Upload
shabbir-akhtar
View
191
Download
16
Embed Size (px)
Citation preview
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
20
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
14
16
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