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A new structure and theoretical analysis on leakage and performance of an oil-free R290 rolling piston compressor
Accepted Manuscript A new structure and theoretical analysis on leakage and performance of an oil-free R290 rolling piston compressor Wu JianHua, Chen Ang PII:
S0140-7007(14)00268-0
DOI:
10.1016/j.ijrefrig.2014.10.007
Reference:
JIJR 2895
To appear in:
International Journal of Refrigeration
Received Date: 11 July 2014 Revised Date:
23 August 2014
Accepted Date: 6 October 2014
Please cite this article as: JianHua, W., Ang, C., A new structure and theoretical analysis on leakage and performance of an oil-free R290 rolling piston compressor, International Journal of Refrigeration (2014), doi: 10.1016/j.ijrefrig.2014.10.007. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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A new structure and theoretical analysis on leakage and performance of an oil-free R290 rolling piston compressor Chen Ang
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Wu JianHua *
School of Power and Energy, Xi’an Jiaotong University, Xi’an 710049, P.R. China *
Corresponding author. Tel.: +82 029 8266 3786, +82 13689206050.
E-mail address: [email protected] (Jianhua Wu), [email protected].
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Postal address: No. 28, Xian-ning West Rd., Xi’an City 710049, Shaanxi, PR China.
Abstract
Lubricating oil improves the reliability of compressors and systems, whereas increases the system complexity. Compared with other types of compressors that have oil-free models, a rolling piston compressor has more leakage paths and bigger leakage loss. Therefore, the leakage is an important
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problem to be solved in order to develop an oil-free rolling piston compressor. The paper put forward a new structure of rotary compressor adopting a low pressure shell, connecting the cavities within piston and behind vane to the cavity at suction pressure and using radial compliance mechanisms. Then the leakage models were developed to calculate the mass flow
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rates within both the present rolling piston compressor without any oil as sealant and the new structure of oil free compressor. Results showed that by the new structure, the influences of
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leakage on the performance of a R290 oil free rolling piston compressor can be largely decreased. Key words: R290, rolling piston compressor, oil-free, leakage, performance
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Nomenclature eccentricity of crankshaft (m)
Rc
radius of cylinder (m)
L
length of leakage path (m)
Re
Reynolds number
Lf
equivalent channel length (m)
V
flow velocity(m s-1)
M
Mach numbers at inlet of leakage flow
W
width of leakage path (m)
P
pressure (Pa)
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e
Greek symbols
gap value (m)
k
the adiabatic exponent of refrigerant
µ
dynamic viscosity (Pa s)
v
specific volume of refrigerant vapor (m3kg-1)
ρ
density (kg m-3)
φ
discharge angle (rad)
Subscripts
EP
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δ
inlet of leakage flow
c
compression chamber
2
outlet of leakage flow
s
suction chamber
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1
2
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1. Introduction The production and consumption of R22 currently used in room air conditioners in China have been frozen since 2013 and will be phased out by 2015 due to its impacts on the ozone layer. The
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alternative refrigerant R410A was developed twenty years ago and known as the transitional
refrigerants because of its higher GWP. Despite that, it has been still widely used in many countries. At present, some companies promote several other synthetic alternatives. Their GWPs are lower than that of R410A. The natural working substance propane has been chosen as the main
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alternative for R22 in Chinese room air conditioner industry because of its excellent environmental protection feature and thermal physical properties. The transformation for the
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production line of R290 room air conditioners and compressors has been aided by Multilateral Funds financially.
One of the important measures to improve the safety of R290 room air conditioners is to limit the R290 mass within system. The main way to reduce R290 charge in an air conditioner is to reduce the internal volume of heat exchangers and liquid line pipes, for example, by adopting the small
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diameter tube for condenser. As a result, the R290 content in the compressor of system becomes relatively larger. Therefore, it is necessary to reduce the R290 charge in the compressor (Wu, 2012).
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The compressor used in a room air conditioner now, as shown in Fig. 1, is mainly the rolling piston type rotary compressor with high pressure shell, whose oil sump is exposed to the discharge pressure, therefore a great amount of R290 is dissolved in oil. Two basic approaches to reduce the
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R290 mass in the compressor are to lower oil charge in the compressor (Gao, 2012) and to use oil with low R290 solubility. On the other hand, oil in system can decrease the heat transfer effect of exchangers, increase the flow resistance and thus degrade the system performance (Youbi-Idrissi, 2008). Additionally, to deal with oil return, the system may be more complex. The decomposition of oil, the flow block of micro channel and other issues related to oil will affect the system’s reliability. Therefore, it is of great significance to research and develop the oil free compressor R290 room air conditioning system. 3
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Fig. 1 R290 rolling piston compressor with high pressure shell
In some ways, R290 is favorable for a compressor with an oil-free model. Compared with R22 and R410A, the difference between discharge pressure and suction pressure of R290 compressor is smaller and, as a result, the loads on some friction pairs are smaller, too. The lower discharge
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temperature of R290 compressor reduces the working temperature of its friction pair. And the large specific heat of R290 is beneficial for us to cool the bearings with gas refrigerant. Admittedly, the viscosity of R290 vapor is lower and it would increase the leakage loss of oil-free rolling
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piston compressor. This is also investigated in this paper. At present, reciprocating, scroll and linear type compressors all have their oil-free or semi-oil-free
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models (Bradshaw, 2011). But the rolling piston compressors have no oil-free version yet. For certain displacement, the integrated performances of rolling piston compressors are very excellent, such as: high COP, good reliability, low vibration and noise, and low cost. So they are widely used for room air conditioners now. Nevertheless, there is no paper has discussed technical issues about oil-free rolling piston compressor. Besides lubricating friction pairs, the oil within compressor also plays a pivotal role in sealing leakage clearances, enhancing heat diffusion and thus ensuring the high reliability and performance of a compressor. The main technical problem of oil free compressors is undoubtedly 4
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the friction and wear. Compared with reciprocating or linear compressor, the rolling piston compressor has more leakage path and bigger leakage loss. So once the rolling piston compressor adopts the oil-free model, it is necessary to figure out how much the leakage loss is and how to
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limit the leakage loss and its effect on the compressor performance. There were many studies on the leakage of rolling piston compressor in the past. Earlier at the beginning of developing rolling piston compressor, Pandeya and Soedel（1978）calculated and
analyzed the effect of leakage on the performance of the compressor. Leakages through the
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clearance between cylinder and piston and the clearance between vane ends and cylinder heads
were considered as a gas flow and the mass flows were calculated by nozzle model. A leakage loss
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of approximately 12% of the ideal mass flow was obtained. The reason for why the value was larger was that the effects of oil sealing and refrigerant Couette flow were not considered. For the leakage loss through the radial clearance on rolling piston, Yanagisawa and Shimizu (1985a) took account of the effect of fluid friction on the leakage and the dynamic change of the clearance. For the leakage through the clearances on rolling piston faces, Yanagisawa and Shimizu (1985b) took particular account of special flow characteristics of the leakage flow and the practical distribution
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of the clearance. Rodgers and Nieter (1996) developed two types of leakage flow model: a gaseous flow of refrigerant treated either as nozzle flow or as Fanno flow in a channel, and a liquid flow of oil/refrigerant mixture as an incompressible flow with refrigerant outgassing. The
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appropriate type of leakage flow model for each leakage path was also discussed. Ishii (1998) assumed the leakage flows through the axial gap between blade and thrust plate and through the
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radial gap between piston and cylinder as incompressible and viscous flow and evaluated the leakage flow and volumetric efficiency. Wu (2000) showed a comprehensive model for leakages in rotary compressors. Ooi (2008) assessed the performance of a rolling piston compressor when operating at a transcritical CO2 cycle by considering leakage mass flow and friction loss. Gasche (2012) presented a non-isothermal two-phase model for oil-R134a mixture leakage through the radial clearance of rolling piston compressors. Some studies on the leakage of scroll compressor have high reference value for the calculation of leakage of rolling piston compressor (Ishii, 1996) (Kang, 2002). In addition, Kus (2013) assessed the feasibility of replacing oil-lubricated 5
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compressor in CO2 refrigeration system with oil-free turbo-machinery. The paper proposed some measures to reduce the leakage mass flow rate by adopting a new structure of oil free rolling piston compressor with low pressure shell, discussed the model to
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evaluate the mass rate of leakage flow, compared the mass flow rates through every path within the oil free rolling piston compressors of both conventional and new structure. Finally, the effect of the mass flow rate on compressor performance was analyzed.
