Application of Automated Fault Detection and Diagnostics For Rooftop Air Conditioners in California

Transcription

1 Appication of Automated Faut Detection and Diagnostics For Rooftop Air Conditioners in Caifornia Haorong Li and James E. Braun, Purdue University ABSTRACT The primary goa of the research described in this paper was to appy a decouping-based faut detection and diagnosis (FDD) technique and economic assessment method to ight commercia cooing and heating equipment in Caifornia. The paper describes the decoupingbased FDD methodoogy and the economic assessment method. The methods have been appied to a number of fied sites. Detaied resuts are given for a singe site and summary resuts are given for the other sites. About 7% of the investigated systems were impacted by fauts and about 4% had more than one faut. Service was justified for about 4% of the units. The FDD technique was abe to diagnose most of the existing fauts. The estimated 1-year cost savings for FDD ranged from about $8 to $17 per ton. Introduction Packaged air conditioning equipment is used extensivey throughout sma commercia and institutiona buidings. However, compared to arger systems, they tend to be not we maintained. Widespread appication of automated faut detection and diagnosis (FDD) to packaged equipment has the potentia to significanty reduce energy use and peak eectrica demand, down time and maintenance costs. In the ate 198 s, some researchers investigated common fauts and methods for faut detection and diagnosis in simpe vapor compression cyces, such as a househod refrigerators. With the growing reaization of the benefits brought by FDD, many more papers about HVAC FDD have appeared in the ast ten years, more than 3 since According to the IEA ANNEX 34 fina report (Dexter and Pakanen, 21), twenty-three prototype FDD performance monitoring toos and twenty-six FDD toos have been tested in rea buidings. Rossi and Braun (1997) modified the genera FDD supervision methodoogy first described by Isermann (1984) for non-critica HVAC system and deveoped a statistica ruebased (SRB) FDD technique for vapor compression air conditioners. Foowing this research, Breuker and Braun (1998a and 1998b) first identified important fauts and their impacts on rooftop air conditioners through interactions with industry personne, and then did a detaied aboratory evauation of the performance of the SRB FDD technique for fixed-orifice systems. To keep track of the up-to-date research, Comstock, Chen, and Braun (1999) performed an exhaustive iterature review of FDD in HVAC. This review provided a soid background and guide for ater research. The faut characteristics for a system with a TXV are different from those with a fixed orifice for which Rossi and Braun originay deveoped the SRB FDD technique. Chen (2) modified and evauated the origina SRB FDD technique for a 5-ton rooftop unit with a TXV as the expansion device. Li and Braun (22 and 23) improved the sensitivity and robustness of the SRB method and reduced its computationa requirements by deveoping a new on-ine modeing approach and new detection and diagnosis cassifiers. The resuting method is simper to

2 impement and was shown to have better sensitivity for detecting and diagnosing fauts than the origina method. However, it was not possibe to modify the method to hande mutipesimutaneous fauts, and the appication of this method to fied sites proved to be difficut because of the requirement for training modes using fied data. The method is better suited to impementation in origina equipment than for retrofit to fied appications. Inspired by an effort to formuate mode-based FDD techniques in a genera mathematica way, Li and Braun (24a) deveoped a decouping-based FDD approach to hande mutipe-simutaneous fauts and eiminate the need for mode training using fied data. The abiity to hande mutipe fauts was addressed by identifying features that decoupe the impacts of individua fauts. The need for on-ine modes was eiminated by empoying manufacturers rating data such as compressor and TXV maps (Li and Braun, 24c). These data are readiy avaiabe at no cost and are generic and reasonaby accurate. The performance of the decouping-based FDD method was initiay tested using aboratory data. A prototype software impementation was deveoped and a demonstration was created for iustration purposes using the Purdue fied site with fauts artificiay introduced. To evauate the potentia benefit for justifying service costs and FDD appication, Li and Braun (24b) aso deveoped an economic assessment method associated with FDD. The current paper first summarizes the decouping-based FDD methodoogy and economic assessment method, and then presents an initia appication to packaged commercia vapor compression equipment ocated in Caifornia. Decouping-Based FDD Technique and Economic Assessment Method Decouping-Based FDD Technique Fauts and system state variabes (temperatures, pressures ) are strongy couped. To hande mutipe-simutaneous fauts, the impacts of different fauts on the system shoud be decouped. That is, if one independent feature, which is impacted ony by one faut, can be found for each individua faut, then mutipe-simutaneous fauts are decouped. For a inearized system (see Equation (1)), a transformation matrix P can be found to diagonaize a transfer function matrix J, the inearized system mode representation, to decoupe the system if such a detaied system physica mode is avaiabe. Before transformation, the FDD feature Y is a vector with different fauts corresponding to different vaues of Y. After the interactions between the transformed FDD features Z and fauts X (see Equation (2)) are decouped, each entry of vector Z ony corresponds to a unique faut entry of the faut vector X and vice versa. That is, transformed features are scaars. At this point, the SRB faut detection cassifier deveoped by Li and Braun (23) can be extended to hande mutipe-simutaneous fauts. The origina n dimensiona FDD probem has been decouped to be n 1 dimensiona SRB FDD probems. The decouping-based cassifying method simpifies faut detection from a high-d probem to n 1-D ones (see Equation (3)). Y = JX (1)

