Obiettivo
The aim of the project is to fit precoolers to a centrifugal air compressor system. Three 500 kW compressors provide 7 bar air to an aluminium plant. Precooling air to one compressor will increase its capacity by 25 %. Allowing one of the other machines to be turned off. A smaller screw compressor will supply the rest of tae air required. Innovation relates to the first application of precooling on a centrifugal compressor.
The present was justified on a rationalisation of the air supply which involved pre-cooling the inlet air of one centrifugal compressor to -28 deg. C and installing a pre-cooled screw compressor for the instrument air.
This was later changed to include a low pressure blower installation as the total air demand increased to 145 m3/min.
By supercharging the centrifugal compressor from 76 to 90 m3/min., this would enable the operation of only one machine instead of two, producing an energy cost saving.
A.A.L. are working on a Strategic Business Plan that will hopefully push up the production capacity by 60% from original plant design. This highlighted a number of inadequacies in the air supply. Presently there are two centrifugal compressors operating with a demand averaging 150 m3/min. and peaking at 175 m3/min. In order to allow for future growth the air supply capacity needs to be 180 m3/min.; this compares with the original project at 105-114 m3/min.
During the re-evaluation of the project certain concerns needed to be addressed namely:
- Possible icing damaging compressor blades.
- Motor capacity under pre-cooled conditions.
The possibility of ice reaching the compressor and damaging the blades was given serious thought. Tests on centrifugal machines under sub zero ambient temperatures indicated that this was not a problem, however, as these were only short runs the concern remained leaving doubt for operation under high ambient humidity conditions. A satisfactory conclusion was reached when it was decided that a temporary filter could be installed in the intake to the compressor downstream of the pre-cooler. This could be removed after the proven satisfactory operation of the plant.
Regarding motor capacity : a detailed appraisal indicated that the compressor drive motor was not large enough to handle the additional load arising through pre-cooling. The measured electrical power is currently 486 kW at an ambient temperature of approximately 20 deg. C. By pre-cooling down to -28 deg. C, the calculated power will rise to 541 kW shaft power. The existing motors are rated at 500 kW at 20 deg. C and have a service factor of 1.15 ie. they are rated at 575 kW at 30 deg. C. The motors are manufactured by Lawerence Scott and cannot operate continuously at more than 50 kW.
A review of the local meterological records indicated that ambient temperatures seldom exceeded 23 deg. C in any given year.
Therefore, it was considered possible to accept a risk and operate with the existing motors, provided the compressor load was automatically reduced by limiting the degree of pre-cooling. This control would be accomplished through Aughinish's existing Distributive Control System.
Different options were evaluated to continue with the project including the pre-cooling of 2 centrifugal compressors. However, the options which involve pre-cooling are economically unfavourable compared with a stand alone auxiliary compressor.
The compressed air requirements for the Aughinish Alumina site are served by 3 - centrifugal compressors, each having a free air delivery capacity of 4,038 m3/hr at 7 bar.
The air consumption is as follows :
Plant Air - 70 to 120 m3/min (average 90 M3/min).
Instrument Air - 15 to 20 m3/min and with expansion could reach 24 m3/min.
The proposal is to introduce a new technology by pre-cooling the intake air to below - 25 deg. C. With pre-cooled intake air the mass flow increase improves the capacity such that one machine should satisfy most of the plant air demands. However, to produce the air for the instruments at approximately 20 m3/min would require a separate unit. For energy efficiency the screw compressor has been selected and complete with a second pre-cooler.
The project therefore involves the introduction of the pre-cooling for both centrifugal and screw compressor intake air on a large installation.
With the application of an air pre-cooler, the centrifugal compressor will require an increased power input with increased air density whereas the screw compressor does not.
However, there is still a distinct advantage in using this technology in conjunction with centrifugal machines as this demonstration will show.
The present efficiency will increase to :
Centrifugal Compression - 8.136 m3/kWh
Screw Compression - 16 m3/kWh
The annual benefit will be in the saving of 2,600,000 kWh of electricity. With present costs of 4.05 p/kWh, the financial saving is UKL 105,300/year.
Campo scientifico (EuroSciVoc)
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CORDIS classifica i progetti con EuroSciVoc, una tassonomia multilingue dei campi scientifici, attraverso un processo semi-automatico basato su tecniche NLP. Cfr.: Il Vocabolario Scientifico Europeo.
Questo progetto non è ancora stato classificato con EuroSciVoc.
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Coordinatore
CO LIMERICK
Irlanda
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