1、4745 (RP-1137) Field Performance Assessment of VAV Control Systems to Determine the Longevity of Recommissioning Wayne Klaczek Pat Fleming, PE Member ASHRAE Mark Ackerman, PE Member ASHRAE Brian Fleck, PhD, PE Member ASHRAE ABSTRACT This paper summarizes key results of ASHRAE Research Project RP-113
2、7, which sought to quantzfi the benej2s and longevity of the recommissioning process on variable air volume (VAV) systems that incorporate direct digital control (DDC). Field testing was completed in three diverse facilities over aperiod of up to two years, generating three data trends. General perf
3、ormance indicators were compared using the data collected prior to commissioning, immediately after commissioning, and long after commissioning was completed (a minimum of six months later). Key performance indicators included: improved DDC system accuracy, indoor air quality (IAQ), and energy efici
4、encyhavings. It is important to note that RP-I 13 7 did not seek to determine allpossible recommis- sioning benejits, only to quantch some of the benejts using jeld testing and laboratovy experiments. This research indi- cates that commissioning is benejcial and that system recom- missioning is typi
5、cally justified within a time period similar to the capitalpaybackperiod. Conclusions are limited to the time period specified; thus, the longevity of recommissioning is at least equal to the payback period but cannot be predicted beyond this within RP-1137. INTRODUCTION Variable air volume (VAV) sy
6、stems with direct digital control (DDC) are generally implemented because of added economy, control, and operational efficiency when compared to conventional systems. Due to the variability with which these systems respond to operating conditions, system commissioning has evolved as an essential too
7、l to ensure HVAC systems operate as they were originally designed (Piette and Nordman 1996; Ellis 1996; Cappellin 1997; Elov- itz 1992). The general benefits of VAV systems with DDC have been documented in the last 30 (or so) years within ASHRAE Transactions. Therefore, for the purposes of this pape
8、r, VAV systems with DDC are generally treated as the best alternative for HVAC systems that are intended to minimize energy costs and maximize individual comfort. However, it is important to acknowledge that most sources that claim VAV systems are superior often refer to the importance of the commis
9、sioning process. Thus, the consensus seems to be that although VAV systems are an excellent option, these systems may behave quite poorly if commissioning is neglected or completed improperly. The problems associated with these systems often result in compromised comfort, lost energy e%- ciency, and
10、 indoor air quality (IAQ) concerns that must be addressed during the commissioning stages. Although the benefits of building commissioning and recommissioning have been recognized for some time, little formal work has been found dealing with the longevity of the recommissioning process. For instance
11、, if a system is re- commissioned such that the components are working perfectly to the original design intent, how long will the system operate in an acceptable manner? Six months? One year? Perhaps 10 years? These types of systems rely on DDC to adjust the system response to deal with varying envi
12、ron- mental conditions; therefore, the proper response of the system is highly influenced by the accuracy of the various control system sensors. If the sensors are not calibrated correctly, the control system will deviate from the design intent. Wayne Klaczek is a graduate student, Mark Ackerman is
13、the faculty service officer, and Brian Fleck is an associate professor in the Mechan- ical Engineering Department, University of Alberta, Edmonton, Alberta, Canada. Pat Fleming is a mechanical engineer at Hemisphere Engi- neering, Inc., Edmonton. 02005 ASHRAE. 37 ASHRAE Research Project 1137 at leas
14、t two years without any formal commissioning process. The objective of RP-1137 was to quantify the benefits, if any, of a recommissioning procedure on VAV systems with DDC at three different installations of varying design (although all of the systems were obviously VAV-based with DDC). The benefits
15、 of calibration were evaluated from a control performance perspective and were assessed before (BC) and after (AC) the recommissioning process with comparisons made to the original design intent. After a mini- mum of six months (AAC) an additional assessment was made to quantifi any deterioration in
16、 performance. Examples of specific performance indicators include energy efficiency, thermal comfort, and IAQ related to an adequate outdoor air flow rate into each zone. For the purposes of this research project, it is also impor- tant to note that the formal commissioning process, as outlined by A
17、SHRAE Guideline 1-1996, was not implemented. Rather a component of formal commissioning, the process of calibra- tions and operational checks, was performed on the systems considered in RP-1137. Formal commissioning is a detailed quality-control process that ensures a building system complies with a
