Dust Control

[JackoH]

Some time ago,I think on this forum, there was a fair bit of discussion on dust control and the fire hazards associated with the installation of same. My fred(f#**^~* ridiculous electronic device) doesn't seem to be able to go back beyond about last May,so am unable to find it. Can someone please advise on the concensus reached. I read an article the other day in an old W.W. mag which said that you must run an earth wire through the extraction piping anchored to a screw at each end to grounded metal to disperse static. Stripped copper electrical wire was suggested.Sorry to start all this again , but perhaps the newer contributors to this BB may have some useful comments to add. John H. http://ubb.ubeaut.com.au/ubb/confused.gif

[fcm]

John, very quickly here is some of it:

" The outside wire is coiled around the length of the ducting whilst the inside is earthed to each take-off or machine"

&

"The average extractor does not have the capacity to get enough dust up the chute to get a combustible ratio. Also, to ignite the interior dust where is it going to arc to? It can't arc to itself in the pipe because PVC is an insulator. If it arcs to the outside, it can't ignite the dust inside the pipe because it is sealed."

When I get a minute next week I will add some further comments of my own - having worked for CSIRO in fire engineering I should be able to give you some useful info http://ubb.ubeaut.com.au/ubb/eek.gif
There have been a few fires that I know of, but I'll have to research it further and get back to you. I also have a cheap, sure-fire http://ubb.ubeaut.com.au/ubb/biggrin.gif method to prevent explosion
damage to equipment should this occur.

Best wishes, Mark

[This message has been edited by fcm (edited 24 August 2001).]

[ubeaut]

John - to go beyond last May click on Show all topics in the topics box and then on GO. At the right hand, bottom of the page there will be a series of page numbers 1 2 3 4 5 6 etc. click on these links to go to earlier pages.

You could also try a search using the search link at the top of the page. Try a few different words and also other forums.

Hope this helps.

Cheers - Neil

[JackoH]

Thanks Neil, for your help,(once again,)I found the posts on this topic (all 17 of them!) on page three of the archives of this forum. Having read them I am still pretty confused.(I'm old, whats your excuse?)
http://ubb.ubeaut.com.au/ubb/confused.gif
Seems to me that the main conclusion is that the dust fire hazard is a bit of a furphy and that if you use metal ducting which is about the same price as plastic then the system is self earthing anyway.
By the way I have appologised to Fred! As usual it was the operator not the tool! Thanks also to you fcm. Regardless of the above I look forward to your comments in due course.
Thanks again, John H.

[This message has been edited by John Hambly (edited 25 August 2001).]

[This message has been edited by John Hambly (edited 25 August 2001).]

[Tristan Croll]

Seems to me that if the duct's PVC, then if you were going to worry about anything, you'd worry about static electricity.
Quite a few years ago now, there was a problem with a new type of plane using a plastic (composite?) fuel tank - they kept blowing up during refuelling. Turned out that the fuel flowing over the plastic caused enough static buildup to create a spark between the tank and fuel, leading to a rather large problem.
Not that it's likely to be a problem in the average case, but in general you're safer with metal.
Just my 2.2c.

Tristan

[Rod Smith]

G'day
Similar story for you, the plastic ute inserts. There have been some instances in the US where metal fuel tanks have exploded. Ute moving (air flow) causes static charge, as filler gets close spark jumps, you can work out the rest. Moral, take the fuel tank out when filling.
Cheers
Rod

[Richard Casey]

Hi guys, wonderfull topic that has been bugging me for ages. As an Electrical Inspector the fear of static scares the hell out of me. A few years ago I replaced the messy lower bag on my dust extractor with a sausage skin barrel that has a snap on lid. I fitted a few perspex windows and it has been a pleasure to empty. The worry started when I placed my hand near the barrel whilst it was going and all the hairs on my arm stood on end.I enquired of all the gov, departments that might have info, tried the local uni, looked up the Aus Standards on static, but am still none the wiser on how big the risk of an explosion there is.The best piece of info I found was to hold a neon bulb against it and if it did not light up then there is a low risk. I am not sure if this is the best way and I would like to know for sure not just hearsay. Regards Richard.

