Volumetric Efficiency Improvement by Overflow in Rolling Piston Compressor

Wen WANG Gensheng HUANG Yilin ZHANG Shanghai Jiao Tong University  Background

 Thermodynamic cycle with overflow

 Theoretical analysis of overflow

 Discussion on rotary compressor

 Conclusion

July 13, 2014 Footer Here 2 1. background

 mini cooling system, movable cooling system, electronic components cooling; micro climate ; compact, mini and micro cooling  micro and meso compressor; Compressor minimization

Minimized machine, high surface - volume rate；lubrication and leakage; high compression ratio compressor @ heat pump。

 Clearance volume is hard to reduced  High volumetric efficiency • Clearance volume In fact，the clearance volume is inevitable。

Clearance volume

Cylinder displacement

VVcsVV14 3  V 4 V  11 VVVh13 VVV 133 V 11  1111c nn     Vh    clearance volume always is important on the performance of compressors; Especially operating with high compression ratio, the effective cylinder displacement tends to decrease. For compact and minimized compressors, the relative clearance volume is larger compared with the conventional scale compressor. Decreasing the clearance volume is limited by compressor’s structure and manufacture accuracy and assemble clearance.

Combined the structure with operation, we focus on how to use of the high-pressure gas in the clearance volume to improve the volume efficiency of multi-cylinder compressor. Rolling piston type rotary compressor with 2 chambers (clearance ratio 3.7%)

Evaporating T 5Ԩ, condensing T 50Ԩ 2. Thermodynamic processes with overflow

• What is Cross flow ?

The multi-cylinder compressor has more than one cylinders, and each cylinder run same stroke by turn with a certain phase difference. One cylinder is at the end of the exhaust process, while one of the other cylinders must be at the end of the intake process or at the initial stage of compression process. The high-pressure gas in the clearance volume could be conducted into the compression cylinder via a overflow channel.

Dankwart, E. Rotary piston compressor, United states patent, 1984, 4487561. The process 1–1′ and 1″–1′ represents the mixing process that results in the rapid rise in pressure of the compression cylinder through the overflow channel. Lines 3–3′ represents the process of the pressure rapid reduction by overflow. The stroke volume or cylinder displacement is enlarged effectively.

(a) Even cylinder compressor (b) Odd cylinder compressor 3. Theoretical analysis of overflow

V P   3   2 VV 13 P1

1 Ve n volumetric efficiency V  11  Vh 

1  n cooling capacity and COP qmheva14213 h eva V  V h eva 14  h eva In a cooling cycle 

q  hh  COP 21eva eva 4 W 1 C n n p1   n 1  The ratio of volumetric efficiency, cylinder displacements and COPs between the compressor with overflow and the conventional compressor are expressed as: 1  1 1 1  n  11 1 n 1 n n  V           V d  COP  1 1  V  11 V n d n COP 1 11    1n nn    1  

1n 1n 1n V3  V1                   V 1  1 V1      Sample design

Parameters of the compressor for discussion

Cavity radius 12mm Roller radius 10mm Cavity thickness 11mm Vane thickness 3mm Suction port radius 2.5mm Discharge port radius 1.5mm Suction port volume 96mm3 Discharge port volume 80mm3 Cylinder displacement 14315mm3 Clearance volume 150mm3

Conclusion

 Overflow is a method to reduce the mass in clearance volume and improve the volumetric efficiency of a compressor. In a rolling piston type rotary compressor, the overflow is possible to realize by specially designed structure. Once the overflow is employed, the volumetric efficiency would be increased as well, especially in high pressure ratio operation, the more the pressure boost in compressor and the more the clearance rate, the more the improvement by overflow.

 In this paper, based on the compressor and with pressure ratios 3.8 to 5.4 in this paper, the overflow could provide 6%-10% improvement rate of volumetric efficiency with 1% clearance rate and, and 6%-12% improvement rate of volumetric efficiency with 3.7% clearance rate.

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July 13, 2014 Compressor Short Course 16