Transforming the energy efficiency of gas compressors and expanders across the world
Submitting InstitutionCity University, London
Unit of AssessmentGeneral Engineering
Summary Impact TypeTechnological
Research Subject Area(s)
Mathematical Sciences: Applied Mathematics
Engineering: Interdisciplinary Engineering
Summary of the impact
City University London's patented rack generation mechanism, the `"N"
rotor profile', offers substantial improvements on the rotary screw
compressors widely used in industry at present. It allows for an increase
in compressor capacity and makes compressors more energy efficient.
Significantly, it can be easily retrofitted to existing compressors,
thereby delivering immediate benefit. The mechanism has already been
licensed to 27 major manufacturers around the world, resulting in savings
of 0.1% of global electricity consumption and an equivalent reduction in
global CO2 emissions. City academic staff have worked with an
existing firm (Howden Compressors Ltd) to raise funds and train
researchers for the development of this technology. In addition, a new
spin- out company (Heliex Power Ltd) has been formed to build on research
developed since 2009. Important examples of the impact include:
- An increase in annual turnover at Howden Compressors Ltd. from £15M in
2009 to £39M in 2011 and a 40 % increase in the number of staff
- The spin-out company Heliex Power Ltd is valued at £15M and has
employed an additional 23 people following investment of £7M from BP
Alternative Energy and ESB Novusmodus. A further £3M will have been
invested by the end of November 2013. Heliex has been named in the 2013
Global Cleantech 100 list.
- An estimated reduction in carbon dioxide emissions worldwide from
energy efficiency improvements totalling 30 Mt (0.1% of total
This research has enabled companies that operate worldwide in air
compression, refrigeration, air conditioning, oil, gas and process
industries to improve their economic and environmental performance in
response to demanding governmental demands.
Twin screw compressors are positive displacement rotary machines used in
air compression, refrigeration, air conditioning, oil, gas and process
industries. They represent approximately 80% of the millions of industrial
positive displacement compressors produced globally each year, A
derivative of the concept operates as an expander to generate power.
Compressors consume approximately 15% of the world's electricity, so an
improvement in their efficiency, however slight, can have a significant
impact on the economy and the environment.
The research on these machines at City began in 1993 with a grant from
the Science and Engineering Research Council awarded to Professor Ian
Smith (a member of academic staff City since 1962) to study power
generation from low grade heat sources, using twin screw machines.
Based on his expertise in modelling screw compressors, Professor Nikola
Stosic (a member of academic staff since 1995) developed a software
package called SCORPATH (Screw Compressor Rotor Profiling and
Thermodynamics). This work led to a full understanding of the processes at
work in screw expander machines, which can generate power from low grade
heat sources such as the expansion of liquid and vapour.
With support from the Royal Academy of Engineering and the rotor
manufacturer Holroyd, Professors Stosic and Smith established the `Centre
for Positive Displacement Compressor Technology' at City in 1996, with the
aim of assisting industry in the design and development of screw machines.
Backed by the Institution of Mechanical Engineers (IMechE), City now hosts
a biennial international conference on Compressors and their Systems which
has become a major meeting point for the compressor industry.
In 1996, building on his earlier work on the mathematical modelling of
gears, Professor Stosic developed and patented a new type of rotor profile
(the "N" rotor profile) that was fundamentally superior to existing types.
It increased the efficiency of screw compressors by at least 8%, reduced
noise generation and made lubrication requirements less onerous. The "N"
rotor profile was consequently used extensively in industry and the income
received from this and other work for industry was used to extend research
at the University on screw compressors.
Professor Ahmed Kovacevic joined City in 1998 and by 2003 had pioneered
the application of Computational Fluid Dynamics (CFD) to analyse screw
compressors. His breakthroughs in this field were later validated
experimentally by the use of Laser Doppler Velocimetry. Kovacevic created
a software suite for the analysis, called SCORG (Screw Compressor Rotor
Grid Generation), which is now widely used by many academic and industrial
institutions. It has contributed to better understanding of phenomena such
as leakage flows, noise generation in compressor ports and the effects of
thermal and pressure loads on compressor elements.