[fcm]

The low down on static:
The generation or presence of static does not necessarily create a danger of fire or explosion, but will initiate it only when all of the following four conditions are fulfilled:
(a) An effective means of static generation.
(b) Accumulating the separated charges and attaining sufficient potential difference.
(c) A static discharge having sufficient energy for ignition occurring between the separated charges.
(d) The discharge occurring in or on an ignitable material.

In general flowing mixtures of dust and air generate charge because of the physical contact and separation of the dust particles against each other and the surfaces of other bodies, e.g. ductwalls.. In particular, dusts from plastics materials, shellac, carbide dust, and quartz sand are particularly prone to static generation.

When dust is dispersed from a surface, static generation cannot be absolutely prevented and will not be eliminated by high relative humidity or earthing of the surface from which the dust is dispersed. The accumulation of static on dust-handling equipment used in dust-enriched atmosphere can be minimized by using low flow velocities and small heights of fall.

However, standards also say:
Outlet nozzles: Nozzles from which dusts are ejected should be made wholly from conducting materials, the separate parts of which must be bonded and earthed.
This is because electrostatic charges on conducting objects can be rapidly dissipated by earthing the objects. (i guess the same does for the other end of the system)

Objects in dust streams: Objects should not be positioned in the dust stream, but where this is unavoidable the object should be made wholly from conducting material and be bonded and earthed.

Bags and containers for collecting and transporting dust: Bags and containers used for collecting or transporting dust should be conducting and be earthed and/or bonded during filling, emptying and transport. Bags and containers are sufficiently conducting for this purpose if they are—
(a) constructed entirely from conducting material; or
(b) rendered conducting by rinsing in an antistatic solution as often as necessary to maintain sufficient conductivity.
(similar to using anti-static fabric softener when washing synthetic clothes)
Earthing and Bonding

Bonding can be used to connect electrically isolated sections of a plant, thus enabling static on such sections to be dissipated through a single earth connection,
Earthing and bonding conductors for dissipating static need not be insulated, but must be sufficiently robust to withstand all mechanical and corrosive influences likely to occur. Attachment must be by soldering, welding or suitable screwed terminations. Earthed conductors should be made of high conductivity copper

WIND-UP
This is a highly technical area of mechanical/electrical engineering. Even with a mechanical and fire engineering background I am stumped and have to admit I don't necessarily have the current working knowledge to fully deal with static problems ! (damn, back to Uni for me)

I'm aware of fires involving wood work extraction systems and dust collection hoppers, however to my knowledge they are not common occurrence. I also believe the majority of fires involving wood work dust extraction systems are caused by the generation of mechanical heat somewhere within the system, not by static build-up.

The above info has been sourced from standards and technical reference material and may assist you in designing your system. I can't say with any certainty that there are any great risks of using conductive ductwork as opposed to PVC pipe, however earthing should be designed according to the application.
As for PVC, some mistake PVC as being non-conductive, which is only half true. PVC is generally a non-conductive material, however some forms of PVC may have a conductive surface(ie electrons can flow along the surface, but not within the material). This is why it is possible to get a "shock" from PVC.

My best advice is:
1. Suitably earth the system components
2. Keep filters, fans etc well maintained and ducts free from obstructions (same as a chimney, it should be checked/cleaned periodically)
3. Calculate the $ value of your workshop/shed, tools and equipment and insure it !


Regards, Mark



[This message has been edited by fcm (edited 28 August 2001).]

[JackoH]

Thanks Mark, (much more friendly than fcm, I couldn't pronounce fcm anyway, my attempts sounded rather rude and would be censored if tried here!) I think I will follow your advice,particularly the bit about insurance, and possibly use metal ducting wich according to some info would solve the problem anyway.
Regards. John H.

[Richard Casey]

Thanks also Mark. During the wee small hours, I went back into the files and read all the previous notes on the subject and then I picked up the latest Aus Woodworker and there is a add for a dust extractor sitting on a wheelie bin. I guess these guys have done their homework and I will stop worrying about dust and go and attack the Qld Maple stump that was delivered this arvo. Keep up the good work guys. I wish I had known about this site a long time ago. Regards Richard.