Centre for Compressor Technology

Our vision

To extend the boundaries of understanding of machines for compression and expansion processes, thereby:

1. enabling them to operate more efficiently over a wider range of conditions, with aim to reducing both environmental impact and manufacturing costs;
2. utilising such machines in novel configurations for improved recovery of power from low grade heat;
3. serving as an aid to industry in the design, manufacture and operation of such machines, both in existing and novel forms.

Expertise and capabilities

Funded mainly by industrial contracts, but with some input from public funding, members of the centre have developed software for improved analytical modelling of both compression and expansion processes, the use of computational fluid dynamics for more detailed studies, including solid-fluid interaction and noise reduction and test facilities for both air and refrigerant compressors and detailed internal flow measurements.

By this means, apart from its ongoing research activities, the centre is able to investigate and solve industrial problems, offer courses in machine design both to registered students and industrial engineers, license proprietary software and, if required, carry out the complete design, build and testing of prototype machines for industry.

Additionally, the centre organises and hosts a biennial International Conference on Compressors and their Systems, which is sponsored by the Institution of Mechanical Engineers (IMechE), the Institute of Refrigeration (IOR) and leading industrial manufacturers. This has been functioning since 1999 and is a leading forum for academic, research and industrial organisations concerned with the development of fluid machinery.

Centre members have been granted 12 patents, published three monographs, 85 journal papers, 166 conference publications and over 200 industrial reports, for which they have received 13 professional awards and prizes.

Key research activities

• Rotor profiling and screw machine performance calculation
• Utilisation of low temperature heat sources by using screw expanders
• Research in multifunctional screw machines
• Multistage screw compressors for high pressure difference applications
• Computational fluid dynamics (CFD) in screw machines
• Laboratory investigation of screw machine working processes
• Investigation of multiphase and leakage flows in screw machines
• Noise in rotating machines
• Multiphase flow modelling and experimentation

Services offered

• Project evaluation
• Software development
• Fault investigation
• Thermodynamic and fluid flow studies
• Stress analysis
• Instrumentation and control
• Laboratory measurements
• Noise suppression
• Complete product design and development

Software program suites

• SCORPATH – Screw Compressor Optimal Rotor Profiling and Thermodynamics
• SCORG – Unique grid generation and analysis of screw machines
• DISCO – Design integration interface for Screw Compressors

History

Following three years of collaborative work between Professor Ian Smith, who was attempting to develop screw expanders, for recovery of power from low grade heat, at City, University of London, and Professor Nikola Stosic of the University of Sarajevo, who had significant experience in the development of similar machines as compressors, the centre was established in 1995, when Nikola Stosic was awarded a Royal Academy Professorship, with industrial co-sponsorship by Holroyd, of Milnrow, Lancs.

Professor Ahmed Kovacevic, who had been working on screw compressors since 1986, joined them in 1996 and pioneered the application of Computational Fluid Dynamics to evaluate the screw machine performance.

Latterly, they were joined by Dr Elvedin Mujic, who made a significant contribution to noise reduction and who now holds a senior position in industry, and subsequently by Dr Ashvin Dhunput, who also now holds a senior industrial position. More recently, they have been joined by Dr Matthew Read. Dr Read is now a lecturer in the School of Mathematics, Computer Science and Engineering. He makes a significant contribution to integrating expanders into power recovery.

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Important contributions have been made in various aspects of application of compression and expansion technologies and educations such as:

