Selection of plate type

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

Bubble-cap plates are appreciably more expensive than sieve or valve plates. 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. 

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

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 

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

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 plates have a positive liquid seal and can therefore operate efficiently at very low vapour rates. 

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