1、NA-04-3-2 (RP-1026) Pressure Drop and Acoustical Application Guidelines for HVAC Plenums Karl Peterman, P.E. Member ASHRAE ABSTRACT New equations derived from the recent ASHRAE-spon- sored research project RP-1026 will enable HVAC system designers to betterpredict aero-acoustic performance of sheet
2、metal plenums. This paper deals primarily with the aerody- namic performance offlow-through plenums andpresents new total pressure drop equations for some of the most common plenum con$gurations. Comparisons are made between these equations, computationalfluid dynamics (CFD) analyses, and hand-calcu
3、lation methods. The practical uses ofplenums are discussed along with prescriptive rules of thumb to help designers implementplenums effectively into their systems for aerodynamic and acoustical benefits. INTRODUCTION Plenums have been used in HVAC designs since fans have been connected to ductwork.
4、 There are many different kinds of plenums encountered in typical HVAC designs, but they all consist of an enclosed volume under a positive or negative pressure and at least two openings to allow the passage of airflow. A plenum with a fan located inside is referred to as a fan plenum; all other kin
5、ds are flow-through plenums. Plenums are typically rectangular, constant cross-section boxes with multiple connections of smaller sizes. In commer- cial HVAC design, they are usually made using standard duct- work materials such as galvanized or stainless-steel sheet metal of an appropriate gauge or
6、 thickness as prescribed by SMACNA guidelines. Plenum walls are often covered with an acoustically absorptive material on their interior surfaces to reduce noise transmission down the duct path(s) andfor to provide thermal insulation. Emanuel Mouratidis, P.Eng. Member ASHRAE Flow-through HVAC sheet
7、metal plenums are found in various places in duct systems. Discharge and exhaust plenums are probably the most common type of HVAC plenums and are typically connected to one fan and multiple duct runs. Return air plenums are used in low-pressure duct systems as a means to collect multiple return air
8、 ducts into a common location before entering an air-handling unit. Mixing plenums are often attached to the inlet side of air-handling units to blend outside air and return air as an effective means to reduce the heating or cooling load on the coils and to help with humidity control. Plenums can al
9、so be placed behind louvers or grilles as a means to distribute the airflow over the entire opening. These plenums are often sized to allow access for maintenance personnel to inspect or clean out any foreign debris. HVAC systems also make use of architectural plenums that are constructed using buil
10、ding materials other than sheet metal ductwork. Architectural plenums are often used in lieu of sheet metal plenums typically due to cost, space, or location constraints. A room can serve as a plenum for an air-handling unit with an unducted inlet that draws air into the room from other openings suc
11、h as fresh air or outdoor air inlets and build- ing return air inlets. A ceiling plenum is the volume of space defined by a suspended ceiling and the structure above and is often used in HVAC design as a means to provide paths for air to return to an air-handling unit. Architectural plenums can be u
12、sed to provide low-velociy air distribution, as in concert halls, where generated noise from air movement must not be audible. Raised floor systems are used in many data centers as plenums to provide underfloor air distribution, which allows flexibility in equipment location coupled with an effectiv
13、e wire management system. Karl Peterman and Emanuel Mouratidis are with Vibro-Acoustics, Toronto, Ontario, Canada. o2004 ASHRAE. 607 Plenums found favor with acoustical engineers who real- ized their potential for providing low-frequency attenuation with the advent of Wells work (1 958). An acoustic
14、al plenum is a type of passive silencer that is most effective in the lower frequencies. HVAC system designers and acoustical engineers alike do not tend to use them in their designs for various reasons. it is the authors opinion that these reasons include a lack of understanding of or comfort level
15、 with their perfor- mance. Previous work on acoustical plenums has indicated a significant margin of error ofhl0 decibels in anticipated acous- tical performance. Recent work on acoustical plenums has reduced that margin of error substantially (Mouratidis 2003). Plenums are commonplace and integral
16、components in many designs. They are used for a variety of purposes but primarily as aerodynamic devices that distribute airflow through multiple duct paths within a limited space. Very little has been written about plenums in duct design texts, and the authors are not aware of any previous studies
