Mist Collector Installation Part 3

June 22, 2009
The mist-collector system has been installed. Then, problems arise that weren’t there before, or that have been there ever since the initial installation. What’s next? When a shop receives a new mist collector, it will have an ...

The mist-collector system has been installed. Then, problems arise that weren’t there before, or that have been there ever since the initial installation. What’s next?

When a shop receives a new mist collector, it will have an installation/operation manual that includes a basic troubleshooting section specific to that collector. But in addition to that information, there are some common, frequently asked troubleshooting questions and answers that can supplement those manuals.

For example: Why, all of a sudden, do system filters need changing two or three times per year while they used to be changed only once yearly. The answer may be:

• A facility has changed its operations from running one shift per day to two shifts. Operating twice as many hours per week will lead to changing filters over twice as often. If a shop wants less-frequent filter changes, it may be necessary to install additional filtration systems, splitting the airflow between the existing and new systems.

• A facility has changed to operating three shifts per day. Most gravity draining mist collectors are not designed for operating three shifts per day (continuous duty) unless they were originally designed and installed with that in mind. Mist collectors often need to have their airflow de-rated 33 percent to 50 percent of maximum design to provide acceptable filter life on continuous operations. Contact the manufacturer of the collector to discuss continuous-duty requirements.

• A shop has changed its process from using watersoluble coolant to straight oil. Straight oil, if delivered at pressures over 800 psi, used on very hot applications, or on machines with high-velocity through spindle nozzles, can create smaller mist particles than water-solubles. Smaller mist particles take longer to coalesce into large enough droplets that can drain, which will keep more mist on the filter longer, causing higher pressure drops. Smaller mist particles can pass through the primary filter media also, and cause shorter life of the final filter, such as with a HEPA. De-rating the airflow down to 75 percent of the current airflow on the collector is recommended if a shop’s primary filter requires replacing sooner than before. If a shop’s HEPA filter is plugging, switching to 95 percent DOP final filters should help. For most mist applications, the 95 percent DOP will provide similar air quality and last three to five times longer.

• A shop has changed its process from using straight oil, synthetic, or semi-synthetic to a water-soluble coolant. Water-soluble coolants and some semi-synthetic coolants contain up to 20 percent paraffin wax. As water evaporates out of the coolant, it leaves behind the paraffin wax that builds up on the filter media fibers, shortening the filter’s life. This is most common in applications running less than 20 hours per day, where collected mist amounts vary constantly, or where the mist collector runs for more than one hour without any liquid mist being collected. A warm water spray can re-liquefy the paraffin wax in some instances. Warm water spray is most helpful in restoring the primary filter of a centrifugal mist collector back to “new.” Another recommendation is to consider a machine tool interlock that turns off the mist collector when no mist needs to be collected.

• A shop is now machining different metals or metal alloys than it was handling previously, or not in such volumes. Harder metal alloys can create more heat, causing finer mist particles and smoke. This will cause primary filters to become more saturated with mist as smaller particles take more time to coalesce into larger droplets that can drain. Using a machine tool interlock to maximize “off time” of the collector to promote draining will improve primary filter life. If there is sufficient airflow, de-rating the collector to 75 percent of its current airflow also will increase the primary filter life. If a HEPA filter is plugged, try switching to 95 percent DOP final filters. For most applications, the 95 percent DOP will provide similar air quality and last three to five times longer.

• When using straight oil, a shop has increased the coolant pressure recently. Higher coolant pressures, especially with straight oils, cause smaller and finer mist particles to form. This will cause primary filters to become more saturated with mist as smaller particles take more time to coalesce into larger droplets that can drain. Again, use a machine tool interlock to maximize “off time” and de-rate the collector to 75 percent of its current airflow, 95 percent for plugged HEPAs, to increase filter life.

• A shop is purchasing filters from a low-cost, knock-off distributor claiming to supply filters of the same quality as the original manufacturer, instead of using original manufacturer’s replacement filters or superior quality filters. When a shop purchases filters for half the price, but gets only a third or half the service life, the savings are minimal. Purchase filters from the original manufacturer and see if filter life returns to original.

Another common troubleshooting question asks why the filter life, from the time the collector was installed, has been considerably shorter than expected.

