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Energy efficiency is a vital consideration in modern compressed air supply. An optimally configured compressed air supply can easily cut
energy costs by 50 %. Investing in product quality also results in a longer service life of the system. This is where the wheat is sorted from the chaff. When it comes to compressed air technology,
technical progress really does count as an additional benefit in the overall system. A modern, technically optimised compressed air station soon compensates for its higher initial investment by a reduced
life cycle cost. Additionally, reduced energy consumption is clearly the greatest return on investment.
Modern electronic controllers have become more powerful to combine both optimised and
conventional compressors to ensure the system works in the most energy efficient manner. Each compressor is programmed into these predictive controllers to ensure that the correct combination is used at
all times to provide the required amount of compressed air at the optimum pressure. This intelligent adaptation of volume and pressure optimises the energy requirement reducing stop-starts thus saving
electrical costs. Speed controlled drives also make it possible to operate continuously at the right pressure avoiding unnecessary electrical expense. Here again the additional investment cost of speed
control is quickly returned in saved energy costs.
Frequency controlled compressors come into their own when a system has fluctuating demand patterns ensuring that peak loads can be handled
efficiently. Any system with a number of fixed speed compressors in place to achieve periodic peak demands require the modern frequency controlled combination to optimise and run the system efficiently.
For this reason more and more businesses are looking to electronic speed regulation as an energy efficient solution.
So, what does frequency control mean, in respect of a compressor? How does it
function? And what, are the specific advantages?
Frequency control allows output control from 25 to 100 % whilst maintaining the exact pressure requirement. The invertor allows infinite speed
adjustment of the drive motor which in turn adjusts the compressor air end to the exact output required. The frequency convertor is designed for soft starts and stops of the drive motor. This means that
during start-up the current does not exceed its rated capacity also providing additional energy saving advantages.
If the output requirement falls below 25 % the compressor works uneconomically.
Therefore the compressor either shuts down or continues to run in idle mode. Because of the requirement to adapt to the varying air demand shut down or idle mode are to be avoided in order to enable the
compressor to work in the most efficient manner.
At the same time if the compressor is expected to work at full load the absorbed power of the invertor would be between 3 to 5 % higher than that
of a fixed speed compressor working at full load. It follows therefore that a frequency controlled compressor must operate at optimum efficiency between 25 to 90 % of its rated capacity.
Frequency
control can also provide advantages when it comes to smaller flow rates particularly where proportional regulation of suction control can be rather inefficient. Looking at the two graphs in Figure 1, it
is obvious that proportional regulation can come into its own between 85 % to 100 % flow capacity, whereas, for lower values as already identified frequency control has considerable advantages. In
summary frequency control, is best suited for use at low flow rates with notably varying demand.
Due to the wide range of adjustment in the invertor it is possible, even in the case of small air
consumption, to adapt the air delivery to the demand and avoid shut down or idle running.
Frequency controlled compressors can maintain system pressure at 0.1 bar tolerance as opposed to the
traditional 0.5 bar tolerances in fixed speed compressors.
In traditional installations with fixed speed compressors the cascading system to maintain a system pressure means that the compressors have
to generate at a higher pressure than is required in the system. Each 1 bar pressure constitutes between 6 and 10 % energy cost. Consequently by utilising a frequency controlled compressor in a multi
compressor system will save energy by generating at the optimum pressure.
The potential energy savings resulting from the use of frequency control are therefore significant.
Integrated
solutions are gaining more and more importance. Furthermore, the footprint size in the air house design is also becoming more and more critical. At BOGE, the integrated compressor system comprising of a
frequency controlled compressor, refrigerant dryer and condensate management system, all ‘under one roof’ has therefore become a big hit! As a direct result customers have a much more flexible approach
to installation possibilities.
With frequency control customers can save in three areas all at once; energy consumption, footprint and capital cost. Frequency controlled compressors are much less
expensive at BOGE, for example and depending on the model, prices are reduced by 10 to 15 %.
Advantages associated with frequency control
1. Soft starts and stops within seconds
2. Continuous volume flow regulation of 25 to 100 %
3. Flexible adaptation of f.a.d.
4. Minimized of wear and maintenance cost
5. No start up current peaks (reduced energy consumption!)
6. Constant network pressure +/- 0.1 bar (reduced energy consumption!)
7. Practically no idling operation (reduced energy consumption!)
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