Clear liquid height

Fv = valve tray F-factor, ft- /min/valve Fvm = valve tray F-factor at the beginning of the valve open region, fr /min/valve g = gravitational constant, ft/s he = clear liquid height, in. ho = dry tray pressure drop, in. 

The pressure drop of these trays is usually quite low. They can be operated at an effective bubbling condition wnth acceptable efficiencies and low pressure drops. For more efficient operation the clear liquid height on the tray appears to be. similar to the sieve tray, i.e., 1.5-2-in. minimum. This is peculiar to each system, and some operate at 1 in. with as good an efficiency as when a 2-in. is used. When data is not available, 2 in. is recommended as a median design point. 

Although Sutherland did not obtain an equation for total tray pressure drop, correlation at this time indicates that it follows the effective head concept of Hughmark. This is a limited evaluation because the data available did not indicate any clear liquid heights over about 0.75 in. 

These results cannot be expected to correlate for a tray just becoming active (very lowUiquid on tray, 0.1 in. ), but have been satisfactory at 0.2-in. for clear liquid height, hd-... 

Assume clear liquid height on tray = 1 in. (note, 1 in. may be a slightly better ralue than the 1.5 in. assumed when determining e,v). 

A simple additive model is normally used to predict the total pressure drop. The total is taken as the sum of the pressure drop calculated for the flow of vapour through the dry plate (the dry plate drop hj) the head of clear liquid on the plate (hw + how) and a term to account for other, minor, sources of pressure loss, the so-called residual loss hr. The residual loss is the difference between the observed experimental pressure drop and the simple sum of the dry-plate drop and the clear-liquid height. It accounts for the two effects the energy to form the vapour bubbles and the fact that on an operating plate the liquid head will not be clear liquid but a head of aerated liquid froth, and the froth density and height will be different from that of the clear liquid. 

If the hydraulic gradient is significant, half its value is added to the clear liquid height. 

A 30 cm-diameter bubble column containing water (clear liquid height 2 m) is aerated at a flow rate of 10 m h . Estimate the volumetric coefficient of oxygen transfer and the average bubble diameter. The values of water viscosity = 0.001 kg m s , density p = 1000 kgm , and surface tension cr = 75 dyne cm" can be used. The oxygen diffusivity in water is 2.10 X 10 ... 

The clear liquid back-up is obtained from a tray-pressure balance and is normally taken to be the sum of the tray-pressure-drop, the clear liquid height on the active area of the tray, and the pressure-drop of liquid flowing under the downcomer apron onto the active area. 

The recommended range of application of the correlation is given in Table 14-10. The clear liquid height at the froth-to-spray transition ha is calculated using the corrected Jeronimo and Sawistowski [Trans. Inst. Chem. Engnrs. 51,265 (1973)] correlation as per Eqs. (14-82) to (14-84). 

Some trial and error is required in this calculation because the clear liquid height hc and the froth density 0, depend on each other, and the weep fraction f. depends on the clear liquid height hc. Clear liquid height is related to froth height and froth density By... 

The terms in Eqs. (14-123) to (14-126) are in English units and are explained in the Nomenclature. For sieve trays, m= 1.94 and C = 0.79. Note that the constants are a slight revision of those presented in the original paper (C. L. Hsieh, private communication, 1991). Clear liquid height is calculated from Colwell s correlation [Eqs. (14-115) to (14-122)]. The Hsieh and McNulty correlation applies to trays with 9 percent and larger fractional hole area. For trays with smaller hole area, Hsieh and McNulty expect the weeping rate to be smaller than predicted. 

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. 

The clear liquid height is equal to the sum of the weir height, the height over the weir, and hah1 the hydraulic gradient, giving... 

Most weep point correlations are based on a pressure balance between the static head of the clear liquid, and the tray pressure drop. Lockett (12) reviewed the performance of several weep point correlations, and noted that their success often depends on how the clear liquid height is estimated. Lockett (12) also presents Mayfield s (37),... 

It was recommended (12,58) that the clear liquid height hc in Eq. >6.33) be calculated using Colwell s (68) clear liquid height correlation (Sec. 6.3.3). The denominator of Eq. (8.33) contains the difference between liquid and vapor densities instead of the liquid density in the original Lockett and Banik correlation (56). This modification, incorporated by Colwell and O Bara (68), has negligible effect at low pressure, but makes the correlation less conservative at high pressure. 

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-... 

Strictly, the Jeronimo and Sawistowski correlation predicts clear liquid heights at the froth to spray regime transition. However, it has been shown (27,92) that clear liquid height in the spray regime is much the same as clear liquid height at that transition. 

Valve trays. Development of a clear liquid height correlation for valve trays has been inhibited by difficulties in measurement of clear liquid heights on these trays. A number of correlations have been proposed (86,93-95), but questions have been raised about their applicability (12,86,87). None has been tested against a sufficiently large data base... 

Flow regime. Since the trays are unlikely to operate in the spray regime (Sec. 6.4.2), it is best to first examine the froth-emulsion transition. This check requires using the Hofhuis correlation for clear liquid height (Sec. 6.3.5). 

Seal check. In Sec. 6.5.7, it was decided to go to clearances under the downcomers that exceed the outlet weir heights. For such designs, it has been recommended (1) that the clear liquid height in the downcomer under turndown conditions exceeds the downcomer clearance by at least 2 in. This will be checked here. 

The Chan and Fair correlation uses the Bennett et al. correlation i Sec. 6.3.3) for calculating the clear liquid height hc and the froth density [Eqe. (6.56) and (6.58)]. For calculating kLait Chan and Fair use a correlation by Foss and Gerster (133),... 

Coefficient in the clear liquid height correlation, defined by Eq. (6.62). 

Clear liquid height at the transition from the froth to spray regime, in of liquid. 

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