18、 given design, operating within the accepted parameters of the design intent. This process is generally imbedded within the construction of a facility and forms part ofthe construction obligations of aproject. RP-1137 sought to isolate the quantifiable effects of system calibration and fault detecti
19、on without the involved process of forming a detailed design intent for a building system. Throughout RP-1137 the original design intent was assumed to be the current system setpoints; thus, a formal commissioning process was not completed, as it is beyond both the scope and the requirements of RP-1
20、137. The design intent was assumed to exercise the recommissioning procedure; thus, it was possible to evaluate the effects and the longevity of recommissioning VAV systems with DDC over time. METHODOLOGY AND DATA COLLECTION ASHRAE RP- 1 137 was extremely reliant on data collec- tion and the trendin
21、g capabilities of DDC systems. The choices of suitable test locations and the use of the control systems data trending functions and of external instrumenta- tion to make measurements were all vital considerations. The following section briefly describes the test locations consid- ered as well as th
22、e data trend techniques and the basic meth- odology for the project. Field Test Locations The test locations considered within RP-I 137 had to be within separate buildings and provide a suitable variation in system configuration, mechanical equipment, operations, maintenance practices, and control m
23、ethodology. Facilities were selected that had a minimum flow of 20,000 CFM (9439 L/s), approximately 20 or more VAV terminals, and varying brands of DDC systems. Finally, it was specified that each facility considered in the study must have been operating for Three suitable facilities were located i
24、n the city of Edmonton and surrounding area. The facilities included a clinical wing at a large medical facility (5 1 supply VAV terminals), an institu- tional building located at the University of Alberta (51 supply VAV terminals), and a municipal library (17 supply VAV terminals). Data Trends and
25、Common Instrumentation The existing control systems at each facility were used to collect extensive trend log data for RP-1137 that were often facilitated using the original building control systems. Addi- tional instrumentation included standard instruments such as pitot tubes, flow arrays, pressur
26、e transducers, thermocouples, and a custom built data acquisition system (DAS) with tracer gas-monitoring capabilities. The calibration of project equip- ment was checked regularly and compared to known stan- dards. For instance, pitot tubes and airflow measurement devices were calibrated using a wi
27、nd tunnel to verifi the accu- racy and repeatability. Measurements were made over the course of RP-1137, relying primarily on the existing DDC systems at each of the facilities, including both terminal data and system data. Data trends were completed for a minimum two-week period; however, the trend
28、ing period was increased within RP- 1 137, sometimes to an excess of two months. The second data trend was taken immediately after the commissioning process, while the third and final data trend was to be completed at least six months later to allow the system to reach a new equilibrium (and possibl
29、y deviate). These final data trends were typically completed 12 to 18 months after recommissioning. In all cases, the DDC indicated airflow rate, airflow setpoint rate, damper position, and the minimum and maximum airflow setpoints, which were continuously monitored and trended. System variables typ
30、ically included all of the available trend system variables for the air-handling unit of interest, including (at a minimum) return, supply, outdoor, and mixed air temper- atures and air flow rates, fan speeds, system static pressure, supply fan pressure, end-of-line static pressure, building pres- s
31、urization, energy utilization of the major components, valve operations (for each heatingkooling coil), and air humidity. The most important trended data included the VAV terminal variables (particularly the indicated flow rates), the energy utilization of the major components (particularly the supp
32、ly and return fans), the zone temperatures, and the air-handling unit operation (for minimum outdoor air and supply air flow rates). Ten-minute time intervals were chosen to provide a suitable amount of detail within the trended data. Tracer Gas Testing In order to monitor the outdoor air flow rate,
33、 a tracer gas system was used to introduce a set amount of SF, into the supply airstream. The preferential way to measure the outdoor air flow is to use a hot-wire anemometer grid or a multi-point pitot tube station; however, in reality it is often very difficult 38 ASHRAE Transactions: Research to
