Downdraft Suggestions

[AgentWD40]

I've got a roughly 4'x4' light duty diy plasma table (52"x51"). I'm running a Hypertherm Powermax 45xp. I was hoping I could get some design tips for an *effective* downdraft table. Nothing fancy, just needs to work!

I've attached preliminary sketch #1 for what I have in mind. Just some thin sheet metal sloped 30 degrees below horizontal with an 8" duct. And per several posts I've seen by Mr Colt I'm shooting for a 3200 - 3600 cfm fan.

Any input will be greatly appreciated!

PS - It's unclear to me what advantages a hybrid water/downdraft table have over just a downdraft? I'm open to implementing that instead as well.

[weldguy]

That looks great and will work well. One thing I would change is the box at the base of your funnel, it leaves an opportunity for small drops and molten slag buildup to be sucked through your fan. A deeper box with the air inlet left up high where it is and screen to cover it would be beneficial. Also a trap door at the bottom like a sandblast cabinet would also be a good idea.
Typically the calculation for air volume takes into account the size of your cutting bed but if you have a full sheet of material that area is covered allowing the use of a smaller fan. If you have a partial sheet you can cover the open areas with scrap material or welding curtain which will also allow you to use a smaller fan. In my experience a 2000cfm fan will be more than enough.

[AgentWD40]

Great! Thanks for the input.

I was thinking about incorporating one of those galvanized ash buckets into the bottom of the 'collector' that slides in and out for easy clean out.

What kind of screen are thinking? How much is too much that would disrupt the air flow from the fan?

[weldguy]

You could use any metal screen or perforated metal and make the inlet diameter where the screen is an appropriate size to accommodate for the % of blockage from the screen material. May be easier to make a louver or hood over the top of the inlet so parts are less likely to be sucked in. The buckets a good idea but I would leave the bottom blocked off during use so all your air flow is drawing down through the top of the table, it will really help to keep the smoke down.

[robertspark]

2c from an HVAC design engineer.

allow for the air to change velocity from a high pressure zone to a low pressure zone and back up to a high pressure zone, that way your particulate matter trapped in the airstream loses its velocity and drops into a catchment bucket before being sucked out through the higher velocity duct to the fan.

think air cyclone separator..... or at least the general principle of one

https://www.google.com/search?q=air+cyc ... 60&bih=641

or on a bigger scale (similar principle) (when designing lots of welding , grinding or cutting fume extracts)

https://www.eurovac.com/wp-content/uplo ... 86x156.jpg

[AgentWD40]

Robert, whats a quick/easy way to implement that? Looking online it looks like a cyclone separators get really expensive really fast. What are other folks doing? I was just planning on shooting what doesn't fall to the bottom out of the side of the shop.

[AgentWD40]

Would something like this work better?

[robertspark]

this shows the air pressure gradients of how they work

https://www.mdpi.com/processes/processe ... 8-g001.png

there is a technical paper on them here
https://www.mdpi.com/2227-9717/8/2/228/htm

normally they are used on wood dust collection
there are some design solutions on line as to how to make one.
http://forum.driveonwood.com/uploads/de ... 14aa82.jpg
https://kentlaursen.dk/wp-content/uploa ... onePDF.pdf
http://billpentz.com/woodworking/cyclon ... eetPg2.gif

here are a load of "DIY" ideas.... again mainly for wood, but you could make one with a couple of small oil drums and some internal baffles, that way you can unclip the lower section (and put wheels on it....) and empty if of debris from time to time.
https://www.google.com/search?q=cyclone ... 83&bih=757

However, I would suggest what you want to try to replicate is the change in pressure / velocity as this will get most of the particulate to settle in the chamber you can then make a bucket which clips into place

what you have drawn in my opinion is restrictive on the airflow inlet, given you have a baffle in the front of it and a large boundary area will restrict airflow.

2021-04-06 20_17_15-Window.png

it is recommended to put a taper or cone where there is an air inlet which will reduce the boundary condition (called the vena contracta)

2021-04-06 20_19_19-Window.png

2021-04-06 20_37_13-Window.png

you can search for a really good (technical) book using:
air hygienists manual pdf
on google and it should link you to an industrial ventilation manual. unfortunately it is copyright and given most of you are law abiding Americans, I don't think I should link to it, but I am sure you can find a link to it and it provides a lot of practical solutions with diagrams and flow rate calculations etc.

I can't provide you with a design solution as there are a lot of factors that come into an effective design (table size, air flow, fan pressure (make, model, current draw, technical manual)) ...... I get paid a lot of money for detailed designs (and they can take some time too) ...... you can however follow the principles given above and develop an efficient and practical solution that works for your table, and collects most of the particulate in a removable bucket (hopper) and provides fairly clean air through the fan so that you don't blow metal particulate all over nearby cars / whatever that you end up sweeping up later when it turns red and rusts over time

Or you can just put a duct and fan outside, from the lowest point in your table as shown in one of the earlier sketches.

[AgentWD40]

what you have drawn in my opinion is restrictive on the airflow inlet, given you have a baffle in the front of it and a large boundary area will restrict airflow.

Are you talking about my second/latest version? Could I simply increase the distance between inlet and the baffle? I only put the baffle there as a [s]rain[/s] "slag cap".

