Cocurrent crossflow

Analysis of a Multi-Unit Cocurrent Crossflow Cooling Tower... 

An analysis of multi-unit cocurrent crossflow cooling towers is made. Towers were placed in series and the results obtained were compared with that of multi-unit countercurrent crossflow cooling towers. The experimental data on the multi-unit cocurrent crossflow cooling tower were, analyzed in terms of enthalpy efficiency of single-unit, heat capacity ratio and number of transfer units. An outline of design calculations of multi-unit cocurrent crossflow cooling towers is presented. 4 refs, cited. 

Cooling towers are broadly classified on the basis of the type of draft natural draft (natural convection), mechanical draft (forced convection) and mechanical and natural. Further distinction is made based on (1) the type of flow i.e. - crossflow, counterflow, cocurrent flow (2) the type of heat dissipation-wet (evaporative cooling), dry, wet-dry and (3) the type of application-industrial or power plant. Each of the major types of cooling towers has a distinct configuration. The major designs are summarized in Figures 1 through 8 and a brief description of each follows. 

In its simplest form, a heat exchanger may consist of two passages, with the cooling fluid in one passage and the warming fluid in the other. The flow direction of each of the fluids relative to one another may be countercurrent, cocurrent, or crossflow. 

It was seen from the discussion of heat exchangers that the fluid streams are not strictly countercurrent. Baffles on the shell side induce crossflow, and in a two-tube-pass heat exchanger both countercurrent and cocurrent flow occur. To account for deviations from countercurrent flow, the logarithmic-mean teri5)erature difference is multiplied by a correction factor, F. Thus,... 

There are also numerous other special designs for heat exchangers which will not be discussed here. It is possible to combine the three basic flow regimes of countercurrent, cocurrent and crossflow in a number of different ways, which leads to complex calculation procedures. 

In addition to the cases of countercurrent, cocurrent and crossflow, further flow configurations are possible. These are also applied in industry and have been investigated by many different authors, cf. the compilation of W. Roetzel and B. Spang [1.16]. The operating characteristics F eilNilCi) = 0 are often complex mathematical expressions, so it seems reasonable to represent the results graphically. 

The gas flows horizontally, contacting by downflowing liquid. The effective driving force for mass transfer is between that for counter- and cocurrent contactors. Crossflow scrubbers have low pressure drop and usually require a lower liquid/gas ratio than either counter- or cocurrent scrubbers. The time of contact between gas and liquid is relatively low, and crossflow units are not reconunended for most chemical absorptions. Design procedures follow a finite-element approach the scrubber volume is divided into cubes, each of which is assumed to reach equilibrium. 

Continuous band crossflow <3-mm thick bed area 20 to 100 m evaporative capacity 2 to 13 g water evaporated/s m residence time 0.2 to 1.3 h steam 1.7 to 1.9 kg/kg water evaporated fan power 35 to 1300 kJ/kg belt drive power 1 kW/m belt. Continuous band gas flow through the bed 1 to 4 g water evaporated/s-m steam 2 to 5 kg steam/kg water evaporated 1.25 m/s gas velocity throngh the bed area 5 to 25 m. Rotary cascading dryer/kiln roto-louvre area 10 to 1000 nC evaporative capacity 18 g water evaporated/s m or 7 g water/s m of peripheral area 5 to 10% solids, residence time 0.1 V/volumetric feedrate gas velocity 1 to 1.5 m/s or 1/2 terminal velocity of particles peripheral velocity 0.1 to 0.5 m/s, 4 to 5 rpm rpm times diameter = 3 to 12 rpm m L/D = 4 to 15/1 average temperatnre of evaporation thronghont = 3°C above the inlet wet-bulb temperature. Countercurrent exit air temperature about 100°C for cocurrent exit air, temperature = 10 to... 

The methods used to analyze mass transfer in leaching systems depend on the methods of contacting used and how they are combined in multistage equipment. Appropriate methods of analysis may be the same for two superficially different types of contacting. Thus, crossflow extraction can be analyzed in terms of fixed-bed extraction and, if the two-phase flow in cocurrent slurry extraction is sufficiently pluglike, it... 

In Example 9.2, perfect mixing was assumed on both sides of the membrane. Three other idealized flow patterns, common to other mass-transfer processes, have been studied countercurrent flow, cocurrent flow, and crossflow. For a given cut, the flow pattern can significantly affect the degree of separation achieved and the membrane area required. For a given membrane module geometry, it is not always obvious which idealized flow pattern to assume. Hollow-fiber modules are the most versatile since they may be designed to approximate any of the three flow patterns mentioned above. 

Calculation of the degree of separation of a binary mixture in a membrane module for cocurrent or countercurrent flow patterns involves the numerical solution of a system of two nonlinear, coupled, ordinary differential equations (Walawender and Stem, 1972). For a given cut, the best separation is achieved with countercurrent flow, followed by crossflow, cocurrent flow, and perfect mixing, in that order. The crossflow case is considered to be a good, conservative estimate of module membrane performance (Seader and Henley, 2006). 

Countercurrent flow Parallel (cocurrent) flow Crossflow... 

Phase I flows through each stage in turn. Phase II is fresh feed to each separation stage. Within the stages, both phases are in cocurrent flow, with mass transfer from phase I into phase II. A mass balance over the first stage of the crossflow cascade, according to Eq. (1-178) and the notation in Fig. 1-64, gives... 

In general, it has been concluded that countercurrent flow is the most efficient flow pattern, requiring the lowest membrane area and producing the highest degree of separation, at the same operating conditions. The order of efficiency for the other three flow patterns is crossflow > cocurrent flow > perfect mixing. 

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