## Nh 654

The friction factor is obtained from Fig. 6.24 (18).

For valve trays, the hydraulic gradient is somewhat larger than that of sieve trays, probably not by much (18). In a similar manner to sieve trays, it is often neglected in the pressure drop calculation (7-9,71,80). In cases of a long flow path of liquid, it should be checked using the Hughmark and O'Connell correlation above.

The Bennett et al. correlation. This correlation was shown (81) to predict experimental sieve tray pressure drop data more accurately than Fair's correlation. The correlation is based on froth regime considerations and is not applicable to the spray regime. The Bennett et al. calculation of dry pressure drop is identical to Fair's, using Eqs. (6.42) and (6.43) and the Liebson et al. correlation (Fig. 6.21a). To calculate the h; term in Eq. (6.41), Bennett et al. depart from the concept of clear liquid flow corrected for aeration effects [Eq. (6.47a)]. Instead, they use Eq. (6.476) and a model of froth flow across the weir. Their residual pressure drop, hR, is a surface tension head loss term, which is important for trays with very small holes (<Vs in. diameter). The Bennett et al. correlation is hR =

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