Mesh and Vane Mist Eliminators

THE EFFICIENCY OF VANE mist eliminators is generally acceptable only for droplets larger than 10 or 20 microns in the case of air and water at ambient conditions. (Compare efficiency curves on Pages 14 and 15.) Furthermore, a vane unit is generally more expensive than a mesh pad in the same application. However, vanes have certain advantages that dictate their selection over mesh in some situations.

1. High velocity: Being less susceptible to re-entrainment and flooding than mesh pads, vane units can operate at velocities 30 to 40 percent higher in both vertical and horizontal flow. (See Table 2.) Higher velocity helps close the efficiency gap with mesh.

2. High liquid load: Vane units typically handle loads about 5 to 10 times greater than mesh pads: up to 10 gpm/ft2 for VNM-50-6 vanes, versus 1 gpm/ft2 for TM-1109 mesh (horizontal flow, air and water, ambient conditions).

3. Fouling and clogging: Solid particles and debris that would lodge in a mesh pad, eventually requiring replacement or cleaning, pass through the much larger apertures of a vane unit. In applications that are subject
to buildup of deposits, vane units can operate for much longer intervals without cleaning and can be cleaned much more readily than mesh pads.

4. Longer corrosion life: The thickness of vanes gives them a substantially greater service life than mesh with the same corrosion rate. In a given corrosive service, a vane unit made of sheet metal will last much longer than a mesh pad made of the same alloy.

5. Low pressure drop: The relative openness of vanes gives them an edge over mesh in applications where pressure drops of a few inches of water column are crucial. (See graphs on Pages 14 and 15.)

6. High liquid viscosity: There are a few applications in which high viscosity impedes liquid drainage so severely that a mesh pad would flood at prohibitively low velocities and liquid loads. Vanes can handle much higher liquid viscosities.

7. Rugged construction: When properly secured in place, a typical vane unit withstands violent surges and liquid slugs that would dislodge and even destroy the most rugged mesh pad.

8. Foam accommodation: Because of liquid agitation in mesh pads, those devices are not generally recommended in applications subject to foaming. Vane units, by contrast, not only drain without foaming, but can actually break foam generated upstream. In view of Items 3, 4, and 7 above, vane units are especially attractive in applications that require high reliability for long periods without maintenance or replacement. Offshore platforms and long-running processes are prime examples.

Vane units can be especially valuable in certain applications when used immediately upstream or downstream of mesh pads. Figures 20 and 21 illustrate these concepts with horizontal flow. With vertical flow, capacity will be reduced as explained before for mesh pads and vane units alone. Mounting a vane unit downstream of a mesh pad as in Figure 20 combines the superior efficiency of the mesh with the superior K-factor of the vanes. The typical K-factor for horizontal flow is raised from 0.42 for mesh alone (Table 2) to 0.65 for vanes. When operated at or above the resulting design velocity, the mesh pad serves as an agglomerator or coalescer of fine mist droplets. Most liquid captured in the mesh pad is re-entrained as larger droplets whose sizes are well above the lower limit of the vane unit. Higher velocity also improves the mist elimination efficiency of the mesh. In applications of co-knit mesh where the re-entrainment velocity is exceptionally low, a downstream vane unit is indispensible. On the other hand, mounting a vane unit upstream of a mesh pad as in Figure 21 combines the superior efficiency of mesh with the superior load and solids-handling ability of vanes. The K-factor of the combination is that of the mesh pad.