Acoustical Design Guidelines for HVAC Systems in Schools

Transcription

1 Acoustical Design Guidelines for HVAC Systems in Schools Seminar 9, ASHRAE Summer Meeting Albuquerque, New Mexico Sponsor: TC 2.6 Sound and Vibration Robert M. Lilkendey Associate Principal Consultant Siebein Associates, Inc. Gainesville, Florida

2 CLASSIFICATION OF NOISE SOURCES IN HVAC SYSTEMS Airborne Noise 2. Self-generated Noise 3. Duct-borne Noise 4. Structure-borne Noise 5. Break-out Noise

3 Guideline #1: Coordinate mechanical equipment selection with space planning.

4 Consider: Locate Fan Coil Units and Small Heat Pumps in fully enclosed closets of relatively high mass, doors to corridor or outside Locate Rooftop Units over non-critical spaces with concrete on roof and adequate distance to spaces served Locate Central Station AHUs in Mechanical Rooms separated from classrooms by buffer spaces, VAV Units in corridors

5 Return Air Supply Air Classroom Classroom Restroom Restroom Janitor Closet Mechanical Room Plan view: Mechanical room buffered from quiet space

6 Existing Small RTU Over High School Classroom 50 dba STANDARD ROOF CURB NO CONCRETE INSIDE CURB

7 Recommended Acoustical Modifications to Meet <40 dba

8 Accepted Acoustical Modifications to Meet <45 dba

9 80 70 Sound Pressure Level (db) re: 20 micropascals dba/nc 50 pre-renovation NC 65 NC 60 NC 55 NC 50 NC dba/nc 40 with gypsum board only NC 40 NC 35 NC NC 25 NC 20 NC Octave Band Center Frequency (Hz)

10 Guideline #2: Construct Mechanical Equipment Room enclosure of sufficient mass to isolate airborne noise.

11 Mechanical Room Provide adequate mass in walls and floors Avoid locating near critical occupancies Watch for doors and other sound leaks Chapter 47 of the 2007 ASHRAE Handbook HVAC Applications, contains recommended assemblies

12 Provide project specific construction assemblies at critical adjacencies based on: Sound levels of Equipment in Mechanical Room Transmission Loss of wall types Sound absorbing characteristics of the receiving room Background NC level requirement in receiving room

13 Guideline #3: Reduce source sound levels to the extent possible.

14 Consider: Avoiding Forward Curved Centrifugal Fans in high static pressure systems Using Plenum Fans in lieu of Centrifugal Specify Sound Levels of Equipment AHRI Standard 260, Sound Rating of Ducted Air Moving and Conditioning Equipment

15 Comparison of Airfoil and Plenum Fan Acoustic Data on Recent School Project: Octave Band Center Frequency, in Hz Fan Type Airfoil Plenum Difference AHU: 11,350 CFM, 6.18 TSP Discharge Sound Levels Based on AHRI Standard 260 Test Data

16 Guideline #4: Install duct borne noise control devices as required.

17 Summary of General Duct Length, Silencer Length, and Flexible Duct Length Required to Achieve Various dba Levels Design NC/dBA Level of Room Served Velocity in Duct near Terminal Supply/ Return Duct Silencer Length Duct Length to First Inlet or Outlet Acoustic Lined Flex Duct NC 25 to 30/ 30 to 35 dba NC 30 to 35/ 35 to 40 dba NC 35 to 40/ 40 to 45 dba 350 / 425 fpm 7-10 ft 90 ft 6-8 ft 400 / 500 fpm 3-7 ft ft 6-8 ft 500 / 600 fpm 3-5 ft 50 ft 6-8 ft Chapter 47 of ASHRAE Handbook - HVAC Applications, describes system analysis process

18 CASE STUDY: AHU in Mechanical Penthouse with VAV Terminal Units in Corridors NC dba Classroom AHU Classroom Silencers in Supply and Return Ducts Floating Concrete Floor Slab in Penthouse Mechanical Room Spring Hung Multi-Layered Gypsum Board Ceiling Classroom Classroom Classroom Classroom Main Ducts to VAVs in Corridor Beyond Classrooms Masonry Chase Walls 6' of Acoustic Flex Duct Supply from Air Handling Unit Return to Air Handling Unit

19 CASE STUDY: AHU in Mechanical Penthouse with VAV Terminal Units in Corridors VAV Terminal Units in Corridor Radiused Elbows Self-Balancing Supply Air System Masonry Chase Walls to Reduce Breakout From High Velocity Main Ducts Supply from Air Handling Unit Return to Air Handling Unit

20 Guideline #5: Follow guidelines in the literature related to air velocity, air flow, and air balancing to reduce flow generated noise.

