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, [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 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
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
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
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
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
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)
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)
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