Selecting a CNC Mist Collector

The Tri-Mist 850 and Tri-Mist 1400

Finding the right high-efficiency mist collector for your application to accomplish these goals is important to ensure employee health and a productive work environment.

Which technology?
Mist collectors generally utilize one of three technologies: centrifugal, media or electrostatic precipitation.

Centrifugal Mist Collectors
Centrifugal Mist Collectors operate using centrifugal force. They spin the filter elements which trap the mist and agglomerate it back into larger droplets. These larger droplets are then spun out onto the wall of the filter drum where they are collected, drained, and removed from the mist collector. Centrifugal mist collectors have been around for a long time and it’s is older technology. In theory, they work, but in today’s practical applications they can present numerous challenges to efficient and productive operations. In some cases, they are no longer effective on applications in modern day manufacturing operations.

First, a centrifugal mist collector can be a high maintenance piece of equipment. When the spinning elements start to become dirty they may tend to become unbalanced. This creates vibrations that can be transmitted to the machine tool. Extraneous vibrations introduced into the machine tool operation can make holding close tolerances impossible. If machining precision parts such as precision medical or aerospace components, you may encounter difficulties. For precision machining, a precision balance impeller is desirable. Be prepared for maintenance to keep a centrifugal collector operating properly. If high-pressure coolant pumps are being used in conjunction with the metalworking fluid, extreme atomization of the fluid will occur creating very small submicron particles which do not lend themselves very well to centrifugal separation. In this case, HEPA post filtration will be required.

While centrifugal technology may be adequate for low-pressure light machining it will not remove any smoke created in the process. Smoke is comprised of submicron particles and can be prevalent in modern machining operations where high-pressure coolant and aggressive cuts are common. A centrifugal collector in this application will also require the addition of a HEPA filter.

The airflow volume is not adjustable on a centrifugal mist collector; they run at maximum output all the time. As we will see below, the ability to vary the output of the impeller is important and advantageous. Centrifugal mist collectors tend to be noisy and may require frequent maintenance and rebuilding as previously noted. Because they will not capture very small particles (smoke and very finely atomized mist), they generally require the addition of a HEPA filter to capture these contaminants.

Media Mist Collector Technology
A media-based mist collector employs barrier technology. There is a container for the components and they basically consist of a blower/impeller, perhaps tracks for some kind of pre-filtration, the main filter compartment, and perhaps some type of post-filtration if it is offered. The blower/impeller is often oversized to compensate for an ever-declining airflow volume due to increasing restriction as the filter or filters becomes dirty and clogged. Thus, while a media collector’s retention efficiency increases over time (due to contaminates clogging the filter), its airflow volume decreases. Different media materials with different retention values are available and in many instances can be tailored to the specific application. Replacement filters are an ongoing expense. When they get dirty the must be replaced. If one looks at the cost over the life of the collector, a media mist collector in a source capture application can be expensive to own and operate. The cost of filter disposal when factored in increases the cost of operation, and ownership even more.

Because media mist collectors tend to be oversized to compensate for the loss of airflow volume due to clogging, they tend to use larger blowers making them less energy efficient and increasing the operating cost. Oversizing a media mist collector can make the problem worse, not better. A fixed speed oversized blower can cause excessive air flow that tends to draw up large droplets of mist which would normally drain back into the coolant reservoir leading to media clogging which shortens the life of the filter and causes unnecessary maintenance and filter replacement costs. Media air cleaning systems in many instances are better left to ambient air cleaning applications. Please note however that in certain applications properly designed media mist collectors are an entirely appropriate choice.

Smaller machine mounted media mist collectors often lack sufficient media area to provide for adequate filter life. It is important to learn how many square feet of collection area is included with this type of collector, what its retention level is, and an estimate of the life of the filter because you are going to be replacing those filters for a long time.

Smaller media mist collectors that are promoted for machine mounting often have less than 40 sq. ft. of media. That is not much especially when you consider the hours of operation. If a machine tool is running two shifts, you can count on replacing the filters on a frequent basis. If using straight oil, you should be sure you have sufficient pre-filtration and media collection area. A water-soluble coolant will tend to be more forgiving.

If considering a centrally ducted system, there are some issues you should be aware of. First, ductwork is expensive to purchase, install and maintain. Over time, machining fluids will tend to pool in horizontal duct runs and these runs will eventually leak. Ductwork should also be cleaned on a regular basis. Overhead ductwork will interfere with lighting, create shadows and can be unsightly in general. In a modern clean manufacturing facility, these are negative factors for employees and the shop environment. When it is time to change filters on ceiling mounted centrally ducted mist collectors, ladders or lifts will be needed because the mist collector will not be very accessible.

The objective when installing a mist collector on a machine tool is to clear the air and contain the coolant mist within the cabinet. To do this, we need only enough air flow to draw negative pressure in the machine tool cabinet. Greater airflow than that is not required and could cause blow thru of contamination. The airflow should be adjusted to attain negative pressure in the cabinet and prevent contaminants from escaping through places such as the door, bar feeder, and the chip drag. The mist and smoke that would have exited these areas it evacuated up and through the ESP collector and clean air is returned to the work area. In the case of a mist collector, more is not always better and often can make things worse.

Be sure to understand your warranty. How many years? What does it cover? What kind of service and support can you expect? Are parts readily available? If you have questions, will they be readily answered? Can you recycle the machining fluid? What provisions are made to return the collected coolant to the machine tool sump?