Optimising a compressed air system
Published: 18 February, 2020
Central controllers are the backbone of an optimised compressor installation, managing the whole compressor room, matching the air supply to demand while offering pressure stability, and contributing significantly to energy efficiency. Ken Revell, business line manager, compressor technique service division, Atlas Copco Compressors UK, says it is a proven fact that a 1bar pressure band reduction can achieve as much as 7% in direct energy cost savings.
To take full advantage of optimisation benefits, it is necessary to look beyond individual machines and consider the compressor installation as a whole. It is essential to be prepared by establishing demand patterns, detecting and rectifying weak points such as air leaks in the supply system, and defining potential areas for improvement.
Few air systems operate at full load all the time. Part-load performance is therefore critical and is primarily influenced by the compressor type, the air demand profile, and the control strategy.
A simple control system may be appropriate to a facility with a single compressor and a very steady air demand. However, a more complex system with multiple compressors and dryers, varying demand, and many types of end users, will require a more sophisticated strategy of compressor control since this function is the important factor affecting system performance and efficiency.
Put simply, with a compressed air system comprising more than one compressor or dryer it may be difficult to determine which unit to run and when. This dilemma is especially true if demand fluctuates daily. A multiple system that isn’t properly controlled risks the production of too much air, which in turn has a negative impact on production costs. Even a compressor running unloaded uses up to 20% of the energy consumed at full load.
Sequence control
For example, in a system that relies on start sequence control the load/unload pressure of each compressor can be set to react to changes in air demand and, if the system pressure drops, an additional compressor will switch to loaded running. However, the sequence will always be the same, and any economic advantages offered by VSD-powered compressors in the installation will not be fully utilised. It is likely that there will be relatively large steps between different compressors’ loading and off-loading which means the system operates across a relatively broad pressure band. The net result will be higher pressures than required, too much unloaded running and a punitive energy bill.
When it comes to compressor co-ordination, conventional network controls use the cascade set point method to operate the system as a whole. Although this procedure avoids selecting part-load compressors, it still presents the problem of approaching a minimum pressure requirement as more and more compressors are added and the range of compressor load and unload set points increases.
Optimised performance
More sophisticated network control systems use single set-point logic to make their operational decisions to start/stop, but the ultimate solution is a centralised controller that can supervise and orchestrate a complete compressed air installation irrespective of compressor type, equipment brand, and plant room location. Such an intelligent system will be capable of linking all compressors and dryers, lowering overall pressure band, optimising performance, and boosting energy efficiency.
With a sequence controller, if pressure drops it will select the next compressor in line regardless of capacity. An optimised unit will select the best machine combination to deliver the required air output in the most efficient way. If the pressure drops rapidly, it will select the largest capacity compressor(s) in the system to compensate. If pressure drops slowly, a smaller rated unit will be triggered to balance the required pressure level.
Optimised central control
Here is how a multiple compressor installation benefits from optimised central control:
Flexible compressor sequencing – Controllers ensure that compressors work in groups and adjust to meet demand. In a fixed system, there might be a specific order in which units are employed to meet air demand. In a flexible system with a central controller, the optimum combination of machines is assured.
Workload equalisation – By spreading the workload over multiple compressors, a central controller avoids unnecessary overloading of individual machines. A controller can also prioritise the selection of newer, more economic compressors over older, and less efficient versions.
Continuous use of VSD machines – Without doubt, VSD compressors offer the most energy-efficient solution for variable load operations. A central controller ensures that in a mixed unit installation VSD compressors are prioritised over baseload or fixed-speed compressors.
In multiple VSD compressor installations, equaliser controllers will maintain compressors in their optimum performance zone, but advanced optimising controllers are able to keep the VSD units to their precise optimum performance point.
Regulation of system pressure – With a central controller, machines can operate within a pre-defined and narrow pressure band. This allows the system to meet demand while still optimising energy efficiency. In a multiple compressor installation, the controller regulates the system pressure by starting and stopping compressors and controlling VSD speed. Thanks to the controller’s built-in intelligence, compressor performance is optimised in all conditions. A tighter pressure band is achieved, resulting in lower energy costs and increased system stability. Multiple pressure bands can be set to avoid wasting energy during non-productive hours and stable pressure contributes to sustained production quality. System status and operating parameters of each connected compressor can be presented on a clear interface at all times.
Shutdown scheduling – Central controllers can help operators schedule maintenance periods for non-working hours to avoid incurring extra costs. Scheduled shutdowns should be part of any predictive maintenance plan.
Fewer service visits – By utilising flexible machine sequencing, workload equalisation and shutdown scheduling, central controllers help machines reduce premature wear. This leads to fewer service visits, and, subsequently, lower service and maintenance costs.
Production security − System pressure not only depends upon the compressor output, but also on other components such as filters, dryers, and receivers Without the system pressure control offered by a central control unit, there is the risk that pressure anomaly alerts may be overlooked and application process output affected as a consequence.
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Advanced controllers
In recent times there have been major advances in the concept and capabilities of centralised controllers. Depending on needs, users can benefit from conventional compressor room sequence control or take the opportunity to opt for the top-of-the-range energy optimisers. These provide advanced priority management, smart machine selection based on continuous monitoring of air demand, control of ancillary equipment, as well as logging and export of critical data.
Atlas Copco says it Optimizer 4 controllers, the successor to its ES units, offer opportunities to select the most efficient combination of compressor units at defined working pressures while balancing unit running hours for all types of rotary stationary compressors − VSD, fixed-speed, oil-lubricated, oil-free, turbo or low-pressure. Connectivity is not confined to same brand units or to a single location within a production site, enabling remote control via Wi-Fi, Ethernet, communication bridges and repeaters.
In support of the of users’ ISO 50001 energy management system continuous improvement system targets, the latest optimiser controllers enable LAN and SCADA system connection to any device for data control and extraction, together with local visualisation and analysis of energy consumption and efficiency. What is more, advanced central control optimisers are designed from the outset to meet the smart factory’s future Industry 4.0 connectivity requirements.
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