3 Z = PY = PJX λ1 = λ 2 O X λn (2) ω1: norma 2 ( µ ) 1 z i σ i, norma 2 i, norma > ω2: fauty 2 ( χ ) {(1 α),1} (3) where, λ i are the transformed scaing factors, µ i, norma and σ i, norma are the mean and standard 2 1 deviation of feature z i at norma operation, ( χ ) {}, is the inverse chi-square cumuative distribution function, α is the fase aarm rate, and i = 1,2, L, n. Cass ω 1, norma operation is seected if the eft-hand-side is ess than right-hand-side and cass ω 2, fauty operation, is seected otherwise. Faut diagnosis automaticay is achieved without any extra computation immediatey after faut detection is finished, so a faut diagnosis cassifier is not required. This approach overcomes the primary drawback of the SRB diagnosis method in that it handes mutipesimutaneous faut diagnosis. The method is aso more generic and system-independent than the SRB method and does not require compicated rues, which depend on the system type. Mathematicay, there exist an infinite number of decouping features for steady-state operation. However, to obtain a detaied physica mode taking fauts into account for a rooftop unit system is extremey difficut and ony those with intuitive physica meaning and those that are readiy avaiabe (ow-cost) are practica. Li and Braun (24a) deveoped a methodoogy with guideines to decoupe the system, which unfods the back-box representing the rooftop unit system. This means that rooftop unit system is viewed from a microscopic point of view rather than treated as a system-eve mode reating inputs and outputs. Some independent features with physica meaning for component-eve fauts were found, and service fauts were isoated from operation fauts. There is an important and practica restriction for the independence features. They shoud be abe to be expressed as functions of ow-cost measurements such as temperatures and pressures. Equation (4) formuates the decouping scheme and resuts of a the rooftop fauts that was deveoped. Each eement of vector Z corresponds to one feature for one singe faut in terms of residua, which is defined as the difference between actua vaue and norma vaue. Strengths about this method are that 1) norma vaues are readiy avaiabe without the need of costy norma modes and 2) actua vaues are obtained either directy by ow-cost sensors or indirecty by virtua sensors based on ow-cost measurements. For exampe, the norma vaue of the condenser air fow rate is the nomina vaue given by manufacturer whie the actua vaue is estimated by an energy baance mode, and the norma vaue of pressure drop across the fiter/drier is zero in a statistica sense whie the actua vaue is estimated by an expansion device mode.

