Transcript PPT - Department of Mechanical Engineering

Acoustic Treatment Two – Noise and Vibration Control
MEBS 6008
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Balanced Flow
Balanced vs unbalanced air flows
4.7 cub.m
31.9 deg C
25.6 deg C
Unbalanced flow increase effectiveness of heat exchanger
Heat exchanger transfer less overall heat, why?
35.7 deg C
The following example illustrates the reasons:
4.7 cub.m.
Exhaust air flow/
supply air flow
(EAF/SAF)
Sensible
Effectiveness
1
0.5
90% or 0.9
2
0.6
85% or 0.85
3
0.7
79% or 0.79
4
0.8
76% or 0.76
5
0.9
70% or 0.7
6
1.0
66% or 0.66
EAF/SAF = 4.7cub.m. /4.7cub.m. = 1
Unbalanced Flow
33.2 deg C
3.3 cub.m.
25.6 deg C
35.7 deg C
4.7 cub.m.
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28.9 deg C
30.0 deg C
EAF/SAF = 3.3 cub.m./4.7 cub.m. = 0.7 2
Addition of two sound pressure levels – one example to illustrate
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Sound Power Level
The reference value used for calculating sound-power level
is 10-12 watts.
Sound-power level (Lw) in dB is calculated using the upper
left equation
Sound Pressure
The reference value used for calculating sound-pressure
level is 2 ×10-5 Pa.
Sound-pressure level (Lp) in dB is shown on the lower left
equation
Note the unit of
the equation
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Reference values are the threshold of hearing.
Sound power is proportional to the square of sound
pressure  multiplier 20 is used (not 10).
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Why 42dBA as stated in previous lecture???
SPL (dBA)
37
36
36
35
31
25
17
10
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dBA/10
10^(dBA/10)
3.7 5011.872336
3.6 3981.071706
3.6 3981.071706
3.5 3162.27766
3.1 1258.925412
2.5 316.227766
1.7 50.11872336
1
10
Sum 17771.56531
Log Sum 4.249725682
10* Log Sum 42.49725682
say 42 dBA
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Noise Control in Ventilation System
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Noise Control in Ventilation System
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Noise Control in Ventilation System
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Noise Control in Ventilation System
End reflection loss
The change in propagation medium (when sound
travels from duct termination into a room) reflection
of sound back up the duct.
The effect is greatest at long wavelength (i.e. low
frequencies)
This leads to a contribution to the control of low
frequency noise from the system.
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Noise Control in Ventilation System
Determination of sound level at a receiver point
When a source of sound operates in a room, energy travels from a source to the room boundaries,
where some is absorbed and some of it is reflected back into the room.
The relation between sound pressure level and sound power level in
real room may be found by
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Noise Control in Ventilation System
Determination of sound level at a receiver point (continued)
For a normally furnished room with regular proportions and acoustical characteristics
between `average’ and `medium-dead’ and room volume < 430 m3, a point source of
source could be found by: -
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FUNDAMENTALS OF VIBRATION
A rigidly mounted machine transmits its internal vibratory forces
directly to the supporting structure.
Vibration isolators is resilient mountings
By inserting isolators between the machine and supporting structure,
the magnitude of transmitted vibration can be reduced (%).
Vibration isolators can also be used to protect sensitive equipment from
disturbing vibrations.
Vibration energy from mechanical equipment  transmitted to the
building structure  radiated as structure-borne noise.
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Vibration isolation
Any residual, out-of-balance force in the rotating parts as a weight located eccentrically.
eventually appear as noise energy
The weight rotates  each part of the machine structure subjected to a cyclic force from
inertia of the rotating off-centre height
Vertical component of the force is concerned acting alternately upwards and downwards,
at a frequency equal to the shaft rotational frequency.
Assumption : machine is rigid for every point (includes mounting feet).
Other case of cyclic force example: Combustion loads in reciprocating engines
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SINGLE-DEGREE-OF-FREEDOM MODEL
The simplest example is the single-degree-of-freedom model.
Only motion along the vertical axis is considered
Damping is disregarded
Valid only when the stiffness of the supporting structure >> the stiffness of the vibration isolator.