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2. A new structure of oil-free rolling piston compressor with low pressure shell
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To reduce the leakage loss of an oil-free rolling piston compressor, a new structure with low pressure shell is proposed in this paper, as shown in Fig. 2. Compared to those of a conventional structure, the compressor has not a suction accumulator as shown in Fig. 1 and the motor is placed under the pump body. The inhaled liquid refrigerant could reserve within the cavity below motor. The motor and cylinder and main bearing and the back of vane are surrounded by refrigerant at
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suction pressure . A plastic suction muffler is used to prevent the cylinder and main bearings from overheating the refrigerant. Since the suction pipe is not connected to the muffler, some of the refrigerant enters the cylinder directly through the muffler, while the other goes down into the low pressure cavity and cool the motor. The distance between suction pipe and the inlet of muffler
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helps to control the distribution of refrigerant. On the other hand, the discharged gas, which only takes a small space, is sealed by the sub bearing.
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With this new structure, the refrigerant in discharge chamber will leak through the clearances on piston ends to the cavity in piston and then leak to suction chamber. In addition, the refrigerant leaking through axial clearances on piston ends can be easily used to cool the friction pair between shaft and piston.
Since the gas behind the vane is at suction pressure, the conventional structure of piston–vane is not suitable anymore due to the lack of press force. Therefore, the other types of vane, such as swing piston-blade type, need be adopted accordingly. 6
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Also, as there is no oil needed to lubricate the friction pair between piston and cylinder wall, the radial clearance between them could be decreased, hence two types of radial compliance
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mechanisms are proposed as follows.
Fig.2 A rolling piston compressor with low pressure shell
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3. Leakage model
Due to its special structure, the refrigerant leakage through the clearances between relatively
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moving parts has a great effect on the performance of a rolling piston compressor. The leakage loss reduces the refrigerant mass rate delivered by the compressor, it also lowers the cooling capacity and volumetric efficiency which lead to a lower COP. Leakage paths in a rolling piston compressor could be classified to 4 different paths as shown in Fig. 3: (1) the radial clearance between rolling piston and cylinder that separates suction chamber from compression chamber; (2) the axial clearances on vane upper and lower ends; (3) the clearances of suction side of vane with slot and of compression side of vane with slot; (4) the axial clearances of on piston upper and
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lower ends. During the normal operation of rolling piston compressors, the clearance between
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vane tip and roller outer surface is so small that the leakage through it will be neglected.
Fig. 3 Leakages in a rolling piston compressor with high pressure shell For a rolling piston compressor with oil as shown Fig.1, the difficulties of leakage model is how to deal with the sealing effect of oil and how to estimate the various solubility of refrigerant in oil
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(Yanagiswa, 1985b) (Nieter, 1996) (Wu, 2000) (Gasche, 2012). For a rolling piston compressor without oil, the calculation of leakage mass flow rate is relatively easy. Three types of leakage models could be used to calculate the mass flow rate through each clearance or leakage path within an oil-free rolling piston compressor: nozzle flow model (Paudeya, 1978) (Nieter, 1996)
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(Kang, 2002) (Ooi, 2008), Fanno flow model (Yanagisawa, 1985a) (Nieter,1996) (Wu,2000)
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(Kang, 2002) (Ooi, 2008) and incompressible viscous flow (Ishii，1996，1998). The ratio of characteristic length to gap width of leakage flow path within rolling piston compressors is about 100-1000. The friction effect of the leakage flow needs to be taken into account. So an empirical friction correction factor to the isentropic nozzle model is required. Although the nozzle model is simple, it is difficult to determine the empirical factor since it is relevant to many other factors. In addition, the characteristic of leakage path of rolling piston compressors is similar to that of scroll compressors. With the study on leakage flow model of a scroll compressor, Kang(2002) showed that the isentropic nozzle model overly predicted the 8
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leakage flow. The Fanno flow model draws on compressive and viscous theory and describes an adiabatic flow with friction through a path with a constant cross-sectional area. This model is more often used to
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calculate the leakage flow for rolling piston compressors. The velocity V and mass rate of leakage
& are obtained by the equations of Fanno flow(Yanagisawa, 1985a): flow m
k −1
λ L k + 1 M 12 1 + 2 M 2 1 1 1 ln[ 2 ( )] + ( 2 − 2 ) = k 1 − 2δ 2k M 2 1+ k M1 M 2 M2 2
Re =
Re ≤ 3560
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96 R , λ= e 0.3164 , Re0.25
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1
2
(1)
(2)
Re > 3560
2δV 2m& = µv µ W
(3)
where L is the length of leakage path, W is the width of leakage path, δ is its gap value, k is the
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adiabatic exponent of refrigerant, µ is dynamic viscosity of refrigerant vapor; v is specific volume of refrigerant vapor; M1 and M2 are Mach numbers at inlet or outlet of leakage flow, respectively, P1 and P2 are pressure at inlet or outlet of leakage flow respectively.
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For the leakage flow through radial clearance between cylinder and piston, Yanagisawa(1985b) presented an equivalent channel length:
δ ⋅ Rc ⋅ (τ 2 − τ 1 ) δ 2
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Lf =
(4)
e 1 − (1 − ) e
δ δ τ = sin −1{[1 − 1 − (1 − ) 2 ]sin φ / [1 + (1 − ) cos φ ]} e
5 6
φ1 = π ,
e
7 6
φ2 = π
(5)
(6)
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where Rc and e are the radius of cylinder and the eccentricity of crankshaft, respectively. Compared with the other leakage path within a rolling piston compressor, the channel length is short, Mach number M1 at inlet is large and the refrigerant acceleration effect cannot be neglected.
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So a convergent nozzle model should be added to the front of straight channel. However, some hypotheses are involved in the equations of Fanno model and their solution
process. Ishii(1996,1998) considered that the refrigerant leakage flows through the axial and radial clearances in scroll or rolling piston compressors can be simulated by a simple incompressible and
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viscous flow model assuming an entire turbulent flow. The results calculated by the model were compared with those measured by tests of refrigerant leaking through an axial gap or a radial gap
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from a pressurized closed vessel. So besides Fanno model, the incompressible viscous flow model is also used to evaluate the leakage mass flow rate in the paper. For leakages through clearances on vane ends and vane sides, the equation of the incompressible viscous flow model is (Ishii, 1996, 1998):
ρ
=λ
L V2 2δ 2
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∆P
λ = 0.35 Re −0.35
(7)
(8)
For radial clearance of piston, the equation is (Ishii, 1996, 1998):
=∫
ϕ0
λ
Vϕ 2 Rc dϕ 2 2hϕ
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Pc − Ps
ρ
−ϕ0
(9)
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where Pc and Ps are the pressure in compression and suction chamber, ρ is the density of refrigerant and V is flow velocity.
4. Calculation and analysis of leakage of an oil-free rolling piston compressor with conventional structure
At present, the rolling piston compressors used in room air conditioners and refrigerators are mostly lubricated by oil and with a high pressure shell as shown Fig. 1. That is, their oil sumps are 10
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under discharge pressure. If these compressors are oil free, refrigerant gas will leak along the directions as shown in Fig.3. The leakage mass flow rates and their effects on performance of compressor could be computed by the leakage models above.
