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International Compressor Engineering Conference School of Mechanical Engineering

1986

Experimental Analysis of Screw Compressor Noiseand Vibration

A. Fujiwara

N. Sakurai

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Fujiwara, A. and Sakurai, N., "Experimental Analysis of Screw Compressor Noise and Vibration" (1986). International Compressor

Engineering Conference. Paper 553.hp://docs.lib.purdue.edu/icec/553

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EXPERIMENTAL ANALYSIS OF SCREW COMPRESSOR

NOISE AND VIBRATION

Akinori Fujiwara. Chief Engineer Noriyoshi Sakurai. Engineer

Compressor Divis ion . Mayekawa Mfg . Co., Ltd. <MYOOM)

Okubo. Moriya-Machi. Ibaraki-Ken, Japan 3 0 2 ~ 0 1

ABSTRACT

Very few reports have been presented to date on

the noise and v ibrational charac ter is t ics of o il in

jected screw compressors.

This time. a se ries of extensive experimental

analysis i s performed. I t includes the measurements

and analysis of compressor casing vibra tions . ro to r

shafts v ibrations, to rs iona l <rotational) vibra tions.

suction and discharge gas pulsations. pressure t r a n s ~ien t in one thread of ro tors ana compressor noise.

These experiments are done mainly with R-22 gas,

and the operating speed of compressor continuously

varied up to 4400 RPM in some te s t conditions. The

te sted compressors are mainly 1SOL<163mm , long ro to r) .

Analyzed re su lts of these experiments help to

explain the major noise and 'v ibrational charac ter is

t ic s of standard o i l in jec ted screw compressors.

Nomenclature

Fo = undamental screw frequency, Hz

Fm male ro tor operating frequency, Hz

Ff female ro to r operating frequency, Hz

k spec ific heat ra t io

n operating speed. RPM

Pd = discharge pressure. PaPs suction pressure. Pa

Vi bUilt-in volume ra t io

Pi bu il t - in pressure r a t io = Vik(adiabatic change)

Zm male tooth number Zf female tooth number

D ro tor diameter. mm

L ro tor length . mm

566

I



INTRODUCTION

Today, the o i l in je c ted screw compressors areknown as high-performanced and highly durable compre

sso r and th e i r app l ica t ion range has become very wide.In sp i te of the amount of in fo rmation concern ing i t sapp l ica t ions and performances, te chn ica l papers dea ling with th e fundamental ch a ra c te r is t ic s of noise andv ib ra t ion of the compressors seem to be very few. In order to eva lua te and reduce the noise and v ib ra t ion of the compressors. th e i r fundamental ch a ra c te r is t ic s

must be well understood. So, to a id the b e t te r understanding of th ese , we w ill p resen t th is summerizedrepo r t of our se r ie s of experiments concerning noise and v ib ra t ion of screw compressors.

Compression Mechanism

The screw compressor i s c la s s i f ied as a po s it ivedisplacement ro ta ry compressor. The noise and v i-b ra tions generated Ro to r

by a screw compressorhave a d is t in c tre la t io n sh ip to i t s gas compression

mechanism. So it i s important to unders tand th i s mechanism. A genera l arrangementof an o i l in jec tedscrew compressor i s shown in Fig. 1.

a Sll'CTlOU PHASE

a: As "':h.e pa.i.:r of lo'oe

<Male) ana. &-rooveCFeaa.1c) 'becin tounmesh. & s :pacevol uae j_s created

and razr. i.s d::t&Wn in

tllroui 'h suction :g o:rt.Unti l the moment at,.hich .suctio npo:r t c lose s , tl'ie

i . : r s:oace i s

!ill el u p wi. th cas

as t o : r rota te .c

'b: The tra.p:ped poc:ke't .o! r. as a . ' t ~ Q !':rollthe i. nl@ 't a.nd ou"tlf!t,i5 •c ved ei:rcu!l l!eren"t i .a l lY under :rotation a."': 't ll ec: ons ' tant suc:'tion pressure.