21 Follow recommended air velocity guidelines Design NC/dBA of Room Served NC 25 to 30/ 30 to 35 dba NC 30 to 35/ 35 to 40 dba NC 35 to 40/ 40 to 45 dba Supply vs. Return Through the Air Terminal Device (free area) Device to 10', Including the Neck 11' to 20' 21' to 30' Supply Return Supply Return Supply Return

22 Follow recommended air velocity guidelines Design NC/dBA of Room Served NC 25 to 30/ 30 to 35 dba NC 30 to 35/ 35 to 40 dba NC 35 to 40/ 40 to 45 dba Supply vs. Return Through the Air Terminal Device (free area) Device to 10', Including the Neck 11' to 20' 21' to 30' Supply Return Supply Return Supply Return

23 Use radiused elbows with several duct diameters of straight duct before and after elbows near AHU inlets and outlets Radius elbow typ. Resilient hangers Return Flexible collar typ. Supply Air Handling Unit (AHU) 3 duct diameters min. Duct silencer typ. Seal penetrations air tight Housekeeping pad External Springs

24 Design and install flex ducts without kinks, especially near outlets (From ASHRAE 2007 Handbook HVAC Applications, Pg ) +0 db +0 db +12 db +0 db db

25 Use volume dampers or design self-balancing systems, not OBDs at air terminal devices VAV Terminal Units in Corridor Radiused Elbows Self-Balancing Supply Air System Masonry Chase Walls to Reduce Breakout From High Velocity Main Ducts Supply from Air Handling Unit Return to Air Handling Unit

26 Guideline #6: Avoid common duct routing pitfalls.

27 Pitfall #1: Un-ducted/Plenum Returns Although it may be possible to meet the HVAC system sound level requirements, plenum returns make it difficult to meet the STC requirements for walls.

28 Pitfall #2: Routing Noisy Ducts above Sound Sensitive Spaces NC to 46 dba - Break-out noise from main supply and return - Airborne sound through gypsum board MER wall - Duct-borne noise from short return duct

29 Pitfall #3: Routing Common Ducts above Adjacent Sound Sensitive Spaces (Crosstalk) Design Problem (Schaffer, Mark E. - A Practical Guide to Noise and Vibration Control for HVAC Systems, 2005) Any use of materials in this presentation, including reproduction, modification, distribution, or republication, (Schaffer, without Mark E. the - A prior Practical written Guide to Noise and Vibration Control for HVAC Systems, 2005)

30 Pitfall #3: Routing Common Ducts above Adjacent Sound Sensitive Spaces (Crosstalk) Design Solution VAV Terminal Units in Corridor Radiused Elbows Self-Balancing Supply Air System Masonry Chase Walls to Reduce Breakout From High Velocity Main Ducts Supply from Air Handling Unit Return to Air Handling Unit

31 Pitfall #3: Routing Common Ducts above Adjacent Sound Sensitive Spaces (Crosstalk) Retrofit Problem - Teachers voices plainly audible in adjacent rooms through common return duct - No internal duct liner allowed - Minimal space available

32 Pitfall #3: Routing Common Ducts above Adjacent Sound Sensitive Spaces (Crosstalk) Retrofit Solution - Close off return opening on bottom of duct - Add elbow opening toward deck above - Install 2 thick sound absorbing acoustical panel on underside of deck - Repeat for each room

33 Guideline #7: Beware of hidden noise sources.

34 Such As: Leaky Dampers in Exposed Ducts Condenser Fans and Compressors in RTUs Exhaust Fans Reheat Coils Fume Hoods Chases with High Velocity Ducts Condensing Units outside Windows

35 Guideline #8: Install vibration isolation devices on HVAC equipment.

36 In accordance with ASHRAE 2007 HVAC Applications Handbook At minimum include: Vibration isolators for all rotating or vibrating equipment (AHU s, RTUs, pumps, chillers, etc.) Flex connectors on duct and pipes Slab breaks at ground floor MERs

37 Guideline #9: Monitor the Value Engineering process.

38 Watch out for: Elimination of duct liner, silencers, etc. Cheaper (noisier) fans. Re-location of RTUs to top of space served. Reduction in ceiling cavity space. Shorter duct runs

39 Guideline #10: Review acoustical submittals and monitor the implementation of acoustical details during the construction process.

40 This includes: Review submittals for vibration isolators, equipment sound levels, duct sizes, duct silencers, etc. On sound critical projects, conduct acoustical kickoff meeting with contractors to review specific details and areas requiring coordination Conduct site reviews

41 5 Common Acoustical Misconceptions/Mistakes 1. Sound attenuators not required on return air side. 2. Making the Contractor or Equipment Manufacturer responsible for HVAC system sound levels in rooms. 3. Duct silencers cause more harm than good. 4. Duct elbows take care of fan noise problems. 5. Relying on published NC ratings of VAV s and Air Terminal Devices.