1、4688 (RP-1045) Thermal Mixing of Outdoor and Return Airflows in Typical Air-Handling Units Miiind Mainkar Fathi Finaish Associate Member ASHRAE Harry J. Sauer, Jr., Ph.D., P.E. FellowlLife Member ASHRAE Member ASHRAE Robert Van Beceiaere ABSTRACT Thispaper examines thermal mixingofoutdoor andreturn
2、airstreams in typical air-handling units equipped with parallel blade dampers. The mixing ofthe two airstreams in rectangu- lar and square mixing chambers is studiedfor eight diferent dampers and blade orientations. Testing is conducted at a total supply air of I6000 CFM with outdoor flow percentage
3、s of 15% and 30%. The temperature diferential between the outdoor and return airflows is kept at 40F Damper blades with chords of4 and 6 inches are tested. To examine the mixing ofthe two airstreams, temperature distributions in the mixed region are measured by an array ofthermocouples and utilized
4、to compute the thermal mixing eflectiveness of the mixedjlow. It appears that the relative flow momentum ofthe two streams plays an important role in the mixing. Values ofrange mixing efectiveness increase signijcantly with increase in the return air velocity. However, this conclusion is not applica
5、ble to conjigurations with minimum outside air in which cold spots can occur downstream in the mixing box. Retesting of conjig- urations producing the least stratijcation at 50% total supply airflow shows that thermal mixing is degraded, particularlyfor the cases ofl5% outside air: INTRODUCTION Mixi
6、ng of two airstreams has attracted considerable attention in the HVAC industry. Increased awareness about indoor air quality (IAQ) among the end-users and stringent standards governing IAQ in buildings dictate an increase in the amount of outdoor air introduced in the building. Moreover, mixing of a
7、 cold OA stream with a hot RA stream in winter poses serious problems of stratification (Haines I 980; Buchko 1999), which further leads to coil freeze-ups (Alyea 1958), sensor errors, and nuisance trips. The most well known effects of stratification are repeated shutdowns due to low-limit ther- mos
8、tat (freeze-stat) trips and fi-ozen cooling coils (Delaney et al. 1984). Another effect is increased sensor error (Ka0 1985). With the newly revised ASHRAE Standard 62-200 1, the seri- ousness of solving the problems associated with stratification has increased due to increased interest in adding ve
9、ntilation air to combat IAQ problems. The gravity of this issue is further increased due to lack of sufficient information about stratifi- cation in the HVAC environment in textbooks or journals. Under such circumstances, researchers and professionals in the field often tend to analyze this problem
10、using empirical observations. One common method suggested to improve mixing has been to introduce outdoor air at the top of the mixing box. The reasoning behind this suggestion is that the cold outside air will drop due to density difference and mix with the warmer return air that will be introduced
11、 from the side. In reality, the momentum of the return airstream is typically greater than the force due to density difference and the cold air introduced from the top is simply carried downstream before it has time to drop (Robinson 1999). Lower air velocities through the dampers and duct configura
12、tion were also suspected to be the cause of lower mixing effectiveness (Robinson 1998). With regard to the air-handling unit/mixing box design, Haines (1980) has presented a discussion of practices that may be used to improve mixing; however, no information concerning the performance of the suggeste
13、d methods was included in the discussion. Experimental work done by Robinson (1999) on air-handling unit combination mixing/filter boxes revealed that the placement of filters within the mixing box decreases the mixing effectiveness of an AHU mixing box. Even with the Milind Mainkar is a mechanical
14、engineer at Bums and McDonnell, Kansas City, Mo. Fathi Finaish and Hank Sauer are professors in the Department of Mechanical, Aerospace Engineering and Engineering Mechanics, University of Missouri-Rolla, Rolla, Mo. Robert Van Becelaere is the vice president of engineering at Ruskin Dampers and Louv
15、ers, Kansas City, Mo. 194 02004 ASHRAE filters removed from the mixing box, the increase in the mixing effectiveness obtained is not sufficient to reduce the stratification to acceptable levels. Mixing needs to be increased to avoid frozen coils and freeze-stat trips in the future. Hence, it was pro
16、posed to either increase the length of the mixing box to provide additional mixing length or to install static air mixers. Static mixers enhance the mixing effective- ness-but at the cost of additional pressure loss. Also, space is a major constraint in design of AHU. In this paper, we present resul
17、ts from a series of tests to measure the thermal mixing for eight configurations with different outdoor and return damper orientations, blade size, and aspect ratios for 15% and 30% outdoor air. Special atten- tion was paid to the parametric combination that results in the most and the least mixing.
