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
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.
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.