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Oil Mist Collection Technologies

Here, I will be discussing technologies used in oil mist collection.  There are really three major technologies being used today for coolant mist collectors:  electrostatic precipitation, media filtration, and centrifugal separation.

The latest and greatest, ideal technology for metalworking fluids mist is electrostatic.  The other two choices, media and centrifugal have mechanical problems when they interact with a mixture of liquid and solid particles found in machine oil mist and smoke.  We get calls all day long from facilities with media or centrifugal wanting to scrap all their equipment to replace it with electrostatic.  Centrifugal and media mist filtration are a huge drain for your company’s money and maintenance resources.  Regardless of whether its water, synthetic or oil based, electrostatic mist collection works best.

Within the history of filtration, media came first.  Filter Media is simply a woven or spun material that catches particles like a dense mesh net.  A common example of media filtration is a furnace filter.  We’ve all seen a dirty furnace filter and have felt the effect it has on your heat or air conditioning.  Especially in the summer time, when the air is humid, the furnace filter gets coated with dust that piles up thicker and thicker.  In humid weather, the dust is slightly moist, which "blinds" the filter.  This blockage is especially worse in the humid months.  If the layer of dust were dry, air can still pass through, between the dust particles, while more dust piles up.  But when moisture is present, it binds the dust particles together in a "clump" preventing airflow.

Now apply this to your mist producing industrial process.  There will be some smoke particles of what you are machining, along with tiny liquid mist droplets from the coolant.  For most applications, there will be more mist than anything else, followed by smoke particles.  The filtration method must accommodate both liquid and solid particles.

Media was developed for solid particles.  It works great with dust, yet has been altered to accommodate liquid.  To do so, they space the mesh out and use a material that is less likely to absorb liquid.  But in order to catch anything, the spacing between fibers must remain close enough to retain and physically prevent from passing whatever is being collected.  The mesh must be tight enough to catch submicron particles, much smaller than the eye can see.  This works fine with dry dust because the air can still pass through the accumulation of solid dust particles.  Unless you can breath under water, air cannot pass through an accumulation of liquid. Remember from high school physics liquid properties of adhesion and cohesion?  Adhesion is what makes the liquid stick to the media filter and solid smoke particles.  Cohesion is what makes the liquid stick together as a droplet or puddle in a blob-like mass.  It is because of these properties that media, no matter how refined the material, will never work beyond an initial particle load to the filter.  The filter becomes wet and does not allow air to easily pass.  Anyone who had tried using media filtration for oil mist eliminators will tell you, each filter lasts a very short time before it must be thrown away and replaced with a new one. That’s where big money is lost.

Electrostatic precipitation works great on liquid and solid particles.  It uses 2 stages, the first stage (ionizing) passes contaminated air through a magnetic field that gives all liquid and solid particles an electric charge.  The second stage (collection) moves the air, which is now full of charged mist and smoke particles between metal plates.  These plates have an opposite electrical charge that pulls the particles out of the air stream.  Unlike the woven mesh of media filtration, these plates do not affect the air flow.  Compare the spacing between fibers of the media mesh with the spacing between collection plates.  You can’t even see through the media filters mesh. Collection plates in an electrostatic mist collector can be up to 1/8 of an inch apart!  This gives more than enough space for months worth of microscopic solid particles to build up, while they are saturated with oil or coolant.  The excess liquid, clean of particles, drains off, to be recycled.  And obviously, there are no filters to dispose and replace, just run the cells through your parts washer.  For oil mist collectors, there is no comparison between media filters and electrostatic precipitators.

Now let’s talk about Centrifugal Mist Collectors.

Centrifugal Oil Mist Separators are pretty cool, in the way they operate… but they just don’t work well in real world applications.  They’re very touchy and go out of balance easily.  To keep them working properly, the manufacture provides a huge checklist of maintenance that they don’t tell you about during the sales process.  Here is a direct quote from a major manufacturer of centrifugal mist eliminators:

…For safety purposes, it is necessary to periodically inspect the [manufacturer] unit’s main components for wear or damage, especially if loud noises or vibration is present. Additionally, we recommend these simple maintenance procedures be carried out once a year:

  • Inspect and replace the motor mounts and torque-limiting straps if worn or damaged.
  • Inspect and replace silencer if necessary.
  • Remove drum and thoroughly clean. Inspect for any damage.
  • Replace or clean drum pads. (Sometimes the pads can be cleaned with soapy water. Remember, the pads are not filters so they don’t have to be perfectly white, but it is important that they are not packed with solid buildup which could cause vibration and wear problems.)
  • Replace lid seal.
  • Check all other accessories for wear and damage, ie: ducting, flange adapters, etc. Repair or replace as necessary.
  • Check all mounting fixtures such as stands and suspension kits for damage or signs of fatigue. Repair or replace as necessary.

Some aspects of this program may have to be carried out on a more frequent basis, depending upon the application. Applications which typically require more frequent maintenance are those in which very fine chips are produced, such as cast iron machining and grinding…

Their suggested maintenance downplays the problems by saying "if loud noises or vibration is present".  And they suggest performing this rebuild procedure once a year "if neccessary".  The complaints we’ve heard from centrifugal mist collector owners is that complete disassembly and meticulous cleaning is necessary on a monthly basis.  Total rebuilds are usually necessary once a year.  Which means, basically replacing all the equipment every year, plus a day of downtime every month.

Vibration from the centrifugal mist collector transferring to the machine tool has also been a big complaint.  The spinning centrifugal mist collector throws off the precision of the machine tool, causing more out of spec parts for the discard bin!

So why does this happen?  The centrifugal mist collector uses a large spinning drum to collect mist and smoke, and fling the mist outward into an outer chamber.  The problem is that there are so many high speed, moving, spinning parts that are in the contaminated air stream.  Moving parts being coated in coolant mist and particles leads to "gummed up" spinning parts.  The dirtiest part of a dirty environment is constantly blowing through the spinning parts of a centrifugal mist collector.

Besides, OscarAir has chosen to feature the Trion electrostatic Mini Mist Eliminator as the "Best Value in Class" …And, manufacturers give OscarAir exclusive pricing, making the highest quality machine-mount mist collector one of the lowest priced mist collectors on the market.

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