Downcomer clear liquid backup

This value is also taken as the mean density of the fluid in the downcomer, which means that for safe design, the clear liquid backup, calculated from equation 11.91, should not exceed half the plate spacing h to avoid flooding. 

If a clearance greater than the weir height is desired, the author recommends that it be made as low as permitted by the downcomer pressure drop and fouling criteria that the clearance not exceed weir height by more than 1 in and that the clearance be made at least 2 in less than the clear liquid backup in the downcomer at minimum vapor and liquid loads. 

Does downcomer flooding occur when the downcomer clear liquid level reaches the tray above As a matter of fact, the flooding occurs earher than the point of clear liquid reaching the tray above. This is because there is a layer of liquid froth on top of the clear Uquid. As soon as the froth reaches the tray above, flooding happens as froth carries significant amount of liquid to the tray above. Thus, the total liquid height including froth level is H = Ha/ < 1)> where is the downcomer froth density. To be conservative, we assume the downcomer backup limit is 80% and thus downcomer flood capacity is expressed as... 

As noted, hdc U calciilated in terms of equivalent clear liquid. Actu-ahy, the liquid in the downcomer may be aerated and ac tual backup is... 

The backup of clear liquid during flowing conditions must be determined in order to set the proper tray spacing. Tray spacing is usually set at twice the liquid height in the downcomer. This can be adjusted to suit the particular system conditions. 

The tray may flood. Water and hydrocarbon mixing on the tray deck, stirred up by the flowing gas, creates an emulsion. The emulsion does not separate as readily as clear liquid from the gas. Premature downcomer backup, followed by tray deck flooding, result. 

The heights of head losses in Eq. (14-92) should be in consistent units, e.g., millimeters or inches of liquid under operating conditions on the tray As noted, hdc is calculated in terms of equivalent clear liquid. Actually the liquid in the downcomer is aerated and actual backup is... 

Downcomer backup flooding occurs when the backup of aerated liquid in the downcomer exceeds the available tray spacing. Downcomer backup can be calculated by adding the clear liquid height on the tray, the liquid backup caused by the tray pressure drop, and the liquid backup caused by the friction loss at the downcomer outlet. The downcomer backup is then divided by an aeration factor to give the aerated liquid backup. 

Downcomer backup. The factors that resist liquid flow from the downcomer onto the tray below are the froth height on the tray, the pressure drop on the tray, and the friction loss under the downcomer apron. These factors cause liquid to back up in the downcomer. Each of these factors can be expressed in terms of clear liquid heads. A tray pressure balance gives... 

The clear liquid height, or the liquid holdup, is the height to which the aerated mass would collapse in the absence of vapor flow. The clear liquid height gives a measure of the liquid level on the tray, and is used in efficiency, flooding, pressure drop, downcomer backup, weep-... 

Downcomer seal at turned down conditions (downcomer backup less downcomer clearance, in clear liquid) 3,8 3.0 3,1... 

In terms of clear liquid the downcomer backup is given by... 

Downcomer backup in terms of clear liquid head... 

Flooding is much more sensitive to vapor rate than to liquid rate. Trays can, however, definitely be flooded by high liquid rates. Prediction of maximum liquid capacity is based on hydraulic calculation of liquid backup in the downcomers. For low-foaming systems, it is often assumed that liquid backup cannot be more than half of the downcomer height, calculated as clear liquid. For high vapor density, e.g., S.Olb/ft the limiting backup is even less. 

The downcomer backup (clear liquid height basis) should be 40% or less of the tray spacing for high-vapor-density systems (>3.01b/ft, 48.1 kg/m ), 50% or less for medium-vapor-density systems, and 60% or less for low-vapor-density systems (<1.01b/ft, 16.0 kg/m ). 

The liquid and froth flowing over the weir partially fill the downcomer, creating a backup of height h f (Figure 14.1). If the overall relative froth density in the downcomer is (t),i, the equivalent clear liquid height is... 

Equivalent clear liquid downcomer backup height... 

The backup relationship [Equation (12.65)] is used with pressure head on a clear liquid basis. The actual height in the downcomer is... 

Returning to Fig. 5.7-16, we see that the backup relationship [Eq. (5,7-29)[ is based on clear liquid heads. Actually, the fluid in the downcomer contains a large amount of entrapped vapor and can be represented as a froth with an average density of 0. . Accordingly, the actual downcomer backup is... 

To avoid excessive downcomer backup, the clearance under the downcomer is usually set so that clear liquid pressure drop at the downcomer outlet does not exceed 1 in of hot liquid (61,172,192, 249). Alternatively, some designers recommend outlet pressure drops not exceeding IV2 in of hot liquid (211), or liquid velocity at downcomer outlet not exceeding 1 to 1.5 ft/s (123,243), or area between the bottom of the downcomer and the tray floor one-third to one-half the area at... 

Clearly, downcomer backup can be estimated if we know the clear liquid height H. However, it is not straightforward in predicting H. As consists of three components, namely, clear liquid height h, tray pressure drop /r and downcomer apron pressure drop h, let us look into each individual components one at a time. 

With relatively high outlet weir height hw, the clear liquid height h increases and vapor and liquid contact time increases. This improves distillation efficiency. However, a too high outlet weir height could affect the downcomer backup and tray capacity. 

Downcomer Backup% According to equation (12.22), the total clear liquid height in downcomer is... 

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