Efficient compressor control
May 1st 2006 Philip McArragher, Technical Manager for HPC Compressed Air Systems, explores how efficient compressor control techniques can reduce energy costs
Despite all its benefits, compressed air is a relatively expensive energy medium. This means that cost savings should be investigated and considered whenever and wherever possible. In many applications one of the main causes of increased costs is a mismatch of compressed air delivery to fluctuating air demand. Often, the duty cycle of a given compressor is only 50%. A large number of users are not conscious of this fact because their compressors have an indicator showing only the hours in operation but not the hours under load.
Well matched control systems can help by increasing the load factor to over 90%, achieving power savings of up to 20% and more.
Internal control systems
There are two standard internal control philosophies for screw compressors; loaded/unloaded control and variable frequency drive.
Loaded/Unloaded control: Most compressors are fitted with three-phase asynchronous drive motors. The permissible starting frequency of these drive motors is lower the higher the drive power. It does not correspond to the starting frequency necessary to cut in and cut out compressors with narrower starting differentials that meet the actual air demand.
These switching cycles would only unload the pressurised end of the compressor. The drive motor, on the other hand, must carry on running for a certain period to avoid exceeding its starting frequency. The power needed to turn the motor during this off-load period must be regarded as a loss. The power consumption of a compressor switched to off-load running is still 20% of full load drive power.
Variable frequency drive: The efficiency of compressors that are speedcontrolled by a frequency converter is not constant over their whole range of control.
In the control range between 30 and 100% it reduces from 94 to 86% for a 90kW motor, for example. Added to this are the losses in the frequency converter and the non-linear power characteristic of the compressors.
If variable speed compressors are wrongly used, they can turn into power eaters without the user being aware of the fact. This means that variable frequency drive is not a universal remedy in the search for maximum efficiency and energy-saving operation.
Classification of air demand
Generally, compressors can be classified according to function into base load, medium load, peak load and standby units.
The base load air demand is the volume of air constantly needed by a production facility.
In contrast, the peak load is the volume of air demanded at certain peak load times. It varies because of the differing demand from various consumers.
To meet the diverse functions of load as well as possible, each compressor must be controlled differently by an internal controller. These slave controllers must be capable of upholding compressor operation, and, therefore, the supply of compressed air should a defect occur on the master controller.
Master controllers
Master controllers coordinate the operation of the compressors in a compressed air system and cut the compressors in or out according to the air demand, for example HPC's SIGMA Air Manager system.
System splitting is the division of compressors of equal or differing capacities and type of control according to base load and peak load air demand of a production facility.
Coordination of compressor operation is a demanding and comprehensive task.
Modern master controllers must not only be able to put compressors of differing make and size into operation at the same time, they must also be capable of monitoring the system for maintenance purposes, balancing the operating hours of the machines and recording malfunctions to bring down servicing costs and increase reliability.
Correct grading
An important condition for an efficient, power-saving master controller is perfect grading of the compressors. The sum of the air capacities of the peak load machines must therefore be larger than that of the next base load machine to be cut in. If a peak load machine with variable frequency drive is used, the control range must be larger than the capacity of the next compressor to be cut in, otherwise the efficiency of the compressed air supply cannot be guaranteed.
Safe data transfer
Another important requirement for perfect function and efficiency of a master controller is safe data transfer. It must be ensured that messages are transferable between each compressor and between the compressors and the master controller.
In addition, the signal paths must be monitored so that faults such as loss of continuity in a connecting cable are immediately recognised.
Normal transfer methods include voltfree contacts, analogue signals of 4 - 20 mA or electronic interfaces, e.g. RS 232, RS 485 or Profibus DP.
The most modern method of transfer is the Profibus. This bus can pass large volumes of data over long distances in a very short time (illustration available). This means that master controllers do not have to be located in the compressed air installation itself.
Analysis reveals potential
Only a thorough air system analysis can obtain the information necessary to optimise system performance. For example, compressor manufacturers like HPC Compressed Air Systems can perform an Air Demand Analysis where the compressed air demand over time and actual system pressure against required system pressure over time can be measured. It is important to realise that each application and compressed air installation is different and has to be treated accordingly. The path to maximum energy savings is the efficient and proper application of compressor control systems.
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