To make adsorption dehumidification run on heat supply temperatures as low as 50°C, and allow an effective use of solar thermal collectors, a new system concept is developed. The aim is to achieve a quasi-isothermal adsorption of water vapour in solid desiccant by carrying out the process in a finned-tube heat exchanger. Heat is removed or supplied by a water circuit. Air flows through a silica gel bed, packed between the fins. A key issue in this solution is to optimize the bed porosity, in

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... er to minimize the pressure drop on the air side. The present experimental study adapts the Ergun equation to packed beds between two flat plates, where the ratio of wall distance to spheres diameter is within the scarcely investigated range of 1÷2. Packed bed porosity estimation is achieved by adapting transitional structure theory in thin layers of spheres to the present case, and extending it to multi-diameter spheres packing.