Providence, Rhode Island Acoustical Society of America 5-9 June 2006

Cooling Tower Noise Control

Tom Paige, P.Eng. Products Application Engineer Kinetics Noise Control, Inc. Vibron Products Group Mississauga, Ontario Canada [email protected]

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

80

75

70

65

60

55 Sound Pressure Level (dB) Level Sound Pressure 50 63 125 250 500 1000 2000 4000 Octave Band (Hz)

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

Induced-Draft Propeller-Fan Cooling Tower Directivity Index (dB) Octave 63 125 250 500 1000 2000 4000 8000 Band (Hz) Front (Air 00122233 Inlet) Side -3 -3 -3 -3 -3 -4 -5 -6 (Enclosed) Top +3 +3 +3 +3 +4 +4 +5 +5 (Discharge)

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

Standard Blades Wide-Chord Blades

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

Discharge Silencers Octave 63 125 250 500 1000 2000 4000 8000 Band (Hz) Insertion 4 142225116 2 1 Loss (dB) Intake Silencers Octave 63 125 250 500 1000 2000 4000 8000 Band (Hz) Insertion 6 8 16 29 24 12 11 10 Loss (dB) 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.