Research on Self Adjusting Back-Pressure Mechanism of Scroll Compressor

427 Pages·2017·14.27 MB·English

Purdue University
Purdue e-Pubs
International Compressor Engineering Conference School of Mechanical Engineering
1992
Research on Self Adjusting Back-Pressure
Mechanism of Scroll Compressor
J. Zhu
Xi’an Jiaotong University; P. R. China
D. Wang
Xi’an Jiaotong University; P. R. China
D. Zhang
Xi’an Jiaotong University; P. R. China
Follow this and additional works at:https://docs.lib.purdue.edu/icec
Zhu, J.; Wang, D.; and Zhang, D., "Research on Self Adjusting Back-Pressure Mechanism of Scroll Compressor" (1992).International
Compressor Engineering Conference.Paper 908.
https://docs.lib.purdue.edu/icec/908
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Herrick/Events/orderlit.html
RESEARCH ON SELF ADJUSTING BACK-PRESSURE
MECHANISM OF SCROLL COMPRESSOR
Zhu Jie, Waq& Disheng and Zhang Dongjun
Dt:panment of Power Machinery Engincerina
Xi'an Jiaotong University, P.R. CHINA
ABSI'RACT
Fon:es actina on the orbitinc scroll arc analysed in detail on the basis of the principle of
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INTRODUCTION
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BACK-PRESSURE CALCULATION
l!J7
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sbhitoiwnsn sicnr oflils .b aIs,e tphlea tfeo cacned f t0h e isl ixenedsc nscdrcorlel ds ubryf atchee, pitR isss uthree osef atlhineg c lfeoarrcaen. ceA cbceotwrdeienng tthoe tohre
refCRilcc [I], the distribution of the pRssure in this clearance is given by
6•.
dp dm1
-;;·"' u6' dt
I
then intqrated
611 r dm
P=P • +n-:o=; -IRn, -d-r- 1
where •. is the viscosity factor of the oil-gas mixture, 1\ 1s the back-pressure, <11 is the
dm
clearance value, ddmr1 is the leakage rate, according to the expression ofT m reference (1],
F is obtained as follows:
0
where P, is the sucuon pressure.
When the friction forces arc neglected, the force equilibriums arc given by
M •g+F, +F0 +F1 +F,-F._=O
F,-R,-MEm1 1 0, -0"=0
R,-F,+01,-0~>-=0
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given as follows:
F.=M•g+F, +F0 +F, +F,
R, = F, - M F.w 2
R, :F,
the moment equlilbriums arc giVen as follows:
·:hz
M,(F ,F )==f, +R, •/11
1 1
1138
M 1{F1 F'2) =F ,·h2 +F •.2§. +R 'h 2 -ME'"~z,h
then
M,(F ,F )=F,( ~ +h,)
1 2
M, (F 1 , F2 ) and M, (F1 , F2 ) are the unbalanced overturn moments, they must be bal·
anced by the moment of the forces F1 and F2 , as shown in Fig. 2.
(F1 -F2)RsinP=M,(F,,F2)
(F, -F2)RcosP=M,(F1,F2)
M,
where P= arctgM '
then
When F, and F2 are the positive value, their directions are downward, this means the
orbiting scroll meshes with the fixed one properly. If one of them is zero, this indicates that one
side of the orbiting scroll baseplate will separate from the fixed scroll surface, and the orbiting
. scroll will incline to this side. so the stable operating condition of the orbiting scroll is as foJ.
lows:
Min(F ,F )>0
1 2
. . J
or (F1 +F2).,,. = M:(F,.F2)R+ M:(F,.F2)
therefore F •. ,,. =M • g+F. +F.+ j M'' (F1,F2)R+ M'2 (F1,F2)
There two pressure distributed in the back of the orbiting scroll baseplate, one is the
oil-supply pressure which is equal to the discharge one approximately, the other is the
back-pressure. so
r:)
F..,,. = P •",.~ + P •••"(R~-
o btat.n ed P .,. = F • .,,.-Pd,.,.~
10(R22 -r21 )
where Pblh is the minimum back-pressure to support the orbiting scroll, which is computed in
light of the orbiting scroll dynamic equilibrium.