F-ig. Oil In jec ted

Screw Compressor the suction end.A.s the in ter-lobellle5h poJ.n-e move-s

towa:rcl th ! diech arr :eend ax i&l l; v th e

volume o! tr apped :pocke t i s g: rad.uall;v:redue l!d and the::Pressure of the gas

o n s e e n t ~ in -c:reasl!!d.. In th i sph ase , Oil i.sin jectl!':d . !or cco l-

OOMPRESSION PHASl!: 1 sealing and

fo :r lu'b:rication.

d:At a. momen"':determ ined bY

: r e a e r m i Vi(bui lt in

I ratio) 1 'tl:le pocke"!:

o:f gas i.S .te l ease dthrough Q ..echar i'l !

c: As rotation port and t) 'l . l!

b TRANSFER PHASE

l;J roceeds. an ot he r C:OID:Pl'e5sed te .s J.s male lo b!. engages full:Y a. isc hargea.a te•a.l! : c-roove a$ th ro ug h the :P ort by thes l! St:al"t •eshin " d D!SCHAROE PHASE !urth e:r l'O tation ,

F ig . 2 OompTession Mechanism <Ref. [1 J >

567



The compression of a gas i s ~ t t a i n e d by the

a ~ r e c t volume reduc tion c f the in te r - lobe space as

the ro to rs ro ta te . This i s i l lu s t ra te d in Fig. 2.

EXPERIMENTAL TECHNIQU:E:

Instrum ents <Re£.[2])

A number of ana ly t ica l too ls and procedures are

ava i lab le to analyze noise and v ib ra t ion . The FFT

<Fast Fourier Transform> ana lys is i s considered as a

ve rsa t i le technique with in ~ h e s e methods. For a prac

t i c a l po in t o f view. s tand-a lone FFT analyzers a re

used as the main instrum ent to analyze s igna ls from

compressors. The fo llow ings are the s igna ls s ~ u d i e d in de ta i led experiments .

- pressure t r an s ien t in one th read of female ro to r

- discharge and suc tion gas pu lsa t ions

- ro to r sha f t rad ia l v ib ra t ion s ( re la t iv e to casing)

- ro to r sh a f t axiaJ. v ib ra t ions < re la t iv e to casing)

- to rs ion a l v ib ra t ion

- re la t iv e ro ta t io na l v ib ra t ion <male and female>

- casing v ib ra t ion acce le ra t ion Cin th ree direc tions>

- compressor noise (normally a t 1 meter>

Fig. 3

shows genera l se t-up of instrum ents and t ran s

ducers .

.a b:: PreS!i iUt"e T-r an sduce r

e; So\lnd Level Mete r d:C ha l. "ge /..mp. e. DC Amp. f:Dat. a P.e.co:rde'r

g FFT Analy:.er h :P lo t t e r

Fig. 3 Instrument General Set-Up

Test F ac i l i t ie s

Fig. 4 Test Fac il i tY Arrangement

568



Deta i led experiments inc lud ing var iab le -speed ope ra tion a re performed in labo ra to ry t e s t f a c i l i t i e s mainly on 1601. Also some measurements a re taken on opera ting compressor in ac tua l p lan t . Shown in Fig . 4 i s th e genera l arrangement of th e t e s t f a c i

l i ty .

FUNDAMENTALS

This sec tion w ill Presen t mate r ia ls found by experiments on th e fundamental c h a ra c te r i s t ic s ofscrew compressor no ise and v ib ra t io n .

Fundamental Screw Frequency

The fundamental frequency of no ise and v ib ra t io n can be determ ined by th e fo llow ing equa tions .

For male drive Fo ~ n/60 X Zm For female d rive Fo ~ n/60 X Zf

These equa tion s ind ica te th a t fundamental frequency is s imply determ ined by th e d rive ro to r opera ting speed and its number of lobes . In th i s repo r t we c a l l th is frequency lFoJ as "fundamental screw frequency".

Another fundamental f requenc ies a re ope ra ting speed of male ro to r and th a t o f female ro to r . They a re ca lcu la ted by next equations .

For male d rive Fro n/60 Ff n/60 X Zm/Zf Fm X Zm/Zf

For female drive Ff n/60 Fm n/60 X Zf/Zm Ff X Zf/Zm

Pressu re Trans ien t in Rotor Thread

Fig . 5 i l lu s t r a te s an example of in ope ra t iona l period r e la t io n sh ip between male ro to r tu rn ing angle and pressu re t ra n s ie n t in one ro to r th read . pu lsa tion of d ischarge and th a t of suc t ion . All these a re pro -

.

"

.

li l

a

~ ~ ~ ~ , 1 . ~ . ~ + . , . ~ ~ ~ ~ . ~ . + . , ~ ~ , . . MFILI:: TURN INC FINCI..E: t

TE! :i j 16 0 L

.c .. D ~ l 6 3 TE - c. 11 .L L/Dal. 65

o.;.R-22 Gas F'd P;a- RELATioN V i ~ ) • 65

a :P res su re T rans ien t b :Suct ion Pu lsa t ion c :D ischarge Pu lsa t io n ffi :D ischarge P o r ~ Open

Fig .S Opera tion -Period of P ressu re & Pu lsa t io n

569



bable causes of no ise . The pressure t ran s ien t domina

te s gas fo rces induced on ro to rs and thereby con tro ls

the fundamental noise and v ib ra t ion ch a ra c te r is t ic s .

