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Posted By holmezy
Afternoon All,
just had a query regarding the face velocity of a "wet" dust extraction unit. The technical spec says it "sucks" 12,183 cubic meteres per hour.
I know that dividing by 3600 gives me per sec. The aperture is tapered, larger at the front edge (2m x .8m), smaller at the point that the dust actually hits the water mist (2m x .36).
How do I calculate the face velocity? Is it simply the velocity divided by the area? In the meantime, I'm off to look through more blurb and seek the answer elsewhere also.
Also, the HSE COSHH essentials indicate that I should be looking at a rate of 1 to 1.5 m/sec to extract stone dust, anyone know better?
Holmezy
nearly time for a beer and a choccy egg!!
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Posted By Smurfer
area of face = 0.8 x 2 = 1.6m2
flow = 12183 m3/hr = 3.38 m3/s
face velocity = 3.38/1.6 = 2.1 m/s
I would check this with an anemometer though rather than relying on manufacturer's specification since you can get losses/pressure drops due to duct length/size/bends/restrictions/etc etc.
HTH
Andy
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Posted By Smurfer
PS. It's the capture velocity which will 'capture' the stone dust and this will depend on how far away from the face the cutting head is.
Look at HSG37 for recommended capture velocities.
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Posted By Merv Newman
Capture velocities are notoriously difficult to measure. As smerfur says the distance and angle from generation and the direction of generation should all be taken into account.
Ideally, for me, the generation point should be within the hood walls and directed straight at the water curtain (if that is what you have)
Direct horizontal extraction away from the worker is a great deal better than vertical extraction straight past the nose (this is also good for chemical fume hoods)
If critical, make-up air should be supplied from the front of the hood or at least from between the worker and the generation point.
Merv
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Posted By Chris Packham
Capture velocity requires also an understanding of the dynamics of the work. For example, if the dust that is being created is being thrown away from the capture system, as might happen with a rotary tool, then the capture velocity will have to overcome the dynamic forces acting on the dust particles before the system can start to draw the particles into the hood.
As Merv says, drawing away from the worker makes sense, but why do so many systems draw upwards when the particles are heavier than air? You then are fighting gravity. Usin a downdraft system means gravity is helping you.
There's a lot more to designing an effective extraction system that capture velocity.
Chris
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Posted By Adrian Watson
Holmezy,
Get competent advice! Designing and evaluating ventilation systems is difficult. If you cannot identify and evaluate the sources of exposure and working practices properly, you cannot design an effective ventilation system.
A substantial number of the many systems that I have seen did not and could not work, despite having been designed by engineers who did the calculations properly, because the system design failed to take account of the the sources of exposure and the working practices of the users.
All systems, upon installation, must be thoroughly examined to ensure that they have been built as designed and tested to show that they are effective. The initial testing must always include air monitoring to assess capture of the contaminants. This air monitoring serves a different function from personal air monitoring. The routine measurement and monitoring of air velocities, static pressures etc, are surrogates for repeating this air monitoring.
Regards Adrian.
PS Merv, supplying air close to an exhaust point is not a good idea as it causes turbulence.
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Posted By Adrian Watson
Holmezy,
Get competent advice! Designing and evaluating ventilation systems is difficult. If you cannot identify and evaluate the sources of exposure and working practices properly, you cannot design an effective ventilation system.
A substantial number of the many systems that I have seen did not and could not work, despite having been designed by engineers, because the system design failed to take account of the the sources of exposure and the working practices of the users.
All systems, upon installation, must be thoroughly examined to ensure that they have been built as designed and tested to show that they are effective. The initial testing must always include air monitoring to assess capture of the contaminants. This air monitoring serves a different function from personal air monitoring. The routine measurement and monitoring of air velocities, static pressures etc, are surrogates for repeating this air monitoring.
Regards Adrian.
PS Merv, supplying air close to an exhaust point is not a good idea as it causes turbulence.
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Posted By Merv Newman
Adrian,
Correct. The input-output balance has to be designed to avoid turbulence. I didn't mean "make-up" as the total and unique supply of air to the extraction system
"Make-up" air should be designed just to make up for the air flow blocked by the users body. THAT really does cause turbulence.
The source of make-up air is between the worker and the work-piece or place.
Holmezy, if it's not too late, get a good designer. If it IS too late then get the air flows (and turbulence) measured. I have occasionally found it worthwhile to have employees wear an atmospheric sampling pump for half a dozen shifts.
Merv (winding down from the gardening)
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