Ip

Figure 2. Two-section Paraflow with connector plate.

Figure 3. Cutaway of Paraflow plate shows turbulence during passage of product and service liquids.

important in heat-recovery processes with close temperature approaches and even in cases with temperature crossovers.

Whenever the thermal duty permits, it is desirable to use single-pass, countercurrent flow for an extremely efficient performance. Since the flow is pure counterflow, correction factors required on the LMTD approach unity. Furthermore, with all connections located at the head, the follower is easily moved and plates are more readily accessible.

Mixing and Variable-Length Plates

To obtain optimum thermal and pressure drop performance while using a minimum number of heat-exchanger plates, mixing and variable-length plates are available for several APV Paraflow plate heat-exchanger models. These

Figure 3. Cutaway of Paraflow plate shows turbulence during passage of product and service liquids.

on a shell, however, can be related to the number of plate passes and since the number of passages/pass for a plate is an indication of the flow area, this can be equated to the shell diameter. This is not a perfect comparison but it does show the relative parameters for each exchanger.

With regard to flow patterns, the Paraflow advantage over shell and tube designs is the ability to have equal passes on each side in full countercurrent flow, thus obtaining maximum utilization of the temperature difference between the two fluids. This feature is particularly

Plate Construction

Depending on type, some plates employ diagonal flow while others are designed for vertical flow (Fig. 8). Plates are pressed in thicknesses between 0.020 and 0.036 in. (0.5-0.9 mm) and the degree of mechanical loading is important. The most severe case occurs when one process liquid is operating at the highest working pressure and the other at zero pressure. The maximum pressure differential is applied across the plate and results in a considerable unbalanced load that tends to close the typical 0.1-0.2-in. gap. It is essential, therefore, that some form of interplate support be provided to maintain the gap and this is done by two different plate forms.

One method is to press pips into a plate with deep washboard corrugations to provide contact points for about every 1-3 in.2 of heat-transfer surface (Fig. 9). Another is the chevron plate of relatively shallow corrugations with support maintained by the peak-peak contact (Fig. 10). Alternate plates are arranged so that corrugations cross to provide a contact point for every 0.2-1 in.2 of area. The plate then can handle a large differential pressure, and the cross pattern forms a tortuous path that promotes substantial liquid turbulence and thus a very high heat transfer coefficient.

Figure 4. Single-pass countercurrent flow.

Vents molded into gasket

First liquid

Plate Heat Exchanger Washboard

Figure 5. Gasket showing separation of throughport and flow areas.

Vents molded into gasket

First liquid

Gasket

Interspace open to atmosphere

Second liquid

Gasket

Interspace open to atmosphere

Second liquid

Plate Heat Exchanger Washboard

Figure 8. Diagonal and vertical flow patterns.

Outside plate edge

Shell and Tube Plate Equivalent

Tube side

No. of passes

No. of passes Side 1

Shell side

No. of tubes/pass No. of cross passes (No. of baffles + 1)

No. of passages/pass

No. of passes Side 2

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