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The main sizes and clearances of a rolling piston compressor are shown in Table 1 and Table 2. Since the clearances have a great impact on the performance of a rolling piston compressor, two
groups of clearances are specified. The working conditions for evaluation are shown in Table 3.The crankshaft speed, the ideal mass rate of the rolling piston compressor and the estimated
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discharge temperature for calculating the heating effect of refrigerant leakage are also in Table 3. Table 1 Main sizes of compressor (mm) Term
value
cylinder diameter
48
24
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cylinder height eccentricity
4.8
piston thickness
6.2
vane thickness
4
vane tip radius
6
vane side sealing length
10
Table 2 clearances of compressor (µm big
small
15
11
axial clearances on vane ends
18
11
clearances on vane sides
27
18
axial clearances on piston ends
15
10
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Terms
radial clearance of piston
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Table 3 working condition analyzed
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Evaporating
Term
value
temperature
10.0℃
Condensing temperature
46.0℃
Suction temperature
18 ℃
Shaft speed
2880rpm
Discharge temperature
R22 R410A
11
81.0℃ 79.5℃
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R290 Ideal mass rate of
R22
19.90 g/s
R410A
28.47 g/s
R290
9.510 g/s
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compressor
62.0℃
For the oil free rotary compressor using R22, R410A or R290 as working fluid with conventional structure and under the conditions shown in Table 3, the mass flow rate through each leakage path
within the compressor is calculated by Fanno model(FM) and incompressible viscous flow
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model(IM). The results are shown in Table 4. For bigger clearances, the results calculated by the incompressible viscous flow model are larger than those by Fanno model, same as that in
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Reference (Ishii, 1996). And the result difference for radial clearance between cylinder and piston is larger than that for other leakage clearances since its channel length is shorter and its flow velocity is larger. However, for small clearances, some results calculated by Fanno model are larger than those by incompressible viscous flow model, and generally the differences between results by two models are not very great. Thus their mean value will be used for following analysis in the paper.
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Table 4 mass flow rate of leakage through various clearances with high pressure shell(g/s) Big clearances
radial clearance of piston
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axial clearance on vane
small clearances
R22
R410A
R290
R22
R410A
R290
FM
1.43
2.13
0.925
0.940
1.40
0.609
IM
1.81
2,88
1.20
1.02
1.61
0.674
FM
0.230
0.352
0.151
0.104
0.163
0.068
0.287
0.455
0.190
0.109
0.173
0.072
FM
0.481
0.758
0.345
0.247
0.391
0.164
suction side
IM
0.488
0.773
0.358
0.243
0.386
0.163
clearance on vane
FM
0.683
1.07
0.488
0.363
0.571
0.313
discharge side
IM
0.711
1.13
0.527
0.355
0.563
0.317
axial clearance on piston
FM
2.20
3.43
1.45
1.23
2.01
0.846
to suction chamber
IM
2.45
3.88
1.64
1.32
2.10
0.886
axial clearance on piston
FM
1.60
2.50
1.06
0.918
1.45
0.614
to compression chamber
IM
1.67
2.64
1.12
0.902
1.43
0.607
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IM
clearance on vane
Note : FM means the results calculated by Fanno model and IM means those by incompressible viscous flow model.
The leakages through various paths affect the performance of rotary compressor differently. For 12
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the conventional rotary compressor with high pressure shell shown in Fig.3, the leakages through the radial clearance of piston(1), axial clearances on vane(2u, 2l), clearances of vane suction side(3s) and axial clearances of piston to suction chamber(4su, 4sl) would decrease the cooling
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capacity and volumetric efficiency of compressor. In the paper, the effects on the cooling capacity or volumetric efficiency are calculated by the volume effect and the heating effect of leakage. The volume effect is estimated by the ratio of the leakage mass flow rate to the mass flow rate of compressor, while the heating effect is estimated by the ratio of the enthalpy increment caused by
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leakage to the suction enthalpy rate of compressor. The leakages from compression chamber to
suction chamber through the radial clearance of piston(1) and axial clearances on vane(2u,2l) decrease the actual gas compression power, i.e. indicated power of compressor, whereas the
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leakages from chambers with discharge pressure to suction chamber through the clearances of vane suction side(3s) and axial clearances of piston(4su, 4sl) do not affect indicated power of compressor directly. So even when they have equal effect on the volumetric efficiency of compressor, leakage 1 and leakage 2 still have greater effect on the indicated efficiency of compressor, which is defined as the ratio of the isentropic compression power to actual gas
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compression power in the paper, than the latter two leakages(3,4). However, it is difficult to calculate the effects of the four leakages on the indicated efficiency of compressor. So in the Table 5, the effects of the four leakages on efficiency are represented by volumetric efficiency of compressor. The refrigerant leakages to compression chamber through the clearance on vane
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discharge side(3d) and through the axial clearances on piston ends(4cu, 4cl) do not directly influence a compressor’s cooling capacity and volumetric efficiency, but increase the indicated
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power of compressor and decrease the indicated efficiency and COP of compressor. The effects of the leakages(3d, 4cu, 4cl) on indicated efficiency are only evaluated by ratio of the leakage mass flow rate to the mass flow rate of compressor in the paper since the heating effect is relatively small and difficult to calculate. The results are also shown in Table 5. On the other hand, for a rotary compressor with low pressure shell shown in Fig.4, the leakages through the clearance on vane discharge side(3d) and through the axial clearances on piston ends(4cu, 4cl) directly influence the volumetric efficiency of compressor, as the leakages through the radial clearance of piston(1) and axial clearances on vane(2u,2l) do. 13
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Table 5 Effect of leakage on efficiency of oil free compressor with high pressure shell (%) Big clearances
Small clearances
R410A
R290
R22
R410A
R290
radial clearance of piston
8.55
9.27
11.79
5.16
5.59
7.11
axial clearance on vane
1.36
1.49
1.88
0.561
0.622
0.775
clearance on vane
2.58
2.87
3.61
1.31
1.45
1.83
3.51
3.86
4.93
1.80
1.99
2.54
12.36
13.7
17.3
6.93
7.69
9.69
8.19
9.02
11.5
4.57
5.05
6.41
suction side clearance on vane discharge side axial clearance on piston to axial clearance on piston to
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compression chamber
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suction chamber
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R22
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Fig. 4 Leakages in a rolling piston compressor with low pressure shell
The leakage loss through the clearances on vane ends in the oil free compressor remains unchanged and relatively small compared with that in the conventional compressor. Except that, the leakage losses through other clearances greatly increase, as shown in Table 5. For the present rolling piston compressor, which has high pressure shell and is lubricated and sealed by oil, the effect of leakage loss on volumetric efficiency or indicated efficiency is about 3-10%. The calculated results in Table 5 show that the effect is about 36% and 20% for R22 oil free rolling piston compressor with same structure when the clearances are big and small, respectively. The 14
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mean dynamic viscosity at suction condition and discharge condition of R22 and R290 is 15.1×10-6 Pa•s and 9.11×10-6 Pa•s, respectively. So, the effect expands to about 45% and 25% for R290 oil free rolling piston compressor although the pressure difference is smaller. Therefore, to
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lower the leakage loss is one of the great challenges to develop oil free rolling piston compressor against reciprocating or scroll ones.
5. Calculation and decrease of leakage loss through axial clearances
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on piston
The axial clearances on piston ends are sealed by liquid oil and refrigerant mixture within present
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rolling piston compressor. In order to have sufficient oil to seal the radial clearance of piston, the clearance value is set bigger. But in an oil free compressor, even for smaller clearance, the leakage loss through the path could not be tolerated. If the sealing strips on two ends of piston are put up, the sealing strips would bear a great force since the pressure difference between discharge pressure and suction pressure directly act on them. The friction loss would increase. Besides, the working stability of the sealing strip will be poor since the forces on it constantly change their
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magnitudes and, especially, directions.
