationThe phi-psi and lambda-psi regression curves identified on these points arepresented in Figure 4 and Figure 5. 6
    • Figure 4 Identification of the phi-psi characteristic Figure 5 Identification of the lambda-psi characteristicA phi – psi characteristic appears to be accurate enough for airflow ratemeasurements, when the fan has backward-curved blades (as in the present case).The airflow rate can be currently determined with an accuracy of about 5 %. In thecase of fans with forward-curved blades, the pressure rise is relatively insensitive toairflow rate and a more accurate result can be obtained by using the efficiencycharacteristic. The electrical consumption is then a better indicator of the flow rate.However, this second approach requires a correct identification of all electrical losses(electrical motor and frequency driver, if any). Experimental validation The fanThe validation is performed on the fan whose characteristics were already identified.The fan considered in this study was originally installed in a box, downstream of a“radiator”(Figure 6). 7
    • Figure 6 the tested fan Tightness of the fan boxA first experimental arrangement (Figure 7) was made in order to verify the tightnessof the fan box.The tightness test consisted in injecting air inside the box, thanks to an auxiliary fan,after having closed both (supply and exhaust) openings.Injected airflow rate and box-ambient over-pressure were simultaneously measured.This allows identifying a fictitious leakage area.An example of measuring result is shown in Figure 8. In the case considered, theleakage flow rate is estimated to 16.8 g/s, which corresponds to a leakage (fictitiousisentropic nozzle throat) diameter of 27.3 mm. Fan box Small fan Figure 7 Experimental identification of the fan box leakage area 8
    • φ 30/60 mm. Fan box tamb = 20,5°C HRamb = 54 % ∆P = 604 Pa Patm = 981 mbar ∆P = 350 Pa Figure 8 Leakage measurement Experimental characterization of the fanThe fan system is characterised at different rotation speeds; it’s equipped with afrequency inverter working between 8.5 and 50.5 Hz.The experimental arrangement is shown in Figures 9 to 11.Three tests series have been performed: they correspond to three pressure dropcharacteristics of the air circuit: 1) With coil and with a diaphragm used to measure the flow rate (highest pressure drop) 2) With coil and with a nozzle used to measure the flow rate (medium pressure drop) 3) Without coil and with a nozzle used to measure the flow rate (lowest pressure drop). ∆Prad ∆Paf ∆Pop tamb HRamb Patm & Win Ninv finv & Wm I Figure 9 Experimental characterization of the fan (principle schema) 9
    • Figure 10 Experimental arrangement (back view) Figure 11 Experimental arrangement (front view)The inverter is provided with direct measurements of the frequency and of theelectrical power supplied to the fan motor. The electrical power supplied to theinverter and the fan rotation speeds are also measured in these tests. This makespossible to identify the inverter loss and the frequency “sliding” of the electric motor.The tests results are presented in Figure 12 and Figure 13. Figure 12 Inverter loss and efficiency 10
    • Figure 13 Motor slidingAs it can be seen here, both the inverter loss and the motor sliding can be identifiedthrough linear correlations.By combining these measuring results with the characteristics already identified (frommanufacturer data), it’s also possible to identify: 1) The global electromechanical loss and corresponding efficiency of the inverter- motor subsystem (Figure 14); 2) The same terms for the whole inverter-motor-fan system (Figure 15).Such characteristics would be easy to combine with on site measurements. Figure 14 Fan shaft power as function of motor electrical power 11
    • Figure 15 Global loss and global efficiencyThe test results were also used to validate the laws identified from manufacturerdata.An example of such checking is presented in Figure 16: simulated and measuredtotal pressure differences are compared for same flow rates and rotation speeds. Theagreement is considered as satisfactory (the actual total pressure at fan exhaust isnot directly measured here, but re-calculated by reference to a hypothetical exhaustarea). Figure 16 Comparison among simulated and measured pressure differences Validation of a much more expedient methodA much more expedient method is proposed by some manufacturers (as the presentone).The fan selected for this study was equipped with openings and connecting pipes fordifferential pressure measurements as shown in Figure 17 and Figure 18. 12
    • Figure 17 Differential pressure measuring device (principle) Figure 18 Measuring device (outside the fan) Figure 19 Measuring device (inside the fan)The measuring principle is described in the manufacturer catalogue as follows: 13
    • This means that the fan inlet is used as a measuring nozzle. The “K” constant“contains” the flow contraction effect.The method can be validated with the test results available. A fictitious nozzleexhaust diameter is identified at each regime.Examples of results are presented in Figure 20. Figure 20 Exhaust diameter of the (fictitious) fan supply nozzleA (fairly constant) fictitious diameter of 193 mm is here identified. The actual diameteris 257 mm. This corresponds to a contraction factor of (193/257)2 = 0.564 and is ingood agreement with the “K” value indicated by the manufacturer.Using refrigeration compressor as enthalpy flow meter The principleDetermining on site the cooling power actually provided by a chiller is also a delicatematter. That power should correspond to the enthalpy flow of the secondary fluid(usually water or brine) supplying the evaporator. But neither the flow rate, nor thesupply-exhaust temperature difference are easy to measure.The measurements are not easier on refrigerant side. 14