34、use these instruments because they require a long section (greater than at least 1 O straight, unobstructed duct diameters) to work effectively. For this reason a custom DAS was used to directly measure the outdoor and return airflows using the concentration of tracer gas in the appropriate airstrea
35、m, as shown in Figure 1. During the course of RP-1137 both CO2 and SF, were utilized; however, problems were encountered early in producing a suitable CO, concentration signai over background levels. Providing a consistent background concentration of CO, was not possible in the field test loca- tion
36、s; thus, SF, was used exclusively. The literature seems to indicate mixed results using SF, tracer gas injection. Fisk and Faulkner (1 992) estimate the measurement error of tracer gas injection to be in the neighborhood of 10%. More recent stud- ies (Fisk et al. 1999) utilized SF, tracer gas to mea
37、sure the airflow within two commercial buildings and concluded that even with a well-developed tracer gas measurement system, the overall flow rate uncertainty was approximately *7%, largely due to problems with adequate mixing (although they even incorporated a fan to improve mixing in the duct, wh
38、ich was not completed in RP-1137). The tracer gas technique seems to be one of the preferential methods to monitor system performance; however, the results of this research project indi- cate that tracer gas injection is not always beneficial, espe- cially at field test locations. Extremely precise
39、measurements and long setup times are required that significantly reduce the benefitskime ratio of this type of measurement. These conclu- sions do not state that tracer gas injection is a poor technique for all research (indeed this is untrue); however, the tracer gas method was found to be general
40、ly unsuitable for use within this research project and will be largely ignored in the subse- quent analysis. -i/ I/ i- + 1 )(Solenoid Vaiv i I I +- + Solenoid I valve/ Solenoi 01 d Valve . d -*- Valve Calibration G (1PPm SFs) as - 1 Static Pressure Transducers . il t 1 9 cfm Vacuum Pump at 30C surro
41、unding the Analyzer I P running an algorithm in Quick Basic Figure 1 General schematic of the tracer gas injection system for a simplistic field test case. Note that in this conjguration SF, is utilized with the constant injection technique to experimentally determine the O/A during the course of th
42、e trend. SF, is injected into the supply air duct and measured in each of the return, supply, and outdoor ducts (recall outdoor concentration should remain zero, and this was ver$ed experimentally). Samples are taken using solenoid valves and a vacuum pump; the samples are drawn into the gas analyze
43、r. The DAS simultaneously measures temperature and duct pressure at several locations in 1 O-minute loops. ASHRAE Transactions: Research 39 Key Performance Indicators Used within RP-1137 The key performance indicators used to gauge the longev- ity of the recommissioning process were a major consider
44、ation within RP-1137 and were based both on existing work and general suitability for this project. Useful work completed by Elovitz (1 992), Mumma and Bolin (1 994), Piette and Nord- man (1 996), Ellis (1 996), Kjellman et al. (1996), Cappellin (1 997), Maki et al. (1 997), Bearg (1 999), Krarti et
45、 al. (2000), and LaBauve et al. (2002) was especially relevant. Based on this past work, it was possible to select a number of possibly useful performance indicators when commissioning VAV systems and to outline the likely benefits of the recommission- ing process. Performance indicators useful with
46、in RP- 1 137 have been compiled based on how well these indicators could be monitored over time. For instance, comfort is generally improved during the recommissioning process, but it is a rela- tively subjective term that is difficult, in general, to exactly quanti; thus, comfort was neglected as a
47、 performance indi- cator in RP-1137. By contrast, the thermal response of each zone was easily determined from the DDC trends and could be used to quantify deviation in system response over time; thus, thermal response was considered although it is not discussed here. A similar strategy was used to
48、determine energy- and economics-related performance indicators; thus, the list of energy indicators considered in this paper is not meant to represent an exhaustive list of all the benefits associated with the recommissioning process-or even the most significant ones. These indicators are the ones t
49、hat could most accurately be quantified during the course of RP-1137. Important indi- cators considered in RP-1137 and discussed here include: improved DDC accuracy, IAQ variations using trend data, and energy savings. The Recommissioning Procedure Recommissioning, specifically focused on the airflow aspect of the systems, was completed at each of the three test facilities. Commissioning is a broad term and is often used to describe everything from a full commissioning procedure to routine system maintenance. Typically, this involves testing the operation of a buildings HVAC systems