4 T T 2 P m& m& Tsh cond dis ca ea sc = Z = LX = NonCond CompLeak LLRestr CondFou EvapFou RefCharge where, Tcond is the temperature difference between the condensing temperature and saturated temperature based on condensing pressure, m& ca is condenser air mass fow rate residua, m& ea 2 is evaporator air mass fow rate residua, P is the iquid ine pressure drop residua, Tsh sc is the difference between suction ine superheat and iquid ine subcooing, NonCond denotes noncondensabe gas faut, CompLeak denotes compressor vave eakage faut, LLRestr denotes iquid-ine restriction faut, CondFou denotes condenser fouing faut, EvapFou denotes evaporator fouing faut, and RefCharge denotes refrigerant charge fauts incuding ow charge and overcharge. Matrix L is sparse and ower trianguar, which means that the system is uniateray decouped. That s, some fauts are ony decouped in one way. For exampe, compressor eakage faut affects condenser fouing faut but does not vice versa. The equation can be soved without iteration as proposed by Li and Braun (24a). Based on the decouping residuas, the cassifier described in Equation (3) can be appied to do FDD. A demonstration prototype was made for Purdue fied emuation site. Figure 1 shows the output at a point where four fauts had been introduced. The bar chart in the upper-eft quadrant shows individua faut indicators reative to a FDD threshod (.2, corresponding to a 4% of capacity degradation), which is chosen based on the sensitivity of the method and economic necessity. Each of the faut indicators have been normaized so that fu scae (i.e., 1.) corresponds to an individua faut causing 2% degradation in cooing capacity. The graph in the ower-eft quadrant shows impacts of the fauts on performance and safety factors as a function of time during the demonstration. The factors incude cooing capacity, COP, and overheating of the compressor. The capacity and COP are reductions reative to vaues for equipment operating normay. The compressor overheating is the difference between the current and norma compressor discharge temperature normaized by a vaue considered to be harmfu to the compressor ife. The tabe in the ower right quadrant summarizes the current vaues of the faut indicators and performance and safety factors. Aso shown are current recommendations provided by the FDD method. The demonstration has been very usefu in testing the FDD method for singe and mutipe fauts and for iustrating the potentia for appication of FDD. Economic Assessment Method In contrast to critica systems, the primary consequences of fauts in HVAC systems are economic rather than safety-reated. Therefore, FDD systems appied to HVAC systems must be assessed based upon economic considerations. Li and Braun (24b) investigated four major aspects of savings associated with FDD appication to rooftop units (RTU) quaitativey and quantitativey. They incude preventive maintenance inspection savings (PMIS), operationa cost savings (OCS) incuding utiity cost savings (UCS) and equipment ife savings (ELS), faut (4)

5 detection and diagnosis savings (FDDS) incuding unnecessary service savings and faut diagnosis savings, and smart service schedue savings (SSSS). Savings are functions of various factors such as ocation, buiding type, appication and system ratings. For exampe, UCS can be cacuated from equation (5) and ELS are quantified as equation (6). The tota OCS is the sum of UCS and ELS, which is especiay important for justifying service as we. UCS α = Q& Cap,Norma (1 ) T ) Ce (1 α) EER Norma (5) β ELS =.625 Q& Cap, Norma T 1 β (6) where, EER Norma is energy efficiency ratio under norma operation; α is the degradation in EER due to the fauts, Q & Cap, Norma is the norma cooing capacity; β is degradation in cooing capacity; C e is the cost of eectricity ($/kwh); and T is the actua runtime. Figure 1. FDD Demonstration Output Fied Appication to Packaged Commercia Equipment in Caifornia Description of Caifornia Sites A the fied-sites in Caifornia are sma commercia buidings that utiize packaged air conditioning and heating equipment. The criteria used for seecting the fied-sites incuded: 1)