18、 Temperature distributions in the mixed region were measured and utilized to compute thermal mixing effectiveness. TEST CONFIGURATIONS AND FACILITY As seen in Figure 1, eight flow configurations are tested. All the tests are conducted using parallel blade dampers with blade sizes of 4 and 6 inches.
19、Two damper aspect ratios-3: 1 and 1:l-are tested. The damper blades are oriented to direct the two airstreams into each other for configurations 4, 5, 7, and 8. The blades are oriented in the same directions for configurations 1, 2, 3, and 6. The testing was conducted at the RUSKIN Research and Deve
20、lopment Laboratory located in Kansas City, Mo. A 7.5 ft test chamber, constructed in accordance with AMCA standard _ /t 2 XED IR O 500-89, is used as the air supply for the tests. The flow is main- tained by a blower at the end of the 7.5 ft chamber, which draws air through the chamber. This blower
21、is controlled by a frequency-control drive and is capable of producing 16000 CFM flow. Tests are conducted at 15% and 30% outside air by volume, which corresponds to 2,400 CFM and 4,800 CFM, respectively. The outside air dampers are sized for velocities of 500 and 1500 fpm. Similarly, the return air
22、 dampers are sized at 85% of total supply air and velocities of 1 O00 and 2000 fpm. The temperature differential between the return and outside airstreams is maintained constant at 4OoF, irrespective of the outside air temperature, by heating the return airstream. The 7.5 ft chamber has nine accurat
23、ely calibrated flow- measuring orifices, which are used to maintain flow control. Suitable instrumentation (manometers, differential pressure gauges, etc.) is used to monitor nozzle pressure drop, static pressure, and airflow monitoring station pressure drop. The return air damper is mounted in a sl
24、eeve containing an airflow monitoring station. The fresh air dampers are attached to a ducted system, which is connected to an outside air source. Figure 2 shows a three-dimensional view of the test facility with a broad classification of its major components. The outside airstream and the return ai
25、rstream enter the mixing box through the respective dampers-the outside air damper and the return air damper. As seen, the outside air damper is connected to the outside air source through a ducted system. The return air damper is mounted in the reurn air duct, which consists of an airflow monitorin
26、g station calibrated on AMCA tunnel, Both dampers are modulated manually to control the K 3 RA XED IR (jl O MIXED AIR FLWI . 160C CFM CASE I. 151 OA RETURN AIR FLOW 13600 CFM WTSIDE AIR FLOW 2400 CFM CASE 2. 30% OA RCIURN AIR FLOW 11200 CFM XED XED IR IR I WTSIDE AIR FLOW : 4800 CFU II I Figure I Es
27、t conJigurations. ASHRAE Transactions: Research 1 95 NOTE - I. OUTSIDE AIR DUCT OPEN OUTSIDE THE ROOM. 2. RETURN AIR IS TAKEN FROM THE ROOM. 3. PDAO UNIT RECORDS TEMPERATURE AT ALL STATIONS SIMULTANEOUSLY. AiL OIMENSIONS ARE IN INCHES SCALE 1:4 Figure 2 Test facilis, at Ruskin S Research Laboratory
28、respective airflow. The mixing box houses a temperature measurement grid installed 12 in. downstream in the box. The return air duct connects the blower exhaust back to the mixing box via the return air damper, thereby forming a re- circulating loop. The duct is insulated to avoid any heat loss from
29、 the hot return air flowing through it. A relief air damper is installed in this duct, toward the blower end of the return air duct. This damper is mounted downstream of the blower, and it opens up in the test lab. It is used to discharge the relief air, which in quantity is equivalent to the air th
30、at is brought into the test facility through the outside air damper. The return airflow is measured by an airflow monitoring station, calibrated on the AMCA tunnel, which is installed in the return air duct. A 5 in. long piece of honeycomb mesh is installed upstream of the airflow monitoring station
31、. This mesh serves the purpose of a flow straightener. Several turning vanes are installed in the bends to help direct the flow and reduce the elbow effect and corresponding pressure loss. To establish the required temperature differential of 40F between the return and outside airstreams, six heater
32、s are installed downstream of the flow measuring orifices and upstream of the blower in the 7.5 ft test chamber. The thermal breakout of the heaters is bypassed in order to achieve the 40F temperature difference when conducting the tests with higher outside air temperatures. Five heaters are connect
33、ed directly to the source and can be controlled by disconnecting them using on-off control. The temperatures of the return airstream are monitored by four thermocouples installed in the return air duct. A part of the hot mixed air is dumped in the test lab through the relief damper, and the remainin
