Case Study: ACR News Submission for July 2017

Energy Effective Compression - ThermOzone

Over the past two decades the Refrigeration and Air-Conditioning industry has had to face up to some enormous challenges to deal with perceived environmental issues

- Ozone Depletion - with the resultant phase out of CFCs, then HCFCs and now even the HFCs that replaced them

- Equipment Reliability - the rapid development of the refrigeration industry throughout the 1980’s and 90’s led to a massive increase in use of refrigeration, but this tended to leave effective product development behind, however the past 10-15 years has seen the industry catch up.

- Energy Efficiency - whereas much equipment from the 1990’s was reliable, efficient use of electrical power was less prominent a concern of equipment designers and users.

- Global Warming Impact - more recently concern for the environment has switched from Ozone depletion to Global Warming affect the gases we routinely use has. This has led to the now familiar F Gas Directives and the measures forcing us all to improve the loss and leakage of refrigerant gases into the atmosphere.

Vapour Compression still King

Throughout this period the base operating principles for the vast majority of cooling systems has not changed very much. Primary cooling still relies upon the Vapour Compression system of refrigeration, however modern technology has improved energy efficiency, but perhaps less so equipment longevity.

The piston compressor which dominated refrigeration for ~ 100 years has now been relegated to the sidelines, with only Carbon Dioxide plant more reliant upon it, along with small domestic equipment, for which there isn’t much scope for development anyway.

Into the piston compressor’s place have grown two now established modes of compression - Scroll serving the smaller to middle end ~1 - 250 kW, with Screw compressors serving the larger end, ~100 - 2,000 kW. n

Rotary Machines

Both technologies have a great deal in common - both are a form of helical mechanism, one way of considering the scroll is as a flattened screw, and importantly both offer continuous forward motion and gas flow, without the re-expansion losses, prevalent in piston compressors with each stroke of the pistons.

Both have considerably fewer moving parts, and can utilise modern rolling element bearings to support the rotating shafts offering a longer life and reliability.


Part Load

In their raw form neither screws nor scrolls are particularly efficient for partially loaded systems, and for most equipment that means a majority of the time. Cooling equipment is generally sized to provide a nominal cooling capacity in the most arduous conditions expected in the height of Summer, yet this is mostly jump for a couple of weeks in July or August. The rest of the time the systems are not working as hard.

nan 2000 a newcomer has arrived serving the chilled water industry in the form of a compact centrifugal turbo compressor offering almost unrivalled efficiency, using magnetic levitated bearings, and consequently no oil. Importantly these compressors offer improving energy efficiency at part load, whereas scroll compressors were more or less fixed efficiency ~ 3.5, and screw compressors, with their slide valve mode of control showed the opposite characteristic of becoming less efficient as load reduced.

Slide valve load control in screw compressors is also technically challenging for the associated controller, and often chillers can operate in an unstable mode, hunting up and down instead of achieving equilibrium control of a system and its thermal load.

Screw Compressor with Variable Frequency Drive Inverter

However, the Screw compressor has come of age with the advent of large Variable Frequency (Speed) Drive Inverters capable of controlling high torque, large capacity induction motors.

This has allowed the Screw compressor to achieve much the same part load characteristic as the magnetic turbo compressors. However, for the screw compressor this means a far wider scope of application rather than a limited operating application range for magnetic turbos which generally require a constant base load and are only useful for water chiller applications.


Screw compressors with inverters operate with very low start up current surge, and can achieve part load efficiency >6, but are happy to be stopped and re-started in conditions of light load demand as many as six times per hour. Screw compressors offer a long service life, and can achieve 75 - 100,000 run hours with regular servicing at 25-30,000 hour intervals. Indeed many screw compressors can be effectively serviced ‘in situ’.

For smaller range equipment the Scroll compressor offers a lower cost for primary assembly bulk procurement, and is generally considered throw away at failure after ~ 6 - 10 years, although service replacements are typically ~ 50% more expensive than screw compressors when considered against comparable cooling capacity.

Scroll compressors of which the majority are fully hermetically sealed, do offer an advantage with higher pressure refrigerants such as R410A and even R32, but notably such equipment generally has a lower effective life span due to the stresses imposed by operating at higher pressures causes for the rest of the equipment, so in this regard a typical 8 year lifespan matches that offered by the compressor, although some will find having to renew expensive and often large plant every 8 years or so a somewhat expensive exercise.

Screw compressors are better assigned where longevity is concerned with typical well built screw equipped chillers offering service life spans of 15-20 years.


Screw compressors are also effective for retrofit to older systems using either piston compressors or multiple scrolls where reliability and / or energy efficiency issues are prevalent, and even the odd site still operating R22 equipment, where screw retrofit has been thoroughly proven as an effective recycling techniques.