(mechanical equipment on G/F or basement locations)
Natural frequency of the isolator : deflect the spring a little more + suddenly release it  the machine
oscillate vertically about its rest position at natural frequency.
The natural frequency fn of the system is
where k is the stiffness of vibration isolator
(force per unit deflection)
M is mass of the equipment supported by isolator.
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This equation simplifies to
(try yourself, noting the unit of g)
where δst is the isolator static deflection in mm, k/M = g /δst.
Static deflection = incremental distance the isolator spring compressed under the
equipment weight.
Isolator static deflection & supporting load  achieve the appropriate system natural
frequency
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Undamped Vibration
Use the steel spring as vibration isolator.
The machine settles under its own weight
The machine deflects the spring by a certain amount  static deflection of the isolator
Static deflection determines the eventual performance of the spring as an isolator when
the machine is running.
Static deflection depends only upon the static stiffness of the spring, and weight of the
machine.
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Damped Vibration
Real isolators have a certain degree of internal
damping
Energy is progressively removed from the system
Amplitude of vibration steadily reduces
Large amount of damping movement of the
mass back to its rest position after initial deflection
will be very sluggish
Neither overshoot nor oscillate
Critical damping
Amount of damping just sufficient for mass to
return to its mean position in min. time without
overshoot
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Damping equation
Figure shown D=20% and D=100%
Effects of Damping
Frequency ratio for maximum transmissibility <
Equivalent undamped amount
At high forcing frequency, transmissibility varies with
amount of damping
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Transmissibility T and Displacement x
Transmissibility T is inversely proportional to the square of the ratio of the disturbing
frequency fd to the system natural frequency fn, or
x
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F /k
1  fd / fn 
2
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Vibration isolation begin after fd /fn > 1.4 .
At fd = fn, resonance occurs (the denominator of
Equation equals zero)
Vibration
transmissibility rapidly
decreases.
At resonance theoretically infinite transmission of vibration.
In practice, some limit on the transmission at resonance exists due to some
inherent damping.
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A frequency ratio of at least 4.5 is often specified, which corresponds to an isolation efficiency
of about 90%, or 10% transmissibility.
Higher ratios may be specified, but in practice this is difficult to achieve.
Nonlinear characteristics cause typical isolators to depart from the theoretical curve.
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Equipment mass increased  the resonance
frequency decreases increasing the isolation.
In practice, the load-carrying capacity of isolators
requires their stiffness or number be increased.
The use of stiffer springs leads, however, to smaller
vibration amplitudes—less movement of the
equipment.
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TWO-DEGREE-OF-FREEDOM MODEL
When heavy mechanical equipment is installed on a
structural floor( especially roof), the stiffness of the supporting
structure may NOT be >> the stiffness of the vibration isolator.
Significantly “softer” vibration isolators are usually required in
this case.
Two-degree-of-freedom model for the design of vibration
isolation in upper-floor locations.
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The precise behavior of this system with respect to
vibration isolation is difficult to determine.
The objective is to minimize the motion of the supporting
floor Mf in response to the exciting force F.
Evaluating the interaction between two system natural
frequencies and the frequency of the exciting force
complicated
Fraction of vibratory force transmitted across an isolator
to the building structure (transmissibility) depends in part
the isolator stiffness comparing with that of supporting
floor.
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Stiffness is inversely proportional to deflection under the applied load, this relationship is shown
as a ratio of deflections.
To optimize isolation efficiencystatic deflection of the loaded isolator>> incremental static
deflection of the floor under added equipment weight.
Floor deflectionexcessive
vibration is attributable to upper
floor or rooftop mechanical
installations.
Static deflection of the vibration isolator is =
incremental deflection of the supporting
floor under the added weight of the
equipment
50% of the vibratory force
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Ideally, this ratio should be on the
order of 10:1 to approach an isolation
efficiency of about 90%.
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Selection of vibration isolators on the basis of the single-degree-of-freedom model
neglected floor stiffness inadequate
Steps to choose vibration isolators with consideration of floor stiffness
Asking structural engineer to estimate the incremental static deflection of the floor due
to the added weight of the equipment at the point of loading
Choose an isolator that will provide a static deflection of 8 to 10 times that of the
estimated incremental floor deflection.