Pressure Pu lsa tions

Pu lsa t ions are caused bY in te rm it ten t gas d is -

charge and suction accoring to the compression mech

anism. The discharged gas pu lsa t ion i s in fluenced by

the pressu re d iffe rence between opera ting discharge

pressure and th read p ressu re a t the moment of the be

ginning of the discharge phase. Example of opera ting

pu lsa tions from 1BOL i s shown in Fig. B. According to

the F igure. th e i r waveforms are s im ila r to sawtooth It r iangu la r wave and these periods ind ica te th a t th e i r frequencies are a t the fundamental screw frequency.

w H E*-!.i; l

C/l

~ REAL ,_. Po.

p,

-.981 E+5

.l ilm SEC TIME

1601

D ~ 1 6 3 L / D ~ 1 . 6 5 R-22 Gas

a:D ischarge

b :Suction

4400RPM

Fig . 6 Operating Pu lsa tion Waveform

Torsional Vibra t ion CTorque Fluctuation>

F ig . 7 shows a ty p ic a l example of opera t ing to r -

s iona l v ib ra tion s igna l from the output of the to rque

meter. The v ib ra t ion i s caused by dynamic gas to rque

and i t s frequency i s a lso a t the fundamental screw

frequency. In genera l. the measured to rs ion a l ampli

tudes from to rque meter would be well below 5% of the

s ta t ic to rque when opera ted under normal conditions

with R-·22 gas.

N 1601 D ~ 1 6 3 L / D ~ 1 . 6 5 R-22Gas

fsr=====J oo ;r r6mSec

1me

Fig . 7 Operating Torsional Vibra tion

Rotor Shaft V ibra tions

Resulting £rom the gas forces and ro to r contac t

570



fo rces . dynamic fo rces are induced on the ro to rs andthereby rad ia l and ax ia l sh a f t v ib ra t ions of thero to rs occur. These v ib ra t ion s are t ran sm itted through the bearings to the cas ing of compressor.The

fundamental frequencY component of ro to r sha f t v ib ra -t ion s both in rad ia l and ax ia l d irec t ion s are iden-t i c a l to the fundamental screw frequency. A ty p ica l example of opera ting ax ia l sha f t v ib ra t ion i s shownin Fig . 8.

~ 6 - z z . - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - , c:

"'"'" "'--<

• p .

.< > til

..........

><=>

160L

o ~ l 6 3 L/0=1.65 R-22

Female

~ 6 . 2 2 L - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - 8 - 0 ~ m S e cTime

Fig . 8 Operating Axial Shaft V ibra tion

General Noise Charac te r is t ic s

Fig . 9 shows the noise spectrum from 250L opera ting a t 3000 RPM. Several d isc re te peaks. well above 1kHz. can be noted . and dB

other peaks are broad •

and not well defined .The do tted l in e in the S Fig. 9 ln d ica te s a

broad-band random noiserad iated by gas £low. E

In genera l . the . ,oo

broad-band random noise gof a frequency spectrum :

i s heard as a rushing

250L

D=255

L /D • l . 65R-22 30 0DRPM

0 2 6 .e 10 1:2 1& 18soqnd.frequency kHz

Fig. 1 0 shows

spectrum of another pressor . 200L. opera t in g a t 3000 RPM.

This compressor again genera-cesnoise a t d isc re te frequencies and are a l : harmonically re la ted .They are ~ i g h e rharmonics of the

fundamental screw frequency \200 Hz>

and the broad-bandrandom noise

the com--

dBA

~ 1 0 0"'"'~ 8 0

Fig . 3 Overa ll Operatlng

Noise Spectrum

200L Dz2Q4

Harmonics L/Dml. 6 5

// \ ~ 2 Gas~ kHz

Fig.10 2kHz Range Operating Noise Frequency Spectrum

571



~ n d ~ c a t e s gas flow n o ~ s e . I t should be noted th a t

the magnitudes of harmonics are a p p r o x ~ m a t e l y a t a

constan t leve l up to severa l order h a r m o n ~ c s .

General Vibration Charac te r is t ic s

F ~ g . 1 1 shows examples of v ib ra t ion from 3201 op

era t ing a t 3600 R P ~ employing a multip ly ing gear box.

