Mist extractors can be made of wire mesh, vanes, centrifugal force devices, or packing. Wire mesh pads (Figure 4-10) are made of finely woven mats of stainless steel wire wrapped into a tightly packed cylinder. The liquid droplets impinge on the matted wires and coalesce. The effectiveness of wire mesh depends largely on the gas being in the proper velocity range. If the velocities are too high, the liquids knocked out will be re-entrained. If the velocities are low, the vapor just drifts through the mesh element without the droplets impinging and coalescing.
The construction is often specified by calling for a certain thickness( usually 3 to 7 inches) and mesh density (usually 10 to 12 pounds per cubic foot). Experience has indicated that a properly sized wire mesh eliminator can remove 99% of 10-micron and larger droplets. Although wire mesh eliminators are inexpensive they are more easily plugged than the other types. Vane eliminators (Figure 4-11) force the gas flow to be laminar between parallel plates that contain directional changes. Figure 4-12 shows a vane mist extractor made from angle iron. In vane eliminators, droplets impinge on the plate surface where they coalesce and fall to a liquid collecting spot. They are routed to the liquid collection section of the vessel. Vane-type eliminators are sized by their manufacturers to assure both laminar flow and a certain minimum pressure drop.
Some separators have centrifugal mist eliminators (as shown in Figure 4-13) that cause the liquid drops to be separated by centrifugal force. These can be more efficient than either wire mesh or vanes and are the least susceptible to plugging. However, they are not in common use in production operations because their removal efficiencies are sensitive to small changes in flow. In addition, they require relatively large pressure drops to create the centrifugal force. To a lesser extent, random packing is sometimes used for mist extraction, as shown in Figure 4-14. The packing acts as a coalescer.