• The mist collector is machine mounted without ducting. Most machine-mountable mist collectors have a small-diameter inlet connected to the machining center. This small inlet also may be the same hole through which collected mist drains, via calculating or measuring air velocity through that inlet. Any mist collected on the filter is unable to drain through an upward velocity of 5,000 fpm. The fix is to add a hopper with legs or inlet option that separates the air inlet and drain. Short life caused by poor drainage is typically due to operating 24 hours per day without sufficient time to drain. A machine tool interlock installed between the machine tool and mist collector can significantly increase filter life, as it will provide more draining time.

• A shop’s system may have been designed with machine tool connection velocities over 3,000 fpm. Mist collection, when done on a fairly well enclosed machine, is about mist containment — not sucking like a vacuum trying to capture every bit of large mist spray. A capture velocity below 2,000 fpm at the machine tool will minimize the amount of mist captured, while providing the same level of mist containment. Having less mist drawn into the ducting means there is less mist that the mist collector has to handle, which results in longer filter life. The solution is to increase the size of the pick-up point on the machine tool to provide a lower velocity inlet and modify ducting as appropriate. The capture velocity of the hood at 2,000 fpm or below should also be maintained for at least the first 5 ft of the ducting.

• A shop’s system was designed with duct velocities greater than 3,000 fpm, and once mist is captured in the ducting, a shop has two options. The first is to transport the mist at a high velocity to ensure that almost all of it will be airborne when it arrives at the mist collector. The second is to slow the air velocity, so a majority of larger mist particles settle out of the air and drain down the ducting. To do this, increase the size of ducting such that the air velocity in the ducting is between 2,000 and 3,000 fpm, averaging 2,500 fpm throughout the entire system. Not only will less mist be airborne when it gets to the mist collector, but increasing the diameter of ducting will decrease the amount of static pressure required to move the air and mist from the machine tool to the mist collector, allowing additional static pressure for filter loading. Make sure to slope the ducting a few degrees to allow any mist that drops out to run toward the machine tool or back to the machine enclosure.

• Spray generated by the machine tool is being sprayed directly into the mist collector inlet, or chips are being thrown directly into the mist collector inlet. To remedy this, add a deflector to the pick-up point inlet to reduce the amount of spray and chips that can be pulled directly into the mist collector inlet. If there is insufficient room, look for another location to install the mist collector inlet such that the spray and chips generated will not be pulled into the mist collector. Or, contact the manufacturer and ask about higher efficiency prefilters that can remove more mist than the current pre-filters do.

• A shop is using water-soluble coolant. Again, these and some semisynthetic coolants can contain up to 20 percent paraffin wax. (Follow the recommendations for dealing with paraffin wax in water-soluble or semi-synthetic coolants given in the first troubleshooting question.)

A final common mist-collector troubleshooting question asks why mist is passing directly through filters and creating visible emission at the discharge and/or creating stains on walls.

• A shop’s process is generating a lot of heat, vaporizing the oil or water used in the process. Mist collectors cannot stop oil vapor or steam until it condenses to a liquid. To fix the problem, cool the air stream to below 104 degrees F at least 15 ft before the inlet of the mist collector by drawing in cooler plant air through a “Y” or “T” fitting.

The mist collector was not designed to handle the increased airflow and duty cycle resulting from a shop boosting its operating schedule, causing filters to become over saturated. Once at 100 percent saturation, any additional mist collected pushes mist through the filter and causes a visible emission. This is most common in V-bag-style units that are unforgiving when not designed properly. Even with filter static pressure drops reading less than 1-in. H2O, saturation can be reached and visible emissions occur.

A process is generating smoke, and the mist collection system does not have a final filter installed. Contact the manufacturer for recommendations on installing a 95-percent DOP or HEPA filter on the mist collector for handling smoke.

• A mist collector letting mist through its filters does not necessarily cause staining on walls. Any fan that blows air will push ambient shop air around, and if the ambient air is not clean due to other sources of mist, smoke or dust in the shop, it will impact a wall surface and may leave a visible stain after several months of operation.

This is the final installment of a series. For Part 1, see American Machinist April 2009, p. 20; for Part 2, see American Machinist May 2009, p. 18.

Information for this article supplied by Donaldson Co. Inc.

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