• Patents for the “N” rotor profile in 1996 and 2013.  The “N” rotor profile is now the industry standard and has generated significant income for City.
• Trilateral Flash Cycle and Wet Steam Cycle for improving the efficiency and reducing the cost of power recovery from low grade heat sources.
• Four fully equipped laboratory test rigs for compressors and expanders, built to meet highest industrial standards.
• Holding a Royal Academy Chair in Positive Displacement Technology between 1995 and 2000.
• Introduction of the Howden Chair in Engineering Design and Compressor Technology in 2008.
• Supporting Electratherm, Reno, Nevada, USA since 2005 in their development and marketing of screw driven Organic Rankine Cycle (ORC) systems for waste heat utilisation.
• Establishing Heliex Power Limited in 2009, as a spin out company from City, University of London for use of steam driven screw expanders for waste heat utilisation.
• Establishing postgraduate courses in Mechanical Engineering at City, University of London, providing a Master of Science education to more than 300 young engineers in screw machine technology through the unique theory and laboratory courses.
• PhD students have been supervised by centre members, leading to their successful doctoral grades in area of Thermodynamics, Fluid Mechanics, Computational Fluid Dynamics, Metrology and Tribology in positive displacement technology.

Selected funded research projects

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“N” rotor profile

The City “N” profile has been developed as a result of 30 years' research and development in screw compressor technology and world patents are held on it.  It confers the following advantages over other profiles:

• Greater flow area for a given rotor diameter
• Smaller leakage area
• Stronger gate rotor lobes, which reduces lobe deflection both during manufacture and operation
• Involute form on the contact band, leading to nearly pure rolling relative motion between rotors
• Lower rotor contact forces without the risk of “rotor rattle” through local torque reversal
• Avoiding seizure in the event of rotor direct contact.

"N" rotor profile diagram. View full-size image.

Expanders

Two-phase process lubricated screw expander on test rig. View full-size image.

Expanders have a variety of applications but for positive displacement machines, their biggest potential is for two-phase processes where they may be used to replace throttle valves and for large scale power recovery from low grade heat sources.

The centre holds vital patents on the use of any type of positive displacement machine for this purpose and has developed low cost, process lubricated twin-screw expanders with adiabatic efficiencies greater than 75%; the highest value ever attained in a two-phase expansion process.

Machine design

Given the required performance data, the centre offers full design services either for a complete compressor or expander, or for rotor and porting only, given the details of the machine.  The output is then delivered to the customer in the form of drawings, which are also accessible electronically from a secure site.

The centre is also able to arrange prototype manufacture and has test facilities complying with full international standards for the testing of air and refrigeration compressors.

SCORPATH

The centre has developed an extensive range of software to aid the design of compressors, which is continually being upgraded.  The main package is known as SCORPATH (Screw Compressor Optimum Rotor Profile and THermodynamics).  Starting from the specified compressor duty and the minimum number of assumptions, this  program determines the optimum machine geometry and rotor profile to obtain the desired results.  The output includes full details of the rotors and their porting which can be imported directly into a CAD system, together with performance predictions, bearing loads and suggested choice and details of the tool profile required to manufacture the rotors.  User licences are available for this package.

Screenshot from the SCORPATH software. View full-size image.

Download the software

Computational Fluid Dynamics and Analysis

Computational Fluid Dynamics (CFD) is widely used today as an aid for optimising the design of fluid machinery.  However, due to the complexity of the geometry and other associated problems, this analytical tool was not used for screw compressors.

The centre has developed SCORG (Screw Compressor Rotor Geometry), a grid generator, together with associated supporting functions, which enables screw compressor flow and performance to be estimated when attached to commercial CFD and CCM (Computational Continuum Mechanics) solvers.  In the latter case, solid-fluid interactions can be estimated so that component distortion and its effect on compressor or expander performance can be estimated.

As in the case of SCORPATH, user licences are available for this software package.

Screenshot from the SCORG software. View full-size image.

Download the software

Use within industry

Reference list of major industrial users of the centre's research, advisory and design services:

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Potential PhD Topics

We invite high-calibre students with a passion for research to join us and study for a PhD. Potential PhD topics are outlined below. If you are interested in one of these, please contact the named supervisor or Dr Yan Youyou, Senior Tutor for Research in SMCSE. Note that there is no funding attached to these topics: applicants must make separate arrangements to fund their studies.

Full details of the application process are available online. On your application form, please state that you wish to be considered for admission to the Centre for Compressor Technology, Department of Mechanical and Aeronautical Engineering.

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