    • An interesting alternative consists in using the chiller compressor as enthalpy flowmeter. But, as seen hereafter, this would require a better dialog with manufacturers…Data provided by the manufacturersAnalysisEach manufacturer seems having his own way to present data in his catalogue.Unfortunately, this “personal” presentation can be a source of confusion.Data available are nor fully clear, neither complete and the reader is, most of thetime, recommended to“…contact the local manufacturer office…” as solution.Unfortunately, these offices are not always able to help: it may occur that theinformation is just no more available, because of too old machines, no moreproduced or replaced by new models.Some examples of machine designations and physical data presentations used bysome manufacturers are presented hereafter, outlining the differences found amongthem.ExamplesFigure 21 gives the designation data found in the catalogue data of manufacturer A. Manufacturer A Evaporator cooling capacity Figure 21 Chiller designation of manufacturer A.Figure 22 gives the designation data found in the catalogue of manufacturer B. 15
    • Reference evaporator cooling capacity – Nominal Tons Figure 22: Chiller designation of manufacturer BThe presentation of physical data is not standardized, as seen in Figure 21 andFigure 22. Here also, each manufacturer has his personalised presentation andattention must be paid to each item presented (nominal loads, refrigerant type, fancapacity, flow rates, etc.), when manipulating catalogues coming from differentmanufacturers.The units must be carefully identified: Some manufacturers use a “semi- SI” systemwith kW, bar, kg/s, m³/h, etc. Other ones are still using Imperial units.But other elementary questions must be answered, as, for example, if the data refersto one, or to several devices (e.g. one or two condenser fans), to partial or global airflow rate, etc. 16
    • ! n ntioAtteFigure 23 Chiller physical data given by manufacturer A 17
    • Figure 24 Chiller, physical data given by manufacturer BARI and Eurovent standardsHopefully, standardized ARI and/or Eurovent reference data are usually alsoavailable in main manufacturer catalogues and can be freely downloaded fromInternet. This is a great advantage for the users. There exist other standards, butthey are not free! 18
    • Chiller modelling and parameter identificationModellingVarious simulation models are currently available. An example of information flowdiagram used for chiller modelling is presented in Figure 22. It corresponds to themodel of a chiller with scroll compressor(s) and with air-cooled condenser. In thismodel, the condenser and the evaporator are modelled as fictitious semi-isothermalheat exchangers. A hypothetical proportional control law is applied to the condenserfan. Figure 25 Example information flow diagramThe same model can be used in two steps: 1) Parameter identification on one or several reference points; 2) Simulation in all other conditions of use.Attention must be paid to the domain of validity of such model, after tuning: accordingto the manufacturer: no extrapolation should be done outside the domain covered bythe catalogue.Parameter identificationThis is most delicate operation. If well done, simulation is no more a problem. Figure26 shows the data usually found in the manufacturer catalogue. A “block diagram” ofthe identification procedure is presented in Figure 27. The parameters identified areindicated inFigure 28.The identification process can be done “manually” and iteratively, by considering theresult trends after each step. Default values are used as first guesses for eachcomponent separately at the nominal point. These values are tuned in order to obtainresults that fit to all orders of magnitude. Finally the parameters are tuned again, inorder to obtain a better agreement with all experimental results and/or allmanufacturer data available. 19
    • The parameter identification is considered as satisfactory if all thermal and electricalpowers are predicted with accuracy of the order of ± 2 %. 20
    • cd – condenser ev – evaporator taex_cd cp – compressor e v – expansion valve Air tex_cd Condenser tsu_cd taex_cp Compressor tsu_e v tasu_cdExpansionvalve & Wcp Manufacturer data & Q ev tsu_cp tex_e v tsu_ev tex_ev R22 Evaporator twex_ev twsu_ev & Mw Figure 26 Data usually found in a manufacturer catalogue Figure 27 “Block diagram” of the identification procedure 21
    • Figure 28 Parameters identified for chiller modelSimulationAt this stage, any user can introduce his proper data, in other to calculate the chillerperformances, as shown in Figure 29. Figure 29 Chiller performances calculation 22
    • Examples of results The examples of results presented hereafter are obtained with data provided by both manufacturers (A and B) already selected. The agreement between simulation and catalogue data is very satisfactory. 60000 20000 Qev Qev,ARI Qev,part-load,50% Wcp Wcp,ARI Wcp,part-load,50% 56000 52000 16000 48000 44000 40000 12000 Wcp [W] Qev [W] 36000 32000 8000 28000 24000 20000 4000 16000 12000 0 12000 20000 28000 36000 44000 52000 60000 0 4000 8000 12000 16000 20000 Qev,man [W] Wcp,man [W] Figure 30 Calculated versus catalogue data (manufacturer A) 50000 20000 Qev Qev ,ARI W WARI 45000 18000 40000 16000Qev [W] [W] 35000 14000 30000 W 12000 25000 10000 20000 8000 20000 25000 30000 35000 40000 45000 50000 8000 10000 12000 14000 16000 18000 20000 Qev,man [W] Wman [W] Figure 31 Calculated versus catalogue data (manufacturer A) Illustration of the measuring method by simulation The compressor model (contained in the chiller model already presented) can be used for an easy and accurate determination the chiller cooling power on site. The two examples of simulation results presented in Figure 32 and Figure 33 demonstrate that very simple (linear) relationships could be used to determine the chiller cooling power as function of the refrigerant pressure measured at compressor supply. In full load and at constant rotation speed, this supply pressure is the almost unique variable to be considered. A slight shift can be applied to this law as function 23
    • of a second variable: the temperature of the secondary fluid at condenser supply(Figure 32) or, more directly, the condensing pressure (Figure 33). Figure 32 Chiller cooling power as function of the compressor supply pressure (with condenser supply air temperature as second independent variable) Figure 33 Chiller cooling power as function of the compressor supply pressure (with compressor exhaust pressure as second independent variable)Such procedure is easy to apply on site, because, most of the time, both (evaporationand condensation) pressures are actually given on the chiller control board… 24