5.5 GAS COMPRESSOR NOISE

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Many gas compressors are not designed with low noise emission as the

primary design criterion. Such factors as high efficiency, durability and

price are usually more important initially than low noise levels. Noise control

procedures are often applied after the compressor has been constructed,

Noise Sources 173

FIGURE 5-2 Schematic for Example 5-2.

Copyright © 2003 Marcel Dekker, Inc.

rather than implemented during the design of the unit. There are some

design factors that are usually not within the control of the designer, however.

These factors include (a) compressor power input (determined by the

required pressure rise and flow rate through the compressor), (b) the fluid

turbulence levels, and (c) the type of gas compressed.

Some design factors that can be adjusted in the design stage for the

compressor include (a) rotor–stator interaction, (b) impeller–diffuser

spacing, (c) compressor rotational speed, and (d) number of compression

stages (Diehl, 1972).

When a rotor blade passes a stator blade in a compressor, the gas is

given an impulse, and noise is generated from this impulsive action. The

peak in the noise generation curve occurs at the blade pass frequency, which

is given by the following expression:

fB ј

NrNsnr

Kf

(5-20)

The quantities Nr and Ns are the number of rotating and stationary blades,

respectively, and nr is the rotational speed of the compressor. The term Kf is

the greatest common factor of Nr and Ns. For example, if the compressor

has 6 rotating blades and 9 stationary blades, the greatest common factor

between 6 and 9 is Kf ј 3. If the compressor operates at a rotational speed

of 6000 rpm, the blade pass frequency would have the following value:

fB ј р6Юр9Юр6000 rev=min=60 s=minЮ

р3Ю ј 1800 Hz

The value of the greatest common factor рKf ј 3Ю means that 3 rotor blades

line up with 3 stator blades during each revolution of the rotor. The blade

passing noise is 3 times more intense than it would be if only one rotor blade

was matched to one stator blade рKf ј 1Ю during each rotation.

Note that if the number of rotating blades were increased to 7 in this

example, the greatest common factor would be Kf ј 1. The blade pass

frequency would be 6300 Hz. Noise at this higher frequency could be

more easily controlled (by damping, for example) than noise at the lower

frequency.

Increasing the radial distance between the impeller and diffuser vanes

results in lower noise levels. In particular, the blade pass noise is significantly

reduced by this tactic. Unfortunately, increasing the spacing also decreases

the efficiency of the compressor. The designer must compromise between

high noise levels and high efficiency for very close impeller–stator spacing

and low noise levels and low efficiency for wide spacing.

The rotational speed of the compressor has a strong influence on the

noise generated by the unit, because the sound power radiated from a com-

174 Chapter 5

Copyright © 2003 Marcel Dekker, Inc.

pressor is proportional to the rotational speed raised to a power between 2

and 5, depending on the type of compressor. A design compromise must be

made between a low-speed quieter unit, which generally requires a large size

for a given flow rate, and a high-speed more noisy unit, which is smaller in

size.

The following correlations may be used to estimate the overall sound

power level for rotating compressors (Heitner 1968). For centrifugal compressors:

LW ј 20 log10рhp=hpoЮю50log10рUt=UoЮю81 (5-21)

The termhp is the compressor power input, and hpo ј 1hp ј 745:7W. The

quantity Ut is the blade tip velocity, and Uo ј 800ft=sec ј 243:8m=s.

For axial compressors, the corresponding correlation is as follows:

LW ј 20 log10рhp=hpoЮю76 (5-22)

The noise spectra for centrifugal and axial compressors is broadband,

with the peak (maximum) in the sound power level occurring at a frequency

fm given by the following expressions:

fm ј 1000рUt=UoЮ (centrifugal compressors) (5-23)

fm ј 2Nbnr (axial compressorsЮ (5-24)

The quantity Nb is the number of blades in one stage of the axial compressor,

and nr is the rotational speed of the compressor.

The sound power level in each octave band for a compressor may be

calculated from the following expression:

LW(octave band) ј LW _CF3 (5-25)

The values of the conversion factor CF3 are given in Table 5-5.

Example 5-3. A centrifugal compressor has an overall blade tip diameter

of 1.20m (47.2 in) and operates at a rotational speed of 4800 rpm. The

power input to the compressor is 500 kW. The compressor is located in a

room having a room constant of 1500m2. The directivity factor for the

compressor is Q ј 2:00. Determine the sound pressure level at a distance

of 15m (49.2 ft) from the compressor and the sound level spectrum at the

same point.

The compressor blade tip velocity is calculated as follows:

Ut ј _nrD ј р_Юр4800=60Юр1:20Ю ј 301:6m=s р989 ft=secЮ

The overall sound power level is determined from Eq. (5-21) for a centrifugal

compressor:

Noise Sources 175

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LW ј 20 log10р500=0:7457Юю50 log10р301:6=243:8Юю81

LW ј 56:5ю4:6ю81 ј 142:1dB

The overall sound pressure level may be determined fromEq. (5-6) for

sound propagated indoors:

Lp ј 142:1ю10log10

4

1500 ю р2:00Ю

р4_Юр15Ю2

_ _

ю0:1

Lp ј 142:1ю10log10р0:002667ю0:0007074Юю0:1

Lp ј 142:1юр_24:7Юю0:1 ј 117:5dB

The peak in the noise level spectrum occurs at a frequency given by

Eq. (5-23):

fm ј 1000рUt=UoЮ ј р1000Юр301:6=243:8Ю ј 1237 Hz

This frequency lies in the 1000Hz octave band (between 707Hz and

1414 Hz); therefore, the octave band sound power level for the 1000 Hz

octave band is given by Eq. (5-25):

LWрoctave bandЮ ј 142:1_4 ј 138:1dB

The octave band sound pressure level for the 1000Hz octave band may be

calculated from Eq. (5-6):

Lpрoctave bandЮ ј 138:1_24:7ю0:1 ј 113:5dB

The results for the other octave bands are given in Table 5-6.

176 Chapter 5

TABLE 5-5 Conversion Factors CF3 (dB) to Convert

from the Overall Sound Power Level for a Compressor

to the Octave Band Sound Power Levels

Frequency, Hz CF3, dB Frequency, dB CF3, dB

fm=32 36 fm 4

fm=16 25 2fm 8

fm=8a 18 4fm 14

fm=4 12 8fm 21

fm=2 7

aThe table entry fm=8, for example, refers to the octave band

that includes the frequency fm=8, where fm is given by Eq.

(5-23) or (5-24).

Copyright © 2003 Marcel Dekker, Inc.

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