1、676 2009 ASHRAEABSTRACT In this study, an energy balance and uncertainty analysis was performed on a standard chilled water cooling coil mounted in a commercial Air Handling Unit operating under typical conditions with a conventional PID loop control. Two different sets of relative humidity transmit
2、ters and temperature sensors (high and low accuracy) were evaluated for measuring relative humidity and temperature of the moist air entering and exiting a cooling coil. The impact of the different errors in these sensors and installation on the uncertainty in the energy calcu-lation is presented. I
3、n addition, the affects of the transient behavior inherent in the cooling coil with respect to the energy balance was evaluated. This study gives insight to how an energy balance test coupled with an uncertainty analysis could be used to verify the cooling coil system performance and instrumentation
4、 output. Experimental results showed that the transient behavior inherent to the cooling coil had a negligible affect on the energy balance calculations and that by employ-ing high accuracy instrumentation and careful installation, expected energy balance results could be attained.INTRODUCTIONThe in
5、strumentation used in a control system of an HVAC application can have a profound effect on the operation and overall energy use of the system. Adequate performance and control of the HVAC system can be achieved through proper selection, installation and operation of the instrumentation used to cont
6、rol the system. Within this study, energy balance and uncertainty analy-ses were performed on a standard chilled water cooling coil mounted in a commercial Air Handling Unit operating under typical conditions with a conventional PID loop control. This study gives insight to how an energy balance tes
7、t coupled with an uncertainty analysis could be used to verify the cooling coil system performance and instrumentation output.For heating and cooling applications, if the energy balance between the moist air and water-side are found to agree, then it is likely that the instrumentation selection, ins
8、tal-lation and operation are as expected. The use of energy balance testing can validate theoretical models, confirm the installation of the equipment and instrumentation, aid in commissioning and substantiate design parameters and stan-dards for HVAC equipment.There are many opportunities for error
9、s in an experimen-tal measurement to arise. These errors can be divided into two categories: errors due to the physical hardware that is perform-ing the measurement (e.g., linearity, repeatability, hysteresis, sensitivity, distortion, responsiveness, etc.), and errors due to the placement/installati
10、on of this hardware. In addition, after the measurement is taken by the instrument, the signal from the sensor could be subject to error caused by distortion due to the wiring and physical terminations, the accuracy of the trans-ducer converting the signal, the accuracy of analog to digital conversi
11、on, and the ability of the software to process, display, and record the signals. For the experimental work in this study, several approaches were undertaken to minimize the error due to the instrument placement and include the use of the sampling tubes for the high accuracy (HA) exiting air relative
12、 humidity and temperature measurements, four point averaging temper-ature sensors for low accuracy (LA) exiting air temperature and direct contact liquid temperature sensors located in the agitated water flow stream at a pipe elbow. Experimental Measurement and Uncertainty Analysis on the Energy Per
13、formance of a Chilled Water Cooling CoilRyan D. Warren, PhD Rahul L. Navale, PhDStudent Member ASHRAE Student Member ASHRAERon M. Nelson, PhD, PE Curtis J. Klassen, PEMember ASHRAE Member ASHRAERyan D. Warren is a senior project engineer in the Energy and Carbon Management division of Nexant, Inc.,
14、Madison, WI. Ron M. Nelsonis a professor in the Department of Mechanical Engineering, Iowa State University, Ames, IA. Rahul L. Navale is a senior project engineer with Eaton Corporation, Eden Prairie, MN. Curtis J. Klaassen is a manager at Iowa Energy CenterEnergy Resource Station, Ankeny, IA.LO-09
15、-064 2009, American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc. (www.ashrae.org). Published in ASHRAE Transactions 2009, vol. 115, part 2. For personal use only. Additional reproduction, distribution, or transmission in either print or digital form is not permitted without
16、 ASHRAEs prior written permission.ASHRAE Transactions 677Literature ReviewThe need for an uncertainty analysis in general is docu-mented in numerous sources (Coleman and Steele 1989, Dieck 1992). Past studies have been done showing the use of uncertainty analysis in cool storage inventory, quantifyi
17、ng operational energy and cost savings, and evaluating HVAC loads. In addition, research has been done evaluating instru-mentation performance relative to the manufacturers specifi-cations and substantiating the importance of correct instrument selection for system control. However, no studies were
