Session: 02-01-02: Product and Process Design 2

Paper Number: 164049

Improving Screw Compressor Displacement and Efficiency by Increasing Rotor Profile Depth 

Abstract: This study examines the impact of rotor profile depth on the performance of screw compressors across various main and gate rotor configurations, all maintaining a consistent main rotor diameter. Both dry and oil-flooded operational modes were analysed. The findings indicate that increasing the rotor profile depth by reducing the inner radius leads to enhanced flow rates, as well as improved volumetric and adiabatic efficiencies. As a result, a new series of N and N Silent rotors has been developed incorporating this feature, which has already been integrated into numerous modern rotor designs at the Centre for Compressor Technology at City StGeorge’s University of London. 

To optimize the performance of a screw compressor, it is essential to design the rotor profile to maximize machine displacement while minimizing the length of the sealing line, which is best understood through the characterisation of the screw compressor volumetric efficiency. If the clearances have a constant size, the volumetric efficiency is proportional to the inverse of the rotor length scale. However, if the clearances are dependent upon the rotor size, the leakage losses are reduced by the length scale inverse squared and the volumetric efficiency increases with rotor size.

The actual volumetric efficiency is likely to fall between the two extreme positions. It is clear that the adiabatic efficiency of a compressor is significantly influenced by its volumetric efficiency. Assuming all other factors remain constant, it can be inferred that adiabatic efficiency is directly proportional to volumetric efficiency. It is crucial to identify which rotor dimension should be designated as the scale length L. If the outer diameter of the main rotor is fixed, increasing the number of lobes will lead to an increase in the inner, root rotor diameter, consequently reducing the throughput. In the extreme case, as the number of lobes approaches infinity, the throughput will approach zero, while the sealing line will still maintain a finite value. Therefore, for a specific rotor size, decreasing the number of lobes will result in a greater rotor profile depth compared to a configuration with a higher number of lobes. This indicates that the rotor profile depth, defined as the difference between the outer and inner radii of the rotor, serves as a more accurate indicator of the rotor scale length than the outer diameter or rotor length when assessing the impact of size on volumetric efficiency. If the centre distance is decreased for rotors with a specified main rotor outer diameter, the rotor profile depth will increase. Consequently, rotor profile depth should be regarded as the preferred criterion for profile design aimed at producing rotors with substantial displacement and high efficiency. However, this may not hold true when it is critical to preserve other design parameters, such as rotor rigidity to minimize deflection and ensuring adequate space for bearings within the centre distance.

 

 

Presenting Author: Nikola Stosic City St George's University of London

Presenting Author Biography: Nikola Stosic graduated in mechanical engineering at the
University of Sarajevo. He then obtained an MSc at the
University of Zagreb followed by a doctorate at the University
of Sarajevo, where he continued as a member of staff. There,
as a result of successive promotions, he became Professor of
Applied Thermodynamics, specialising in steam boilers. He
began working with Ian Smith at City University in 1992,
and in 1995 he was awarded a Royal Academy of Engineering
chair in Positive Displacement Compressor Technology, thus
enabling the two of them to establish the Centre for Positive
Displacement Compressor Technology. Since 2001, Professor
Stosic has been Professor of Positive Displacement
Compressor Technology at City University, London.
Professor Stosic has carried out research work on a range
of applications of heat and fl uid fl ow, including boilers,
furnaces and compressors. Arising from this work, he is the
author and co- author of over 230 peer- reviewed publications
and more than 200 major industrial reports, two textbooks
on screw compressors, a textbook on computers in
engineering which ran to three impressions in 4 years, and
a major text on boilers and furnaces with two editions in
5 years.
In addition to gaining student silver and gold medals,
Professor Stosic has received 11 professional awards and
prizes.

Authors:

Nikola Stosic City St George's University of London
Ian K Smith City St George's, University of London
Ahmed Kovacevic City St George's, University of London