Tonal Noise in Screw Chillers
ASHRAE Seminar 33
Sponsored by TC 2.6
Patrick Marks
Product Engineer
Johnson Controls, York, PA
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
Sometimes called “Helical” or “Rotary Helical” screw.
Positive Displacement Machine
Gas is compressed due to shape of rotors.
Seal between rotors typically provided by oil.
Excellent part load efficiency.
Seminar 33, 26 June 2006
Screw Compressor Tone Generation
Tone Generation
o Gas-borne path: Pressure
Î As male lobe passes
pulsations in discharge vapor.
discharge port, gas slug
is released. (Lobe
o Structure-borne path:
Passage Frequency –
Unsteady bearing forces.
LPF)
Î VFD equipped
compressors and geared
compressors can
increase the LPF:
⎡ RPM × Number of Lobes ⎤
LPF = ⎢ ⎥⎦
⎣ 60
4 lobes 237 Hz 5 lobes 297 Hz 6 lobes 356 Hz
Seminar 33, 26 June 2006
Screw Compressor Tone Generation
Harmonic Content
Î In addition to the LPF,
higher multiples will be
present (Harmonics).
Î Degree of silencing, load
conditions and geometric
effects in the discharge
plenum will effect the
amplitudes of the
harmonics.
5 lobes 297 Hz
594 Hz, -10 dB
891 Hz, -10 dB
5 lobes 297 Hz 1188 Hz, -5 dB
1485 Hz, -5 dB
1782 Hz, -5 dB
Seminar 33, 26 June 2006
Screw Chiller Noise Sources
Chiller Noise Sources
ÎTypically the compressor is not the primary source
of sound radiation.
ÎShell and tube heat exchangers are typically very
responsive at the LPF frequencies and their
harmonics.
ÎPulsation and vibration typically travel from the
compressor to other system components.
Lw
Lw
Lw
Lw
Lw
Lw
Lw
Lw
Vibration Vibration Vibration
Compressor Oil Sep. Condenser Compressor Oil Separator Fan
Pulsation Pulsation Pulsation
Pulsation
Pulsation
Vibration
Vibration
Pulsation
Pulsation
Vibration
Vibration
Vibration Vibration
Evaporator
Evaporator Piping Piping
Pulsation Pulsation
Lw
Lw
Lw
Lw
Vibration Vibration
Seminar 33, 26 June 2006
Pulsation Reduction
Similar to IC engine
exhaust:
Î High sound pressure
levels
Î Duct acoustics
Differences:
Î Refrigerant working fluid
Î Longer design life
Î Unique material issues
Î Pressure vessel
requirements
Seminar 33, 26 June 2006
Pulsation Reduction Methods
Absorptive refrigerant
mufflers
Î Analogous to “Glass Packs”
in the automotive industry.
Î Material requirements
Refrigerant and oil
compatibility
Fiber containment
Pulsation Reduction Methods
Reactive refrigerant
mufflers
Î Analogous to free flowing
mufflers in the
automotive industry.
Seminar 33, 26 June 2006
Pulsation Reduction Methods
Unequal length
discharge/suction lines
Î Analogous to unequal
length intake manifold
runners.
Seminar 33, 26 June 2006
Structure-Borne Excitation Reduction
Compressor Vibration
Transmitted to Other
Components
Î Multiple, flexible lines
Î Decoupled piping
Î Structural modifications
Stiffness and/or mass
modifications
Numerical analysis
Seminar 33, 26 June 2006
System Level Noise Control - Lagging
Limp Mass with Absorption
Only for f > 250 Hz
2 2
Mass > 1.0 lb./ft (4 kg/m )
3 3
0.5 < ρ < 2.0 lb./ft (8 to 32 kg/m )
Perform a source ranking prior to installation.
10
Impervious Outer Layer 8
Typical IL (dB)
6
2
Absorptive Material
Adhere this side to chiller
0
100 200 400 800 1600 3150 6300
One-Third Octave Band Center Frequency (Hz) A
Seminar 33, 26 June 2006
System Level Noise Control - Enclosures
20
Enclosures 18
16
Î Rigid
Typical Insertion Loss (dB)
14
Panels 12
10
Absorption 8
Î “Flexible” 6
4
Quilted Barrier 2
Absorption and TL 0
125 250 500 1000 2000 4000 8000 A
Combined Octave Band Center Frequency (Hz)
Î Sealing of leaks is
critical!
Seminar 33, 26 June 2006
System Level Noise Control – De-Coupling
Use flex joints where
possible.
Î Water Piping
Î Conduit
Î Properly size vibration
isolation – remember
floor deflection in your
analysis!
Î Be cognizant of all
relevant codes when
installing
Seminar 33, 26 June 2006
System Level Noise Control - Air Cooled Chiller
Noise Mitigation
Partial Enclosure (Fans and Refrigerant System Noise)
ÎLouvers
ÎSound Traps above Fan Deck
ÎBarriers
20
18
Low Noise Fans 16
Typical Insertion Loss (dB)
14
ÎSlower speed 12
10
ÎReduce inflow distortions 8
ÎReduce system losses 4
ÎSwept blades 0
125 250 500 1000 2000 4000 8000 A
Octave Band Center Frequency (Hz)
Seminar 33, 26 June 2006
Tonal Noise From Screw Chillers - Summary
Screw 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:
ÎStructure-borne
ÎGas-borne
Job specific noise control:
ÎComponents - Lagging
ÎWater-Cooled System - Enclosure
ÎAir-Cooled System - Partial Enclosure
Seminar 33, 26 June 2006
