Local heat transfer coefficients were measured during the forced convective flow vaporization of a 60% v/v methyl- cyclohexane/40% v/v n-hexane mixture in a 48 Kw electrically heated horizontal furnace tube (15 feet long, 3/4 inch I.D.). Experimental single-phase heat transfer coefficients were a factor of 1.18 higher than those predicted by the Dittus-Boelter equation. In two-phase flow a large variation occurred in circumferential heat transfer coefficients (up to 250%) and large departures were observed from values predicted by the Chen correlation. For example, in the subcooled nucleate boiling regime, measured heat transfer coefficients were as much as three times the predicted values. Mandhane et al's flow pattern map only provided an indication of observed flow regimes. In an attempt to predict fouling a single-phase model was developed based upon a two step reaction in which all diffusional and kinetic resistances were included. Removal was considered to be by mass transfer and fluid shear. Considerable success was obtained in applying the model to Watkinson's (2) data on gas oils. The model was applied in a semi-quantitative manner to data on two-phase heat transfer coefficients to assist in the prediction of fouling in vaporizers. Even a slight variation in coefficients had a significant effect on fouling.
|Date of Award||1977|