An aggregates site has achieved a 61 per cent energy saving on compressed air at DSM plants by using an EnergAir intelligent air management system
A compressed air audit conducted on a Dry Silo Mortar (DSM) production site in Scotland revealed that 56.07 per cent of the input energy used was actually non-productive — the result of an inefficient, but common cascade control system, originally installed to manage three 75kW Atlas Copco compressors. The audit also highlighted that using three compressors of the same capacity did not allow the system to efficiently match compressed air output to demand.
To solve the problem an EnergAir Metacentre compressed air management system was installed: added to this was a variable speed drive (VSD) and a change in the compressor line-up to include a smaller 30kW machine. Offload energy usage was reduced by over 90 per cent: when combined with additional operating efficiencies achieved by the system, total energy savings have reached 61 per cent, or 182,728kW/h.
Said compressor control specialist Peter Tomlins, of EnergAir: “The savings made at the aggregate site have prompted the business to install EnergAir’s Metacentre management system at all of its UK DSM sites. There are also three more sites to be completed in other sectors within the business. The beauty of this system is that we can accurately audit a site prior to installation so we know what savings to expect. In every case so far, the actual savings have matched or indeed exceeded our initial calculations.”
Expanding DSM production
This particular aggregate producer is currently investing more than £20m over five years in the strategic development of its national mortar business, including the expansion of DSM production. Each of the DSM plants is capable of producing between 400 and 800 tonnes a day, or 100,000 to 200,000 tonnes per annum. Flexible production capacity means that they can be highly responsive to customers needs and tend to run close to 15 hours a day, making energy efficiency close to the top of the operational agenda for each site.
There are a number of reasons that air compressors pose such a problem for energy efficiency, says EnergAir. Many sites still use fixed-speed compressors operating on a cascaded pressure switch control system. While this method is seen as the norm for running compressors, it is massively inefficient. And with relentless energy price rises it is essential that sites work to develop more efficient methods.
When operating fixed speed compressors with a cascade control system the exact requirement of compressed air for a plant often falls between the combined capacities of the installed compressors — as one compressor’s limit is reached, the output is then doubled as the next compressor comes online. This arrangement uses far more energy than is required and leads to an increase in off load running, with the compressor system creating more pressure than is needed.
On the audit trail
In total, the compressed air audit conducted at the aggregate site ran continuously for 311.4 hours, with a high-frequency data log rate of every five seconds. The site consisted of three 75kW compressors, all fixed speed. Of the total energy consumed by each compressor, 56.07 per cent of this energy was ‘non-productive’ — adding thousands of pounds to the sites energy costs.
Site compressed air demand often fluctuated depending on the time of day, week and even year. This meant that with the fixed-speed compressors operating with a rigid cascade method the site was rarely running at the ideal capacity in terms of efficient generation.
There were two recommendations followed to improve the running of the plant, Combined, they were able to reduce energy costs by well over 50 per cent.
Two major changes to the existing compressor line-up were made. The first was to install a retrofit variable speed drive, giving a fixed-speed compressor variable speed capability. This allowed compressed air generation to be varied more precisely, without reducing compressor efficiency. The second step was to exchange one of the 75kW compressors with a 30kW compressor. The demand profile that had been recorded during the audit showed a large gap between the varying demand of the plant and the generation capacity. A smaller compressor, combined with the new variable speed capacity of one of the original compressors ensured that air generation could be matched closely to demand, making huge gains in energy efficiency.
Sensors provide pressure feedback
Finally, a new EnergAir Metacentre compressed air management system was installed. This system is connected to sensitive pressure sensors on the air ring main which provided feedback on system pressure; it uses this data in real time to assess demand, while controlling all three compressors. The system selects the most efficient combination of compressors and compressor speed to accurately match demand.
Now installed, the management system also provides maintenance updates every few seconds, helping maintenance teams to identify faults in the compressor system far quicker than normal and reducing the risk of running a faulty compressor for many hours.
EnergAir's Metacentre system keeps air pressure at the required 6.0 bar — a reduction from the 7.5 bar it was running at before. Coupled with the correct use of the VSD, the site has now reduced its offload (non-productive) energy usage to just one per cent rather than the previous 56.07 per cent. With this dramatic increase in efficiency the total potential energy savings on this site amount to 182,728 kW/hrs based on a 3000-hr year.
Not only has this installation helped to reduce non-productive energy, in turn helping the site to reduce its carbon footprint, but with the savings of over 50 per cent in energy costs the Metacentre management system will have paid for itself far sooner than was expected, and has reduced maintenance costs significantly.