Integrating SCORG with SCORPATH, Professor Kovacevic went on to create a
new software suite called DISCO© (Design Integration for Screw
Compressors) which offers CAD tools with advanced CFD analysis. It has
become the new industry standard in this field of engineering.
In 2008 Professor Kovacevic applied his experience of designing screw
machines to form and train a team of 15 researchers at Howden Compressors
Ltd. in Glasgow. He raised around £6.5M of industrial funds within the
company to establish improved Design Principles for Screw Compressors,
boosting company turnover, profit and employment.
In 2009, research on a new system for the recovery of power from
industrial processes and engine exhaust gases, using the expansion of wet
steam in screw machines, led to the formation of the spin-out company
Heliex Power Ltd.
References to the research
1. Kovacevic A., Stosic N., Smith I.K., Mujic E. & Gueratto D.
(2011). Extending the Role of Computational Fluid Dynamics in Screw
Machines, Proc. IMechE, Part E: J. Process Mechanical Engineering,
0954408910397586 Winner of the IMechE Moss Prize 2011 for the
2. Mujic E., Kovacevic A., Stosic N. & Smith I.K. (2011). Noise
generation and suppression in twin screw compressors Proc. IMechE, Part
E: J. Process Mechanical Engineering, 225(2) 127-148 10.1177/1464419311403875
Most read paper in JPME in February 2013
4. Smith I.K., Stosic N., Kovacevic A. & Mujic E. (2011). Steam as
the working fluid for power recovery from exhaust gases by means of screw
expanders Proc. IMechE, Part E: J. Process Mechanical Engineering,
225(2), 117-126 10.1177/2041300910393429
Winner of the IMechE Ludwig Mond Prize for 2011
5. Stosic N., Smith I.K., Kovacevic A. & Mujic E. (2011). Review of
Mathematical Models in Performance Calculation of Screw Compressors
Int. J. Fluid Machinery and Systems, 4(2) 10.5293/IJFMS.2011.4.2.271
6. Stosic N., Smith I.K., Kovacevic A. & Mujic E. (2011). Geometry of
screw compressor rotors and their tools Journal of Zhejiang
University-SCIENCE A, Applied Physics & Engineering, 12(4),
The Proceedings of the IMechE are well-established peer-reviewed
journals. The IMechE Moss Prize (output 1) was awarded for the best paper
on a process industries mechanical engineering subject published by the
Institution in the previous year. The Ludwig Mond Prize (output 4) was
awarded for the best contribution made during the previous year to the
progress of mechanical engineering of interest to the Chemical Industry.
Details of the impact
City's Compressors Grouphas developed a worldwide reputation. It has
advised or assisted 68 organisations in 26 countries on compressor and
expander design.1 It is believed that more than half the
world's screw compressors, currently manufactured, contain our patented
"N" profile rotors.
(1) Economic Impacts
(a) Profiling and Design of Screw Compressors
Our 1996 "N" rotor profile patent (GB9610289) has allowed the design of
screw machines that are cheaper and more efficient, with a broader range
of applications, than earlier technologies. In many cases, the Group
designed, built and tested machines in order to validate their predicted
performance. In others licensees, after training at City, have used this
profile to design new products in conjunction with technical advice from
academic staff. It has been licensed to 27 manufacturers, including
world-leading companies. Three examples of its use since 2008 are listed
Holroyd, the original sponsors of our Compressor Centre, were
able to penetrate the Chinese market in the face of strong competition
from foreign manufacturers. This led to their acquisition by the CQME
Group, valued at $1 billion, in June 2010.2
Howden Compressors Ltd is the major compressor manufacturer in
the UK (www.howden.com). The use of
our "N" rotor profile and new design principles allowed the company to
secure the single largest contract in its history (£14M), raise its
turnover, from £15M in 2009 to £39M in 2011 and expand its staff by 40%.