6 buiding occupancy type and size; 2) HVAC system instaed, and 3) cimate region. The types of buidings incude smaer retai stores, pay areas for fastfood restaurants and moduar schoorooms. The HVAC systems instaed incude different rooftop and wa mounted units with different capacities and from different manufacturers. The cimates incude two different macrocimate types: coasta and inand. The foowing system state variabes were measured: high and ow-side pressures, suction and discharge ine temperatures, evaporating and condensing temperatures, iquid ine temperatures before and after fiter/driers, condenser inet and outet air temperatures, return and outdoor and suppy air temperatures, return and outdoor air humidities, and current and votage and power consumption of the compressor. Appication of FDD to a Fastfood Restaurant Pay Area This site is ocated in Oakand, Caifornia. A 6-ton rooftop unit was instaed for the pay pace of the fastfood restaurant. A scro compressor and a TXV are used in this RTU. Data coected from Apri to October in 22 were used to perform FDD. After fitering the transient data by a steady-state detector and removing the bad data corrupted by the acquisition equipment, 1119 data points (one data point every five minutes) were retained. Since the RTU has been instaed for severa years, fauts have been fuy deveoped. Statistica resuts are presented in terms of histogram bar pots. Figure 2 shows normaized faut indicators for the five different fauts considered in this study. Figure 2a gives normaized faut indicators for a iquid-ine restriction faut. A the steady-state data points are ocated at the right of the red dotted FDD threshod ine and the mean vaue is around.8. That is, a steady-state points indicate that the iquid-ine was restricted. Most ikey the fiter or drier was cogged by debris. If this faut happened individuay, it woud resut in about a 16% cooing capacity degradation. Figure 2b pots the normaized faut indicator for refrigerant charge fauts. Simiar to Figure 2a, a the steady-state data points are ocated to the right of the FDD threshod and the mean vaue is about 1.6, which means that the system charge was very ow. If this faut happened individuay, it woud resut in about a 32% cooing capacity degradation. Figure 2c pots the normaized faut indicator for a condenser fouing faut. Most of the steady-state data points (>95%) are to the right of the FDD threshod and the mean vaue is about.5, which indicates that the condenser was a itte dirty. If this faut happened individuay, it woud resut in about 1% cooing capacity degradation. Figure 2d pots the normaized faut indicator for a compressor vave eakage faut. A the steady-state data points are to the eft of the FDD threshod and the mean vaue is about -.7, which indicates that the compressor worked propery and the compressor had about 15% heat oss. However, according to heat transfer anaysis and our experience with aboratory data, compressors instaed in packaged RTUs have very sma heat oss, ess than 5% of the power input and even gain some heat at some operating conditions. The expanation for this discrepancy is probaby that the discharge ine temperature is not measured accuratey using the RTD temperature sensor. Braun and Li (23) investigated the RTD measuring issue and presented a correction approach. However, there is evidence that the discharge ine temperature sensor was not probaby instaed or insuated. The normaized faut indicator for the evaporator fouing faut is potted in Figure 2e. Most of the steady-state data points are at the right of the FDD threshod and the mean vaue is

7 25 Figure 2. Normaized Faut Indicators for Different Fauts (a) Liquid-ine Restriction (b) Refrigerant Low Charge 25 Number of Data Points Normaized Faut Indicator Normaized Faut Indicator (c) Condenser Fouing (d) Compressor Leakage Number of Data Points Normaized Faut Indicator Normaized Faut Indicator 5 (e) Evaporator Fouing Number of Data Points Normaized Faut Indicator

8 about.96, which indicates that the evaporator was dirty. If this faut occurs individuay, it woud resut in about 19% cooing capacity degradation. In summary, the system had four simutaneous fauts, ow refrigerant charge, iquid ine restriction, condenser fouing and evaporator fouing. Economic Savings Assessment for the Fastfood Restaurant Pay Area Figure 3a pots the cooing capacity degradation associated with the combination of these fauts for the case study site. The system cooing capacity was degraded 23~45% (average of about 32%), which was consistent with the vaue indicated by the refrigerant charge faut indicator. The ow charge faut is a system-eve faut which is not competey decouped from other component-eve fauts and thus its indicator refects the impact of other fauts. The cooing capacity degradation is consistent with the return air temperature and system running time. It can be seen from Figure 3b that the average return air temperature was around 78 F and the highest was 88 F, which means that this equipment did not maintain comfortabe space conditions much of the time. From Figure 3c, it can be seen that the system ran continuousy for a ong time (average of 2.5 hours, maximum of 9 hours). So, based on a comfort criterion, service shoud have been done to correct the diagnosed fauts. Figure 4a pots an economic criterion for service, EER degradation. It can be seen that the system EER degraded between about 1~4%, depending on the operating conditions. Compared with the cooing capacity degradation, the EER degradation was a itte smaer. Figure 4b pots the system power consumption reduction. The average power consumption reduction was about 15%, which was smaer than the average cooing capacity degradation ( β ) of 32%. The average degradation in EER (α ) was 21%. Roughy, the weighted average utiity rate ( C e ) between on-peak and mid-peak periods in North Caifornia is.8$/kwh. For this restaurant appication in North Caifornia, the average runtime was 9 hour/year. With automated FDD, it is estimated that direct operationa cost savings (OCS) for this site woud be about $252/year with UCS of $17/year and ELS of $145/year. This estimate does not consider the direct impact of fauts on reducing the runtime before faiure of the compressor, but ony their impact on ife due to an increase in runtime. It woud cost about $1 to repace the compressor in this air conditioner, so this coud be an important effect that is not considered. Service costs to add some refrigerant, repace the fiter/drier, and cean the heat exchanger cois woud be around $5 if they were performed in one trip. Based upon the OCS aone, the economic payback for service to correct the fauts woud be around 2 years. Thus, service appears to be justified based upon both comfort and economic criteria for this case study. There are some other significant savings associated with the appication of automated FDD that need to be considered in evauating overa economics. For a singe RTU, the foowing estimates were obtained using data from service providers: PMIS of $23/year, FDDS of $137/year, and SSSS of $7/year. PMIS comes from reduction of costy preventive maintenance visits. Appication of automated FDD woud reduce or eiminate the costy and time-consuming manua diagnostic time. For this particuar case study, it woud be difficut for an experienced technician to identify a four fauts in one visit without the hep of an automated FDD system. It is estimated that smart service scheduing for mutipe fauts woud contribute about $7/year in savings. The tota estimated savings associated with the appication of automated FDD to this site woud be about $662/year. It appears that this eve of savings woud provide reasonabe paybacks for impementation of automated FDD.

