Sizing a MistFix unit
Applying the design velocity to size a MistFix unit (Step 5 above) is a bit more tricky than with a conventional flat mesh pad. In a flat pad, the velocity is the same throughout the pad—at least ideally, to a first approximation. That velocity is simply the volumetric flow rate divided by the cross-sectional area of the pad, which is the same throughout the thickness of the pad. However, in the cylindrical mesh pad of a MistFix unit, the cross-sectional area through which the gas enters the mesh at the outer surface is larger than the area of the inner surface.
(See Figure 5.) Thus, the velocity increases as the gas flows radially inward.
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FIGURE 5. A special consideration in selecting a MistFix unit: higher velocity at the inner surface than at the outer
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Depending on the judgment of the designer, the design velocity may be established at the inner or outer surface or anywhere in between. The most conservative and cautious approach is to set the inner-surface velocity at the design value, to make sure re-entrainment is avoided. Velocities farther out toward the outer surface will be progressively lower.
Figure 6 depicts the application of Steps 4 and 5 above to a typical simplified design case. A knockout drum with design throughput of 3,600 actual cubic feet per minute has an 18-inch gas exit nozzle. A MistFix unit is to be inserted in the nozzle, and the proper length is to be determined. The pressure is less than 85 psig. The flowing materials are hydrocarbon vapor and liquid with densities of 0.31 and 45.2 pounds per cubic foot.
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FIGURE 6. Simplified sizing example
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Using a design K factor of 0.42 as discussed before, the design velocity turns out to be 4.91 feet per second. The cross-sectional area required to achieve that velocity is 12.2 square feet. For this MistFix unit, the length to achieve that design velocity at the outer surface is 31.1 inches. By comparison, if a conventional round horizontal mesh pad were used, that design cross-section would correspond to a diameter of 47.3 inches.
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