WORKING PROCESS MODEL
1139
Compression Pocket
The meshing condition .of the scroll wraps {N =3 ) is shown in Fig. 3, when the crank angle
increases from o to 2n, the unsealed volumes in the outsied become the closed ones. When the
crank angle increases from 2 n to [~{N-1)+ 9"'] (9" is the discharge angle), the sucked me
dium is compressed in working pocket 3 and 2 successively. The compression pockets 3 and 2
communicate with the back-pressure chamber in a certain angle range respectively, the analyti
cal model of the working porccss is shown in Fig. 4, The crank angle corresponding to the
back-pressure port postion is 9b • when the angle is in between (2x+9b ) and 4x, the pocket 3
communicates with the back-pressure chamber, the pocket 2 docs not. When the angle is in be
tween 4x to (4n+9b ), the pocket 2 communicates with the chamber, the pocket 3 does not. Be
cause the diameter of the back-pressure port is small, the port may be covered in a certain
range.
Because there are too many factors influencing the working process. In order to simplify
the calculation, the following assumptions arc made.
(I) The medium is treated as the ideal· gas with a constant specific heat and its state is
homogenous.
(2) The compression process is regarded as the adiabatic one, the flow of the medium is
instaneous steady.
(3) Gravitational and kine!ftatic energies of the medium are neglected.
The following fundamental equations discribing the medium properties are given accord
ing to the laws of conservation of energy and mass.
dP = kmRv L{T u. - T)dm Ill - dvP dv
dV=mdv+vdm
dm=dm -dm ••J
IJII
dP +dV =dm +dT
P V m T
P(O)=P,
T(O)= T,
V(O)=o
where k is the adiabatic exponent, R is the gas constant, T, is the suction temperature, m;.
and m arc the leakage masses which are obtained in light of the leak model in reference [2].
001
Back-Pressure Chamber
The analytical model of the back-pressure chamber is given in reference [I).
SIMULATIVE CALCULATION
1140
The simulative calculation is carried out on the basis of the analytical models of the work
ing process and the back-pressure chamber, the flow diagram of the computer program is
shown in Fig. 5, The input parameters of this program arc the scroll basic dimensions , the op..
crating condition. the properties of the media and so on. The differential equations are solved
by means of the Rungc-kutta method. The function of the program is to caluclate the state
parameters of the working media. the back-pressure and the compressor efficiency, etc. The
propcnics of the media in the adiabtic process without leakage and back-pressure port arc re
garded as the initial value, the pressures are regarded as the convergent criteria. The mass con
servations of the gas and oil in a cycle are additional criteria to the back-pressure chamber.
RESULT ANALYSIS
In order to analyze the effect of the back-pressure chamber on the compressor efficiency
and the effect of the structure parameters of the chamber on the back-pressure, the ell:periment
is carried out. The variations of the back-pressure with the pon position and diameter arc
measured, the influences of back-pressure on the compressor displacement and power arc
presented.
Effect of the Back-Pressure Chamber.
The pressure curve of the working pocket is shown· in Fig. 6, the dotted line P represents
1
the compression process without the back-pressure chamber, the real line P represents one
2
with the chamber. These two lines duplicate each other in.the range from 0 to 9b 'this means
that the chamber does not communicate with the analysed pocket in this range. The line P in
creases abruptly in the range from 9; to a. , the back-pressure chamber communicates 2w ith
the compression pocket, the media flow from the chamber to the pocket, so the media mass and
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the media flow from the compression pocket to the chamber in the rang<: ca. - a. ), owing to
the mass decrease, the compression line increases slowly. The variations of the line P and P
arc similar each other after e. , but P2 is lower than P1 , the reason is that the med1i a cornin2g
from the compression pocket leak out through the clearance of the back-pressure chamber. As
shown in this figure, the compression power with back-pressure is greater than that without
the pressure.
Flow through the Back-Pressure Port
The flow condition is discribed in Fig. 7, the square between the curve and the abscissa is
the product of the mass flow rate and the angular velocity w. when the square is positive, it
means that the media flow int(l the back-pressure chamber, when the area is negative, the flow
direction is reverse. During a cycle, the positive square is greater than the absolute value of the
negative o!'-e, the difference is the product of the chamber leakage and w. There arc two pons in
1141
orbiting scroll baseplate, owing to the effect of the scroll wrap thickness, there is the difference
between the communicating anr;les of two ports, so only one port is communicatinr; at eb , the
other port is open at ed . The closing condition is similar to the opening's, only one port is close
at e. , two ports are both close at e. . The ports are covered in the range from e, to (2it+9b ).