Income to the University resulting from work with Howden has totalled
over £1M to date.3
RotorComp, a major German manufacturer, developed a new family
of air compressors, also based on our "N" rotor profile, and
consequently increased their sales from 19,000 units to 50,000 units per
annum between 2008 and 2010.
(b) Screw Expanders for Power Recovery from Low Grade Heat
A market survey by BP Alternative Energy International on the use of
City's concepts for power recovery found that the potential world market
was in excess of $5 billion per annum. Two examples on the use of these
concepts in practice are given below:
Industrial partnership with ELECTRATHERM
ELECTRATHERM (www.electratherm.com) is
a US manufacturer of small scale Organic Rankine Cycle Systems (ORC) for
power generation from low grade heat sources. In 2006 they were granted a
licence to use the City expander technology for power recovery in their
"Green Machine". The company has since received investment of more than
Since 2011, more than 50 "Green Machine" systems which utilise expanders
manufactured by Howden Compressors Ltd in the UK have been installed in
the field. These recently reached a cumulative 87,000 hours of operation,
thereby generating approximately 300 MWh of electrical energy so far. The
demand for such machines is rapidly rising.
New spin-out: HELIEX Power Ltd
HELIEX Power Limited (www.heliexpower.com)
is a spin-out company from City University London established in 2009 to
commercialise innovative steam screw expander technology to utilise waste
energy from industrial steam.
BP Alternative Energy International made an initial investment of £2M at
the end of 2010 to get HELIEX started.2 A further £5M has since
been raised from BP and Greencoat Capital, the investment arm of the
Electricity Supply Board of Ireland. A further £3M investment from these
companies will have been made by the end of November 2013. Pre-production
machines have been successfully tested and the first commercial machines
have been installed, including one in a steelworks in Italy. Currently the
company is valued at £15M and employs 23 people. Heliex has established a
factory at East Kilbride near Glasgow with a capacity of 2,000 steam screw
expander generator sets per year and production is under way. The company
has also built a test and development centre which incorporates 160 and
600kW dynamometers and a 0.5MW connection to the national grid. Each
generator set sells for between £110,000 and £240,000 depending on size. A
European distributor network is being established and the company is
exploring joint ventures in China (market for 1M units) and India.6
Heliex has been named in the 2013 Global Cleantech 100 list.7
(2) Impacts on the environment
According to the International Energy Agency, approximately 15% of global
electrical energy is consumed by motors driving compressors, while
approximately 80% of all industrial compressors are of the twin screw type
and would benefit from the fitting of our more efficient `N' rotor design.8
Such retrofitting would improve the energy efficiency of these widely
used devices by up to 8%, resulting in a reduction in global power
consumption of at least 0.1% and an equivalent reduction in global CO2
emissions. Based on Enerdata's Global Energy Statistical Yearbook 2013,
this accounts for around 30 Mega Tons (Mt) of CO2 per annum.9
(3) Impacts on practitioners and professional services
To maximise uptake of our analytical software, we have run training
courses for engineers and designers in more than 30 organisations over the
past few years.
For example, our work with Gardner Denver, the world's largest
manufacturer of air screw compressors, has ensured that the firm now
exclusively use our "N" profile rotors and our DISCO© and SCORPATH
software. Dr Elvedin Mujic, a former PhD student in the Group, is now a
leading researcher in Bitzer, a major German manufacturer of refrigeration
City organises a biennial International conference on `Compressors and
their Systems' which is backed by the IMechE. It is the main meeting point
of leading engineers and managers from more than 100 major world
compressor manufacturers. This serves as a forum for dissemination of
results achieved at City and has had a big impact on industry since its
inception in 1999 (http://www.city.ac.uk/compressorsconference).10
Sources to corroborate the impact
1) List of selected major research and consultancy projects of the
Compressor Centre available on request
2) Holroyd, Head of R&D - www.holroyd.com
3) Howden, CEO Compressor Division - www.howden.com
5) Applications of screw expanders video: BP Ventures - Heliex http://vimeo.com/36842177
6) HeliexPower, CEO - www.heliexpower.com
10) Biennial International conference on `Compressors and their Systems'