Evaporators will ice up when humid air is circulated through the evaporators in cold stores, and defrosting of evaporators is important to secure a steady process at cold stores keeping temperatures low. One mm of ice covering the evaporator coils will reduce the efficiency of heat transfer between the evaporator and the cold store air by 5%. Many cold stores use time-based defrost management. Timers, however, do not take into account variations in weather (humidity), evaporation (warm and cold products) and traffic in the cold store (distribution of humidity at evaporators). A Defrost on Demand sensor can measure the exact layer of ice build-up, and defrost as often as needed and as short as possible. HB Products A/S offers this technology, in which a sensor measures rime build up on the surface of the air cooler’s cooling ribs caused by the freezing of moisture. The HBDF sensor defrosts depending on rime build-up on the air cooler during operation. It measures the amount of rime build-up based on capacitive measurement. A change in the electrical capacitance between the electrodes is detected when they are surrounded by a medium. The capacitance depends on the dielectric constant of the surrounding medium – which in this case is air or water. If the measured electrical capacitance changes, the sensor can calculate how far the electrodes are immersed in the respective medium. Thus the HBDF sensor measures ice build-up on the cooling ribs. The product is available in India from Metalex Cryogenics Ltd., Pune. For more information, visit www.hbproducts.dk
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