Design Criteria

The trays should be designed for maximum throughput. However, owing to inaccuracies in the design equations and fluctuation of process conditions (e.g. flow rates, temperature and pressure), safety factors are needed to ensure stable column operation at all times (see Figure 4).

Jet Flood Safety Factor

The jet flood safety factor (JFSF) is defined as the ratio of vapour velocity required to entrain the entire liquid flow (Umax) to the operating velocity (Uop). It is a useful measure of entrainment and hydraulic stability. The typical JFSF value is 1.2.

Turndown Ratio

For various reasons, the column may be operated at a reduced throughput. Weeping is encountered if the vapour velocity can no longer support the liquid on the tray. Although flow dynamics permit stable operation as long as dumping is avoided, tray efficiency suffers because weeping reduces the vapour-liquid contact. The turndown ratio is the ratio of the design vapour flow rate to the flow rate that permits some weeping without seriously affecting the tray efficiency. Recommended weepages at turndown conditions for vacuum and pressure operations are 3% and 7%, respectively.

Downcomer Area Safety Factor (DCASF)

and Downcomer Backup Safety Factor (DCBUSF)

The liquid handling capacity of a tray is determined by downcomer design and tray spacing. The DCASF determines the approach of the top downcomer area to the minimum area to the minimum area required for vapour-liquid disengagement. The DCBUSF determines the approach of the downcomer froth height to the downcomer depth (= tray spacing + outlet weir height). Safety factors in the range of 1.5-2.0 are recommended.

Pressure Drop

The pressure drop across an operating tray should be specified if it affects the number of equilibrium stage requirements for the separation. This is often the case for vacuum applications. Stable operation can be obtained at a pressure drop of 1-3 in (2.5-7.6 cm) of liquid per tray for vacuum and 2-5 in (5.1-12.7 cm) for pressure operations.

Solar Panel Basics

Solar Panel Basics

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