Case Study: Servicing Air-Conditioning on Modern Trains

Air Conditioning Servicing and Maintenance - ThermaCom

Modern train carriages are effectively sealed capsules. Maintaining a comfortable environment is essential and is the principal reason for passenger complaints after late trains.

For the well known Virgin Pendolino’s serving the West Coast Mainline air-conditioning is provided by a module situated atop the carriage, forming part of the roof structure. At the heart of the module is a Screw compressor, providing the driving force for the refrigeration cooling system. This compressor reliably provides an equivalent to around a million track miles service.

The expected lifespan of a typical train car is at least 25 years and during this time it will travel many million miles and will accumulate some 150,000 track travel hours. During this travel period the compressor will be operating almost continuously.

train 1


Screw Compressor Service Life

The Screw compressor used in the Pendolino AC module is manufactured in Germany. As the carriage is sealed, it maintains the re-circulated & make-up airflow at a comfort temperature around 22oC. However, like any machine with rolling bearings, it has a finite service life, and typically the compressor will provide reliable service for around 7-8 years or 50,000 run-hours.

In this way the train makers have designed a very successful and effective cooling system, but it is clear these compressors will not provide service for the lifespan of the carriage itself, indeed achieving an estimated 50,000 run hours is astounding. Comparatively compressors of similar design typically expect to achieve around 25,000 hours in other industrial applications.

train 2


Service Life Extension

The common service failure of a Screw compressor is primarily due to bearing fatigue. The problem for the maintainer is knowing just when that fatigue life-point is approaching. Get it right and change these bearings before failure and the service life is effectively renewed. However, once the first bearing fails, total failure is not long away, and the time period from initial distress to complete failure is comparatively short, as little as a few days or hours operation, and for machines intended to operate away from direct engineering scrutiny for many months at a time, this presents a major challenge, especially if inconvenience to the train user is to be avoided.

ThermaCom have been anticipating such a fate for the Pendolino rolling stock since 2005. These trains first entered service around 2002, and by 2009 we were observing an increase in service failures and nature of such failures increasingly showed our predictions that these compressors could all reach life-point in an alarmingly short period.

This was not good news for the train maintainer, who is required to provide a high level of serviceability to the train operator. Random failures are at best inconvenient, but when they start happening on a weekly then daily basis then what in the earlier days was an occasional drama truly becomes a crisis.

Quality & Procedures

The Pendolino Fleet comprises some 500 individual carriages, and the whole upgrade project has been timed to conclude over an 18 month period. For ThermaCom this has meant a rigorous training procedure to satisfy the exacting standard of the Railway, streamlining of supply lines for Bearings, and other parts. To comply with the rigid safety and compliance standards expected the whole programme is documented and fully traceable.

Operating Safety and Reliability are key for train operators. At ThermaCom we provide the highest levels of overhaul reliability: Tooling, Calibration, Welding to EN standards and most importantly a diligent training regime of our fitters, with dedicated overhaul procedure for the specific compressors.

train 3


Full Simulation Test Facility

ThermaCom have created a refrigeration based test facility. This is adaptable to force compressor operation in the most arduous Summer operating conditions, with computerised data records of a procedural pass on all compressor.

Remediation Techniques

A significant proportion are found at or approaching final stage failure. Some are beyond repair, with a further volume suffering loosening of the original bearings such that secondary failure faults would wreck the compressor in short order, but crucially ThermaCom have developed remediation techniques to recover these compressors. Observing that around 25% of the volume would fail within months has borne out the decision for the overhaul investment, and has avoided the train maintainers having to process costly AC failures from the trains in service.

train 4


At the time of writing the Pendolino compressor overhaul programme is halfway through, and the benefits of following an established procedure are self-evident. The throughput is ahead of schedule, and proving highly effective for these trains. The remedial procedures for machines that would otherwise have failed has saved the maintainers a huge sum against the alternative for new compressors and reliability is well proven.

Environmental and Cost Benefits

These area are not complicated every compressor is qualified to meet its original design specification those that have failed or beyond economic remanufacture are held and where appropriate the parts are qualified and again held for use or disposed off.

The end results are a pool of compressor going back into services available to cover tens of thousand of hours and 7-8 years available use. Each compressor is static and dynamically tested in a bespoke test equipment. Not just tested for function the feed back on the failure of the original compressor are built into the test to enable a high quality and reliable machine.

train 5


The result 500 compressor returned to a full design specification a reduction of 60% on compressor cost the largest saving throughout the total maintenance project and a project given a rotation of materials allowing the flexibility required for the train availability for maintenance. The project team gain valuable knowledge enabling diagnostic and incite to compressor failure given a new prospected of there maintenances program.

Case Study: Chiller Repair & Refurbishment

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Happy New Year

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