9 Figure 3. Capacity Degradation, Return Air Temperature, and Runtime Distributions (a) (b) Number of Data Points Cooing Capacity Degradation Return Air Temperature 5 (c) Number of Data Points Continuous Running Time between off cyces (min) Summary Resuts for Other Sites Since the sites in Caifornia were originay configured for appication of the SRB technique, some necessary information about the compressor, expansion device and system configuration were not avaiabe for appication of the FDD method discussed in this paper. Therefore, the FDD technique was ony partiay appied to the other sites. Simiar to Oakand fastfood restaurant site, data coected from Apri to October in 22 were used to do anaysis for the foowing sites except for the Anaheim retai store site, which is based on data coected in 23. The diagnosis resuts for these sites are summarized in Tabes 1, 2, 3 and 4. In summary, the initia investigation shows that fauts happen very frequenty at the fied sites. Fifteen of the twenty-one systems (71%) are significanty impacted by fauts: eeven (52%) have fiter/drier restrictions, ten (48%) have refrigerant charge fauts, eight have (38%) have more than two simutaneous fauts and nine (43%) justify service immediatey.

10 2 Figure 4. EER Degradation and Power Consumption Reduction (a) (b) 4 Number of Data Points Degradation of EER Power Consumption reduction Tabe 1. FDD Resuts for Moduar Schoo Sites Woodand Oakand RTU1 RTU2 RTU1 RTU2 Refrigerant Charge Norma Norma Norma Over Liquid-ine Restriction Yes Yes Yes No Evaporator Fouing No No No Yes Service Not yet Yes Not yet Yes Based on the statistica assumptions made by Braun and Li (24), the ifetime tota savings per ton are summarized in Tabe 5 for a different types of appications and ocations. The estimated savings over the 1-year ife of the unit range from $8 to $17 per ton. Greater savings are possibe in hotter cimates (South Caifornia) due to arger cooing requirements and higher utiity rates. The savings woud be greater for heat pumps because they operate throughout the whoe year. Concusions A decouping-based FDD technique and economic savings assessment method were appied to packaged air conditioning equipment at Caifornia fied sites. Initia investigation shows that fauts occurred frequenty at the fied sites, which was confirmed by a recent site visit. The FDD technique was abe to diagnose most of the existing fauts even with imited information. The potentia yeary cost savings associated with appication of the FDD technique are significant, ranging from $8 to $17 per ton. The preiminary estimation of impementation costs of the FDD technique approximatey ranges from $25 to $6 per system for individua appication and $175 to $375 for mutipe appications. So, for a midde-sized RTU (say 6-ton),

