M A C H I N E R Y be running at less than capacity. This is especially unfavourable when production facilities are operating for just one or two shifts, as the dryers are left to dry only the compressed air for smaller consumer points or leaks during periods when production activities are not required. To ensure reliable compressed air quality it is recommended that compressed air dryers run continuously. However, the more the compressors are running at anything less than full capacity, the greater the energy waste resulting from a compressed air dryer left running continuously 24 hours per day, configured to deal with maximum temperatures. Under such conditions, the energy requirements for the compressed air dryer can spike dramatically and account for up to 20% of the total energy required for compressed air production. Energy optimisation measures Refrigeration dryers have benefited from technical innovations over the years, including ‘digital scroll’ coolant compressors and thermal mass dryers, which improved the performance of refrigeration dryers operating with air flow rates of less than 50 m³/min in partial load. The digital scroll method involved modifying the clearance losses within the scroll compressor used for the coolant, which in turn regulated the flow rate of coolant to adjust it to the quantity required to cool the compressed air. In addition to a scroll compressor to cover the base load, a controlled scroll compressor which was switched off completely during periods of very low demand was employed. The method allowed for a relatively large control range. Unfortunately technical difficulties of implementing it made it less attractive. Buffer dryers were the preferred technology for compressed air flow rates less than 20 m³/min. Some systems relied on a tank to buffer load fluctuations while keeping the pressure relatively constant and reducing compressor switching to a minimum. This meant the larger the tank, the smaller the pressure fluctuations and therefore, less switching was required. Other systems relied on 54 September 2017 Innovative new refrigeration dryers are reducing space requirements and drastically cutting energy consumption. a thermal mass instead of incorporating a cool air buffer tank. These dryers generally use mineral materials to store the cooling energy. In order to keep the switching frequency of the coolant compressor within economical bounds and to ensure a consistent pressure dew point, the amount of mass required rises in direct proportion to the system capacity. Furthermore, heat distribution within the thermal mass requires precise regulation. Thermal mass systems are extremely reliable and involve no mechanical loads or switching of any type of system. Moreover, when the thermal mass is saturated, the system maintains safety reserves in order to accommodate short-term overload periods. However, weight considerations impose certain restrictions, for example thermal mass dryers suitable for even relatively modest compressed air capacities of 17 to 20 m³/min are extremely heavy. Until recently, larger systems had to be equipped with digital scroll systems and were precluded from benefiting from the advantages of thermal mass dryers. New technology A new technology which has recently entered the market is a refrigeration dryer equipped with a totally different type of thermal mass – a phase changing material (PCM). Phase changing materials can store and release vast quantities of energy if they are harnessed at the precise point at which they undergo a phase change. These materials work according to the same principle by which ice cubes keep a drink cool. The temperature of the drink remains constant as long as the ice cubes remain melting in the glass. They are capable of absorbing a significant amount of heat before melting completely; consider that the same amount of energy is required to change solid ice with a temperature of 0°C to a liquid as is needed to heat water from 0°C to 80°C. These thermal masses are also known as latent heat thermal masses owing to their capacity to store thermal energy virtually invisibly for long periods with only minor losses and their ability to accommodate any desired repetition cycle. Latent heat thermal masses usually employ special salts or types of paraffin as the storage medium since they can absorb huge amounts of thermal energy. When the thermal energy is discharged, the thermal mass solidifies. During this process, the thermal mass returns the large amount of heat it
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