In the paper, the structure shown in Fig. 2 is taken to connect the cavity within piston to that at suction pressure. The pressure within piston is about suction pressure. The refrigerant leakage
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directions through the clearances on piston ends are shown in Fig.4. Assuming that the pressure in suction chamber is always the suction pressure of compressor, the leakage through the clearances
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from the cavity within piston to suction chamber will be zero. The mass flow rate and the effect on volumetric efficiency of the leakage from compression chamber to cavity in piston for small clearance group are shown in Table 6. It could be seen that the effects of the leakage on volumetric efficiency of compressor are greatly reduced. So their effects on the indicated efficiency of compressor also would be greatly reduced. Table 6 Mass flow rate and effect on volumetric efficiency of leakage of oil free compressor with low pressure shell (%)
Mass flow rate (g/s) R22
R410A 15
R290
Efficiency drop (%) R22
R410A
R290
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0.140
0.214
0.091
0.736
0.791
1.01
axial clearance on vane end
0.106
0.168
0.070
0.561
0.622
0.775
0
0
0
0
0
0
0.331
0.524
0.218
1.79
2.00
2.48
0
0
0
0
0
0
0.511
0.814
0.338
2.59
3.09
3.81
1.09
1.72
0.717
5.68
6.50
8.08
clearance on vane suction side clearance on vane discharge side axial clearance on piston ends from/to suction chamber axial clearance on piston ends from compression chamber total
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radial clearance of piston
The cavity within piston may not be connected to other cavities except those of suction chamber
and discharge chamber in cylinder. Thus, the refrigerant in discharge chamber will leak through
mass flow rate will be smaller than that in above situation.
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the clearances on piston ends to the cavity in piston and then leak to suction chamber. The leakage
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In addition, the refrigerant leaking through axial clearances on piston ends could be easily used to cool the friction pair between eccentric shaft and piston, i.e. eccentric bearing. The refrigerant leaking into piston could flow to suction muffler of compressor through a circular groove, one or several radial holes on eccentric shaft and center hole of crankshaft. The temperature of the R290 vapor will be low due to the lower discharge temperature of R290 compressor and the throttling
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effect during leakage process.
6. Calculation and decrease of leakage loss through clearances on
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vane sides
In the paper, the measure to reduce the leakage loss through clearances on vane sides is also to
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connect the cavity behind vane to cavity at suction pressure. It is easy for a rolling piston compressor with low pressure shell shown in Fig.2. The mass flow rate and the effect on volumetric efficiency of the leakage loss through the clearances on vane sides is calculated by above model and shown in Table 6. The reduction in the magnitude of the leakage loss is obvious. But the back pressure of vane is a main factor to force vane to press piston tightly for a conventional rolling piston compressor or a swing piston rotary compressor (Shintaku, 2000). However, swing piston-blade type (Masuda, 1996) and vane hinged on piston type (Okur, 2011) rolling piston compressor shown in Fig. 5 and vane hinged on cylinder (or swing vane) type 16
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rolling piston (Hu, 2013) could be adopted. These types of rolling piston compressor do not need the high pressure at outer end of vane to ensure its normal motion. These kinds of structures also eliminate or decrease the friction and wear on vane tip when there is no oil lubrication. Moreover,
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the sealing length of leakage flow path could be augmented because the vane spring hole is
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canceled.
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Fig.5(a) A swing piston-blade type rolling piston compressor
Fig.5(b) A vane hinged on piston type rolling piston compressor
7. Calculation and decrease of leakage loss through radial clearance of piston For a conventional rolling piston mechanism, it is difficult to control the radial clearance between 17
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cylinder and piston because the dimension chain of the clearance is long and the dynamic clearances of eccentric bearing and main shaft bearing affect the variation of the radial clearance (Yanagisawa, 1985a). In addition, even though the radial clearance is set to a small value (10
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microns), its leakage loss is also great as shown in Table 5. The paper presents two types of radial compliance mechanisms: swing link or eccentric bush type
and slider block or sliding bush type for rolling piston compressors as shown Fig. 6. The radial compliance mechanisms are suitable for aforementioned conventional, swing piston-blade and
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vane hinged on piston type rolling piston compressors. They are different from but basically
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similar to the radial compliance mechanisms of scroll compressor.
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Fig. 6(a) Swing link radial compliance mechanism for rolling piston compressors
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Fig. 6(b) Slider block radial compliance mechanism for rolling piston compressors
By the radial compliance mechanisms, the radial clearance between cylinder and piston will be reduced. When the gap is set to 2 microns, the leakage loss calculated by above models is shown in Table 6. The effect on the volumetric efficiency of the leakage loss through radial clearance of piston could be limited to 1%. Since the leakage is from compression chamber to suction chamber,
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its effect on the indicated efficiency will be less than 1%. However, the radial compliance mechanism for rolling piston compressor need further study since the gas force on piston changes greatly.
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8. Conclusion
Since there are many leakage paths within a rolling piston compressor, the leakage loss of the
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oil-free rolling piston compressor with present structure is high. Adopting a low pressure shell, connecting the cavities within piston and behind vane to cavity at suction pressure and using radial compliance mechanism decreases the leakage loss. By these means, the effects of leakage on the volumetric efficiency of a R290 oil-free rolling piston compressor could be limited to 8%. Considering that the force and thermal induced deformation of a R290 rolling piston compressor are relatively small and its clearances may be set to smaller values, the effect of leakage on cooling capacity and COP of oil free rolling piston compressor will just somewhat bigger than that
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of existing rolling piston compressors with oil lubrication. Compared with an HCFC or HFC compressor, a HC compressor will get more benefits from an oil-free unit because of the decrease of refrigerant charge. However, the lower pressure difference
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between discharge and suction also help develop the friction pairs of compressor. Above considerations are useful for the study on oil-free HC rolling piston compressor used for refrigerator, freezer or cooler and on oil-free rolling piston type vacuum pump or gas compressor
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with low discharge pressure.
However, the radial compliance mechanism for rolling piston compressor need further study since
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the gas force on piston changes greatly.
Reference
Bradshaw, C.R., Groll, E.A., Garimella, S.V., 2011. A comprehensive model of a miniature-scale linear compressor for electronics cooling. Int. J. Refrigeration 34 (1), 63-73. Gao, B., Chen, Z., Gao, Q., 2012. Research of R290 compressor effect on RAC system charge
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amount. In: Proceedings of the International Compressor Engineering Conference. Purdue University, West Lafayette, IN, USA, p. 1332
Gasche, J., L., Andreotto, T., Maia, C., R., M., 2012. A model to predict R134a refrigerant leakage
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through radial clearance of rolling piston compressors. Int. J. Refrigeration 35, 2223-2232. Hu, X., Qu Z., Yang X., Sun J., 2013. Theoretical study on frictional losses of a novel automotive
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swing vane compressor. Int. J. Refrigeration 36, 758-767. Masuda, M., Sakitani, K., Yamamoto, Y., Uematsu, T., Mutoh, A., 1996. Development of swing compressor for alternative refrigerants. In: Proceedings of the International Compressor Engineering Conference. Purdue University, West Lafayette, IN, USA, pp. 499～504.
Ishii, N., Bird, K., Sano, K., Oono, M., Iwamura, S., and Otokura, T., 1996. Refrigerant leakage flow evaluation for scroll compressors. In: Proceedings of the International Compressor
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Engineering Conference. Purdue University, West Lafayette, IN, USA, pp. 633~638. Ishii, N., Bird, K., Yamamoto, S., Matsunaga, H., Sano, K., Hayashi, M., 1998. A Fundamental optimum design for high mechanical and volumetric efficiency of compact rotary compressors. In:
Lafayette, IN, USA, pp. 649~654.
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Proceedings of the International Compressor Engineering Conference. Purdue University, West
Kang, D., J., Kim, J., W., Sohn, C., B., 2002. Effects of leakage flow model on the thermodynamic performance of a scroll compressor. In: Proceedings of the International Compressor Engineering
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Conference. Purdue University, West Lafayette, IN, USA, p. C20-5.
Refrigeration 36, 1576-1583.
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Kus, B., Neksa, P., 2013. Oil free turbo-compressors for CO2 refrigeration application. Int J
Okur, M., Akmandor, I., S., 2011. Experimental investigation of hinged and spring loaded compressors pertaining to a turbo rotary engine. Applied Thermal Engineering 31, 031-1038. Ooi, K., 2008. Assessment of a rotary compressor performance operating at transcritical carbon
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dioxide cycles. Applied Thermal Engineering 28, 1160-1167.
