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| In "Enclosures I," you'll learn about why enclosures are
necessary, what kinds of enclosures are available, what should be considered in the design
of an enclosure, and what to do if existing equipment lacks enclosures. |
| In "Enclosures II,"
ventilation and exhaust ductwork for enclosures are discussed. |
| If you need guidance in designing MRF delivery systems, machine
tools, enclosures, exhaust ductwork from the enclosures, mist collectors, and guidelines
for testing mist collection systems, find out more. |
Why are machine tool enclosures necessary?
| The surface tension and cohesive properties of metal removal fluid cause the metal
removal fluid to form droplets. When these droplets are dispersed into the air, they are
usually called "mist" in the shop. Mist may have negative health effects on
those working in metal removal operations. Mist-generating operations should be enclosed
and ventilated, where possible. New machining and grinding equipment should be equipped
with enclosures and appropriate ventilation that minimizes the release of MRF aerosols
into the workplace. |
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Providing well-designed enclosures and splashguards can
reduce employees’ exposure to MRF, as well as preventing the loss of MRF and
improving the general cleanliness of the operation. Also, where effective splashguards are
lacking, the machine operator may be tempted to avoid being splashed by reducing the flow
of fluid to levels that don't control the heat generated by the machining process.
Overheating of cutting tools and work pieces can damage both and may lead to the
generation of fumes or vapors.
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| Existing enclosures and splashguards should be maintained. Missing
equipment and enclosures should be restored. If
guarding has been removed or the enclosure is not maintained, larger particles may escape
through openings in the enclosure. |
What types of
enclosures are available?
There are a number of designs:
Some designs are more effective than others.
What is the "total
enclosure"?
| As its name indicates, total enclosure means the complete enclosure of a particular
machine or operation and proper exhaust of the enclosure. The enclosure or housing is
designed to allow make-up air into the housing and to limit openings to the minimum
required. This is the preferred method of enclosure. |
What is "close
capture"?
| In this design, a contaminant-capturing enclosure is mounted very close to
the point of mist generation and away from the breathing zone of the operator. This device
is open to the ambient air at some point, has a high air (capture) velocity, and has
relatively lower volume requirements than some systems. This is the preferred method for
contaminant capture when total enclosure is not feasible, if it controls mist emissions
adequately. |
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| It should be noted that, when close capture is used with MRF, there may be
a significant loss of the MRF into the exhaust system, causing excessive make-up fluid to
be added and consequently destabilizing emulsion. |
What is "tunnel enclosure"?
| This is a continuous enclosure encompassing several interconnected
machining operations. The same design considerations applied to close capture and total
enclosure (see below) should also be used here. |
What is a "push-pull
hood"?
| The capture (exhaust) hood is at the side of the machine operator, with
make-up air being directed in front of the worker, past the workpiece, toward the exhaust
hood. |
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| Not recommended unless other capture designs are not effective. |
What is a "side-draft
hood"?
| The capture (exhaust) hood is located to one side of the worker or behind
him or her, but there is no specifically directed stream of make-up air as there is in the
push-pull hood. |
Not recommended.
What is a "canopy hood"?
| The canopy hood is located above the machine operator's breathing zone.
Large volumes of exhaust air are required for the operation of this design. |
Not recommended.
What is a "down-draft hood"?
| This device is located in the floor or at the base of the machine. It is
intended to capture and exhaust contaminants pulled down vertically from the machine and
away from the operator. It requires large volumes of exhaust air and is sensitive to the
effects of the heat generated by metal removal operations. |
Not recommended.
What factors need to be taken into account in designing
enclosures?
