Parallel Flows

Basic Heat-Transfer Equations. Consider a simple, single-pass, parallel-flow heat exchanger in which both hot (heating) and cold (heated) fluids are flowing in the same direction. The temperature profiles of the fluid streams in such a heat exchanger are shown in Figure 2a. 

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

Assuming that U, and are invariant with respect to temperature and space, one can integrate equation 14 subject to equation 19, and obtain, after rearrangement, a basic heat-transfer equation for a parallel-flow heat exchanger (4). 

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. 

A numerical study of the effect of area ratio on the flow distribution in parallel flow manifolds used in a Hquid cooling module for electronic packaging demonstrate the useflilness of such a computational fluid dynamic code. The manifolds have rectangular headers and channels divided with thin baffles, as shown in Figure 12. Because the flow is laminar in small heat exchangers designed for electronic packaging or biochemical process, the inlet Reynolds numbers of 5, 50, and 250 were used for three different area ratio cases, ie, AR = 4, 8, and 16. 

Fig. 8. Combined flow reactor models (a) parallel flow reactors with longitudinal diffusion (diffusivities can differ), (b) internal recycle—cross-flow reactor (the recycle can be in either direction), comprising two countercurrent plug-flow reactors with intercormecting distributed flows, (c) plug-flow and weU-mixed reactors in series, and (d) 2ero-interniixing model, in which plug-flow reactors are parallel and a distribution of residence times dupHcates that...

Fig. 6. Approaches to minimising entrapment and impingement of fish and large aquatic invertebrates, eg, blue crabs, on trash screens at intakes, (a) An inlet pump house with vertical traveling screens mounted flush with a river shoreline to minimise obstmctions to animal movements (b) parallel flow to direct fish to a recovery chamber that returns to the water body (c) a velocity cap atop a vertical, offshore inlet induces a horizontal flow which fish avoid...

Fig. 8. Drying time and rate profiles for leather pasted on glass plates and dried in two temperature stages. Gas velocity = 5 m/s in parallel flow, 71°C in the first stage, 57°C in the second. The falling rate, drying rate is proportional to residual moisture content.

Parallel flow, suction in ends Series flow (basic compressor)... 

Parallel flow, suction in center Series flow, one cooling point... 

Parallel flow. The direction of gas flow is parallel to the surface of the sohds phase. Contacting is primarily at the interface between phases, with possibly some penetration of gas into the voids among the solids near the surface. The solids bed is usually in a static-condition (Fig. 12-30). 

Further, because of the nature of sohds-gas contacting, which is usually by parallel flow and rarely by through circulation, heat transfer and mass transfer are comparatively inefficient. For this reason, use of tray and compartment equipment is restricted primarily to ordinaiy drying and heat-treating operations. Despite these harsh hmitations, when the listed situations do exist, economical alternatives are difficult to develop. 

In order to estimate diying rates from Eq. (12-42) values of the empirical constants are required for the particular geometry under consideration. For flow parallel to plane plates, exponent n has been reported to range from 0.35 to 0.8 [Chu, Lane, and Conklin, Ind. E/ig. Chem., 45, 1856 (1953) Wenzel and White, Ind. Eng. Chem., 51, 275 (1958)]. The differences in exponent have been attributed to differences in flow pattern in the space above the evaporating surface. In the absence of apphcable specific data, the heat-transfer coefficient for the parallel-flow case can be taken, for estimating purposes, as... 

Drops collide with the drop separator and drain down to the lower part of the tower. These drops are large, so their total surface area is small and insignificant. The effective heat transfer process takes place when the drops move with the air flow, so this arrangement has to be treated as a parallel flow heat transfer. 

FIGURE 4.20 Heat transfer according to the parallel flow principle. 

FIGURE 4.21 Water cooling in a cooling tower, which operates according to the parallel flow principle in other words, the water spray turns in the direction of the air flow. 

The direction of flow is important, as it has a pronounced effect on the efficiency of a heat exchanger. The flows may be in the same direction (parallel flow, cocurrent), in the opposite direction (counterflow), or at right angles to each other (cross-flow). The flow may be either single-pass or multipass the latter method reduces the length of the pass. 

A schematic diagram of a parallel-flow (cocurrent) heat exchanger is shown in Fig. 9.4. 

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