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Posted By Kevin Fitzgerald
Hi All,
I am currently doing some research on the safety and effectiveness of recirculating fume hoods with HEPA filters.
I wonder does anyone have any knowledge of these and the pros/cons of using them?
Regards
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Posted By Merv Newman
I did a lot of work on fume hoods a few years ago. Can't remember the numbers but might be able to look them up.
basic recommendations were :
1. To exhaust from the back wall as well as from above. This ensures that fumes are drawn away from user and not up through the breathing area.
2. To ensure adequate make-up air from outside the hood. Equivalent to the exhaust volume. (can cause a draft up the skirt)
3. To install side closing windows rather than (or as well as) the traditional vertical sash. These confine the exposed body area to minimum required for work and (with both horizontal and vertical closings) ensures that the breathing area is always outside the hood.
4. In-ward airflow should be at least 1 foot per second (without looking it up)
5. visible/audible indicater of air-flow failure.
6. What do you do with the vented air ? Is it clean ?
Hope this helps
Merv
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Posted By Diane Thomason
There is a relatively new BS for fume cupboards - if you don't have the details of this I'll be happy to look it up (I don't have a copy of the actual BS though.) I went on a 1-day course on the new fume cupboard standards at the HSL last November so I could check to see if recirc models were discussed.
FWIW I our policy here is not to recommend them: you are too dependent on the chemical filters (which can get saturated after a period of use) and you can't always be sure someone won't use the FC for a chemical it can't cope with. I'm interested that you mention HEPA filters - which will not remove gases, fumes or vapours - does your process need removal of particulates only?
In the past I have used one recirc model, but it was strictly reserved for low-hazard work (e.g. weighing out dusty chemicals of low toxicity.)
When trying to get hard data about safety of these models (a few years ago now) I drew a blank - I hope there's more info around now to help you.
Diane
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Posted By John Donaldson
I would endorse what Diane has said.
We try to avoid the use unless it can be justified.
These is also the problem of what to do with the filter if it becomes contaminated or reaches the end of it life.
Some specialist disposal companies are reluctant to take them and you have to have robust procedures in place when they are removed.
HEPA and chemical filters are also very expensive and can cost in excess of £1000 each.
Sorry if this has gone of on a bit of a tangent
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Posted By Kevin Fitzgerald
My thanks to all of you who replied to my query. Very helpful.
Diane, if you have any information on that BS standard I would greatly appreciate it.
Kevin.
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Posted By Chris Packham
Kevin
I was at a meeting at HSE last Friday where LEV was discussed. The message from HSE seemed to be that they were not in favour of recirculating systems, for the reasons already mentioned. They showed examples where these systems had failed.
Your original message was not entirely clear to me as to whether you were actually considering a fume cabinet or whether this was LEV in a workplace environment. If the latter, then I recommend you obtain a copy of HSG37 An introduction to local exhaust ventilation. As a basic introduction this is excellent. (I undertsand that it is due to be replaced shortly by a new, improved HSE guidance document, but its contents are still valid and useful.)
Depending upon what you are trying to remove, there are many different alternatives. For example, if the particulate is heavier than air a down-draft bench may be more suitable than a hood.
Regards
Chris
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Posted By Diane Thomason
Kevin,
The new-ish standard is BS EN 14175, and is significantly different from the previous BS.
I will see if I can find out whether recirc models were mentioned in the HSL course.
If you are seriously thinking of choosing/purchasing/installing/commissioning/testing a FC I'd recommend you get the BS if you can, even if you will be the user of the kit and other people will be doing the engineering bits. Some people can access BSs via Technical Indexes or similar - I can't, unfortunately.
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