Selection of Tray Type (Tower Internal)

Tower Internal

Selection of Tray Type

The principal factors to consider when comparing the performance of bubble-cap, sieve and valve trays are: cost, capacity, operating range, efficiency and pressure drop.

1.    Cost


Bubble-cap trays are appreciably more expensive than sieve or valve trays. The relative cost will depend on the material of construction used; for mild steel the ratios, bubble-cap: valve: sieve, are approximately 3.0:1.5:1.0. However, comparative quotations over the last few years show the relative costs in dollars per square foot of tray area to be: bubble-cap tray, 20; flexitray, 14; Uniflux, 10; sieve, 10; and turbo grid, 10. These are costs before installation.

2.    Capacity


There is little difference in the capacity rating for the three types (the diameter of the column required for a given flow-rate); the ranking is sieve, valve, and bubble-cap.

3.    Operating range


This is the most significant factor. By operating range is meant the range of vapour and liquid rates over which the plate will operate satisfactorily (the stable operating range). Some flexibility will always be required in an operating plant to allow for changes in production rate, and to cover start-up and shut-down conditions. The ratio of the highest to the lowest flow rates is often referred to as the "turn-down" ratio. Bubble-cap trays have a positive liquid seal and can therefore operate efficiently at very low vapour rates.

Sieve trays rely on the flow of vapour through the holes to hold the liquid on the tray and cannot operate at very low vapour rates, but, with good design, sieve trays can be designed to give a satisfactory operating range; typically, from 50 per cent to 120 per cent of design capacity.

Valve trays are intended to give greater flexibility than sieve trays at a lower cost than bubble-caps.

4.    Efficiency


The Murphree efficiency of the three types of trays will be virtually the same when operating over their design flow range and no real distinction can be made between them.

5.    Pressure Drop


The pressure drop over the trays can be an important design consideration, particularly for vacuum columns.  The trays pressure drop will depend on the detailed design of the tray but, in general, sieve plates give the lowest pressure drop, followed by valves, with bubble-caps giving the highest.

Summary

Sieve trays are the cheapest and are satisfactory for most applications.Valve trays should be considered if the specified turn-down cannot be met with sieve trays. Bubble-caps should only be used where very low vapor (gas) rates have to be handled and a positive liquid seal is essential at all flow-rates