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In thisstudy naturalconvection heat transferofl5firn-tube heat exchangers was measured. There WereJiveJinpitches and three diferent tube numbers. The results show that lower chilled water temperature will give more heat transfel: but water velocity in tubes is a weak factor of heut transfer It was also found that the single-tube heat exchangers have thehighest heat transfer coeflcient, with the three-tube heat exchangers the lowest. The heat transfer coeflcient and the unit area heat transfer rate of the heat exchangers can reach 10 W/m² °C and 1 O0 W/m², respectively. A hydraulic diameter forfin-tube heat exchangers was dejned and used as the length scale of natural convection heat transfer The experimental results were analyzed and the correlaton Nu = 0.43 (RA)^0.25 was obtained.