The a ~ s p l a c e m e n t spectrum. Fig .11-a . c o n t a ~ n s 3

a o m ~ n a n t d isc re te spectrum and the peak occurring a t

240 Hz i s the fundamental screw frequency. The spec

trum a t 50H z shows opera ting speed of inpu t sha f t ana

i t s unbalance<including motor.coupling and drive gear

) . EquallY. the peak a tso Hz

ind ica te s ou·tput sha f t

operating speed and i t s unbalance. And the waveform

of displacement ind ica tes the "bea t" between 5 0 H z and

60 Hz opera ting frequency s ign a ls .

The ve loc i ty spectrum. shown in Fig .11-b , in d i

ca te the fundamental screw frequency and harmonics

decreasing th e i r amplitude with frequency. The wave

form of ve loc i ty shows appearances of sawtooth wave

hence its harmonics may decrease approximately -BdB/

octave in amplitude.In con tras t with the ve loc ity s igna l , the acce l

e ra t ion s igna l shown in Fig .11-c ind ica tes more

def in i te tendency of se r ie s of pu lses . And according

to i t s waveform. the acce le ra t ion spectrum contains

more harmonics. The higher harmonics should co m e

1!1

'. .u} ... .. .. , ! . 1 , . . . . . . ~ - : , ; ; c ; . " " c ; ~ . H J 1

F ig .11-a Displacement

. ,,~ - 1 ~ - - - - - - - - j""'""

H ! ~ u

~ . , l l ~ T1me

~ - z ~ i i J - - - - - - - - - - i " """'"" 1

i

L 4 ~ ~ , , ~ • ~ ~ . ~ ~ ~ . ~ fre quency

Fig .11-b Velocity

Multip ly ing Gear Box~ r - - 1-- Motor

Compresso r - -

-Skid

Fig .11-c Accelera tion F ig .11-d Arrangement

Fig. 11 Operating Vibration Waveform and Spectrum

572



mainly from ro to r meshing. Genera l ly . measured v ib ra -t io n a cce le ra t ion values may not exceed 9 .8 m/sec2 < = 1 G>. The "Axial" d irec t ion shows th e maximum v i-b ra t io n acce le ra t ion lev e l from normallY opera t ing

compressors. It

should be noted th a t . in gene ra l. la rg e r v ib ra t ion leve l r e su l t in h igher sound le v e l .

Vibration Parameter Selec tion

According to ex tens ive v ib ra tio n measurements. da ta ind ica te th a t the v ib ra t ion of screw compressor has seve ra l harmonics of the fundamental screw f r e -quency a t a constan t leve l in acce le ra t ion in c e r ta in range of f requenc ie s . An example i s shown in Fig .12 .

0: ,4 00

E•l ...,<d

HRG ~ m/s>

<llu

u

< Ill .A .. Jtw 0

I

I

I

! I

Jl I fl /R Sf R Ll N

Jww, Ju .... i

2kHz

1601 D ~ l 6 3

L / D ~ l . 65 R-22 Gas

3600RPM

Axia l

Frequency

Fig . 12 O p e r a ~ i n g Accele ra t ion Spectrum

dB So .re

gard ing the 0

. 20

:::"-40 a

""'60

, .Acce:l era t. ion

I I

["--. I

"' ~ Y e l o c H v

N il !

.N l I I ill .........

'

lOO" 1000

se le c t io n of v ib ra tio n measurement parameter. displacement i s not th e p re fe r red parameter except in case of unbalance problem becauseof its low s en s i t iv i ty a t high frequency . as shown in Fig .13 . F ig . 13 Disp l. ,Vel.& Ace.

Relation I t i s recommended th a t one

should choose acce le -ra t ion as a Parameter ~ h e n measuring screw compressor v ib ra t io n in d e ta i l . Care must be taken to avoid mounted resonance of acce leromete rs w ith in the f re -quency range of in t re sT .

As a sepa ra te c o n s i d e r a t ~ o n . the disp lacement probe !p rox im ity probe> i s lhe only t ran sducer when measuring sh a f t v ib ra tio n re la t iv e to the cas ing .

EFFECT OF O P E R ~ T I N G CONDITIONS

Operating Pressu re Condition

The opera ting ~ r e 3 s u r e cond it ion has a d is t in c t

573



~ n f l u e n c e upon compressor n o ~ s e . casing v ib ra t ion s

and performance. O p e r a t ~ n g pressu res a lso a f fe c t

t o r s ~ o n a l v i b r a t ~ o n . sh a f t v ib ra t ion and pressu re

pu lsa tion s . The use of economizer a lso has some

e ffe c t on above mentioned v ib ra t io n s .As -!;he pr·essure in a rotc :r th read , . jus t p r io r to

the s t a r t of d ischarge phase. i s only determined th e

o re t ic a l ly by the m u l t ~ p l i c a t i o n between suc tion

pressu re and bu i l t - in pressu re r a t io ( i . e . Ps X Pi )

without economizer. so the re ex is ts only one ac tua l

operat ing discharge pressu re which w ill exactly agree

with the t h e o r e t i c ~ l l Y determined discharge pressure . Regardless of the magnitude. the re ex is ts a c e r ta in

pressu re d iffe rence between these two discharge pres

sures . In genera l . the opera ting compressor noise and

casing v ib ra t ion leve l ~ i l l inc reaae with the in crease in the pressu re d iffe rence .

