Counterflow

Spreading velocities v are on the order of 15-30 cm/sec on water [39], and v for a homologous series tends to vary linearly with the equilibrium film pressure, it", although in the case of alcohols a minimum seemed to be required for v to be appreciable. Also, as illustrated in Fig. IV-3, substrate water is entrained to some depth (0.5 mm in the case of oleic acid), a compensating counterflow being present at greater depths [40]. Related to this is the observation that v tends to vary inversely with substrate viscosity [41-43]. An analysis of the stress-strain situation led to the equation... 

Fig. 2. Fluid temperature profiles in (a) a parallel flow heat exchanger and (b) a counterflow heat exchanger. Terms are defined in text.

A similar derivation can be made for a siagle-pass counterflow heat exchanger ia which the hot and cold fluids ate flowing ia the opposite direction (see Fig. 2b). The resultiag heat-transfer equation is still... 

The equations for counterflow ate identical to equations for parallel flow except for the definitions of the terminal temperature differences. Counterflow heat exchangers ate much mote efficient, ie, these requite less area, than the parallel flow heat exchangers. Thus the counterflow heat exchangers ate always preferred ia practice. 

For heat exchangers other than the parallel and counterflow types, the basic heat-transfer equations, and particularly the effective fluid-to-fluid temperature differences, become very complex (5). For simplicity, however, the basic heat-transfer equation for general flow arrangement may be written as... 

Fig. 3. Extreme cases of counterflow heat exchangers having infinite surface area (a) Cf and (b) Cf C. Terms are defined ia text.

The expressions for S can be obtained usiag basic governing heat-transfer equations, such as equations 14 and 19 with proper substitutions of equation 36. For simple, siagle-pass, parallel- and counterflow heat exchangers, the foUowiag expressions result ... 

Fig. 4. Heat-exchanger effectiveness where numbers on the curves represent the ratio flow (b) counterflow (c) parallel...

It is quite common ia the designs for fine classification to recontact the coarse stream transversely or ia counterflow with air before dischargiag it (see Fig. 9). This removes dry fine particles not removed ia the primary classification. That is, these particles are swept back iato the feed and given another chance to exit with the fine particles. Such an arrangement iacreases the overall sharpness iadex and reduces the overall apparent bypass. Another variation is to reenter the air from the sohd/gas separation of the coarse stream. 

Plate Columns. There are two general types of plates in use crossflow and counterflow. These names refer to the direction of the Hquid flow relative to the rising vapor flow. On the cross-flow plate the Hquid flows across the plate and from plate to plate via downcomers. On the counterflow plate Hquid flows downward through the same orifices used by the rising vapor. 

Another important plate which has characteristics similar to a counterflow plate is the Multiple Downcomer (MD) plate (52). This is a plate where the active area occupies the full column cross section but with a pluraHty of small downcomers interspersed among the perforations. The downcomers are specially sealed to prevent upflow of vapor through them. The plate has been used successfully in many high Hquid flow cases. 

Heating and Cooling Agitated Batches 1-2 Farallel Elow-Counterflow... 

Figure 12-12 illustrates water and air relationships and the driving potential which exist in a counterflow tower, where air flows parallel but opposite in direction to water flow. An understanding of this diagram is important in visualizing the cooling-tower process. 

Ultimately, the economic choice between counterflow and cross-flow is determined by the effectiveness of the fill, design conditions, and the costs of tower manufacture. 

In counterflow movement, heat from the outgoing sohds is transferred directly to cold incoming solids, reducing heat losses and fuel requirements. Continuous conveyor ovens are employed also for diy-ing refractoiy shapes and for drying and baking enameled pieces. In many of these latter, the parts are suspended from overhead chain conveyors. 

The cross-flow plate (Fig. 14-16 ) utilizes a hquid downcomer and is more generally used than the counterflow plate (Fig. 14-16Z ) because of transfer-efficiency advantages and greater operating range. The liquid-flow pattern on a cross-flow plate can be controlled by placement... 

A counterflow plate often used for contacting gases with liquids containing sohds is the baffle plate, or shower deck (Fig. 14-21). 

These two types of flooding are usuaUy considered separately when a plate column is being rated for capacity. For identification purposes they are caUed entrainment flooding (or priming ) and downflow flooding. When counterflow action is destroyed by either type, transfer efficiency is lost and reasonable design hmits have been exceeded. 

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