Analysis of the working process in an oil-flooded screw compressor by means of an indicator diagram

Abstract

This paper presents the results of an experimental investigation of the thermodynamic processes in an oil-flooded screw compressor. The pressure within the working chamber is measured with a small pressure sensor, embedded in the female rotor on the discharge side. The results so obtained were transformed into an indicator diagram. Based on the indicator diagrams at various operating conditions, the working process is analysed. Owing to oil restricting back-flow of the gas through the discharge port, constant-volume compression is not evident even at substantial under-compression conditions, thus making the compressor maintain a high efficiency over a wide range of pressure ratios. However, the additional power consumption resulting from over-compression is comparatively large at pressure ratios lower than that for which the compressor was designed. So a compressor with fixed volume ratio should be designed with the built-in volume ratio low enough to avoid this effect. At the end of the discharge process, the pressure rises steeply due to increased resistance to oil flow, and therefore, the design of a flow-guiding slot to assist oil discharge is recommended. Higher rotating speeds increase the pressure slightly, but the losses associated with this are compensated by the increased volumetric efficiency. Thus a screw compressor maintains its isentropic efficiency over a wide range of speeds. Larger oil:gas ratio presents a higher pressure level in the indicator diagram, offsetting the improved sealing effect, and test results show that the appropriate oil:gas mass ratio range is between 8:1 and 20:1.

Keywords

screw compressor oil-flooded indicator diagram working process pressure sensor

Get full access to this article

View all access options for this article.

References

Sangfors

Computer simulation of the oil injected twin screw compressor. Proc. 7th International Compressor Engineering Conference at Purdue University, Lafayette, Indiana, USA, 1984, 528–535.

Fujiwara

Kasuya

Computer modeling for performance analysis of rotary screw compressor. Proc. 7th International Compressor Engineering Conference at Purdue University, Lafayette, Indiana, USA, 1984, 536–543.

Singh

P. J.

Patel

G. C.

A generalized performance computer program for oil flooded twin-screw compressors. Proc. 7th International Compressor Engineering Conference at Purdue University, Lafayette, Indiana, USA, 1984, 544–553.

Stosic

Milutinovic

L. J.

Hanjalic

Kovacevic

Investigation of the influence of oil injection upon the compressor working process. Int. J. Refrig., 1992, 15 (4), 206–219.

Fujiwara

Osada

Performance analysis of oil injected screw compressors and their application. Int. J. Refrig., 1995, 18 (4), 220–227.

Hanjalic

Stosic

Development and optimisation of screw machines with a simulation model, part II: Thermodynamic performance simulation and design. ASME Trans. J. Fluids Engng, 1997, 119 (9), 664–670.

Fleming

J. S.

Tang

Cook

The twin helical screw compressor, part 2: A mathematical model of the working process. Proc. Instn Mech. Engrs, Part C: J. Mechanical Engineering Science, 1998, 212 (5), 369–380.

Miyoshi

Analysis of screw compressor performance based on indicated diagrams. Proc. 11th International Compressor Engineering Conference at Purdue University, Lafayette, Indiana, USA, 1992, 259–268.

Haugland

Pressure indication of twin screw compressors. Proc. 10th International Compressor Engineering Conference at Purdue University, Lafayette, Indiana, USA, 1990, 450–456.

10.

Peng

Xing

Cui

Shu

Research on working process of oil-injected twin-screw compressors based on recording of p-V diagrams. IMechEng Conference on Compressors and Their Systems, London, UK, pp. 105–110.

11.

Xing

Screw compressors: Theory, design and application (in Chinese); 2000, pp. 71–78 (China Machine Press, Beijing).