ASHRAE 4842-2006 VAV Airflow Sensor Response in Relation to “Poor” Upstream Duct Geometry《变风量空调气流传感器的反应在有关“欠佳”上游管几何(反相1137)》.pdf
《ASHRAE 4842-2006 VAV Airflow Sensor Response in Relation to “Poor” Upstream Duct Geometry《变风量空调气流传感器的反应在有关“欠佳”上游管几何(反相1137)》.pdf》由会员分享,可在线阅读,更多相关《ASHRAE 4842-2006 VAV Airflow Sensor Response in Relation to “Poor” Upstream Duct Geometry《变风量空调气流传感器的反应在有关“欠佳”上游管几何(反相1137)》.pdf(12页珍藏版)》请在麦多课文档分享上搜索。
1、4842 (RP-11.37) VAV Airflow Sensor Response in Relation to “Poor” Upstream Duct Geometry Wayne Klaczek Student Member ASHRAE Pat Fleming, PE Member ASHRAE Mark Ackerman, PE Member ASHRAE Brian Fleck, PhD, PE ABSTRACT A recent ASHRAE research project, RP-1137, indicated Variable air volume (?AV) term
2、inal placement has typi- cally been considered “good enough in the HVAC industry, regardless of upstream duct geometry that clearly afects the response ofthe flow sensoy. This paper describes the loss of VAV flow sensor amplijcation, un formi, and precision following common upstream duct geometries
3、from a recom- nzissioning standpoint. Two brands of VAV terminals were tested in three different sizes: 250,200, and I50 mm diameters (referred to as 1 0, 8, and 6 in. throughout thispaper). Upstream geometry was modeled after transitions that were deemed Ipoor ” during a recommissioning procedure t
4、hat was completedat three facilities duringASHRAE Research Project I1 3 7. The poor ”upstream conditions included combinations of concentric reducers, expanders, 90” elbows, and S-shape geometries with varying straight duct lengths prior to the sensor: This paper outlines some basic recommendations
5、that HVAC designers, terminal manufacturers, and commissioning agents can use to improve the accuracy of VAVairflow sensors both before installation and as part of a recommissioning procedure. BACKGROUND INFORMATION Variable air volume (VAV) systems with direct digital control (DDC) are commonly imp
6、lemented because of added stability, zone control, and greater energy efficiency. However, the literature indicates that a thorough commissioning proce- dure is required to ensure that VAV systems with DDC operate as they were intended (Elovitz 1992). Recommissioning (or retrocommissioning) VAV syst
7、ems is generally beneficial based on energy savings alone (Piette and Nordman 1996; Kjellman et al. 1996). that the calibration of individual VAV terminals was often the most significant improvement made during recommissioning (Klaczek et al. 2004; Klaczek et al. 2006). Several VAV termi- nals were
8、identified during the course of RP-1137 located downstream of “poor” duct conditions, and questions were developed regarding the response of VAV airflow sensors. It was hypothesized that VAV airflow sensor response is highly dependent on upstream duct geometry, even after a perfect recommissioning p
9、rocedure. Designers are often compelled to use transitions prior to VAV terminals due to constricted ceiling spaces; however, often ductwork is installed with poor geometry contrary to the design intent. Sometimes this is due to space constraints, while in the case of concentric reducers, these tran
10、sitions are some- times added to allow for large ducts and lower static pressure losses prior to the VAV terminals (concentric reducers are often supplied by manufacturers of the box if they do not actu- ally manufacture the specified size, for instance, downsizing a 8 in. duct to a 4 in. box). Unfo
11、rtunately, there is a common misconception that the effects of transitions are minimized if a long straight section of ducting is located prior to the VAV terminal. Variable air volume airflow sensors are designed to provide an amplified pressure signal that can be monitored with a low-cost pressure
12、 transducer. Unfortunately, poor upstream geometry can provide a significant calibration error despite the presence of long straight ducting prior to the VAV terminal. INTRODUCTION This study sought to experimentally determine the effects of poor upstream duct geometry on the response provided by Wa
13、yne Klaczek is a research engineer at C-FER Technologies, Edmonton, Alberta, Canada. Mark Ackerman is the faculty service officer for the Mechanical Engineering Department and Brian Fleck is an associate professor at the University of Alberta, Edmonton. Pat Fleming is a mechanical engineer at Hemisp
14、here Engineering, Inc., Edmonton. 202 O2006 ASHRAE. different VAV airflow sensors from a recommissioning point of view. Several nonideal duct configurations were re-created in a laboratory based on the upstream conditions commonly found in real buildings. However, it is important to note that these
15、upstream conditions are far from the worst types of tran- sitions that are possible. The geometries that were tested included a 40D baseline case (a baseline measurement taken with 40D of straight duct prior to the test location) and combi- nations of 90“ short radius elbows, concentric reducers, ex
16、panders, and S-shape geometries. The theoretical VAV sensor response was determined from basic fluid mechanics and compared to the experimental results. The VAV airflow signal was compared to the true airflow rate to determine which geometries resulted in the greatest sensor amplification loss with
17、a dimensionless flow coefficient (C). The upstream geometries were ranked in terms of the greatest losses in amplification and precision. Flow visualization experiments were also completed for a few common geometries (concentric reducer, 90“ elbow, and the 40D baseline case) to examine internal airf
18、low behavior. The results identified common problems with the current design of VAV airflow sensors and led to recommendations for manu- facturers, system designers, and commissioning agents. THEORY VAV averaging flow sensors are simple in both design and practice; as such, they are a common and cos
19、t-effective way to monitor airflow. VAV airflow sensors utilize a differential pressure signal (U) to estimate the airflow by monitoring both the total pressure (P,) and the wake pressure (Pw), as shown by Equation 1. where QTRUE = K= A AP= P - - - T - - pw - true airflow rate, m3/s a constant, dime
20、nsionless cross-sectional area, m2 differential pressure signal, Pa air density, kg/m3 total pressure, Pa wake pressure from the VAV sensors low-pressure port, Pa All of the terms on the right-hand side of Equation 1, except the differential pressure (AP = PT-Pw), are commonly represented by some co
21、nstant within a typical DDC system, which is a generally safe assumption since the air density (p) is the only variable (and it changes only very slightly with temperature). VAV airflow sensors provide an amplified pres- sure signal because the low-pressure port is located within the wake region (fo
22、llowing the sensor); the presence of a wake region is accomplished with sensor geometry, shape, or with the addition of protrusions to ensure flow separation occurs. The theoretical amplification that can be expected from a VAV airflow sensor will be proportional to the square root of the pressure c
23、oefficient (C,) at the low-pressure port, as shown in Equation 2. Thus, the relationship between the pressure coefficient (C,) and the VAV flow signal is given by Equation 3, which is generated by substituting the Pw term from Equation 2 into Equation 1 and simplifying. where C, = the pressure coeff
24、icient, dimensionless PD = the dynamic pressure, Pa V, = air velocity far upstream, ds QVAV ,/(p-pw)(1 -cp) (3) It is clear from Equation 3 that the more negative the pressure coefficient (C,) the greater the amplification of the VAV flow sensor. The VAV flow sensors that were evaluated within this
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