18、found to show the use uncertainty analysis coupled with an energy balance analysis for commissioning purposes and/or verifying instrumentation performance.Performing a proper uncertainty analysis can be very complex and time consuming. There are several references that address uncertainty analysis a
19、s applied to the evaluation of HVAC systems. ASHRAE Guideline 2-1986 (ASHRAE 1986) provides a guideline for reporting uncertainty in results of experimental data as applied to HVAC equipment. DAlbora et al. (1999) evaluated the uncertainty in cool storage inventory using an energy balance method and
20、 Reddy (1999) has applied engineering uncertainty analysis in the evaluation of energy and cost savings of cooling system alter-natives based on field-monitored data. Thomas (1991) documented a study evaluating various humidity sensors used to measure the humidity of the air enter-ing and exiting a
21、cooling coil, which is required for determin-ing coil loads and supply-air quality. This study evaluates the total air-side and water-side loads; however, there is no mention of the instrumentation uncertainty or how this discrepancy could affect the load analysis. Joshi et al. (2005) evaluated the
22、performance of six differ-ent relative humidity sensors used in building HVAC applica-tions. Within this study, the accuracies of these sensors were assessed by comparison to the manufacturers specifications. It was found that only two sensors performed within the manu-facturers stated specification
23、s over the entire range of testing conditions. The results of this study substantiate the impor-tance of instrument selection and verifying instrument perfor-mance over the entire expected operating range for proper operation. The importance of relative humidity sensors and their impact on energy sa
24、vings and operation is addressed by Corsi (2004). Corsi (2004) documented issues relating to the perfor-mance and selection of relative humidity transmitters for HVAC systems. It is shown that instrumentation performance can significantly impact system control, and thus, energy use of HVAC systems a
25、nd indoor air quality. In addition, perfor-mance parameters of relative humidity transmitters such as accuracy, response time, and calibration requirements are discussed. METHODOLOGYThe main objective of this study was to quantify how accurately an energy balance can be performed on a standard chill
26、ed water cooling coil mounted in a commercial Air Handling Unit with the HVAC system operating under typical control and conditions. In addition, the effects of instrumen-tation error on the energy balance were evaluated using an uncertainty analysis. The entering and exiting temperatures and the fl
27、ow rate of the heat transfer fluid circulated to the coil were measured to establish the water-side heat transfer. The dry-bulb temperature and relative humidity measurements entering and exiting the coil were taken by both high accuracy (HA) and low accuracy (LA) instrumentation. All enthalpy value
28、s required for calculating moist air energy transfers were determined using correlations presented in the American Society of Heating, Refrigerating and Air-Conditioning Engi-neers (ASHRAE) Fundamentals Handbook. Calculated energy transfers using test data were compared on minute, hourly, and daily
29、time intervals using both the high and low accuracy instrumentation. Three main criteria were evaluated based on the results of the experimental measurements: 1. The discrepancy between water-side and air-side calcu-lated energy transfers for both sets of instruments2. The difference in results betw
30、een using high accuracy and low accuracy dry-bulb temperature and relative humidity sensors with their respective installations at the inlet and outlet of the cooling coil3. The affects of transient behavior in the cooling coil and coil responsiveness of energy transfer on an energy balanceExperimen
31、tal SetupThe experimental apparatus for this study included a commercial HVAC air handling system with a nominal capac-ity of 3200 cfm (90.6 m3/min) supply air. System control was accomplished by a commercial direct digital control system programmed to maintain a supply air temperature setpoint by m
32、odulating the chilled water control valve using typical PID algorithms. The cooling coil was subjected to conditions that would be expected in normal operation. The test was set to run for a total time of 24 hours. While in operation, air and water conditions entering and exiting the cooling coil we
33、re moni-tored and data was taken on a one minute time interval. A sche-matic of the air handler unit, relative placement of all instrumentation, and air sampler can be seen in Figure 1. A detailed description of each instrument shown in Figure 1 can be found in Table 3.The cooling coil used for the
