Acoustical Design Guidelines for HVAC Systems in Schools

Acoustical Design Guidelines for HVAC

Systems in Schools

Robert M. Lilkendey

Associate Principal Consultant

Siebein Associates, Inc.

Gainesville, Florida

Seminar 9, ASHRAE Summer Meeting

Albuquerque, New Mexico

CLASSIFICATION OF NOISE SOURCES IN HVAC SYSTEMS

1

5

3

2

4

4

4

4

1.

Airborne Noise

2. Self-generated

Noise

3.

Duct-borne Noise

4.

Structure-borne Noise

5.

Break-out Noise

Guideline #1:

Coordinate mechanical equipment

selection with space planning.

• 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

Plan view:

Mechanical room buffered from quiet space

Classroom

Mechanical

Room

Restroom

Janitor

Closet

Restroom

Classroom

Return Air

Supply Air

Existing Small RTU Over High School Classroom – 50 dBA

STANDARD ROOF CURB

NO CONCRETE INSIDE CURB

10 20 30 40 50 60 70 80 31.5 63 125 250 500 1000 2000 4000 8000 16000 Octave Band Center Frequency (Hz)

S ound P re s s u re Le v e l ( d B ) re : 20 m icr o p asca ls NC 15 NC 20 NC 25 NC 30 NC 35 NC 40 NC 45 NC 50 NC 55 NC 60 NC 65

50 dBA/NC 50

pre-renovation

44 dBA/NC 40

with gypsum

board only

Guideline #2:

Construct Mechanical Equipment

Room enclosure of sufficient mass

to isolate airborne noise.

• 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

• 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

Provide project specific construction

Guideline #3:

Reduce source sound levels to the

extent possible.

• 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

Octave Band Center Frequency, in Hz

Fan Type

63

125

250

500

1000

2000

4000

Airfoil

97

100

99

98

93

86

79

Plenum

91

81

89

91

90

81

74

Difference

6

₁₉

10

7

3

5

5

Comparison of Airfoil and Plenum Fan

Acoustic Data on Recent School Project:

AHU: 11,350 CFM, 6.18” TSP

Guideline #4:

Install duct borne noise control

devices as required.

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

350 / 425 fpm

7-10 ft

90 ft

6-8 ft

NC 30 to 35/

35 to 40 dBA

400 / 500 fpm

3-7 ft

50-70 ft

6-8 ft

NC 35 to 40/

40 to 45 dBA

500 / 600 fpm

3-5 ft

50 ft

6-8 ft

Summary of General Duct Length, Silencer Length,

and Flexible Duct Length Required to Achieve

Various dBA Levels

Chapter 47 of ASHRAE Handbook - HVAC Applications, describes

system analysis process

NC 25-30

35 dBA

CASE STUDY: AHU in Mechanical Penthouse with VAV

Terminal Units in Corridors

Return to Air Handling Unit

Supply from Air Handling Unit

AHU

Classroom Classroom

Masonry Chase Walls

Silencers in Supply and

Return Ducts

Floating Concrete

Floor Slab

in Penthouse

Mechanical Room

Main Ducts to VAVs in

Corridor Beyond

Classrooms

Spring Hung

Multi-Layered

Gypsum Board Ceiling

CASE STUDY: AHU in Mechanical Penthouse with VAV

Terminal Units in Corridors

Return to Air Handling Unit

Supply from Air Handling Unit

Masonry Chase Walls

to Reduce Breakout

From High Velocity

Main Ducts

VAV Terminal Units in Corridor

Radiused Elbows

Self-Balancing Supply

Air System

Guideline #5:

Follow guidelines in the literature

related to air velocity, air flow,

and air balancing to reduce flow

generated noise.

Follow recommended air velocity guidelines

Design NC/dBA

of Room Served

Supply

vs.

Return

Through the

Air Terminal

Device (free

area)

Device to 10',

Including the

Neck

11' to 20'

21' to 30'

NC 25 to 30/

30 to 35 dBA

Supply

350

425

550

700

Return

425

500

650

800

NC 30 to 35/

35 to 40 dBA

Supply

425

500

700

850

Return

500

600

800

1000

NC 35 to 40/

40 to 45 dBA

Supply

500

600

800

1000

Return

600

700

900

1150

Follow recommended air velocity guidelines

Design NC/dBA

of Room Served

Supply

vs.

Return

Through the

Air Terminal

Device (free

area)

Device to 10',

Including the

Neck

11' to 20'

21' to 30'

NC 25 to 30/

30 to 35 dBA

Supply

350

425

550

700

Return

425

500

650

800

NC 30 to 35/

35 to 40 dBA

Supply

425

500

700

850

Return

500

600

800

1000

NC 35 to 40/

40 to 45 dBA

Supply

500

600

800

1000

Return

600

700

900

1150

Use radiused elbows with several duct diameters of

straight duct before and after elbows near AHU inlets and

outlets

3 duct

diameters

min.

Supply

Return

Duct

silencer

typ.

Air

Handling

Unit (AHU)

Seal

penetrations

air tight

External

Radius

elbow typ.

Flexible

collar

typ.

Resilient

hangers

Housekeeping pad

Design and install flex ducts without kinks,

especially near outlets

+0 dB

+0 dB

+12 dB

(From ASHRAE 2007 Handbook – HVAC Applications, Pg. 47.10)

• Use volume dampers or design self-balancing systems,

not OBDs at air terminal devices

Return to Air Handling Unit

Supply from Air Handling Unit

Masonry Chase Walls

to Reduce Breakout

From High Velocity

Main Ducts

VAV Terminal Units in Corridor

Radiused Elbows

Self-Balancing Supply

Air System

Guideline #6:

Avoid common duct routing

pitfalls.

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.

NC 40

44 to 46 dBA

Pitfall #2: Routing Noisy Ducts above

Sound Sensitive Spaces

- Break-out noise from

main supply and return

- Airborne sound through

gypsum board MER wall

- Duct-borne noise from

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)

Supply from Air Handling Unit

Masonry Chase Walls

to Reduce Breakout

From High Velocity

Main Ducts

VAV Terminal Units in Corridor

Radiused Elbows

Self-Balancing Supply

Air System

Pitfall #3: Routing Common Ducts above Adjacent Sound

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

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

Guideline #7:

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

Guideline #8:

Install vibration isolation devices

on HVAC equipment.

• In accordance with ASHRAE 2007 HVAC Applications

Handbook

• At minimum include:

9 Vibration isolators for all rotating or vibrating

equipment (AHU’s, RTUs, pumps, chillers, etc.)

9 Flex connectors on duct and pipes

Guideline #9:

Monitor the Value Engineering

process.

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.

Guideline #10:

Review acoustical submittals and

monitor the implementation of

acoustical details during the

construction process.

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

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.

5 Common Acoustical

Misconceptions/Mistakes