Paudeya, P., Soedel, W., 1978. Rolling Piston Type Rotary Compressors with Special Attention to Friction and Leakage, In: Proceedings of the International Compressor Engineering Conference.
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Purdue University, West Lafayette, IN, USA, pp. 209～218 Rodgers, R.J., Nieter, J.J., 1996. Comprehensive analysis of leakage in rotary compressors. In:
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Proceedings of the International Compressor Engineering Conference. Purdue University, West Lafayette, IN, USA, pp. 287～293. Shintaku, H., Ikoma, M., Hasegawa, H., et al, 2000. Experimental and Theoretical study of an
advanced rotary compressor, In: Proceedings of the International Compressor Engineering Conference. Purdue University, West Lafayette, IN, USA, pp. 753~760. Wu, J., 2000. A mathematical model for internal leakage in a rotary compressor. In: Proceedings of the International Compressor Engineering Conference. Purdue University, West Lafayette, IN, 21
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USA, pp. 649~654. Wu, J., Yang, L., Hou, J., 2012. Experimental performance study of a small wall room air conditioner retrofitted with R290 and R1270. Int. J. Refrigeration 35, 1860-1868.
Radial clearance on the rolling piston. Int. J. Refrigeration 8 (2), 75-84.
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Yanagisawa, T., Shimizu, T., 1985a. Leakage losses with a rolling piston type rotary compressor. I.
Yanagisawa, T., Shimizu, T., 1985b. Leakage losses with a rolling piston type rotary compressor.
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II. Leakage losses through clearance on rolling piston faces. Int. J. of Refrigeration 8 (3), 152-158.
Youbi-Idrissi, M., Bonjour J., 2008. The effect of oil in refrigeration: current research issues and
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critical review of thermodynamic aspects. Int. J. Refrigeration 31, 165-179.
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Highlights
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The potentials and difficulties of oil-free R290 rotary compressor are discussed. A new structure of the oil-free compressor is proposed to reduce its leakage loss. Calculating models are discussed to calculate precisely the leakage flow rate. The leakage loss can be decreased to an acceptable value by the new structure.
S0140-7007(14)00268-0
DOI:
10.1016/j.ijrefrig.2014.10.007
Reference:
JIJR 2895
To appear in:
International Journal of Refrigeration
Received Date: 11 July 2014 Revised Date:
23 August 2014
Accepted Date: 6 October 2014
Please cite this article as: JianHua, W., Ang, C., A new structure and theoretical analysis on leakage and performance of an oil-free R290 rolling piston compressor, International Journal of Refrigeration (2014), doi: 10.1016/j.ijrefrig.2014.10.007. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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A new structure and theoretical analysis on leakage and performance of an oil-free R290 rolling piston compressor Chen Ang
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Wu JianHua *
School of Power and Energy, Xi’an Jiaotong University, Xi’an 710049, P.R. China *
Corresponding author. Tel.: +82 029 8266 3786, +82 13689206050.
E-mail address: [email protected] (Jianhua Wu), [email protected].
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Postal address: No. 28, Xian-ning West Rd., Xi’an City 710049, Shaanxi, PR China.
Abstract
Lubricating oil improves the reliability of compressors and systems, whereas increases the system complexity. Compared with other types of compressors that have oil-free models, a rolling piston compressor has more leakage paths and bigger leakage loss. Therefore, the leakage is an important
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problem to be solved in order to develop an oil-free rolling piston compressor. The paper put forward a new structure of rotary compressor adopting a low pressure shell, connecting the cavities within piston and behind vane to the cavity at suction pressure and using radial compliance mechanisms. Then the leakage models were developed to calculate the mass flow
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rates within both the present rolling piston compressor without any oil as sealant and the new structure of oil free compressor. Results showed that by the new structure, the influences of
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leakage on the performance of a R290 oil free rolling piston compressor can be largely decreased. Key words: R290, rolling piston compressor, oil-free, leakage, performance
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Nomenclature eccentricity of crankshaft (m)
Rc
radius of cylinder (m)
L
length of leakage path (m)
Re
Reynolds number
Lf
equivalent channel length (m)
V
flow velocity(m s-1)
M
Mach numbers at inlet of leakage flow
W
width of leakage path (m)
P
pressure (Pa)
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e
Greek symbols
gap value (m)
k
the adiabatic exponent of refrigerant
µ
dynamic viscosity (Pa s)
v
specific volume of refrigerant vapor (m3kg-1)
ρ
density (kg m-3)
φ
discharge angle (rad)
Subscripts
EP
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δ
inlet of leakage flow
c
compression chamber
2
outlet of leakage flow
s
suction chamber
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1
2
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1. Introduction The production and consumption of R22 currently used in room air conditioners in China have been frozen since 2013 and will be phased out by 2015 due to its impacts on the ozone layer. The
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alternative refrigerant R410A was developed twenty years ago and known as the transitional
refrigerants because of its higher GWP. Despite that, it has been still widely used in many countries. At present, some companies promote several other synthetic alternatives. Their GWPs are lower than that of R410A. The natural working substance propane has been chosen as the main
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alternative for R22 in Chinese room air conditioner industry because of its excellent environmental protection feature and thermal physical properties. The transformation for the
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production line of R290 room air conditioners and compressors has been aided by Multilateral Funds financially.
One of the important measures to improve the safety of R290 room air conditioners is to limit the R290 mass within system. The main way to reduce R290 charge in an air conditioner is to reduce the internal volume of heat exchangers and liquid line pipes, for example, by adopting the small
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diameter tube for condenser. As a result, the R290 content in the compressor of system becomes relatively larger. Therefore, it is necessary to reduce the R290 charge in the compressor (Wu, 2012).
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The compressor used in a room air conditioner now, as shown in Fig. 1, is mainly the rolling piston type rotary compressor with high pressure shell, whose oil sump is exposed to the discharge pressure, therefore a great amount of R290 is dissolved in oil. Two basic approaches to reduce the
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R290 mass in the compressor are to lower oil charge in the compressor (Gao, 2012) and to use oil with low R290 solubility. On the other hand, oil in system can decrease the heat transfer effect of exchangers, increase the flow resistance and thus degrade the system performance (Youbi-Idrissi, 2008). Additionally, to deal with oil return, the system may be more complex. The decomposition of oil, the flow block of micro channel and other issues related to oil will affect the system’s reliability. Therefore, it is of great significance to research and develop the oil free compressor R290 room air conditioning system. 3
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Fig. 1 R290 rolling piston compressor with high pressure shell
In some ways, R290 is favorable for a compressor with an oil-free model. Compared with R22 and R410A, the difference between discharge pressure and suction pressure of R290 compressor is smaller and, as a result, the loads on some friction pairs are smaller, too. The lower discharge
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temperature of R290 compressor reduces the working temperature of its friction pair. And the large specific heat of R290 is beneficial for us to cool the bearings with gas refrigerant. Admittedly, the viscosity of R290 vapor is lower and it would increase the leakage loss of oil-free rolling
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piston compressor. This is also investigated in this paper. At present, reciprocating, scroll and linear type compressors all have their oil-free or semi-oil-free
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models (Bradshaw, 2011). But the rolling piston compressors have no oil-free version yet. For certain displacement, the integrated performances of rolling piston compressors are very excellent, such as: high COP, good reliability, low vibration and noise, and low cost. So they are widely used for room air conditioners now. Nevertheless, there is no paper has discussed technical issues about oil-free rolling piston compressor. Besides lubricating friction pairs, the oil within compressor also plays a pivotal role in sealing leakage clearances, enhancing heat diffusion and thus ensuring the high reliability and performance of a compressor. The main technical problem of oil free compressors is undoubtedly 4
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the friction and wear. Compared with reciprocating or linear compressor, the rolling piston compressor has more leakage path and bigger leakage loss. So once the rolling piston compressor adopts the oil-free model, it is necessary to figure out how much the leakage loss is and how to
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limit the leakage loss and its effect on the compressor performance. There were many studies on the leakage of rolling piston compressor in the past. Earlier at the beginning of developing rolling piston compressor, Pandeya and Soedel（1978）calculated and
analyzed the effect of leakage on the performance of the compressor. Leakages through the
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clearance between cylinder and piston and the clearance between vane ends and cylinder heads
were considered as a gas flow and the mass flows were calculated by nozzle model. A leakage loss
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of approximately 12% of the ideal mass flow was obtained. The reason for why the value was larger was that the effects of oil sealing and refrigerant Couette flow were not considered. For the leakage loss through the radial clearance on rolling piston, Yanagisawa and Shimizu (1985a) took account of the effect of fluid friction on the leakage and the dynamic change of the clearance. For the leakage through the clearances on rolling piston faces, Yanagisawa and Shimizu (1985b) took particular account of special flow characteristics of the leakage flow and the practical distribution
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of the clearance. Rodgers and Nieter (1996) developed two types of leakage flow model: a gaseous flow of refrigerant treated either as nozzle flow or as Fanno flow in a channel, and a liquid flow of oil/refrigerant mixture as an incompressible flow with refrigerant outgassing. The
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appropriate type of leakage flow model for each leakage path was also discussed. Ishii (1998) assumed the leakage flows through the axial gap between blade and thrust plate and through the
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radial gap between piston and cylinder as incompressible and viscous flow and evaluated the leakage flow and volumetric efficiency. Wu (2000) showed a comprehensive model for leakages in rotary compressors. Ooi (2008) assessed the performance of a rolling piston compressor when operating at a transcritical CO2 cycle by considering leakage mass flow and friction loss. Gasche (2012) presented a non-isothermal two-phase model for oil-R134a mixture leakage through the radial clearance of rolling piston compressors. Some studies on the leakage of scroll compressor have high reference value for the calculation of leakage of rolling piston compressor (Ishii, 1996) (Kang, 2002). In addition, Kus (2013) assessed the feasibility of replacing oil-lubricated 5
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compressor in CO2 refrigeration system with oil-free turbo-machinery. The paper proposed some measures to reduce the leakage mass flow rate by adopting a new structure of oil free rolling piston compressor with low pressure shell, discussed the model to
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evaluate the mass rate of leakage flow, compared the mass flow rates through every path within the oil free rolling piston compressors of both conventional and new structure. Finally, the effect of the mass flow rate on compressor performance was analyzed.
