Tonal Noise in Screw Chillers

ASHRAE Seminar 33Sponsored by TC 2.6

Patrick Marks

Product EngineerJohnson Controls, York, PA

26 June 2006

Seminar 33, 26 June 2006

Tonal Noise in Screw Chillers - Agenda

Why do screw chillers produce tones?Types of excitations.Harmonic content.Pulsation control.Structure-borne control.System level mitigation suggestions.

Seminar 33, 26 June 2006

Screw Compressor Function

Gas is compressed due to shape of rotors.Seal between rotors typically provided by oil.Excellent part load efficiency.

Sometimes called “Helical” or “Rotary Helical” screw.Positive Displacement Machine

Seminar 33, 26 June 2006

Screw Compressor Tone Generation

Tone GenerationAs male lobe passes discharge port, gas slug is released. (Lobe Passage Frequency –LPF)

VFD equipped compressors and geared compressors can increase the LPF:

⎥⎦⎤

⎢⎣⎡ ×

=60

LobesofNumberRPMLPF

4 lobes 237 Hz 5 lobes 297 Hz 6 lobes 356 Hz

o Gas-borne path: Pressure pulsations in discharge vapor.

o Structure-borne path: Unsteady bearing forces.

Seminar 33, 26 June 2006

Screw Compressor Tone GenerationHarmonic Content

In addition to the LPF, higher multiples will be present (Harmonics).Degree of silencing, load conditions and geometric effects in the discharge plenum will effect the amplitudes of the harmonics.

5 lobes 297 Hz

5 lobes 297 Hz

594 Hz, -10 dB

891 Hz, -10 dB

1188 Hz, -5 dB

1485 Hz, -5 dB

1782 Hz, -5 dB

Seminar 33, 26 June 2006

Screw Chiller Noise Sources

Compressor Oil Separator

Lw

Evaporator Piping

Lw

Vibration

Pulsation

Vibration

Vibr

atio

n

Puls

atio

n

Vibration

Pulsation

Lw Lw

LwVi

brat

ion

Puls

atio

n

Lw

FanCompressor Oil Sep.

Lw

Evaporator Piping

Lw

Condenser

Vibration

Pulsation

Vibration

Vibr

atio

n

Puls

atio

n

Vibration Vibration

Pulsation Pulsation

Lw Lw

LwLw

Vibr

atio

n

Puls

atio

n

Chiller Noise SourcesTypically the compressor is not the primary source of sound radiation.Shell and tube heat exchangers are typically very responsive at the LPF frequencies and their harmonics.Pulsation and vibration typically travel from the compressor to other system components.

Seminar 33, 26 June 2006

Pulsation Reduction

Similar to IC engine exhaust:

High sound pressure levelsDuct acoustics

Differences:Refrigerant working fluidLonger design lifeUnique material issuesPressure vessel requirements

Pulsation Reduction MethodsAbsorptive refrigerant mufflers

Analogous to “Glass Packs” in the automotive industry.Material requirements

Refrigerant and oil compatibilityFiber containment

Seminar 33, 26 June 2006

Pulsation Reduction Methods

Reactive refrigerant mufflers

Analogous to free flowing mufflers in the automotive industry.

Seminar 33, 26 June 2006

Pulsation Reduction Methods

Unequal length discharge/suction lines

Analogous to unequal length intake manifold runners.

Seminar 33, 26 June 2006

Structure-Borne Excitation Reduction

Compressor Vibration Transmitted to Other Components

Multiple, flexible linesDecoupled pipingStructural modifications

Stiffness and/or mass modificationsNumerical analysis

Seminar 33, 26 June 2006

System Level Noise Control - Lagging

Limp Mass with AbsorptionOnly for f > 250 HzMass > 1.0 lb./ft2 (4 kg/m2)0.5 < ρ < 2.0 lb./ft3 (8 to 32 kg/m3)Perform a source ranking prior to installation.

Impervious Outer Layer

Adhere this side to chiller

Absorptive Material

0

2

4

6

8

10

100 200 400 800 1600 3150 6300

One-Third Octave Band Center Frequency (Hz)

Typi

cal I

L (d

B)

A

Seminar 33, 26 June 2006

System Level Noise Control - Enclosures

EnclosuresRigid

PanelsAbsorption

“Flexible”Quilted BarrierAbsorption and TL Combined

Sealing of leaks is critical!

0

2

4

6

8

10

12

14

16

18

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125 250 500 1000 2000 4000 8000 A

Octave Band Center Frequency (Hz)

Typi

cal I

nser

tion

Loss

(dB

)

Seminar 33, 26 June 2006

System Level Noise Control – De-Coupling

Use flex joints where possible.

Water PipingConduitProperly size vibration isolation – remember floor deflection in your analysis!Be cognizant of all relevant codes when installing

Seminar 33, 26 June 2006

System Level Noise Control - Air Cooled Chiller Noise Mitigation

0

2

4

6

8

10

12

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18

20

125 250 500 1000 2000 4000 8000 A

Octave Band Center Frequency (Hz)

Typi

cal I

nser

tion

Loss

(dB

)

Partial Enclosure (Fans and Refrigerant System Noise)LouversSound Traps above Fan DeckBarriers

Low Noise FansSlower speedReduce inflow distortionsReduce system lossesSwept blades

Seminar 33, 26 June 2006

Tonal Noise From Screw Chillers - SummaryScrew chillers produce tonal noise that is a function of their operating condition.Screw chiller manufacturers have devised a number of ways for reducing screw chiller noise. Screw compressor energy excites the chiller system in two ways:

Structure-borneGas-borne

Job specific noise control:Components - LaggingWater-Cooled System - EnclosureAir-Cooled System - Partial Enclosure