INCORPORATING COLD CHAIN
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Properly considering the system demand characteristics will have a major impact on energy efficiencies and equipment performance .
When people look at plant performance , it tends to always be at peak performance but , that is only two months in a year , and at that , only about 8 hours per day during the period . So , we need to consider part load performance of the system . Screw compressors are large capacity compressors with a very small footprint . The beauty of these machines is that they have an infinitely variable capacity control system allowing them to adjust their capacity to match plant requirements .
Sounds too good to be true ! Well , there is a specific energy consumption ( SEC ) consideration depending on the application where at full speed the part load energy efficiency is drastically reduced . These days we are looking at variable speed drives to keep the compressors running at maximum capacity whilst adjusting the drive speed
Properly considering the system demand characteristics will have a major impact on energy efficiencies and equipment performance . to match the system capacity demand . The variable speed drive uses between 5 to 10 % power to drive it , and does improve the SEC , but only up to a point .
Each compressor , whether screw or reciprocating , has a minimum operational speed at which point we either switch off or we start to unload the compressor .
Properly considering the system demand characteristics will have a major impact on energy efficiencies and equipment performance . A well-designed capacity control system is paramount . I recently was at a plant where there was a glycol chiller that was being controlled by the glycol temperature instead of the system suction back pressure . The load was all over the place with dramatic surges in suction pressures resulting in excessive loads and liquid slug back on the compressors .
The net result was high power usage , massive wear and tear , and maintenance costs . There was a total of four compressors that regularly cycled in and out . Now that we have sorted out the control system , the plant has a stable suction , and only two compressors are operating . To top that there has been additional condensing capacity installed resulting in low head pressures and reducing water and electrical usage on the plant .
Next month I will chat about a report from the International Institute of Ammonia Refrigeration ( IIAR ) on this very subject , I am sure you will find it enlightening . CLA www . coldlinkafrica . co . za
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