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Froth entrainment flooding. Most of the work reported in the literature lumps spray and froth regime entrainment flooding together. Froth entrainment flooding is far less common than spray entrainment flooding, and occurs mainly at close (<18 in.) tray spacing, when the froth envelope can approach the tray above. Some flood data at close tray spacing that pertains to froth entrainment flooding were reported (14,20,37,38). Froth regime entrainment work (37-41) is also relevant to froth entrainment flooding.

The factors affecting froth entrainment flooding differ from those affecting spray entrainment flooding. The critical variable is the distance between the top of the froth and the tray above (20,39,41). This implies that the flood velocity strongly increases as tray spacing is raised, liquid load is lowered and weir height is lowered. Some flood and entrainment data (37-40) confirm this trend. On the other hand, tray geometry variables such as hole diameter and fractional hole area can be expected to have a lesser effect (if any) on froth entrainment flooding. This was confirmed by entrainment data (38-40) but not by flood data.

Froth entrainment flood can be predicted using the Fair or the Smith et al. correlations. Both included froth entrainment flood data in their data base. Testing of Fair's correlation (20,41) against a handful of more recent data suggests that it gives conservative froth entrainment flood predictions. The Smith et al. correlation is claimed 120) to be less conservative. The Kister and Haas correlation is unsuitable for froth entrainment flood prediction.

Froth entrainment flooding is only encountered at close tray spacing. It can be excluded from any capacity checks when tray spacing exceeds 18 in (except when conditions are conducive to vapor cross flow, Sec. 6.2.4). Note that at low tray spacing (< 18 in), either spray or froth entrainment flooding can restrict capacity.

6.2.7 Downcomer backup flooding.

Downcomer backup flooding occurs when the backup in the downcomer exceeds the tray spacing, i.e.,

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