Development of coated, annular fins for adsorption chillers

Abstract The specific cooling power (SCP) of adsorption chillers could be increased by employing zeolite coated, extended heat transfer surfaces in the generator. An experiment featured 50 annular aluminium fins, 76 mm diameter, each coated with 2 g of zeolite CBV901, and pressed onto a 10 mm bore aluminium tube. A large (4 kg), structurally stiff and easily fabricated casing enclosed the finned tube; the resulting generator assembly formed a “thermal compressor”, increasing the pressure of methanol vapour from ∼100 to ∼280 kPa. During a typical thermal cycle the peak-to-peak temperature changes were: casing temperature 35 K; methanol vapour (inside the generator) 45 K; the fin 65 K; heat transfer liquid 80 K. On a Clapeyron diagram, a non-ideal, anisosteric depressurisation stage was attributed to the relatively high mass of refrigerant vapour in the generator (∼1 g, compared with at least ∼8 g held in the zeolite). The radial temperature profile along the fin was modelled by lumping the energy contents of the aluminium and the bonded zeolite into an effective heat capacity. Thereupon, the model was fitted to recorded data – the adjustable coefficients inside the model related to the thermal resistance of the tube-to-fin-root interface. The best-fit incurred a root mean square error of 4.0 K, at which point the interfacial resistance governed heat transfer and hence cooling power. Based on computed heat transfer to the fin, the inferred coefficient of performance was 42%; this could be improved by eliminating the interfacial resistance and optimising the fin thickness.