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2. A new structure of oil-free rolling piston compressor with low pressure shell
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To reduce the leakage loss of an oil-free rolling piston compressor, a new structure with low pressure shell is proposed in this paper, as shown in Fig. 2. Compared to those of a conventional structure, the compressor has not a suction accumulator as shown in Fig. 1 and the motor is placed under the pump body. The inhaled liquid refrigerant could reserve within the cavity below motor. The motor and cylinder and main bearing and the back of vane are surrounded by refrigerant at
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suction pressure . A plastic suction muffler is used to prevent the cylinder and main bearings from overheating the refrigerant. Since the suction pipe is not connected to the muffler, some of the refrigerant enters the cylinder directly through the muffler, while the other goes down into the low pressure cavity and cool the motor. The distance between suction pipe and the inlet of muffler
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helps to control the distribution of refrigerant. On the other hand, the discharged gas, which only takes a small space, is sealed by the sub bearing.
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With this new structure, the refrigerant in discharge chamber will leak through the clearances on piston ends to the cavity in piston and then leak to suction chamber. In addition, the refrigerant leaking through axial clearances on piston ends can be easily used to cool the friction pair between shaft and piston.
Since the gas behind the vane is at suction pressure, the conventional structure of piston–vane is not suitable anymore due to the lack of press force. Therefore, the other types of vane, such as swing piston-blade type, need be adopted accordingly. 6
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Also, as there is no oil needed to lubricate the friction pair between piston and cylinder wall, the radial clearance between them could be decreased, hence two types of radial compliance
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mechanisms are proposed as follows.
Fig.2 A rolling piston compressor with low pressure shell
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3. Leakage model
Due to its special structure, the refrigerant leakage through the clearances between relatively
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moving parts has a great effect on the performance of a rolling piston compressor. The leakage loss reduces the refrigerant mass rate delivered by the compressor, it also lowers the cooling capacity and volumetric efficiency which lead to a lower COP. Leakage paths in a rolling piston compressor could be classified to 4 different paths as shown in Fig. 3: (1) the radial clearance between rolling piston and cylinder that separates suction chamber from compression chamber; (2) the axial clearances on vane upper and lower ends; (3) the clearances of suction side of vane with slot and of compression side of vane with slot; (4) the axial clearances of on piston upper and
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lower ends. During the normal operation of rolling piston compressors, the clearance between
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vane tip and roller outer surface is so small that the leakage through it will be neglected.
Fig. 3 Leakages in a rolling piston compressor with high pressure shell For a rolling piston compressor with oil as shown Fig.1, the difficulties of leakage model is how to deal with the sealing effect of oil and how to estimate the various solubility of refrigerant in oil
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(Yanagiswa, 1985b) (Nieter, 1996) (Wu, 2000) (Gasche, 2012). For a rolling piston compressor without oil, the calculation of leakage mass flow rate is relatively easy. Three types of leakage models could be used to calculate the mass flow rate through each clearance or leakage path within an oil-free rolling piston compressor: nozzle flow model (Paudeya, 1978) (Nieter, 1996)
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(Kang, 2002) (Ooi, 2008), Fanno flow model (Yanagisawa, 1985a) (Nieter,1996) (Wu,2000)
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(Kang, 2002) (Ooi, 2008) and incompressible viscous flow (Ishii，1996，1998). The ratio of characteristic length to gap width of leakage flow path within rolling piston compressors is about 100-1000. The friction effect of the leakage flow needs to be taken into account. So an empirical friction correction factor to the isentropic nozzle model is required. Although the nozzle model is simple, it is difficult to determine the empirical factor since it is relevant to many other factors. In addition, the characteristic of leakage path of rolling piston compressors is similar to that of scroll compressors. With the study on leakage flow model of a scroll compressor, Kang(2002) showed that the isentropic nozzle model overly predicted the 8
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leakage flow. The Fanno flow model draws on compressive and viscous theory and describes an adiabatic flow with friction through a path with a constant cross-sectional area. This model is more often used to
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calculate the leakage flow for rolling piston compressors. The velocity V and mass rate of leakage
& are obtained by the equations of Fanno flow(Yanagisawa, 1985a): flow m
k −1
λ L k + 1 M 12 1 + 2 M 2 1 1 1 ln[ 2 ( )] + ( 2 − 2 ) = k 1 − 2δ 2k M 2 1+ k M1 M 2 M2 2
Re =
Re ≤ 3560
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96 R , λ= e 0.3164 , Re0.25
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1
2
(1)
(2)
Re > 3560
2δV 2m& = µv µ W
(3)
where L is the length of leakage path, W is the width of leakage path, δ is its gap value, k is the
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adiabatic exponent of refrigerant, µ is dynamic viscosity of refrigerant vapor; v is specific volume of refrigerant vapor; M1 and M2 are Mach numbers at inlet or outlet of leakage flow, respectively, P1 and P2 are pressure at inlet or outlet of leakage flow respectively.
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For the leakage flow through radial clearance between cylinder and piston, Yanagisawa(1985b) presented an equivalent channel length:
δ ⋅ Rc ⋅ (τ 2 − τ 1 ) δ 2
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Lf =
(4)
e 1 − (1 − ) e
δ δ τ = sin −1{[1 − 1 − (1 − ) 2 ]sin φ / [1 + (1 − ) cos φ ]} e
5 6
φ1 = π ,
e
7 6
φ2 = π
(5)
(6)
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where Rc and e are the radius of cylinder and the eccentricity of crankshaft, respectively. Compared with the other leakage path within a rolling piston compressor, the channel length is short, Mach number M1 at inlet is large and the refrigerant acceleration effect cannot be neglected.
