system components
multiple stages of filters
glossary terms:
pocket filter
cartridge filter
DOP
HEPA
types of filters for each stage
1st and 2nd stage
2nd stage
3rd stage
effectiveness of filters
location of mist collector discharge
if air is recirculated |
| Mist collectors should be designed and maintained using ANSI B 11 TR 2-1997 as a
guide. |
| Poorly maintained mist collectors may increase metal removal fluid aerosol
concentrations in workplace atmospheres. It is important to inspect air cleaner
filters and clean them regularly or replace them, as appropriate. Collected
aerosol should never be allowed to drain back into the fluid system. |
What are the components of such a filtration system?
| There are many commercial mist collection systems available, but the best
design for a given application depends on many factors of each specific application. |
In general, commercial filters have multiple stages.
 | The first stage is relatively inefficient.
It filters out the largest droplets. |
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| The final stage, using DOP or HEPA (high efficiency particulate
air) filters, usually removes 95% of whatever has not been removed in the first two
stages. |
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| Most filters are effective when new, but they may rapidly lose
effectiveness as they become loaded with liquid. Filters become more efficient as solids
cake onto the filter, but less efficient with liquid loading. Filters with high liquid
loading may release droplets or vapor back out into the air. As filter loading increases,
the pressure drop increases, and the power required to move air through the system
increases, as does operating cost. |
What types of filters are used for the first stage?
Primary and secondary
filters:
| Open cell foam designed to coalesce MRF particles (that is, to collect small, individual
droplets and combine them until they form a liquid that can be drained) |
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| Metal mesh filters of various sizes designed to remove large droplets |
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| Centrifugal filters, which are designed to remove airborne droplets through centrifugal
force and drain them out of the system. |
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| Helical tube systems, with many helical elements housed in a tube and mounted in a
common frame (these elements cause the air to rotate, and centrifugal force throws the
particles onto the tube, where they collect) |
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| Electrostatic precipitators, which electrostatically charge the droplets and attract
them to a collection element, where they coalesce and drain out of the system |
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| Disposable filters designed to be replaced on a regular basis |
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What are the types of second-stage
filters?
The same kinds of filters are
used for the second stage as for the first stage.
What are the types of final-stage
filters?
| Media bags with an air-to-medium (cloth) ratio of no more than 10 CFM/sq. ft.(10 cubic
feet per minute of air to a square foot of medium) |
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| Rigid cell: 250-500 FPM face velocity |
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| Cartridge filters with an air-to-medium ratio of 10 CFM per square foot maximum |
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Are some filters more effective than others?
| For first-stage filters, experimental work has shown that metal
mesh filters were as efficient as the other designs, and they experienced lower pressure
drop. Efficiencies for metal mesh filters can be significantly increased with only modest
increases in pressure drop. |
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| For second-stage filters, electrostatic precipitators have
generally been found to be as efficient as pocket filters. They experience very low
pressure drops, require only moderate maintenance, and provide long service life. Other
filters, such as pocket filters, lose their efficiency more rapidly, but in some
applications can last up to a year. For mineral oil, up to 30% of the mist entering the
system can be expected to evaporate and pass through the filter as vapor. For some mineral
oils, collectors with multiple stages had the lowest overall efficiency (still quite
acceptable), but the electrostatic precipitator provided the highest overall efficiency,
perhaps because the oil droplets were removed from the air stream and drained away before
they could evaporate. |
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| For third-stage filters, all 95% DOP filters have high efficiency
for droplets of all sizes, but the lifetime efficiency of these final-stage filters
depends on the efficiency of the first- and second-stage filters. |
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| A useful discussion of the costs of controlling MRF mists can be
found in the Proceedings of the November 13-16, 1995 AAMA symposium, The Industrial
Metalworking Environment Assessment & Control (page 321). |
| Does it matter where the mist collector
discharge is located? |
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| The discharge of the mist collector can be directed back into the shop or
it can be directed outdoors through the roof or wall of the building. Each method of
discharge has its supporters. |
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| Discharging the cleaned air back into the shop can save energy in some
instances, since the air does not need to be replaced by supply air. Possible
disadvantages of this method are that an improperly operating mist collector will return
some mist to the shop. Also, vapors are not removed by the filters and these may
contribute to mist levels in the shop or contribute to odor problems. |
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| Discharging the mist collector exhaust from the building eliminates the
possibility of increasing the indoor mist level and it eliminates moisture and vapors from
the building. The disadvantages can be increased need for building supply air and
increased energy costs. There may also be a need to obtain an EPA permit for venting the
process air from the building. |
What if air is
recirculated?
Where recirculation of exhaust ventilation is used,
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| The air must be adequately cleaned of contaminants, chemical and
microbial, before it is recirculated. |
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| Equipment used to clean air should be efficient enough to meet rigorous
collection standards and to maintain their efficiency over time. |
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| Monitoring of recirculated air should be performed often enough to assure
that contaminant levels do not exceed established limits. |
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Contact Chris Roman, 