Vane Demister

A vane mist eliminator, or vane demister, is a gas-cleaning device that pulls fine liquid droplets out of a flowing gas. It is made of a stack of corrugated blades, or vanes, set close together in a zigzag pattern, so that gas passing through is forced to change direction again and again along a narrow, winding path. That is the whole trick. The gas can follow the twists easily, but the droplets it carries are heavier and keep going straight, so they slam into the blade surfaces and stick. The little droplets that collect then run together into big ones, and gravity pulls those down the blades and out through a drain, so the caught liquid leaves the unit instead of being blown back into the gas. Three things happen in sequence, then: the droplets impact the vanes, they coalesce, and they drain away. The result is clean gas out of the top and collected liquid out of the bottom.

They do the same job, catching droplets, but in different ways, and they suit different duties. A wire mesh demister is a pad of knitted wire; it catches droplets on the fine wires and is very good at trapping small, fine mist. Its weakness is that the tight mesh clogs easily if the gas is dirty or the liquid is sticky, and it cannot take a high gas velocity before it floods. A vane demister, with its open, robust blades, is the opposite: it shrugs off fouling, drains freely, and handles a much higher gas flow, this vane pack carrying 30 to 100 percent more gas than a wire mesh pad of the same face area. The trade is that vanes catch slightly larger droplets than a fine mesh does. So the rule of thumb is: wire mesh for fine mist in clean service, vanes for high flow, higher velocity or dirty, fouling-prone streams. Tell us the gas and the droplets and we will advise which fits.

This vane pack removes better than 99 percent of droplets five microns and larger, which covers the great majority of the entrained liquid in most gas streams. It does so at a very low pressure drop, a few tenths of a kilopascal rising to only a little over one kilopascal in hard duty, so it costs almost nothing in fan or blower energy. It works best within a band of gas velocity, roughly two and a half to five metres per second: too slow and the droplets are not thrown hard enough to be caught, too fast and captured liquid can be torn off the blades and re-entrained. The blade spacing, offered at 20, 25 or 30 millimetres, is chosen to match the droplet size and the flow. Give us your gas rate, its droplet load and the vessel size and we will size the pack and set the spacing.

The vanes are made in whatever material suits the chemistry: 304 or 316L stainless steel for general and mildly corrosive gas, and polypropylene, fibre-reinforced plastic or PTFE where acids or aggressive vapours would attack metal. That range lets the same design go into almost any process. A vane demister is used wherever a gas leaves a process carrying a mist that must be removed, at the top of scrubbers and absorbers, above evaporators and flash vessels, in knock-out drums and separators, and in the vents of columns and reactors. Its purpose is always the same: to protect downstream equipment, to recover valuable liquid, and to keep droplets and their pollutants out of the discharged gas. The industries are the chemical, gas and general process trades. Send us the service and we will confirm the material and the design.