Over the past three decades a revolution in compressor technology has taken place.
This advance in compressor technology is affecting the vast majority of vapour compression cooling systems used for water chiller applications.
For those manufacturing these water chillers to a tight price, use of throw away technology, such as the scroll compressor, has seen a reduction in expected equipment longevity in order to fulfil a market demand for ever lower outlay costs.
Whereas heavier built chillers have tended to become larger capacity, and for which the Screw compressor has become dominant, offering a long service lifespan and increased reliability. This means a higher outlay cost, but significantly lower lifetime cost. Keeping lifetime cost down directly correlates with the lifetime impact these machines have upon the environment. A machine that operates for 20 years, and with lower energy usage than one that only serves 10 years represents twice the value from an environmental perspective.
Both Scroll and Screw compression technologies utilise continuously rotating machines and these provide a more reliable and significantly more energy efficient mode of compression than was offered by the traditional piston compressors they have largely displaced. A key advantage for these machines is simply fewer moving parts - a scroll compressor has 5 moving parts a screw compressor has just 3, and simple continuous lubrication systems, less affected by the refrigerant fluid being compressed.
However, since ~2000 further developments have been introduced to further improve not only reliability, but significantly further energy efficiency.
Part Load Operation
Most cooling equipment is selected to provide an optimum capacity at a notional arduous condition / high ambient temperature that will be rarely if ever experienced, with most water chillers now sized for 35oC ambient temperatures. For most sites they may only experience close to such temperatures for a few days, or at most 1 - 2 weeks each year.
So what happens the rest of the year ?
The cooling equipment will operate at a varying degree of part load and herein lies a significant issue from an energy perspective.
Screw compressors offer a more efficient performance generally than scroll compressors, when at full load, with the latter simply being turned on and off to provide the desired capacity, whereas with screw compressors they typically use a load modulating slide valve to effectively alter the amount of compressor capacity operating at any moment in time.
As mentioned already Screw compressors are notably more efficient than Piston compressors, but as the screw unloads its efficiency does reduce progressively away from the 100% position. Indeed as load reduces to ~ 50% it loses ~15% of its optimum efficiency, and will equalise with a typical Scroll compressor, but it will reduce still further below 40%, where it ~ equalises efficiency with its traditional rival piston compressor, losing around 25% of its full load efficiency.
This is a significant disadvantage for the Screw compressor especially when compared with the converse part load efficiency of other modern compressors such as magnetically levitated turbine compressors, where the efficiency improves as the load reduces.
Variable Speed (Frequency) Control
So what can be done to alleviate Slide Valve efficiency losses?
Variable speed inverter drives are commonly used throughout industry for pumps, fans and other motor driven applications,. Inverters are proving increasingly effective for Screw compressors also.
Using an Inverter the slide valve is held at full load, with capacity variation provided simply by adjusting the Inverter output frequency (and voltage), and hence speed / output.
Indeed several advantages are achieved...
- Part load efficiency - Positive > optimum efficiency achieved at minimum capacity
- Smaller compressors - Selection is based upon 70hz supply frequency, not 50hz
- Effective turn down ratio - Speed range 30 > 70 hz : turn ratio of 42%
- Lower run accumulation - Average speed 40 - 45 hz = longer compressor life
- Soft start - Compressors start on just 10 - 20 amps
- Soft start - Start is very gentle = longer compressor life
- Simple control strategy - Speed control is simple, effective & precise
- Retrofit technique - Inverter conversion is a bread and butter upgrade
It can be seen that the difference in ESEER provides an effective energy saving with or without an inverter, and this saving is provided for the majority of the time the chiller is operating. Yet the saving for specialised design Magnetic Turbine machines is marginal at part load, when compared with an Inverter driven screw compressor.
Screw compressor conversion has been well proven as an effective life-cycle upgrade option for most chillers > 100 kW capacity.
Retrofit can also take account of other energy improvement techniques, e.g. Control electronics, EC Fans, etc, and also can account for the changing situation with refrigerants. With HFC refrigerants entering their phase down period, increased emphasis upon Global Warming Potential and carbon tonnes means that existing plant can be made not only more GWP compliant, but further significant energy gains can be achieved.
Notably R134a is being used with huge success to replace R407C, achieving an 8% energy improvement, additional to the savings being made form Inverters. R134a already has field proven HFO (low GWP) replacements both commercially viable and available.
This might seem daunting, but has become a routine procedure. Many projects are achieving a Payback within just 2 - 3 years, with our record ROI project achieving payback within just 9 months on an £80 k project.
Low Carbon Project Funding
Any project achieving such fantastic payback can be funded from specialist loan providers, whereby the project is funded from the energy savings, and with Carbon Trust Accreditation approval for funding is straightforward.
If you have water chillers and are concerned about longevity, efficiency and reliability then you can contact ThermOzone Ltd …
Kelvin House, Preston Road, Reading, RG2 0BE
Telephone: 0118 918 1400
ThermOzone are an Accredited Supplier for the Carbon Trust, with their manufacturing bases in the UK at Reading and Nottingham