17、done on aerody- namic performance of HVAC plenums. They have been used primarily by mechanical systems engineers for use as flow- through plenums where they solve a particular aerodynamic problem and by air-handling unit manufacturers as fan plenums to contain un-housed centrifugal fans, or plenum f
18、ans, which have become quite popular in recent times. Even so, they remain misunderstood in their potential to help solve both aerodynamic and acoustical problems. The recent ASHRAE-sponsored research project, RP- 1026, addressed flow-through type plenums (without internal fans) and quantified their
19、 acoustical performance in transmis- sion loss and their aerodynamic performance in total pressure drop (Mouratidis 2003). RP-1026 focused on sheet metal HVAC plenums that are relatively simple and easy to quantifi, unlike architectural plenums that can be complex in shape and construction. Eight di
20、fferent plenums of various sizes were analyzed with different wall types and inletloutlet configura- tions that correspond to typical installations. New equations, replacing those developed by Wells, were developed that more closely predict the acoustical performance of plenums with various configur
21、ations. (See the final project report Mourati- dis 20031 for more information.) DERIVATION OF PRESSURE DROP THROUGH BASIC IN-LINE MODELS RP-1026 measured the aerodynamic performance of many different plenums in various configurations. As a base- line set, “in-line” plenums were constructed with a si
22、ngle inlet opening and a single outlet opening, both the same size and located opposite from each other with their centers lined up. Three square duct sizes were used for openings in this part of the project, measuring 12 x 12,24 x 24, and 36 x 36 in., appli- cable to various plenum sizes. Round and
23、 oval duct sizes were not used in the investigation. All test data were collected in the Vibro-Acoustics labo- ratory, which is an accredited aero-acoustic test facility under the National Voluntary Laboratory Accreditation Program (NVLAP). The facilitys large physical size made it an ideal environm
24、ent for this study on large sheet metal test specimens, Using the ducted test layout in accordance with ASTM standard E477 (ASTM 1999), the total pressure drop (TPD) was determined for 46 unique in-line plenum and duct size configurations. The ASTM standard defines the TPD as the measured difference
25、 between the total pressure two-and-a- half equivalent duct diameters upstream and five equivalent duct diameters downstream of the plenum. Using regression analysis, the TPD for all inline configurations (TPD,) was found to be TPD, = 0.424 * P, , where P, = velocity pressure in the upstream duct me
26、asured in inches of water (in. H20) as defined by P, = (V/ 4005)2, where V= mean duct velocity in feet per minute (fpm). As indicated by the curve in Figure 1 , the above equation produced a very good correlation coefficient of 0.94. Statisti- cal analysis across all applicable test plenums and Equa
27、tion 1 at a normalized inlet duct flow of 2000 fpm (or P, = 0.249 in. H,O) produced a standard deviation of 0.030 in. H,O. There- fore, Equation 1 provides an accurate means to predict in-line TPD, independent of the plenums intemal dimensions, wall type, and opening sizes. COMPARISON OF MEASURED AN
28、D CALCULATED PRESSURE DROP THROUGH A BASIC IN-LINE MODEL The experimental results for one particular plenum analyzed in RP-1026 were compared with four prediction methods: the new derived equation from RP- i 026 (Equation i), computational fluid dynamics (CFD) analysis, and two other hand-calculatio
29、n methods based on ASHRAE (2001) duct fitting tables and a fan engineering book (BFC 1983). Figures 2 and 3 show the CFD model of a 6 ft W x 4 R H x 5 ft L plenum (internal clear dimensions) o O0 I o 20 O 40 O 60 o BO la p. Wl Figure 1 In-line plenum pressure frop model, total pressure drop (TDP) vs
30、. velocity pressure (P,): n = 46 specimens. 608 ASHRAE Transactions: Symposia 07-J for a 45 degree offset, it would be less than 1700 fpm. For elbow configuration plenums, inlet velocities below 1650 fpm should keep the total pres- sure drop below 0.35 in. H20. Keep in mind that a smooth flow veloci
31、ty profile will take a considerable distance to redevelop downstream of an abrupt, sharp contraction, as is typically used in WAC plenums. Additional duct fittings in close proximity to air discharges of such a plenum will have higher than antici- pated pressure drops due to interactive system effec
32、ts. Bell- mouth, pyramidal, or conical transitions on the air outlet opening will help to minimize both the plenum pressure drop and the downstream system effects. 4. Make the plenum internal dimensions at least 1.5 times the dimensions of the inlet and outlet openings to ensure that it behaves as a
33、n acoustical expansion chamber. Plenum pres- sure drop is not dependent on this area change according to RP-iO26. Instead, only the relative position of the outlet opening to the inlet opening seems to determine the total pressure drop. There is no apparent acoustical benefit to offset the inlet and
