Acoustic Treatment Two 1 Acoustic Treatment Two – Noise and Vibration Control MEBS 6008

Acoustic Treatment Two

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Acoustic Treatment Two – Noise and Vibration Control

MEBS 6008


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Balanced vs unbalanced air flows

Exhaust air flow/ supply air flow

(EAF/SAF)(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

Unbalanced flow increase effectiveness of heat exchanger

Heat exchanger transfer less overall heat, why?

The following example illustrates the reasons:

Balanced Flow

Unbalanced Flow

4.7 cub.m.

4.7 cub.m

35.7 deg C

31.9 deg C

28.9 deg C

25.6 deg C

4.7 cub.m.

35.7 deg C

30.0 deg C

33.2 deg C3.3 cub.m.

25.6 deg C

EAF/SAF = 4.7cub.m. /4.7cub.m. = 1EAF/SAF = 4.7cub.m. /4.7cub.m. = 1

EAF/SAF = 3.3 cub.m./4.7 cub.m. = 0.7EAF/SAF = 3.3 cub.m./4.7 cub.m. = 0.7


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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

Sound power is proportional to the square of sound pressure multiplier 20 is used (not 10).

Reference values are the threshold of hearing.Note the unit of the equation


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SPL (dBA) dBA/10 10^(dBA/10)

37 3.7 5011.87233636 3.6 3981.07170636 3.6 3981.07170635 3.5 3162.2776631 3.1 1258.92541225 2.5 316.22776617 1.7 50.1187233610 1 10

Sum 17771.56531Log Sum 4.249725682

10* Log Sum 42.49725682say 42 dBA

Why 42dBA as stated in previous lecture???


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Noise Control in Ventilation System


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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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The relation between sound pressure level and sound power level in real room may be found by

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.



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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: -

Determination of sound level at a receiver point (continued)



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FUNDAMENTALS OF VIBRATION

A rigidly mounted machine transmits its internal vibratory forcesdirectly 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.

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

eventually appear as noise energy


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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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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.

Undamped VibrationUndamped Vibration

Use the steel spring as vibration isolator.


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Damped VibrationDamped VibrationReal isolators have a certain degree of internal dampingEnergy is progressively removed from the systemAmplitude of vibration steadily reducesLarge amount of damping movement of the mass back to its rest position after initial deflection will be very sluggishNeither overshoot nor oscillate Critical dampingCritical dampingAmount of damping just sufficient for mass to return to its mean position in min. time without overshoot


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Damping equationDamping equation

Effects of DampingEffects of Damping

Frequency ratio for maximum transmissibility < Equivalent undamped amount

At high forcing frequency, transmissibility varies with amount of damping

Figure shown D=20% and D=100%


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Transmissibility T is inversely proportional to the square of the ratio of the disturbing frequency fd to the system natural frequency fn, or

Transmissibility T and Displacement x

2/1

/

nd ff

kFx


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Vibration isolation begin after fd /fn > 1.4 .

Vibration transmissibility rapidly decreases.

At fd = fn, resonance occurs (the denominator of Equation equals zero)

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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Ideally, this ratio should be on the

order of 10:1 to approach an isolation

efficiency of about 90%.

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

Stiffness is inversely proportional to deflection under the applied load, this relationship is shown

as a ratio of deflections.

To optimize isolation efficiencystatic deflection of the loaded isolator>> incremental static

deflection of the floor under added equipment weight.

Floor deflectionexcessive vibration is attributable to upper floor or rooftop mechanical installations.


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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 stiffnessSteps 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

RemarksRemarks

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 ANDNOISE IN PIPING SYSTEMS

All piping has mechanical vibration (equipment and flow-induced vibration andNoise) 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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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.

Flow noise in piping


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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 noisedestroy 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 PracticeNoise 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.

Documents

Acoustic Treatment Two 1 Acoustic Treatment Two – Noise and Vibration Control MEBS 6008