11 the yeary savings woud range from $5 to $1 and woud appear to provide reasonabe payback periods for impementation. Tabe 2. FDD Resuts of Pay Areas in Fastfood Restaurant Sites Bradshaw Castro Vaey Watt Avenue Stage 1 Stage 2 Stage 1 Stage 2 Refrigerant Charge Low Norma Norma Low Norma Liquid-ine Restriction Yes No No No No Service Yes NA NA Not NA Tabe 3. FDD Resuts for Retai Store Sites at Riato RTU1 RTU2 RTU3 RTU4 RTU5 Refrigerant Charge Low Low Norma Norma Over Liquid-ine Restriction No Yes No No No Service Yes Yes NA NA No Tabe 4. FDD Resuts for Retai Store Sites at Anaheim RTU1 RTU2 RTU3 RTU4 RTU5 Refrigerant Charge Low Low Norma Low Norma Liquid-ine Restriction Yes Yes Yes Yes Yes Service Yes Yes Not yet Yes Not yet Tabe 5. Estimated Lifetime Tota Savings per Ton of Cooing (based on a 6-ton RTU) Location Buiding Type PMIS OCS FDDS SSSS TELS ($) ($) ($) ($) ($) North Caifornia Moduar Schoo Restaurant Retai Store Moduar Schoo South Caifornia Restaurant Retai Store Acknowedgements This work was supported by the Caifornia Energy Commission (CEC) with additiona cost-sharing provided by Honeywe, Inc and Fied Diagnostic Services, Inc. We are extremey gratefu for the guidance and technica support provided by Chris Scruton at the CEC, Vern Smith at the Architectura Energy Corporation (AEC), Todd Rossi and Doug Dietrich at Fied Diagnostic Services, and Bob Sundberg, Adrienne Thome, and Bi Bray at Honeywe.

12 References Braun, J.E. and Li, Haorong. 23. A Decouping-Based FDD Approach for Mutipe- Simutaneous Fauts. Progress report submitted to Architectura Energy Corporation. Braun, J.E. and Li, Haorong. 24. Automated Faut Detection and Diagnostics of Rooftop Air Conditioners for Caifornia. Fina report submitted to Architectura Energy Corporation. Breuker, M.S. and Braun, J.E., 1998a. Common Fauts and Their Impacts for Rooftop Air Conditioners, HVAC&R Research, Vo. 4, No. 3, pp Breuker, M.S. and Braun, J.E., 1998b. Evauating the performance of a Faut Detection and Diagnostic System for Vapor Compression Equipment, Internationa Journa of Heating, Ventiating, and Air Conditioning and Refrigerating Research, Vo. 4, No. 4, pp Chen, Bin, 2a. Evauating the potentia of on-ine faut detection and diagnosis for rooftop air conditioners, Master s Thesis, Schoo of Mechanica Engineering, Purdue University. Comstock, M.C., J.E. Braun, and B. Chen, 1999, Literature Review for Appication of Faut Detection and Diagnostic Methods to Vapor Compression Cooing Equipment, Purdue University, Ray W. Herrick Laboratories, Report # HL Dexter, Arthur; Pakanen, Jouko (ed.). Demonstrating Automated Faut Detection and Diagnosis Methods in Rea Buidings. VTT Buiding Technoogy, 21. VTT Symposium p. + app. 13 p. Isermann, R Process Faut Detection Based on Modeing and Estimation - A Survey, Automatica, 2(4): Li, H. and Braun, J.E. 23, An Improved Method for Faut Detection and Diagnosis Appied to Packaged Air Conditioners. ASHRAE Transactions, Vo.19, Part 2. Li, H. and Braun, J.E., 22, On-Line Modes for Use in Automated Faut Detection and Diagnosis for HVAC&R Equipment, Proceedings of the 22 ACEEE Conference on Energy Efficiency in Buidings, Monterey, CA, 22. Li, H. and Braun, J.E., 24a, A Methodoogy for Diagnosing Mutipe-Simutaneous Fauts in Rooftop Air Conditioners. Accepted for the Tenth Internationa Refrigeration and Air Conditioning Conference at Purdue to be hed on Juy 12-15, 24. Li, H. and Braun, J.E., 24b, An Economic Evauation of Automated Faut Detection and Diagnosis for Rooftop Air Conditioners. Accepted for the Tenth Internationa Refrigeration and Air Conditioning Conference at Purdue to be hed on Juy 12-15, 24. Li, H. and Braun, J.E., 24c, Modeing Adjustabe Throat-Area Expansion Vaves. Accepted for the Tenth Internationa Refrigeration and Air Conditioning Conference at Purdue to be hed on Juy 12-15, 24. Rossi, T.M. and Braun, J.E., A Statistica, Rue-Based Faut Detection and Diagnostic Method for Vapor Compression Air Conditioners, Internationa Journa of Heating, Ventiating, and Air Conditioning and Refrigerating Research, Vo. 3, No. 1, pp