A measured example of compressor noise and casing

v ib ra t ion shown in Table 1 ind ica tes above mentioned

tendency under an opera ting d ischarge pressure condi

t ion . Another example under a constan t operat ing

suc tion pressure condition i s shown in Table 2.

Table 1 (dB)

Noise & Vibra tion a t Varying Ps No. Ps MPa V.Acc. H,Acc. A.Acc. SPL

1 0.50 +4.1 +2.2 +2.9 +3.1 2 0,30 0 0 0 0

3 0.16 +0.1 +1.1 +3.1 +3.4

*Pd=l,37MPa,N0.2 as a re fe rence value

Table 2 (dB)

Noise a t Varying Pd No Pd M a SPL

1 0.91 +0.2

2 l. 07 0

3 l. 3 7 +2. 4

* P s ~ 0 . 2 4 M P a , N o . 2 as a r e f . value

In add it ion . the discharge port s ize cH. M and

L). H-port genera lly shows the bes t e ffec t on meas

ured noise and v ib ra t io n from re f r ig e ra t io n compres

so rs because of its l e a s t gas mass flow ra te with in

th ree ports under normal opera ting condit ion .

Operating Speed

Operating da ta i s used to determine not onlY the

leve l of noise ana v ib ra t ion but a lso . more importan

t ly . the frequency components o f these s igna ls . By

vary ing the opera ting speed of the compressor. the

e f fec t on frequency spectrum can be seen and the

various resonances. ~ n h e r e n t in the compressor system.

can be determined. Fig . 14 shows an example of a th ree-d im en tiona l

RPM spectrum map of generated sound pressure leve ls

from 160L. The peaks C a t 100 Hz. 220 Hz. 275Hz and

545 Hz) are ind ica t ion s of various resonances.

In th i s case . the maximum amplitude peak a t 545

Hz comes from a re la t iv e ro ta t io n a l resonance between

574



the male and female ro to rs . and can be determined bY the method of de tec t ing the phase d iffe rence between two s inuso ida l too th passing s igna ls £rom invo lu le gears a ttached to each ro to r s .

The peak a t 100Hz in Fig .14 . f ina l lY determined a t 87.5 Hz in h igher frequency re so lu tion analYsis with more f ine p i tch change in the opera ting speed .is

coming from a t o r s ~ o n a l resonance in the system. And the peaks a t 220 Hz and 275 Hz are confirmed

to be resonances of sk id members by h a m m e r ~ n g method .

0

. &27

* not in dB -sca le , in l in e a r sc a le

E+S

~ 4 3 0 0 - 1500

Frequency 2kHz

PWR SP

l60L

D-163 L / D ~ l . 6 5 Vi-5 .8 R-22

.--<:> ::

<UP-<

Q.:.:: 0'-'

·rl"" .u <I)

<U <I)

H p.<I) U)

p.

0

Fig .14 3-D RPM Spectrum Map of Generated N o ~ s e For the discharge gas pressure pu lsa tion . a

typ ica l example i s shown in Fig . 15. The magnitude inc reases with o p e r a t ~ n g speed. but on the b a s ~ s of our data . it cannot be accounted fo r i t s e f fec t on compressor n o ~ s e .

JFo

Pa / 4400

.490 •• E+5

" . " a '"""'.

1200 "' 0 Frequency 2kHz

l60L

L /O al. 65 V1-J.65R-22 Gas

Fig . 15 3-D RPM Pulsa tion Spectrum Map

Rela tive to the casing v ib ra tion acce le ra t ion .a s a r e su l t of our e x p e r i m e n t . ~ t can be assured th a t the acce le ra t ion le v e l w lll i n c r e ~ s e propo rtiona l ly with the increase in opera ting speed when no resonance

575



has occured.

Part Load Condition

An outs tand ing fea tu re of the standard screw

compressor i s the ab i l i ty of s tep le ss capacity con

t ro l with the s l id e valve system. The change in Vi

re su l t ing from the ax ia l movement of the s l id e valve

a f fe c ts not onlY the performance o i compressor but

a lso the ch a ra c te r is t ic s of noise and v ib ra t ion .