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So a convergent nozzle model should be added to the front of straight channel. However, some hypotheses are involved in the equations of Fanno model and their solution
process. Ishii(1996,1998) considered that the refrigerant leakage flows through the axial and radial clearances in scroll or rolling piston compressors can be simulated by a simple incompressible and
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viscous flow model assuming an entire turbulent flow. The results calculated by the model were compared with those measured by tests of refrigerant leaking through an axial gap or a radial gap
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from a pressurized closed vessel. So besides Fanno model, the incompressible viscous flow model is also used to evaluate the leakage mass flow rate in the paper. For leakages through clearances on vane ends and vane sides, the equation of the incompressible viscous flow model is (Ishii, 1996, 1998):
ρ
=λ
L V2 2δ 2
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∆P
λ = 0.35 Re −0.35
(7)
(8)
For radial clearance of piston, the equation is (Ishii, 1996, 1998):
=∫
ϕ0
λ
Vϕ 2 Rc dϕ 2 2hϕ
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Pc − Ps
ρ
−ϕ0
(9)
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where Pc and Ps are the pressure in compression and suction chamber, ρ is the density of refrigerant and V is flow velocity.
4. Calculation and analysis of leakage of an oil-free rolling piston compressor with conventional structure
At present, the rolling piston compressors used in room air conditioners and refrigerators are mostly lubricated by oil and with a high pressure shell as shown Fig. 1. That is, their oil sumps are 10
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under discharge pressure. If these compressors are oil free, refrigerant gas will leak along the directions as shown in Fig.3. The leakage mass flow rates and their effects on performance of compressor could be computed by the leakage models above.
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The main sizes and clearances of a rolling piston compressor are shown in Table 1 and Table 2. Since the clearances have a great impact on the performance of a rolling piston compressor, two
groups of clearances are specified. The working conditions for evaluation are shown in Table 3.The crankshaft speed, the ideal mass rate of the rolling piston compressor and the estimated
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discharge temperature for calculating the heating effect of refrigerant leakage are also in Table 3. Table 1 Main sizes of compressor (mm) Term
value
cylinder diameter
48
24
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cylinder height eccentricity
4.8
piston thickness
6.2
vane thickness
4
vane tip radius
6
vane side sealing length
10
Table 2 clearances of compressor (µm big
small
15
11
axial clearances on vane ends
18
11
clearances on vane sides
27
18
axial clearances on piston ends
15
10
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Terms
radial clearance of piston
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Table 3 working condition analyzed
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Evaporating
Term
value
temperature
10.0℃
Condensing temperature
46.0℃
Suction temperature
18 ℃
Shaft speed
2880rpm
Discharge temperature
R22 R410A
11
81.0℃ 79.5℃
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R290 Ideal mass rate of
R22
19.90 g/s
R410A
28.47 g/s
R290
9.510 g/s
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compressor
62.0℃
For the oil free rotary compressor using R22, R410A or R290 as working fluid with conventional structure and under the conditions shown in Table 3, the mass flow rate through each leakage path
within the compressor is calculated by Fanno model(FM) and incompressible viscous flow
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model(IM). The results are shown in Table 4. For bigger clearances, the results calculated by the incompressible viscous flow model are larger than those by Fanno model, same as that in
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Reference (Ishii, 1996). And the result difference for radial clearance between cylinder and piston is larger than that for other leakage clearances since its channel length is shorter and its flow velocity is larger. However, for small clearances, some results calculated by Fanno model are larger than those by incompressible viscous flow model, and generally the differences between results by two models are not very great. Thus their mean value will be used for following analysis in the paper.
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Table 4 mass flow rate of leakage through various clearances with high pressure shell(g/s) Big clearances
radial clearance of piston
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axial clearance on vane
small clearances
R22
R410A
R290
R22
R410A
R290
FM
1.43
2.13
0.925
0.940
1.40
0.609
IM
1.81
2,88
1.20
1.02
1.61
0.674
FM
0.230
0.352
0.151
0.104
0.163
0.068
0.287
0.455
0.190
0.109
0.173
0.072
FM
0.481
0.758
0.345
0.247
0.391
0.164
suction side
IM
0.488
0.773
0.358
0.243
0.386
0.163
clearance on vane
FM
0.683
1.07
0.488
0.363
0.571
0.313
discharge side
IM
0.711
1.13
0.527
0.355
0.563
0.317
axial clearance on piston
FM
2.20
3.43
1.45
1.23
2.01
0.846
to suction chamber
IM
2.45
3.88
1.64
1.32
2.10
0.886
axial clearance on piston
FM
1.60
2.50
1.06
0.918
1.45
0.614
to compression chamber
IM
1.67
2.64
1.12
0.902
1.43
0.607
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IM
clearance on vane
Note : FM means the results calculated by Fanno model and IM means those by incompressible viscous flow model.
The leakages through various paths affect the performance of rotary compressor differently. For 12
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the conventional rotary compressor with high pressure shell shown in Fig.3, the leakages through the radial clearance of piston(1), axial clearances on vane(2u, 2l), clearances of vane suction side(3s) and axial clearances of piston to suction chamber(4su, 4sl) would decrease the cooling
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capacity and volumetric efficiency of compressor. In the paper, the effects on the cooling capacity or volumetric efficiency are calculated by the volume effect and the heating effect of leakage. The volume effect is estimated by the ratio of the leakage mass flow rate to the mass flow rate of compressor, while the heating effect is estimated by the ratio of the enthalpy increment caused by
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leakage to the suction enthalpy rate of compressor. The leakages from compression chamber to
suction chamber through the radial clearance of piston(1) and axial clearances on vane(2u,2l) decrease the actual gas compression power, i.e. indicated power of compressor, whereas the
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leakages from chambers with discharge pressure to suction chamber through the clearances of vane suction side(3s) and axial clearances of piston(4su, 4sl) do not affect indicated power of compressor directly. So even when they have equal effect on the volumetric efficiency of compressor, leakage 1 and leakage 2 still have greater effect on the indicated efficiency of compressor, which is defined as the ratio of the isentropic compression power to actual gas
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compression power in the paper, than the latter two leakages(3,4). However, it is difficult to calculate the effects of the four leakages on the indicated efficiency of compressor. So in the Table 5, the effects of the four leakages on efficiency are represented by volumetric efficiency of compressor. The refrigerant leakages to compression chamber through the clearance on vane
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discharge side(3d) and through the axial clearances on piston ends(4cu, 4cl) do not directly influence a compressor’s cooling capacity and volumetric efficiency, but increase the indicated
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power of compressor and decrease the indicated efficiency and COP of compressor. The effects of the leakages(3d, 4cu, 4cl) on indicated efficiency are only evaluated by ratio of the leakage mass flow rate to the mass flow rate of compressor in the paper since the heating effect is relatively small and difficult to calculate. The results are also shown in Table 5. On the other hand, for a rotary compressor with low pressure shell shown in Fig.4, the leakages through the clearance on vane discharge side(3d) and through the axial clearances on piston ends(4cu, 4cl) directly influence the volumetric efficiency of compressor, as the leakages through the radial clearance of piston(1) and axial clearances on vane(2u,2l) do. 13
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Table 5 Effect of leakage on efficiency of oil free compressor with high pressure shell (%) Big clearances
Small clearances
R410A
R290
R22
R410A
R290
radial clearance of piston
8.55
9.27
11.79
5.16
5.59
7.11
axial clearance on vane
1.36
1.49
1.88
0.561
0.622
0.775
clearance on vane
2.58
2.87
3.61
1.31
1.45
1.83
3.51
3.86
4.93
1.80
1.99
2.54
12.36
13.7
17.3
6.93
7.69
9.69
8.19
9.02
11.5
4.57
5.05
6.41
suction side clearance on vane discharge side axial clearance on piston to axial clearance on piston to
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compression chamber
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suction chamber
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R22
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Fig. 4 Leakages in a rolling piston compressor with low pressure shell
The leakage loss through the clearances on vane ends in the oil free compressor remains unchanged and relatively small compared with that in the conventional compressor. Except that, the leakage losses through other clearances greatly increase, as shown in Table 5. For the present rolling piston compressor, which has high pressure shell and is lubricated and sealed by oil, the effect of leakage loss on volumetric efficiency or indicated efficiency is about 3-10%. The calculated results in Table 5 show that the effect is about 36% and 20% for R22 oil free rolling piston compressor with same structure when the clearances are big and small, respectively. The 14
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mean dynamic viscosity at suction condition and discharge condition of R22 and R290 is 15.1×10-6 Pa•s and 9.11×10-6 Pa•s, respectively. So, the effect expands to about 45% and 25% for R290 oil free rolling piston compressor although the pressure difference is smaller. Therefore, to
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lower the leakage loss is one of the great challenges to develop oil free rolling piston compressor against reciprocating or scroll ones.