34、 outlet openings in the lower frequencies-in fact, RP-1026 showed a decrease in TL with inlevoutlet offsets. The project results also showed that an offset creates a signifi- cant impact on the total pressure drop-ofien more than three times the in-line base model pressure drop. Multiple outlet open
35、ings also do not have a significant acoustical benefit over a single outlet and have little effect on pressure 5. drop. 6. When using the plenum acoustical equations, use the inside clear dimensions of the plenum, not the exterior dimensions since the wall types may include thick acoustically absorp
36、- tive media. RP-1026 used walls from bare sheet metal to 8 in. thick absorptive walls while maintaining the inside clear dimensions of the plenums. 614 ASHRAE Transactions: Symposia CONCLUSIONS Designers of HVAC systems can have a higher degree of confidence in using plenums in their designs with t
37、he new equations derived from the recent ASHRAE-sponsored research project, RP-I 026. Total pressure drop equations were developed for some of the most common plenum configura- tions. These new equations can effectively replace other hand- calculation methods for determining aerodynamic perfor- manc
38、e. Used together with the acoustical equations, the designer will be able to more accurately predict a plenums performance. Plenums are useful components in HVAC design for solv- ing both aerodynamic and acoustical problems. Some simple rules of thumb have been presented here to help designers imple
39、ment plenums effectively. The dimensional and config- uration constraints discussed in combination with maximum airflow guidelines will help ensure plenum performance. REFERENCES ASHRAE. 2001. 2001 ASHRAE Handbook-Fundamen- tals. Atlanta: American Society of Heating, Rehgerat- ing and Air-conditioni
40、ng Engineers, Inc. ASTM. 1999. ASTM-E477, Standard Method of Testing Duct Liner Materials and Prefabricated Silencers for Acous- tical and Airflow Performance. Conshohoken, Pa.: American Society for Testing and Materials. BFC (Buffalo Forge Company). 1983. Fun Engineering, Eighth Edition. Buffalo, N
41、.Y.: Buffalo Forge Company. CD-adapco Group. 2002. Star-CD V3.150A (Build 520) for ndowsNT/200/XP. 1988-2002. Chen, YS., and S.W. Kim. 1987. Computation of turbulent flows using an extended k-E turbulence closure model. Mouratidis, E. 2003. The Aero-Acoustic Properties of Com- mon HVAC Plena, TRP-10
42、26. Atlanta: American Soci- ety of Heating, Refrigerating and Air-conditioning Engineers, Inc. Wells, R.J. 1958. Acoustical Plenum Chambers. Noise Con- trol (July). NASA CR- 179204. BIBLIOGRAPHY ASHRAE. 1999.1999 ASHRAE Handbook-HVAC Applica- tions. Atlanta: American Society of Heating, Refrigerat-
43、ing and Air-conditioning Engineers, Inc. ASHRAE. 1987. ASHRAE-41.2, Standard Methods for Lab- oratory Airflow Measurement. Atlanta: American Soci- ety of Heating, Refrigerating and Air-conditioning Engineers, Inc. Beranek, L.L., and I.L. Ver. 1992. Noise and fibration Con- trol Engineering, Principl
44、es and Practice. New York: John Wiley and Sons, Inc. SMACNA. 1995. HVAC Duct Construction Standards- Metal and Flexible. Chantilly, Va.: Sheet Metal and Air- Conditioning Contractors National Association, Inc. APPENDIX A ACOUSTICAL EQUATIONS FOR IN-LINE PLENUMS In ducted rectangular HVAC systems, th
45、e cut-off frequency is determined by where d = largest duct dimension, width or height (ft), and co = speed of sound (fvs), approximately i 130 ft/s in most air ducts. Both the reactive and absorptive components, derived from the low-frequency plenum attenuation data, were combined in a model of ple
46、num acoustic performance defined across two distinct frequency ranges. The cut-off frequency of the sound inlet duct (not necessarily the airflow inlet duct) is determined from Equation Al as the demarcation between the two frequency ranges. Excluding the potential effects of offset orientations, su
47、ch as the cose term $1, the prediction of plenum attenuation is as follows: For 50 Hz If If,: TL = A-Factor * S + We (-42) where A-Factor = surface area coefficient, d13/ft2 S w, = wall effect, dB = surface area, P, for a11 acoustic panel surfaces Forf, f 5000 Hz: where b = 3.505 n = -0.359 Kr = att
48、enuation coefficient, as derived by area of outlet section of plenum, fi2 total inside surface area of plenum less the inlet and outlet areas, fi2 distance between centers of inlet and outlet sections, ft directivity factor = 2 (for an inlet open- ing in the center of a wall) average absorption coefficient of ple- num lining at a particular frequency, f - usually by one-third octave bands See the full report (Mouratidis 2003) for further equations to describe acoustical performance of plenums with other configurations (e.g., offsets, elbows). ASHRAE Transactions: Symposia 61 5