34、g air gets recircu- lated in the tunnel as return air. MEASUREMENT SETUPS AND INSTRUMENTATION To collect temperature and flow measurements, two measurement systems are employed. Flow Measurement Setup The mixed airflow is measured by using nine calibrated flow-measuring orifices installed in the 7.5
35、 ft chamber. Pres- sure drop of 0.66 in., maintained across nine calibrated flow- measuring orifices in the 7.5 fi chamber, ensures a total flow of 16000 CFM through it. The pressure drop across the orifices is measured with the help of a differential pressure manometer. The value of pressure drop i
36、s obtained from the calibration charts for the orifices. To measure the return airflow, an airflow monitoring station, calibrated on the AMCA tunnel, is installed upstream of the return air damper. A differential pres- sure manometer is used to monitor the pressure drop across this air monitoring st
37、ation. The value of the pressure drop across the return air damper, corresponding to the desired return airflow, is obtained from the damper characteristic curve. The pressure drop is set by manually modulating the 196 ASHRAE Transactions: Research return and outside air damper. Once the mixed airfl
38、ow and the return airflow are maintained, the balance has to come in through the outside air damper and has to leave through the relief air damper. Temperature Measurement Setup Temperature measurements are acquired by an array of T- type thermocouples distributed at the measuring station 12 in. dow
39、nstream from the return air duct. For the rectangular mixed plenum configurations, the grid consists of 30 thermo- couples placed 12 in. apart from each other in an array of three rows, with ten thermocouples per row. The thermocouples closer to the side are 5 in. apart from the respective wall, and
40、 those on top and bottom are 8 in. apart from the respective walls of the mixing plenum. For the square mixed plenum configurations, the grid consists of 36 thermocouples placed 12 in. apart from each other in an array of six rows, with six thermocouples per row. The thermocouples closer to the duct
41、 walls are 4 in. apart from the respective walls. To measure the temperature of the return and outside airflows, four thermo- couples are installed in each of the two ducts. The thermocou- ples are calibrated by use of a certified mercury-glass thermometer, which allows for measuring the deviation o
42、f readings shown by the thermocouples compared with the ther- mometer. The measurements are collected using the IOTECH “Personal Daq/56” data acquisition module, which is a multi- function module that can be attached to PCs via a universal serial bus (USB), which allows for direct measurements of mu
43、ltiple channels of thermocouples. The software package consists of a spreadsheet-style software for acquisition and real-time display and an analysis package for post-acquisition viewing. The package provides advanced charting capabili- ties, including multiple channels per chart, multiple chart gro
44、ups, and support for up to 100 Personal Daq devices attached to one PC. To examine the inherent behavior of the electronic hardware, a few tests were conducted over a period of 12 hours using two sets of the system, each having the main unit with 10 thermocouples attached to it and add-on module wit
45、h 20 thermocouples attached to it. The main aim of these tests was to investigate the stability of the TC readings as a function of time. The data produced by these tests revealed that the TC readings are stable and reliable, and the data acqui- sition system has a warm-up period of approximately tw
46、o hours. TEST PROCEDURE The data acquisition system for temperature measure- ments is turned on at least two hours prior to the actual data acquisition because the system has a warm-up period of around two hours in order to obtain stable readings. Once the data acquisition systems are ready, the air
47、flow through the tunnel facility is established. Depending on the flow rates to be established, the outside air damper and return air damper angles are adjusted roughly. The relief damper is partially opened for an easy balancing of the flows. For 15% outside air, it is seen that the outside air dam
48、per is roughly 15% open, while the return dampers are 85% open. After setting the dampers, the blower is started and the pressure drop across the flow-measuring orifices in the 7.5 ft test chamber is moni- tored. With all nine orifices open, to maintain a flow of 16000 cfm through the orifices, the
49、pressure drop across is adjusted accordingly. Once the total flow through the test facility is established, adjustments are done to the dampers for setting the return airflow. The pressure drop across the return airflow measuring station is adjusted by manually modulating the outside air damper and return air damper. The value of the pressure drop corresponding to the return airflow is obtained from the calibration curve ofthe damper. The calibration curve is a graph of pressure drop across the damper versus flow through the damper. This adjustment ensures the quantity of recirculated