The variation of the back-pressure is shown in Fig. 8. the slow variation of the back-pressure
is caused by the leakage of the chamber in the range [(-2.:+9, )-9b ].
Effect of the Back-Pressure Port Position
The change of the back-pressure with the port position is shown in Fig. 9, the position is
the main factor effecting the back-pressure. When the port angle 010 ( shown in Fig. 3 ) in·
creases, the back-pressure decreases, the leakage of the back-pressure chamber is small, so the
curve of the back-pressure is concave-upward. The maximum error between the values of
calculation and experiment is 3.5%. The variation of the volumetric efficiency with the port PO·
sition is shown in Fig. 10, The efficiency increases slightly as the port angle rises. one reason is
that the back-pressure decreases and so does the leakag of the chamber, the other is that the oil
flowinr; into the working pocket gets more, therefore the sealing property of the working pock·
et is improved. The comparison between the theoretic value ·Pbth and the experinlental result
Pb at the port anr;le 225 o is shown in Fig. 11, when the pressure ratio of Pb to Pbthmn is in
the range from 1.05 to 1.1 5, the orbiting scroll moves stably and the friction loss is less. when
the port angle is great than or equal to 280 D , the axial froce and overturn moment arc both
unbalanced, the compressor can not work properly. The variation of the work input with the.
port angle is shown in Fig. 12, although the power decreases slightly as the port angle increases,
this increases the unstable degree of the orbiting scroll.
Effect of the Back-Pressure Port Diameter and the Chamber Volume
The variatoins of the back-pressure fluctuation and its average with the port diameter arc
shown in Fig. 13. 14 respectivly, the variations of the back-pressure fluctuation and its aver.
age with the chamber volume are shown in Fig. 15 and 16 respetivcly. According to these fig.
ures, the port diameter and the chamber volume have little influence on the back-pressure av.
crage, but they effect the back-pressure fluctuation, if the port diameter decreases from 2 mm
to 1mm, the fluctuation ratio Pb/ Pbo decreases from 2.8% to 2.2%, but if the back;,-pressure
chamber volume increases from V,b (The displacement volume )to SV,h , the ratio Pb I P"" .
decreases from 10% to 2.5%. According to the results of calculation and experiment, the range
of the back-pressure port diameter and the chamber volume are recommended as follows:
.-:D4-' I (LJ • V,. = (1.4"':4.0) x 10 -• {s I m)
Vb!V,0=2.0-4.0
CONCLUSION
1142
(I). The appropriate range of the back-pressure is (1.05-1.15) times the theoretic maxi·
mum.
(2) The diameter range of the back-pressure port is recommended as follows:
-n4D-l I (IJ. v,. = (1.4-4.0) X 10 -• (slm)
(3) The volume range of the back-pressure chamber is recommended as follows:
v~ .
=2.0-4.0
th
REFERENCE
[I] Zhu jie, ect. "Theoretic Model of Back-Pressure Chamber for Scroll Compressor"
1992 Purdue Compressor Conference.
[2] Zhu jie, ect. • A Research of Scroll Compressor Working Process Computer
Simulation and Testing" 1990 Purdue Compressor Conference.
Fo
F,
Pi~.l Porce anslvsis
Mr
r
nort
~i~.2 BalancinP, force @'ig. 3 Jorking pocket
1143
Pocket 2 Pocket 3
PoCk€t 1
Pd
----Jr
1.__ ____
Back-pressure chamber
Fig.4 Analytical model
CALCULATE POCKET VOLUME
AND PORT AREA
CALCULATE ti!EDIA PROPERTIES
IN WORKING POCKET CALCULATE MEDIA PROPERTIES
I"N BACK-PRESSURE CHAMBER·
?
Fi~.5 Comuuter Pro~am
1144
0,5
Pa
0.4
1:1- I
0.2 I
I
I
),1
I
9c
20) 40) 600 800 lOQ') 12:})
CITA
Fig.6 Pressure of workinR uooket
4,')')
2,')0
'." .'.
.!<
f"o '),'))
.-<
l(
.... -2. )')
~
l)') 300 360 (1')')) ( 2))) ( JQ0)
Fig.7 Flow' throuF,h back-pressure nort
:?.3'5 ,.....---------------..
2. ::>0
12') 240 360
Fi~.B Back-nressure
1145