As shown in Fig. 16. under par t load operations

s . 8 .. ..

H-Por t

~ 0 ~ 0 ~ ~ - - ~ 6 0 ~ ~ . ~ 0 - - ~ 2 ~ 0 - - ~ Load(%)

Fig.16 Vi Change in Unloading

the change in Vi

ranges from 3.0 to

1.25 even in the L

po r t .so the d iffe rence between th eo re t ic a l ly

determined discharge

pressu re (ps X Pi) and

the ac tua l operating

discharge pressure

a lso would be varied

with unloading.

As the re su l t of above . i t appears th a t a complex

change occurs in the cha ra c te r is t ic s of no ise and

v ib ra t ion under pa r t load opera tion .

Fig. 17 shows a 3-D v ib ra tion spectrum p lo t from

160L o p ~ r a t i n g a t 3600RPM under par t load .

4 ..- ! ..,

.. H

.... .,

"

160L

D•l63

L/D•l, 65

Vi•2,63

(100% Load)

d e ~ ; 2 2 Gas

..

.,

.....

" .. " " '" '" '. .. . . 00

< o 1 & ~ ~ ~ ~ ~ ~ ~ ~ 0 . .. " . .. . .,.,

""'

Fig. 17 3-D Axial Vibra tion Spectrum Map

Under Part Load Operation

F rom fu l l unloaded operat ion da ta , it can be

assumed th a t the re tu rn ing o i l from the bearings and

the mechanical sea l w i l l sometimes obstruc t a s tab le

opera tion of compressor under reduced gas flow . More

over the s l id e valve sometimes v ib ra tes a t the fundamental screw frequency and. by impacting on the

casing , it would cause an excessive no ise and I or

v ib ra tion problem in extreme case.

576



Type of Gas

Change in type of gas to be compressed sometimes ind ica te s d is t in c t e ffec t on the compressor no ise and

v ib ra t ion ch a ra c te r is t ic s . Here. as a typ ica l example, Table 3 i s shown as re fe rence . Other than th i s Table, da ta in d ica te s an ex is tence of la rg e d if fe rence in ax ia l sh a f t v ib ra t ion s ana the amplitude under opera-t io n in some t e s t conditions during which the amplitude with helium gas is four times g rea te r than the amplitude w ith a i r .

The accura te cause of the d iffe rence in noise and v ib ra t ion cannot be confirmed now. but i t can be assumed th a t it i s re su l t ing from a change in the molecular weight of the gas to be compressed Which

may a f fe c t the leakage in s id e the compressors as well as the e f fe c t of o i l .

Table 3 Noise and V ibra tion D iffe rence Gas Vert. Ace. H ori. Ace. Axia l Ace, SPL Air 1 . 4 6 m / s ~ 1.53m/s" l .86m/s 2 85.5dB Helium 4 .80 3.50 7.34 93.5 Operating Condition : P s ~ 0 . 0 4 9 M P a , ( P d / P s ) ~ P i , 3000RPM

3201, 0 ~ 3 2 1 , L / 0 ~ 1 . 6 5 , V i ~ 5 . 8 In jec t ion Oil Q uantity

In jec t ion o i l flow ra te sometimes a f fe c ts the noise le v e l experienced in la rge re f r ig e ra t io n compresso rs . I t i s sa id th a t under a reduced in je c t ion opera tion overa l l noise lev e l sometimes can be reduced bY seve ra l dBs. Other than R-22, and some l ig h t molecular weight gas , th i s phenomena i s c lea r ly confirmed by some experiments .

An example shown in Fig .18 dB I 0 4 r - c - ~ ~ ~ - - - - - - - - - - - - - - - - - - - ,

.F u l l ·

4 4 ~ 0 - - - - - - - - ~ - - - - - - - - - - - - - - - - ~ Frequen cy

Fig . 18 In jec t ion Oil Effec t

i s an opera t ing no ise from 320L opera t ing a t 3000RPM with he-lium gas. Although

in th i s spectrum p lo t .reduced in je c -t ion o i l opera t ion i s qu ie te r by 4 dB. in some t e s t s . the a t t ta in ed improvement in sound pressu re le v e l i s well above 8 dB .

AFFECT OF SURROUNDING EQUIPMENTS

In most cases . the measured opera ting no ise from

577



compressors con ta ins many components from various

ind iv idua l sources. The motor. gear box. coupling.

pip ing , o i l separa to r , sk id and so on. They a l l may

combine to y ie ld a complex frequency spec tra p lo t .

Here. some s im p lif ied examples are discussed .

