www.engineeringnews.co.nz 55 previously absorbed back into the environment. The temperature remains constant during the transition from one state of matter to another as all the heat entering the system is invested in the change of state. These innovative refrigeration dryers exploit the analogous principle of liquefying and solidifying for thermal management purposes. At a basic level, these dryers function as follows: when compressed air requires cooling, from e.g. a starting temperature of 5 °C, the coolant compressor is switched on. The refrigeration dryer cools the paraffin to a temperature of around 3 °C while the compressed air cools simultaneously. During this period, the temperature remains constant because the paraffin is undergoing a phase change from fluid to solid. The material is then cooled somewhat more, to around 2 °C. The coolant compressor then switches off the supply current. The compressed air then flows into the heat exchanger, which is surrounded by the solidified paraffin, where the air gradually warms the paraffin, which in turn keeps the compressed air cool as it changes from the solid to fluid state. This process continues until a set maximum temperature threshold is reached, at which point the coolant compressor switches on the supply current and the whole cycle begins anew. These new refrigeration dryers employ a paraffin-based system. This material has a low expansion coefficient as well as 98% better thermal density than the materials previously used as thermal masses. The higher storage density of the PCM has meant that the heat exchanger in the refrigeration dryer could be completely redesigned. While earlier refrigerlow, requiring less than 87 watts per m³/min to dry compressed air. Thanks to new, smaller components as well as intelligent component layout, this new technology additionally means that the entire dryer can also be significantly lighter and more compact. Furthermore, the entire cooling system has been upgraded in these new refrigeration dryers, along with the air heat exchanger. A highly efficient scroll compressor has replaced the previously used reciprocating compressor and, the capillary tube has been replaced by an expansion valve. Expansion valves regulate the coolant quantities dynamically, depending on the load. As a result, significantly less coolant is required and the coolant compressors can run at a much lower output. Overall, these new dryers require 50% less power than comparable conventional equipment. ation dryers used copper spiral heat exchangers, the first thermal mass dryers relied on plate heat exchangers. However, the new refrigeration dryers work with an aluminium heat exchanger that combines these two heat exchanger systems – an air-air heat exchanger as well as another compressed air-PCM heat exchanger. In addition to energy efficiency advantages, this new dual heat exchanger design has also reduced the space requirement. Reduced pressure loss, reduced energy requirement The compact design has allowed pressure losses to be reduced to 0.15 bar in comparison to the 0.20 bar value characteristic of conventional models. The input energy requirements are also exceptionally How the new refrigeration dryer system works Compressor supplies cold coolant to dry compressed air and cool the thermal mass. Thermal mass solidifies, maintaining constant temperature, and channels significant amounts of heat through the coolant. Coolant cools the thermal mass until the switch-off threshold is reached. Coolant compressor switches off. By absorbing heat from the air, thermal mass provides cooling action to dry compressed air. Thermal mass melts, maintaining constant temperature, while absorbing significant amounts of heat from the moist compressed air. Thermal mass warms until a threshold is reached, triggering the compressor to switch on.
ENsep17-eMag
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