Hydraulic gradient pressure drop

The characteristics of the pump relate the applied pressure on the cake to the flowrate at the exit face of the filter medium. The cake resistance determines the pressure drop. During filtration, liquid flows through the porous filter cake in the direction of decreasing hydraulic pressure gradient. The porosity (e) is at a minimum at the point of contact between the cake and filter plate (i.e., where x = 0) and at a maximum at the cake surface (x = L) where sludge enters. A schematic definition of this system is illustrated in Figure 2. 

For stable tray operation, the hydraulic gradient should be less than one half the dry tray pressure drop. For conditions of high weir height and high Vq (Pv) the greater... 

For bubble cap trays the phenomenon is believed to be induced by excessive hydraulic gradient it is recommended to keep hydraulic gradient to less than 40% of the dry pressure drop. 

One of the possible ways to account for the effect of roughness on the pressure drop in a micro-tube is to apply a modified-viscosity model to calculate the velocity distribution. Qu et al. (2000) performed an experimental study of the pressure drop in trapezoidal silicon micro-channels with the relative roughness and hydraulic diameter ranging from 3.5 to 5.7% and 51 to 169 pm, respectively. These experiments showed significant difference between experimental and theoretical pressure gradient. 

FIG. 14-32 Pressure-drop contributions for trays, hd = pressure drop through cap or sieve, equivalent height of tray liquid hw = height of weir how = weir crest hhg = hydraulic gradient hda = loss under downcomer. 

For valve trays, the hydraulic gradient is somewhat larger than that of sieve trays, probably not by much (13). In a similar manner to sieve trays, it is often neglected in the pressure drop calculation (7-9,71,80). In cases of a long flow path of liquid, it should be checked using the Hughmark and O Connell correlation above. 

The wet tray pressure drop is obtained by calculating the dry tray drop, then adding the slot submergence, including the weir crest and half of the average hydraulic gradient. 

Low pressure drop and negligible hydraulic gradient are characteristic of Flexitrays. 

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