5. Calculation and decrease of leakage loss through axial clearances
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on piston
The axial clearances on piston ends are sealed by liquid oil and refrigerant mixture within present
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rolling piston compressor. In order to have sufficient oil to seal the radial clearance of piston, the clearance value is set bigger. But in an oil free compressor, even for smaller clearance, the leakage loss through the path could not be tolerated. If the sealing strips on two ends of piston are put up, the sealing strips would bear a great force since the pressure difference between discharge pressure and suction pressure directly act on them. The friction loss would increase. Besides, the working stability of the sealing strip will be poor since the forces on it constantly change their
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magnitudes and, especially, directions.
In the paper, the structure shown in Fig. 2 is taken to connect the cavity within piston to that at suction pressure. The pressure within piston is about suction pressure. The refrigerant leakage
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directions through the clearances on piston ends are shown in Fig.4. Assuming that the pressure in suction chamber is always the suction pressure of compressor, the leakage through the clearances
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from the cavity within piston to suction chamber will be zero. The mass flow rate and the effect on volumetric efficiency of the leakage from compression chamber to cavity in piston for small clearance group are shown in Table 6. It could be seen that the effects of the leakage on volumetric efficiency of compressor are greatly reduced. So their effects on the indicated efficiency of compressor also would be greatly reduced. Table 6 Mass flow rate and effect on volumetric efficiency of leakage of oil free compressor with low pressure shell (%)
Mass flow rate (g/s) R22
R410A 15
R290
Efficiency drop (%) R22
R410A
R290
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0.140
0.214
0.091
0.736
0.791
1.01
axial clearance on vane end
0.106
0.168
0.070
0.561
0.622
0.775
0
0
0
0
0
0
0.331
0.524
0.218
1.79
2.00
2.48
0
0
0
0
0
0
0.511
0.814
0.338
2.59
3.09
3.81
1.09
1.72
0.717
5.68
6.50
8.08
clearance on vane suction side clearance on vane discharge side axial clearance on piston ends from/to suction chamber axial clearance on piston ends from compression chamber total
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radial clearance of piston
The cavity within piston may not be connected to other cavities except those of suction chamber
and discharge chamber in cylinder. Thus, the refrigerant in discharge chamber will leak through
mass flow rate will be smaller than that in above situation.
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the clearances on piston ends to the cavity in piston and then leak to suction chamber. The leakage
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In addition, the refrigerant leaking through axial clearances on piston ends could be easily used to cool the friction pair between eccentric shaft and piston, i.e. eccentric bearing. The refrigerant leaking into piston could flow to suction muffler of compressor through a circular groove, one or several radial holes on eccentric shaft and center hole of crankshaft. The temperature of the R290 vapor will be low due to the lower discharge temperature of R290 compressor and the throttling
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effect during leakage process.
6. Calculation and decrease of leakage loss through clearances on
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vane sides
In the paper, the measure to reduce the leakage loss through clearances on vane sides is also to
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connect the cavity behind vane to cavity at suction pressure. It is easy for a rolling piston compressor with low pressure shell shown in Fig.2. The mass flow rate and the effect on volumetric efficiency of the leakage loss through the clearances on vane sides is calculated by above model and shown in Table 6. The reduction in the magnitude of the leakage loss is obvious. But the back pressure of vane is a main factor to force vane to press piston tightly for a conventional rolling piston compressor or a swing piston rotary compressor (Shintaku, 2000). However, swing piston-blade type (Masuda, 1996) and vane hinged on piston type (Okur, 2011) rolling piston compressor shown in Fig. 5 and vane hinged on cylinder (or swing vane) type 16
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rolling piston (Hu, 2013) could be adopted. These types of rolling piston compressor do not need the high pressure at outer end of vane to ensure its normal motion. These kinds of structures also eliminate or decrease the friction and wear on vane tip when there is no oil lubrication. Moreover,
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the sealing length of leakage flow path could be augmented because the vane spring hole is
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canceled.
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Fig.5(a) A swing piston-blade type rolling piston compressor
Fig.5(b) A vane hinged on piston type rolling piston compressor
7. Calculation and decrease of leakage loss through radial clearance of piston For a conventional rolling piston mechanism, it is difficult to control the radial clearance between 17
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cylinder and piston because the dimension chain of the clearance is long and the dynamic clearances of eccentric bearing and main shaft bearing affect the variation of the radial clearance (Yanagisawa, 1985a). In addition, even though the radial clearance is set to a small value (10
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microns), its leakage loss is also great as shown in Table 5. The paper presents two types of radial compliance mechanisms: swing link or eccentric bush type
and slider block or sliding bush type for rolling piston compressors as shown Fig. 6. The radial compliance mechanisms are suitable for aforementioned conventional, swing piston-blade and
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vane hinged on piston type rolling piston compressors. They are different from but basically
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similar to the radial compliance mechanisms of scroll compressor.
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Fig. 6(a) Swing link radial compliance mechanism for rolling piston compressors
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Fig. 6(b) Slider block radial compliance mechanism for rolling piston compressors
By the radial compliance mechanisms, the radial clearance between cylinder and piston will be reduced. When the gap is set to 2 microns, the leakage loss calculated by above models is shown in Table 6. The effect on the volumetric efficiency of the leakage loss through radial clearance of piston could be limited to 1%. Since the leakage is from compression chamber to suction chamber,
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its effect on the indicated efficiency will be less than 1%. However, the radial compliance mechanism for rolling piston compressor need further study since the gas force on piston changes greatly.
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8. Conclusion
Since there are many leakage paths within a rolling piston compressor, the leakage loss of the
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oil-free rolling piston compressor with present structure is high. Adopting a low pressure shell, connecting the cavities within piston and behind vane to cavity at suction pressure and using radial compliance mechanism decreases the leakage loss. By these means, the effects of leakage on the volumetric efficiency of a R290 oil-free rolling piston compressor could be limited to 8%. Considering that the force and thermal induced deformation of a R290 rolling piston compressor are relatively small and its clearances may be set to smaller values, the effect of leakage on cooling capacity and COP of oil free rolling piston compressor will just somewhat bigger than that
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of existing rolling piston compressors with oil lubrication. Compared with an HCFC or HFC compressor, a HC compressor will get more benefits from an oil-free unit because of the decrease of refrigerant charge. However, the lower pressure difference
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between discharge and suction also help develop the friction pairs of compressor. Above considerations are useful for the study on oil-free HC rolling piston compressor used for refrigerator, freezer or cooler and on oil-free rolling piston type vacuum pump or gas compressor
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with low discharge pressure.
However, the radial compliance mechanism for rolling piston compressor need further study since
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the gas force on piston changes greatly.
Reference
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Engineering Conference. Purdue University, West Lafayette, IN, USA, pp. 633~638. Ishii, N., Bird, K., Yamamoto, S., Matsunaga, H., Sano, K., Hayashi, M., 1998. A Fundamental optimum design for high mechanical and volumetric efficiency of compact rotary compressors. In:
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USA, pp. 649~654. Wu, J., Yang, L., Hou, J., 2012. Experimental performance study of a small wall room air conditioner retrofitted with R290 and R1270. Int. J. Refrigeration 35, 1860-1868.
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Highlights
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The potentials and difficulties of oil-free R290 rotary compressor are discussed. A new structure of the oil-free compressor is proposed to reduce its leakage loss. Calculating models are discussed to calculate precisely the leakage flow rate. The leakage loss can be decreased to an acceptable value by the new structure.