Oil Separa to r

As already shown in Fig. 9 , the accompanYing

b r o a d ~ b a n d random noise component decreases with

frequency in frequency spectrum p lo t .In con tra s t with

the F igu re .in some cases as the typ ica l example shown

d B r - - - - - - - - - - - - - - - - - - - - - - - - - - - ~ 100 320S

D•J2 l

L/d•l .l

R-22

3000RPM

Noise.

I 6 20kHz

Fig . 19 Separator Noise

Piping

in Fig. 19. a broad

peak appears in o p e r ~ a ting noise spectrum.

The broad peak

presen t i s seen to

emerge from general

background. This may

be due to resonance

noise rad ia ted from a

source o the r than the

compressor. In th i s

opera ting noise spec

trum. th a t peak has

come from sepa ra to r ou t le t resonance.

Fig. 20 shows

320L te s t s tand .

spectrum re su l t ing

suc tion p ip ing , can

ment of the piping .

an example of opera ting no ise from

The contained broad-band noise

from gas flow noise rad ia ted from

be reduced by changing a rrange-

dB

lO j

>

• ..,

•

•..

.,

0

"'44

0 Frequency 5kHz

320L

D•32l L/D •l ,65

3000RPM

Fig. 20 Suction P iping Gas Flow Noise

578

/



Gear Box <Ref. [3 J )

In some app lica t ions . a multiPlYing gear box i s employed to a t ta in la rge r capac ity than th a t of d irec t drive opera tion . Gear s e t with in gear box sometimes produces noise problem because of i t s improper mesh. as shown in Fig. 21. In the F igu re .the

peak a t about 3300 Hz i s corresponding to the mesh .frequency of th i s gear se t . And around the primary mesh .frequency. the re i s a se r ie s of equallY spaced components of sidebands. These sidebands ind ica te ex is t ing gear p itch e rro r .

~ 114

l>

Cll

t-..l

MFrG ~ d B •r-i

0

z 54

Frequency

Fig . 21 Gear Noise Spectrum

DETECTION OF FAILURE

5kHz

200S D"'204

1/D"=l.l

R-12

Thrust Bearing Flaking

In screw compressors. ro ll ing-e lement bearings are usually employed as ro to r th ru s t bearings to ensure prec ise ax ia l posit ion ing . These bearings have

£ in i te £atigue l i£ e and sometimes £a il unexpected lf by £laking o£ raceway as a re su l t o£ an abnormality . The example shown in Fig. 22 i s a typ ica l oper

a ting v ib ra t ion £rom 160L a t 3600 RPM. In the

TIM E fl LIN

ll rwR sr R LIN

u ~ s E c

!CkHz

160L D ~ l 6 3 L / D ~ l . 6 5 R-22 Gas 3600RPM

Ax:ial Ace.

Fig . 22 Operating Abnormal Vibra tion

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v ib ra tion acce le ra t ion spectrum. a c lu s te r of peaks

a t approximately 3 .5 kHz ind ica te s an abnormality in

t h { ~ · compressor. And the waveform ind ica tes th a t the

unusual s igna l i s generated in a se r ie s of impacts

repea ting a t re la t iv e ly low frequency.

In th i s case . the technique o f the abso lu te

averaging in the time domain s igna ls i s employed. and

by ~ t s spectrum.shown in Fig. 23, the repea ting f r e ~ quency i s confirmed a t 322.5 Hz. According to th is

frequency, the damage in a bearing inner raceway.

shown {n Fig. 24. can be de tec ted . Concerning the

frequencies derived by damage of ro ll ing -e lemen t

bearing , re fe r to the Table 4.

u

u

""i"iiR Sf A LIN 11diz

Frequency

Fig . 23 Absolute Averaged Spectrum

c

Fig. 24 Inner Raceway Flaking

Scuffed Rotor Tooth

Table 4 Bearing Frequencies

!yp ee o f Damage Damaged Part

Eccen tr ic ity*1 Inner n·Fr

Rough Spot lnner n·Z•Fi

Outer Raceway

Rolling Element 2•n•F b

* 1 :and / Wear,

Modulation may

Where F1•l /2 •F r·( l+d /D •cos fi)

Fo•l/2 • Fr· ( l -d /D · cosp)

Fb•l /2 •F r·D /d · [1 -(d /D ) '· c o s ' l ' l

Fr . Fll! or ff d•Diameter of Rolling Element

D•P;I,t c.h Diameter

P•Conr.a.c.t Angle

ZcNumber of R oll ing Elements

n • l ·2 ·3 . ...

