A 0.3-m (12-in.) square vertical wind tunnel has been built to provide airflows in the range 2.4–5.8 m/s (490–1140 fpm) for fouling studies. Precipitated calcium carbonate dust can be fed into the airstream (which may be heated) prior to a heat exchanger model. The paper reports results on a four-row, four-pass spiral wound finned tube heat exchanger. Fouling tests have been carried out primarily near the extremes of surface heat flux available (1000 W/m2 to −700 W/m2). The results have been analyzed by the general method of Kays and London to give an effective nondimensional airside heat transfer parameter (StPr2/3) and a friction factor. Tests have covered the Reynolds number range from 1350 to 3800. Normalization of fouling data to fixed Reynolds numbers has given friction factor curves which increase to an asymptotic level between 1.4 and 2.5 times the initial value, whilst StPr2/3 values tend to fall by only 10–20 percent during the same time

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