I can't provide you with a design solution as there are a lot of factors that come into an effective design (table size, air flow, fan pressure (make, model, current draw, technical manual)) ...... I get paid a lot of money for detailed designs (and they can take some time too)

I really appreciate your opinions, time, and effort. You went above and beyond providing this reading material.

you can however follow the principles given above and develop an efficient and practical solution that works for your table, and collects most of the particulate in a removable bucket (hopper) and provides fairly clean air through the fan so that you don't blow metal particulate all over nearby cars / whatever that you end up sweeping up later when it turns red and rusts over time

Or you can just put a duct and fan outside, from the lowest point in your table as shown in one of the earlier sketches.

That's kind of the thing... I didn't expect to be going down this route when I posted this morning! The plan was to run exhaust out the back of the building out into an empty yard. My priority is to keep the dust out of my shop and lungs. Is a dust trap really necessary? I thought I would try and catch the relatively big pieces of slag but the fine stuff I wasn't initially too worried about filtering out of the exhaust. If I should be more worried about it then please let me know.

p.s. On a side note I'm drawing an offset smoker and now you've got me rethinking some of aspects of it's design. lol

[robertspark]

Yes I was talking about the upstand tube with the baffle plate over the top.

Remember the pareto principle..... you can achieve 80% of the result with 20% of the input (i.e. small simple modifications could make a big difference)

achieving that last 20% will cost you 80% of the time and money

applies to many things
https://betterexplained.com/articles/un ... 8020-rule/
https://en.wikipedia.org/wiki/Pareto_principle

don't always need to do all the long calculations to just be in the ball park and get some of the benefits from other designs

[adbuch]

The plan was to run exhaust out the back of the building out into an empty yard. My priority is to keep the dust out of my shop and lungs. Is a dust trap really necessary? I thought I would try and catch the relatively big pieces of slag but the fine stuff I wasn't initially too worried about filtering out of the exhaust. If I should be more worried about it then please let me know.

If you have plenty of land with no neighbors close by, then dumping all the smoke and debris outside in the yard may be ok. The reason I use a draft system in conjunction with a water table is that most of the larger debris goes into the water where it is extinguished and sinks to the bottom of the water table. This water table setup alone probably eliminates over 90% of the smoke. Any of the remaining smoke is eliminated by the draft system - that is it is exhausted to the outside of the building via ducting with exhaust fan. There is very little - if any - visible smoke/particulates coming out of the exhaust fan. If I had to do it over again, I would do it exactly the same way. I have been running this setup for over 4 years now with no problems.

Although my water tray/draft system is set-up for a Plasmacam 4x4 table, this same principle could be adapted to other sizes and configurations of cutting tables. Link to the build process below.
David

https://photos.app.goo.gl/h8LHufi6frttfGNQA

water table 2.jpg

water table 1.jpg

[AgentWD40]

Thanks for sharing adbunch!

I see your using almost a 3000 cfm blower. What's the size of the duct work? How far from the top of the slats is the top of the water? Finally, how deep is the water pan?

Do you need to cover portions of the table not being used to maintain good evacuation?

If I remember correctly I think Jim Colt recommended 6-8" between top of slats and top of water.

[Rodw]

Here is a more ambitious 8 zone design running Linuxcnc. The Mesa 7i76e hes using has 16 outputs so no problems managing the 8 solenoids. Even on a 4x4 table, I'd go with 2 zones.

https://forum.linuxcnc.org/plasma-laser ... ns?start=0

[AgentWD40]

Ambitious indeed. I had skimmed through that thread before. I don't want to go down the road of automating draft doors but I may borrow ideas from that design anyway.

@adbunch, if you're able I'd still like to know those specs on your table I asked about above.

[adbuch]

Thanks for sharing adbunch!

I see your using almost a 3000 cfm blower. What's the size of the duct work? How far from the top of the slats is the top of the water? Finally, how deep is the water pan?

Do you need to cover portions of the table not being used to maintain good evacuation?

If I remember correctly I think Jim Colt recommended 6-8" between top of slats and top of water.

The round ducting is 4" diameter. Distance from top of cutting grates to top of water is about 11 inches. Water tray is 4" deep, constructed from 11 ga. steel with welded seams. Yes - I usually cover any open portions of the table top to promote a good draft (good evacuation). In the video below I am using sheets of 1/8" masonite (hard board) to cover most of the open parts of the table.
David

https://www.youtube.com/watch?v=pLLiygynHaU

Plasmacam cutting F1 battery tray.jpg

[adbuch]

Ambitious indeed. I had skimmed through that thread before. I don't want to go down the road of automating draft doors but I may borrow ideas from that design anyway.

@adbunch, if you're able I'd still like to know those specs on your table I asked about above.

I posted the dimensions above. I hope you saw them.
David

[cstroke]

Biggest suggestion I have is whatever you do, make sure it's easy to clean out. If you start doing a bunch of cutting you'll certainly wish you did.
I have two pans that are approximately 5'x6' x 6" deep and I've cleaned them out 3-4x this year, they roll out and it makes it easier.
I'm so happy I dont have to clean out a water table or reach down and shovel stuff out.