The function of a weir is to maintain a desired liquid level on the tray. Typical weir height is between 2 - 4 inches. Low weirs are frequently used in low pressure or vacuum columns. Notched (rectangular or V-shaped) weirs are commonly used for low liquid loads. see the Figure below for some examples:
The higher the liquid level, the higher the tray pressure drop. Higher liquid level also imply more liquid hold-up on the tray, which may be undesirable if the liquid is toxic or hazardous.
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Downcomers are used to guide liquid flow from an upper tray to a lower tray. The most common is the segmental-type, which can be either straight or sloped (angled). See the Figure below.
The straight, segmental, vertical downcomer is widely used as it provides good utilization of column area for downflow and has cost and simplicity advantage. Sloped downcomer can be used if vapour-liquid disengagement is difficult (e.g. due to foaming). Sloped downcomer also provide a slightly larger active area for vapour-liquid contact, but it is also more expensive.
A downcomer must be sufficiently large to allow liquid to flow smoothly without choking. Sufficient time must also be provided in the downcomer to allow proper vapour disengagement from the down-flowing liquid, so that the liquid is relatively free of vapour by the time it enters the tray below. Inadequate downcomer area will lead to downcomer choking, whereby liquid backs up the downcomer into the tray above and eventually flood the column.
Downcomer sealing can be achieved primarily by 2 means: (1) inlet weirs, and (2) recessed seal pan as shown in the Figure below. These devices provide a positive seal on the tray. The disadvantage is that they create a pocket of stagnant liquid where dirt, sediments, etc can build up. A large amount of such build up can restrict the downcomer outlet area and lead to premature flooding. Thus, the use of these devices is not recommended in fouling or corrosive services.
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The most common tray spacing used in practice is 18 - 24-inch, although in some applications, up to 36-inch tray spacing had been used. Tray spacing is usually set to allow easy access for maintenance.
A spacing of 24-inch or larger makes crawling between trays easier and is often desirable where frequent maintenance is expected, e.g. as in fouling and corrosive service. Increasing tray spacing necessarily adds to the column height requirements. Larger tray spacing may be required where the trays require elaborate support structures (e.g. beams) that may interfere with vapour disengagement.
Tray Layout (Configuration)
The cross-flow tray is the most commonly used. It is simple in design and economic in construction. In most applications, single-pass cross-flow tray design as shown is adequate. 2 or more pass trays are usually used for large diameter columns, where a high liquid load is expected. Use of multi-pass trays can lower the tray or downcomer liquid loading by splitting the liquid into 2 or more paths. For example, a 2-pass tray will halve the liquid load on the tray.
Whenever possible, the number of passes should not exceed 2. This is because increasing the tray passes shortens the path length. Shorter path lengths will reduce the tray efficiency. Multi-pass trays are also more expensive. In addition, trays containing 2 or more passes may be sensitive to liquid and/or vapour mal-distribution if the flow paths are not symmetrical.
Other Considerations for Trays
Materials of Construction
Generally, trays are made of stainless steel, e.g. Type 304, 316 or 410. The main factors affecting the choice of materials of construction of tray parts are:
Compatibility with the chemicals processed
Compatibility with the column materials of construction
Anticipated rate of corrosion
Procedure and expected frequency of cleaning
Cost (e.g. maintenance, replacement, effects on plant operations, etc)
Tray manways allow maintenance workers and inspectors to travel from one tray to another. Manway size should be at least 16 x 20-inch. For multi-pass trays, one tray manway should be provided for each tray pass, as the central downcomers restrict access from one side to the other.
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