Height and flow considerations in the vessel
Special considerations for MistFix units are also involved in Step 8 above, regarding available vessel height and internal flow guidelines. The length of a MistFix device is of course constrained by the vertical space inside the vessel as well as the typical 54-inch maximum. The length limit is to allow good drainage of captured liquid, without excessive flooding in the lower end of the MistFix unit. Four and a half feet is a generally appropriate maximum for typical wire mesh, low-viscosity liquid, and a typical light mist load of about 0.1% volumetric. The length limit must be shortened for high viscosity, heavy mist loads, and high-density mesh.
Flow guidelines in turn, encompass two considerations: maintaining an even velocity profile throughout the length of the unit, and avoiding strong turbulence and fluid shear around and below the MistFix unit. Consultation by Amistco engineers is especially important in this area.
To prevent uneven flow through the mesh, the bottom end of the device should be well clear of the inlet stream—more than about half a vessel radius above the top of the inlet nozzle. In many cases, undesirable turbulence can be diminished by a properly selected inlet diffuser. If a manway is not available for installing such a device, it may be possible to use a diffuser designed for insertion through the inlet nozzle as shown in Figure 7. To control flow inequality along the length of the MistFix unit, variation in thickness or density of the mesh may be incorporated.
 |
|
FIGURE 7. Use of an inlet diffuser to help maintain even flow through a MistFix unit when it extends near the inlet nozzle of the knockout drum
|
|
