Providence, Rhode IslandAcoustical Society of America5-9 June 2006

Cooling Tower Noise ControlTom Paige, P.Eng.Products Application Engineer Kinetics Noise Control, Inc.Vibron Products GroupMississauga, OntarioCanadatpaige@kineticsnoise.com

VIBRON PRODUCTS GROUP

Presentation Overview

• Induced-Draft Cooling Towers• Cooling Tower Noise Sources• Noise Control Options• Silencers for Cooling Towers• Vibration Isolation• Case Study - Silencing

Induced-Draft Cooling Towers

• Most common type• Large propeller fan located on

top of unit discharging upward• Air is drawn into the intakes on

the sides of the unit and passes through a stream of water

• Some water evaporates into the airstream causing cooling

Cooling Tower Basic System

From ASHRAE Publication - HVAC Simplified

Cooling Tower Noise Sources

• Fan noise - low frequency• Blade passage tone below 63 Hz• Spraying and splashing noise of

water - higher frequencies• Motor, belts or gearbox noise• Intake noise level typically 5 dB

less than discharge• Noise increases with fan speed

Typical Noise Spectrum

800-Ton Induced-Draft Propeller-Fan Cooling Tower 50’ from Air Inlet

50

55

60

65

70

75

80

63 125 250 500 1000 2000 4000

Octave Band (Hz)

Soun

d Pr

essu

re L

evel

(dB

)

Directivity

Octave Band (Hz) 63 125 250 500 1000 2000 4000 8000

Front (Air Inlet) 0 0 1 2 2 2 3 3

Side (Enclosed) -3 -3 -3 -3 -3 -4 -5 -6

Top (Discharge) +3 +3 +3 +3 +4 +4 +5 +5

Induced-Draft Propeller-Fan Cooling TowerDirectivity Index (dB)

From ASHRAE Publication - Application of Manufacturers Sound Data

Noise Ratings

• Cooling Technology Institute• Acceptance Test Code ATC-128• Sound pressure level readings

are taken on 4 sides and top at distances 5’(1.5 m) and 50’(15 m)

• Sound power levels calculated from the 50’ SPL readings

Cooling Tower Typical Sound Data

From ASHRAE Publication - Application of Manufacturers Sound Data

Noise Control Options

• Location and orientation• Over-sizing (expensive)• Low noise fans• Motor control options• Noise barrier walls• Silencers

Low Noise Fans

• Aerodynamically shaped blades• Lower tip speed• Larger diameter blades• More blades• Larger chord blades• 12 dBA reduction possible

Low Noise Fan Blades

From ASHRAE Publication - A Practical Guide to Noise and Vibration Control for HVAC Systems

Standard Blades Wide-Chord Blades

Motor Control Options

• Two-speed motor• Variable frequency drive• Day-night operation• On-off cycling noise annoying• Reducing fan speed by 50%

reduces noise by 8 to 10 dBA

Noise Barrier Walls

• Close as possible to unit• Clearance for air movement

(restriction or recirculation)• Maintenance access• Sound absorbing material

facing noise source• Barrier must at least block line-

of-sight at receptor

Roof-Top Noise Barrier Wall

Cooling Tower Silencing

• Fan discharge silencers on top• Intake silencers on sides• Plenums for access and

servicing• Pressure drop very low• System effects

Materials for Silencer Construction

• Discharge air is 100% saturated with water vapor

• G 90 Galvanized steel • 304, 316 Stainless steel (costly)• Fiberglass reinforced polyester• Budget, life expectancy,

environment

Structural Considerations

• Cooling tower cell not usually strong enough to support weight of silencers

• Separate structural frame needed

• Wind-loading on silencers

Vibration Isolation

• Required on cooling towers on building rooftops

• Provide structural beams at base

• Use vertically-restrained spring isolators with 2” to 4” deflection

• Seismic considerations

Restrained Spring Isolator

Case Study - Large Casino

• Large casino located adjacent to Niagara Falls

• 3 double-cell cooling towers• Each unit 1970 tons of cooling• Cooling tower environmental

noise impact on natural waterfall sound a major concern

• Also noise impact on hotel

Cooling Tower Assembly Begins

Cooling Tower Assembly Complete

Silencers - Design Requirements

• Discharge silencers installed on propeller fans on top of cells

• Fan diameter 3300 mm (11 ft)• Intake silencers installed over

air intakes on front and back • Pressure drop to be very low• Separate structural framework• Access for maintenance

Silencer Installation Begins

Intake Silencers - Design Criteria

• Two intake silencers per cell• Total 12 intake silencers • 3500 x 5200 mm (11.5 x 17 ft)• Length 900 mm (3 ft)• Standard rectangular silencers• Airflow 54,250 L/s (115,000 cfm) • Design pressure drop 3 Pa

(0.01” wg)

Cooling Tower Intake Silencers

Discharge Silencers - Design Criteria

• Total 6 discharge silencers• Rectangular with circular bullet • 3500 x 3500 mm (11.5 x 11.5 ft)• Length 3000 mm (10 ft)• Bullet diameter 900 mm (3 ft) • Airflow 108,500 L/s (230,000 cfm) • Design pressure drop 15 Pa

(0.06” wg)

Discharge Silencer Assembly

Discharge Silencer Assembly

Discharge Silencers Installed

Cooling Tower Silencers Complete

Design Insertion Loss

Octave Band (Hz) 63 125 250 500 1000 2000 4000 8000

Insertion Loss (dB) 4 14 22 25 11 6 2 1

Octave Band (Hz) 63 125 250 500 1000 2000 4000 8000

Insertion Loss (dB) 6 8 16 29 24 12 11 10

Discharge Silencers

Intake Silencers

Maintenance Issues

• Short plenums provided between cooling tower fans and silencers for access

• Silencer material - galvanized steel

• Zinc-rich paint on exposed welds• Acoustic media - fiberglass

Casino Cooling Tower Silencers

Cooling Tower Silencing Overview

Summary

• There are several strategies for reducing cooling tower noise.

• Properly designed silencers can reduce noise levels by about 10 to 20 dBA without affecting cooling tower operation.

• Silencers can easily be adapted to induced-draft cooling towers.