This work was planned to study the effects of gas and liquid rates on the true gas-film coefficients of heat and mass transfer between gas and liquid in packed columns. Adiabatic humidification tests were made in a column, 254mm in i.d., dumped-packed with 15, 25 and 35mm porcelain Raschig rings to a height of 320mm. The air, supplied by a blower, passed through an orifice, and then through a preheater to the column. Water was recirculated through the system by means of a pump, and was at the wet-bulb temperature after steady-state conditions had been attained. To minimize the end effects due to spray below and above the packed section, specially designed column as shown in Fig. 1 was employed. Humidities of the incoming and outgoing air were determined by means of wet and drybulb, thermometers; the dry-bulb temperature of air just above the packings was calculated from the humidity and the wet-bulb temperature.
Data and calculated results are shown in Tables 1, 2 and 3. Superficial mass velocity of air G ranged from 700 to 2800 kg/m²·hr; superficial water velocity L ranged from 800 to 20000kg/m²·hr.The gas-film coefficient of heat transfer hGa[kcal/m³·hr·°C]for 25mm Raschig rings, for several water rates, were plotted against G in Fig. 2. Data were fitted by parallel straight lines having slopes of approximately 1.0. The effect of water rate was made clearer by Fig. 3 and Fig. 4, which also included the data for 15 and 35mm rings. It is evident that the increase of hGa with water rate is comparatively large at low water rate, and becomes very small at high water rate. The correlation was represented by
or by
Eq. (7) is sufficient for engineering purposes, for L of 1000 to 20000 kg/m²·hr, and Eq. (8) is considered suitable for extrapolation outside this range of L. Fig. 5 and Fig. 6 show the validity of these equations for the three kinds of Raschig rings.
The effects of G and L on the gas-film coefficient of water vapor transfer k'Ga [kg/m³·hr·ΔH] were quite similar to the effects on hGa. Fig. 7 shows the plot of k'Ga for 25mm Raschig rings against G at several water rates. The data for the three kinds of Raschig rings were plotted in Fig. 8 and Fig. 9, being correlated by
or by
The latter equation may be suitable for extrapolation outside the experimental range of L.
The ratio hGa/k'Ga calculated from Eqs. (7) and (9) or from Eqs. (8) and (10) was 0.26, the "Lewis relation" being nearly satisfied.
(H.T.U.)G for 25mm Raschig rings, for several water rates, was plotted against G in Fig. 10. As evident from Eqs. (6), (9) and (10), (H.T.U.)G is independent of G, at a given value of L.