34、test is a horizontal coil with 0.5 in. (1.27 cm) copper tubes and aluminum plate fins. There are six passes in a single row serpentine configuration with water side turbulators and 115 fins per foot (3.77 fins per centi-meter). The dimensions of the coil are 36 in. (0.91 m) wide by 24 in. (0.61 m) t
35、all. A summary of the cooling coil general description can be seen in Table 1.A summary of the cooling coil performance data can be seen below in Table 2.678 ASHRAE TransactionsTo minimize measurement error of the average air condi-tions exiting the cooling coil, an air sampler was built for the hig
36、h accuracy instrumentation (see Figure 2). The air sampler was constructed from PVC piping and draws air through the device using a small fan. The sampler was designed to move air through a 1-1/2 in. (3.81 cm) main PVC pipe subjecting the sensors to an air velocity of approximately 800 fpm (4.06 mps
37、). As part of the experimental setup, different independent types of relative humidity transmitters with different levels of accuracy were used to determine the cooling coil entering and exiting air relative humidity conditions. On the entering air side of the cooling coil, both high accuracy and lo
38、w accuracy relative humidity transmitters were used. The high accuracy sensor measures and outputs both a relative humidity and dry-bulb temperature reading whereas the low accuracy sensor only measures and outputs a relative humidity reading. On the exiting air side of the coil, different high accu
39、racy and low accuracy transmitters were used. The high accuracy sensor on the exiting air-side measures and outputs dry-bulb tempera-ture and relative humidity whereas and the low accuracy sensor only outputs a relative humidity reading.Two different, independent dry-bulb temperatures were also used
40、 at both the inlet and at the outlet of the cooling coil. The first set of dry-bulb temperatures are designated as the high accuracy sensors. The other set of dry-bulb temperatures Table 1. General Description of Chilled Water Cooling CoilParameter DescriptionCooling coil typeCopper tube per aluminu
41、m plate finCooling coil header Drainable copper headerNumber of rows / passes 6 pass per single row serpentineFin spacing 115 fins/ft (3.77 fins/cm)Finned area36 in. (0.91 m) wide by 24 in. (0.61 m) highTube constructionCopper tube, 1/2 in. (1.27 cm) diameterTurbulators Water-side turbulatorsFigure
42、1 Schematic of experimental set-up.Table 2. Performance Data of Chilled Water Cooling Coil Parameter Value (English) Value (SI)Total cooling capacity 122,100 Btu/hr 35.78 kWEntering air dry bulb temperature82.0 F 27.8 CEntering air wet bulb temperature66.5 F 19.2 COutlet air dry bulb temperature54.5
43、 F 12.5 COutlet air wet bulb temperature54.0 F 12.2 CAir pressure drop 0.78 in. water 76.49 PaWater flow rate 28.0 GPM 106.0 l/minEntering water temperature 44.0 F 6.7 COutlet water temperature 53.7 F 12.1 CASHRAE Transactions 679were measured using an array of four resistance temperature devices (R
44、TDs) configured to obtain an average dry-bulb temperature reading both entering and exiting the coil. A summary of the instrumentation is presented in Table 3.All water temperatures were measured using precision Platinum RTDs in direct contact with the agitated water flow stream installed at a pipe
45、elbow. Further, the water flow rates were measured using magnetic flow meters. The energy balances were performed using both the high and the low accuracy transmitters and temperature sensors. The first energy balance was performed using the high accu-racy instrumentation. The second energy balance
46、was performed using the low accuracy instrumentation and the temperatures obtained using the array of RTDs.Preliminary Instrumentation PreparationBefore beginning the experiment, all of the sensors were checked to verify that their performance was within manufac-turers specifications. The high accur
47、acy relative humidity and dry-bulb temperature sensors were calibrated by the manufac-turer and comply with the National Institute of Standards and Technology (NIST) traceable calibration certificate. The low accuracy relative humidity transmitters were checked at three operating conditions for accu
48、racy using a fundamental princi-pal two pressure humidity generator and were found to be operating within the manufacturers specifications.The water temperature sensors were checked by conduct-ing an operational test. For this test, all fans were shut down to eliminate the load on the coil and water
49、 was circulated through the system for several hours to compare the temperature read-ings. The water temperature sensors all read within 0.25 oF (0.14 oC) of each other validating the temperature difference readings. Similarly, to check the relevant air temperature sensors another operational test was conducted. For this test all water pumps were shut down to eliminate any load on the coil and air was circulated through the system. The temperature outputs of the sensors were checked and all read within 0.25 oF (0.14 oC) of each other validating the air temperature read-in