[Inner Raceway and Oute r i s Stationary]

In a ce r ta in opera ting conditions . scu ff ing o£

ro to rs sometimes take p lace .

One typ ica l example of abnormal noise £rom 200L

opera ting a t 3600 RPM i s shown in Fig . 25. For the

noise lev e l of th is compressor.BS dBA was recorded a t

the beginnings of the p lan t opera tion and it has i n ~

creased well above 100 dBA a t th e time of the analy -

s i s . The overa ll noise lev e l ind ica tes only how

ser ious the problem i s . To de tec t the o r ig in of no ise

a frequency ana lys is i s necessary . The analyzed noise

spectrum con ta ins many sub-harmonics of fundamental

screw frequency, ind ica t ing too th separa tion and

580



co l l i s io n a t the po in t of ro to r mesh con tac t . The waveform a lso shows one co l l i s io n tak es p lace per every two mesh. Afte r in spec t ion ; th e cause of th i s abnormali ty i s confirmed r e su l t in g from scu ffed and worn ro to r too th f lank s . <The scu ffed pa t te rn on the ro to rs can be reproduced by computer s imu la t ion , an example i s shown in F ig . 26 .)

In t h i s ca se .scu ff ing of ro to rs i s caused by o i l compression a t f requen t s ta r t -u p s . Opera ting sequence i s rev ised to avoid unnecessar i ly long o i l pump opera t ion before compressor s t a r t s . and the problem was re so lved .

200L 5 5 6 ~ 0 ~ 2 0 4 RERL.

L /D g l , 65

R-22 Gas

3600RPM -. 35e t

r t.;.s. 3m Sec .aoSEC TIMER Se.00mHC

.Sub-harmonics

X: OVII'r.!l.ll Hz r: 10t.5<B

F ig . 25 Abnormal Noise Signal

Drive Slde Flank

Fig. 26 Simulated Scuffed P a t te rn <Male Rotor)

Unprecise Rotor

The screw ro to rs a re so s t r i c t l y con tro l led and in spec ted in manufacturing th a t the accuracy has been mainta ined over a ce r ta in le v e l .

As a seldom example, i f ever occur . F ig .27 shows an opera ting noise on 2508 a t 3000 RPM with unprecise ro to r s e t . The sauna i s heard as a ~ r a t t l i n g " no ise .

The waveform ind ic a te s th a t th ree meshing pu lses occur repea ted ly in abnormal magn

itude a t every revolu t io n of the female ro to r . Ana the shown spectrum con ta in s a se r ie s o f d isc re te frequency components

581



. , , ~ •S

""' TIME A LIN

. - . 9 7 ~ Fo 2Fo _ 3F o . 4Fo

~ d B -ru ' ."' - -

~ -0

"" 3 7 .

2505

D-255

L/D-1.1

R-22 Gas

8000RPM

0 fWR Sf A LIN lkH

Fig . 27 Operating Noise with U n p r e ~ i s e Rotor

spacing approximately 33 Hz<= F£ ) between harmonics

o£ th e £undamental screw £requency. A£ter in spec ting

th e ro to r s e t . it i s con£irmed th a t abnormal noise i s

caused by uneven too th th ickness o£ th ree o£ th e s ix

£emale ro to r .

SUMMARY

The mate r ia l in th i s repo r t i s d irec ted toward

prov id ing a bas ic understanding o£ the £undamental

ch a ra c te r i s t ic s o£ noise and v ib ra t ion in th e o i l in -

aected screw compressor.

With respec t to £ requency .i t can be assumed th a t

th e £undamental screw £requency <Fa = m X Zm ; £or

male d rive) dominates a l l aspec ts in compressor noise

and v ib ra t ion . From the r e su l t s o£ these experiments

it i s be lieved th a t compression mechanism. design and

manu£acturing parameters . and opera ting cond it ion s

a l l in£1uence on pressu re pu lsa t ions o£ discharge and

suc t ion gas. More im portan tly , they in£1uence on

dynamic gas fo rces induced on ro to rs and th e mechani

ca l con tac t fo rces between ro to rs . th e reby con tro l a l l

th e ch a ra c te r i s t ic s o£ no ise and v ib ra t ion .

In our next report.we w il l p resen t a th eo re t ic a l

ana ly s is o£ screw compressor v ib ra t ion .

REFERENCES

[11 ASHRAE. Herica l Rotary Compressors.

1979 Equipment Handbook,pp. 12.14-12 .17 .

[21 Tanaka. N . . and o the rs . Trans. of JSME. 1984.

vol.SO ,no .458 . pp. 1783-1789. [3] Mitchel. L.D .• Orig ins of Noise.Machine Design.

May 1 .1 969.

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