2. DESCRIPTION AND PRINCIPLE OF OPERA TION
OF THE GRASSOSCREW COMPRESSOR GSL
GRaSSi:i
2.1 GENERAL
-
GAASSOSCAEW GSL
is
the
des1gnal1on fora
series
c'
rotary positNe d1sp!acemen1 comp;esso�s maae unaer
llcense of the SWed1sh firm SRM (Svenska Rotor Mas•
kmer). These so•called "Lysholm" screw compressors.
equIpped with only two main rolating
parts
have been rle
veloped
and
deslgned
for genera!
inoustnru
use as a nor
mal (HP) single-stage machine
or
as (LP) booster In
refngerating, freezmg. arr conditioning and heat pump
installations, normally operatmg w1th !he refrigerants NH
3
(R717) and R22
- These compressors are ma1nly Intended fo• direc1 dqve
via a flex1ble coupling by a three-phase, two-pole eleclnc
motor: consequen1Iy at a speeo approach1ng 3000 rpm or
3600 rpm for 50 Hz and 60 Hz supply froquency respec
trvefy.
- 1 he compressor senes GSL covers a total numbe• of 7
types. charactenzed by d1f1eren1 rotor diameters and
lengths and having a swept volume increasing from 630
up to 4880 m3lh (50 Hz direct drive). These 111d1v1oual
1ypes are Indica1ed by addtng d1reci to GSL the swept vol
ume
m
m
3
/h, d1v1ded by l
o
ano rounded up or down
to
5
In principle each type can be supplied wIth four different
so-called bu1II m volume ratIos, deno1ed by the figures 22,
26, 37 or 48 after the swept volume 1n the type designa•
tlon. lnd1ca:1ng a butlt•in volume ratio of 2 2 2.6 3 7 an<?
4 8 respechvely.
Fxample:
GSL 240·37
Gmssoscrew
Î
Î
T
Large capacity series �
Swept volume (div1ded by 1 O _____ _.
and rounded oft)
Built•1n volume ratio
--------..J
(mlllttphed by 1 O}
In the case of a complete compressor package, the chat·
acter ·p· is added to the type designation·
Example:
G
S
L
.e
2
4
O
-
3 7
Fora complete survey or all compressor rypes. t&ehnlcal
data and lim1ts of operalion, refer to paragrapn 2.9.
- The structure of the Grassoscrew compressor, in I1s moS1
s1mplified form, ts schernatically representeo in figure 2. •.
where the nghl r,and
part
1s a vertical cross-section cll'ld
the left hand part a ver1ical longnud,nal secuon.
This figure shows that the compressor cons1sts of two par•
allel, intermesh1n9 helicatly grooved rotors (i. 2) v11th
equal (outer) diameter and length. supoorted by integral
shaft ends In one horizontal plane and enclosed on all
sIdes by a specially shaped (double-cylmdncal) mner
housing (3) wi1h very sl1ght radial and axial clearance
The so-catled male rotor (1 ). havmg four convex lobes
(thu� tour
o
Ilt:rluoo
spaces. cauea
lutes
or grooves) 1s
dnven mrect from outsIde (9) In the dIrect1or indicated
and drives the so-called female (2) rotor wh1ch has sIx
concave lobes {lhus sIx tlutes or grooves). The male t'Otor
lobes funct,on as pistons that roll and s,1de w1th1n the Ie
male rotor flutes thereby Impart1ng a ·o:ating -notIor w1th
213
(�416) times its speed to the femafe rotor. In any verti
cal cross.secHon both the male and female lobes have
1r,e latesl non•symmemcal profile
One end face of the atorementioned mner houstng (3) Is
prov1ded w1th a spec1ally curved axial 1nlet port (5), which
communIcates
win
the suctIon cl1amber
(7)
inside the
gastIg'11 outer llOLsing (4) At the oppos11e 1ower end of
the retors there ,s an outlel pon (6) in the inner hous1ng,
which cons1sts of
an
ax,al and radial pari and communi
cates with the dIscharge chamber (8) inside the outer
housing.
2.2 PRINCIPLE OF OPERATION
As the Grassoscrew Is a posI1Ive d1splacernent compressor,
the followmg three successIve phases can be d1st1ngu1shed:
Suction phase
A oair o' male lobe and female flute unmesh on the intet
port sroe anc suct1on gas flows ax1ally m the increaslng
volume formed by the cor1espond1ng male•female groove
pair unlll the looe and flute are completely unme�hed and
the groeve pair is no more in communteatIon w,1h the inlet
pon Refer to tne socces::.Ive groeve pair stages (pos1•
11ons)
a
,
b
,
c
and
d
in 111e
top pIctutc
of
fig.
2.2
Compresslon phase
Wner remeshing of lhe male lobe and female llute starts
at tne inlet end, tt'>e trapped gas Is graaually reduced 1n
volume. S,multaneousty, while being compressed in lhis
way, the charge of gas 1s moved hahcally töward !he drs
cMrge end as the lobes' and flutes' mesh point moves
along ax1ally Reler to the sucoessIve groove pair stages
(pos1t1c'lS)
e f
and
gin
the bottom picture of fig
2
2
•
Oischarge
phase
Compress1on ceases. when the groeve pair concemed
comes tnto communicat1on w1th the outlet pon, thus caus
Ing 1he d!Scharge oi the compressed refngeram gai; to
tal(e place un11I the groove pair volume is reduced to ze•o.
See
groove pair
stage
(pos1tion)
h
m 1he bonom picture
of
fig. 2.2,
The complete cycle thus ooscnbed fo, one groove pair.
ta�es place cont1nuously tour times per revolution ol the
male rotor, resulhng in a uniform, hardly pulsating ovorall
gas flow through the compressor.
2.3 BUILT-IN VOLUME AND PRESSURE RATIO
L1ke 01her rotary positive displacement compressors without
suct,on and d1scharge va
l
ve
s
.
the Grassoscrew has aso
called builHr- or ft)ted volume ratio.
Tt11s is 1he ratio of ttie volume of one male•female rotor
groove pair at 1he moment the inner houmng Just scats off
rh1s groove pair !rom the sucl1on side (= volume of begin
mng or compress1on) and lhe volume ol the same groove
p..u, �. ?ho 1 me 1h0t II oomco ,nto commumc.ation Wlth the
oullet oo� (end of compress1on)
r
or each refngerant this "built-in volumt! rat1o
n
corresponds
toa so-called "built-in or r,xod pressure ratio". For the stand
ard built-in volume and pressure ratlos. rere• to the tAble fn
pJ.1.1grapr 2 9
92..01
Compressor Package GSLP
Page2.1