| The enclosure must be designed for efficiency, safety, accessibility, and
compatibility with the metal removal fluid being used. Some general considerations: |
| Enclosure design should take into account access by cranes so that major
parts of the machine tool may be removed for repair or replacement without destruction of
the enclosure. |
| All enclosures intended to capture MRF mist should be designed and
constructed with materials known to effectively resist degradation by the MRF it will be
used with. Seals, gaskets, and sound absorption materials should be known to resist
degradation by the specific kinds of MRF used. Seals should be designed to hold position
and not shift or roll as entry ports are opened and closed. |
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| Enclosures designed to capture MRF mist may also provide considerable
noise abatement depending on the design and the materials used in the construction. |
| There should be a slope or pitch to the top of the enclosure to prevent
dripping. Enclosure design should avoid sharp corners or protrusions. |
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| Windows may be a design requirement and, where splash and spray may impact
them, consideration should be given to the use of automatic wipers to clean the windows
during operation of the machine. |
| Reasonable access should be planned for the operator, the work pieces and
tool changes. Access ports should be either hinged or sliding. Removable panels are
generally not recommended because they are too often not replaced after being removed. |
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| Where access ports are installed, electrically operated interlocks should
be installed to prevent operation while the ports are open. |
| Safety of the operator during normal operation of the machine should be a
primary consideration in design of all enclosures. Safety reviews should be conducted
during design and installation periods. Compliance with confined space regulations and
safety lock-out capability should be given serious consideration where operators or repair
personnel may be required to enter the enclosure. |
|
| Depending on the type of MRF in use, fire alarms and suppression systems
may need to be part of the design considerations. |
| For some metal removal operations, permanent lights may need to be fitted
inside the enclosure. Where lighting is a factor, design should be consistent with NEC and
NFPA as well as applicable local or plant-specific codes. Under some conditions, portable
lighting systems may be the optimum solution. |
Enclosure design should allow for effective removal of chips and swarf.
| An efficient design will limit the free access of outside air into the
enclosure to minimize exhaust volume and to maintain the designed airflow inside the
enclosure. Enclosure openings should be limited to those necessary for operator access,
exhaust make-up air, and utility entries. Special attention should be paid to areas such
as MRF entry and exit, chip removal systems, and other areas where air may enter or leave
the enclosure. |
| Some make-up (replacement) air is necessary for the system to function
properly. However, a good design should, as much as possible, capture contaminants by
filters, etc., at the working face of the tool before they are exhausted into the air
collection system. |
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| Replacement air ducts should not be placed where the airflow will be
interrupted before it reaches the workface by contact with parts of the machine that do
not generate contaminants. |
| Mist collection exhaust ducts should be located where they are unlikely to
capture either liquid MRF or chips. Airflow velocity at the collection duct should not be
so high as to facilitate capture of chips or liquid MRF. |
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| Exhaust ducting should make use of tapered entries to minimize excessive
duct takeoff velocity. Suggested duct entry velocity should be in the vicinity of 2000 FPM
or less for heavy materials such as cast iron, with lower velocities for lighter materials
such as aluminum or plastics. |
| The enclosure should not interfere with the operation of the equipment
through increased vibration or capture of heat. |
| The manual Industrial Ventilation: A Manual for Recommended Practice, available
from the American Conference of Governmental Industrial Hygienists, has much useful
information concerning the proper design and operation of ventilation systems |
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What if existing equipment lacks an enclosure?
| Retrofitting existing equipment should be
considered, using ANSI Technical Report B11 TR 2-1997 as a guide. Unless properly designed
and constructed, retrofits may not significantly capture metal removal fluid aerosols.
With some equipment, retrofitting may not be possible or economically feasible. |
| How effective is retrofitting in comparison with new-equipment enclosures? |
| Enclosure and ventilation are important controls for MRF mist. Where
feasible, machine enclosures should be phased in with new machinery or machinery rebuilds.
There is considerable difference in the effectiveness and cost of for OEM (Original
Equipment Manufacturer) enclosures compared to "retrofit" enclosures. |
OEM enclosures:
 | OEM enclosures are cleaner than retrofit ones, with less evidence of MRF on the floors
of OEM areas and less visible mist in the air. |
|
| Ventilated OEM enclosures have can result lower mist levels, in many cases to 0.5 mg/m3
or less. |
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| Even without ventilation, some OEM enclosures can approach 1.0 mg/m3 of mist.
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Retrofit enclosures:
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| Retrofits are much less efficient; even with ventilation, they have
difficulty lowering mist levels consistently to 1.0 mg/m3. Some studies show
little or no benefit for retrofit enclosures compared to non-enclosed machines. |
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| Retrofitting takes considerable time and effort; it involves
 | disrupting production |
| trying to fit to predetermined "footprints" |
| custom fitting to varied shapes |
| tying into existing ventilation systems |
| working in crowded surroundings
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| The additional effort shows up in costs as well, with retrofits costing
approximately twice as much as OEM enclosures. |
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Contact Chris Roman, 