Consider also building spans, equipment operating speeds, equipment power,
damping and other factors
Remarks
The type of equipment, proximity to noise-sensitive areas, and the type of building
construction may alter these choices.
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Fans and Air-Handling Equipment:
Fans with wheel diameters < or = 560 mm and all fans operating at speeds
to 300 rpm NOT generate large vibratory forces.
For fans operating under 300 rpm, select isolator deflection so that the
isolator natural frequency is 40% or less of the fan speed.
Fan operating at 275 rpm, an isolator natural frequency of 110 rpm (1.8 Hz)
or lower is required (0.4 × 275 = 110 rpm).
A 75-mm deflection isolator can provide this isolation.
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Pumps:
Concrete bases (Type C) should be designed for a thickness of one tenth the
longest dimension with minimum thickness as follows:
For up to 20 kW, 150 mm;
For 30 to 55 kW, 200 mm;
For 75 kW and higher, 300 mm.
Pumps over 55 kW and multistage pumps may exhibit
excessive motion at start-up  supplemental
restraining devices can be installed if necessary.
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ISOLATION OF VIBRATION AND
NOISE IN PIPING SYSTEMS
All piping has mechanical vibration (equipment and flow-induced vibration and
Noise) transmitted by the pipe wall and the water column.
Equipment installed on vibration isolators exhibits motion or movement from
pressure thrusts during operation.
Vibration isolators have even greater movement during start-up and shutdown
(equipment goes through the isolators’ resonant frequency).
The piping system must be flexible enough to
•
Reduce vibration transmission along the connected piping,
•
Permit equipment movement without reducing the performance of
vibration isolators, and
•
Accommodate equipment movement or thermal movement of the
piping at connections without imposing undue strain on the
connections and equipment.
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Flow noise in piping
Minimized by sizing pipe so that
- the velocity is 1.2 m/s maximum for pipe 50 mm and smaller and
- using a pressure drop limitation of 400 Pa per metre of pipe length with a
maximum velocity of 3 m/s for larger pipe sizes.
Flow noise and vibration can be reintroduced by
-turbulence,
-sharp pressure drops, and
-entrained air.
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Resilient Pipe Hangers and Supports
Resilient pipe hangers and supports are necessary to prevent vibration and noise
transmission from the piping to the building structure and to provide flexibility in the piping.
Suspended Piping.
Isolation hangers described in the vibration isolation section should be
used for all piping in equipment rooms.
The first three hangers from the equipment : the same deflection as the
equipment isolators (a max. limitation of 50 mm deflection)
Remaining hangers : spring or combination spring and rubber with 20 mm
deflection.
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Floor Supported Piping.
Floor supports for piping in equipment rooms and adjacent
to isolated equipment:
The first two adjacent floor supports should be the
restrained spring type, with a blocking feature that prevents
load transfer to equipment flanges as the piping is filled or
drained.
Where pipe is subjected to large thermal movement, a slide
plate should be installed on top of the isolator
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Piping Penetrations
Most HVAC systems have many points at which piping must penetrate floors, walls,
and ceilings.
Risk for a path for airborne noisedestroy the acoustical integrity of the occupied
space.
Seal the openings in the pipe sleeves by an acoustical barrier such as fibrous
material and caulking (between noisy areas, such as equipment rooms, and
occupied spaces)
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Flexible Pipe Connectors
(1)
They provide piping flexibility to permit isolators to
function properly,
(2)
They protect equipment from strain from
misalignment and expansion or contraction of piping,
and
(3)
They attenuate noise and vibration transmission
along the piping .
The most common type of connector are arched or
expansion joint type, a short length connector with one or
more large radius arches, of rubber or metal.
All flexible connectors require end restraint to counteract the
pressure thrust.
Overextension will cause failure.
Manufacturers’ recommendations on restraint, pressure, and
temperature limitations should be strictly adhered to.
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Noise Control in Practice
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Noise Control in Practice
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Noise Control in Practice
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Noise Control in Practice
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Exercises would be provided
later as this file size is too
large.
Good Luck in the coming
Exam.
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