Pressure Drop Through the Aerated Liquid

Pressure drop through the aerated liquid [h L, in Eq. (14-100)] is calculated by... 

The pressure drop through the aerated liquid, hL, has been correlated as a function of the calculated height (hw + how + hg) of vapor-free liquid over the perforations by Fair3 as follows... 

The second term takes into account the pressure drop through the aerated liquid over and around the dispenser correspond to the clear liquid height. This pressure drop has a relationship with the liquid height on a tray. The method, commonly used, defines an aeration factor p, such that. 

The pressure drop across the plate comprises two resistances in series, one for friction through the perforations and the other for head loss in flowing through the aerated liquid on the tray ... 

Wet pressure drop h is the pressure drop that the vapor goes through the aerated liquid on the tray. h ean be calculated by using equation (12.35) ... 

The calandria will operate at the condition such that the pressure drop through the piping and equipment is exactly equal to the pressure drop available for the system (Figure 10-4). The available pressure drop is a function of the liquid level in the evaporator and the difference in density between the saturated liquid and the aerated liquid. For a constant vapor rate, increasing liquid rates will reduce the available pressure drop while increasing the pressure drop through the system. Lowering the liquid level in the evaporator reduces the available pressure drop therefore the liquid circulation rate will decrease. 

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. 

As illustrated, liquid accumulates on the low side of this tray. Vapor, taking the path of least resistance, preferentially bubbles up through the high side of the tray deck. To prevent liquid from leaking through the low side of the tray, the dry tray pressure drop must equal or exceed the sum of the weight of the aerated liquid retained on the tray by the weir plus the crest height of liquid over the weir plus the 2-in out-of-levelness of the tray deck. 

Now we turn our attention to wet pressure drop. As h represents the pressure drop that vapor goes through the aerated hquid on the tray, it can be modeled in proportion to the tray clear liquid height as... 

Zg hydraulic gradient on tray (high minus low clear liquid height) in. of liquid h pressure drop through aerate liquid on tray in. of liquid liquid head over the outlet weir in. of liquid downcomer seal in. tray pressure drop in. of liquid... 

Liquid enters from the downcomer at the left, flows through the zone aerated by the upflowing vapor, and departs into the downcomer at the right. A two-phase mixture exists on the tray and it may be either liquid-continuous (a bubbly froth) or vapor-continuous (a spray), or some combination of the two. The objective of the designer is to determine the mass transfer efficiency and pressure drop brought about by this contacting action resultant typical performance profiles are shown in Fig. S.7-S. 

Once the weight of liquid on one portion— the lowest area—of a tray deck exceeds the dry tray pressure drop, the hydraulic balance of the entire tray is ruined. Vapor flow through the low area of the tray deck ceases. The aeration of the liquid retained by the weir on the low area of the tray deck stops, and hence the hydraulic tray pressure drop increases even more. As shown in Fig. 4.3, the liquid now drains largely... 

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