Downdraft design questions

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04 Nov 2020 18:13 #188331 by txtrone
Not sure if this is the right place, but I suspect someone here has experience with this.

I am trying to find the best type and size fan to exhaust the smoke/fumes/dust from my table. The table is 6x12 and has a v-shaped basin. Suspended halfway between the basin bottom and the cutting surface is a 7" ID steel pipe with 8ea 4" square 'ports'. There is only one outlet on the table located at one end. The table will be situated in my shop in such a way that the outlet of this pipe is 8' from the exterior wall, at 90 degrees from where I need to vent.

I have read good things about backward incline blowers. Any input on what size and type fan as well as connection methods will be much appreciated.

Thanks

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04 Nov 2020 20:22 #188342 by rodw
Replied by rodw on topic Downdraft design questions
That looks like a really nice design.

This article suggests 3200-3600 CFM for a 4' x 4' table
www.hypertherm.com/learn/spark-the-blog/fume-control/
So for a table that size you would need a LOT of air. 16000 cfm.
Most commercial tables are zoned

Assuming you are going to use Linuxcnc's Plasmac, I would suggest you add an air activated shutter on each of the 8 outlets that is triggered based on the position of the x & y axis. You don't need seperate compartments to contain the air. Better that you don't
so by using 8 zones, you would reduce the air flow requirement to a much more reasonable 2000 CFM

Here is a bit of a rough demo of mine showing how this can be done


You could use two lincurve components with axis.x.pos-cmd and axis.y.pos-cmd as respective inputs but you would need a bit of hal trickery to merge these into a zone. I think it would be a lot easier to write a custom component which is the path I took for the demo.
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04 Nov 2020 22:05 #188346 by txtrone
Replied by txtrone on topic Downdraft design questions
I am planning to use LinuxCNC. The PC is built and I am about to begin hurting my head learning about how to connect the Mesa cards and all the other electronics. I have one hobby in the queue that I am nearing completion on, so I should be ready to start in earnest on the electronics this week, or early next week. I think I have everything from the original list you made for me. To get the table running in the meantime I connected my CandCNC setup.

So you are saying if I can manage to add automated vents controls then I do NOT need to add zones or baffles?

What type of fan/blower would you recommend? Should I be looking for 2000 CFM at 0 SP?

Lastly, would you be willing to write the custom component for a fee?

Your setup is top shelf! I like the cabinet, very tidy work.

As always, thanks for taking the time to help!
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05 Nov 2020 00:57 #188357 by rodw
Replied by rodw on topic Downdraft design questions
I had in mind something like a pneumatic slide gate valve www.acsvalves.com/options/view/maintenan...-valves-air-operated

I'm not sure if baffles would be needed. I did see one big commercial table that just had a central duct with air ram activated flaps that was not baffled hence my idea. The air rams were inside the duct and they used copper tubing to the rams. I guess so it did not melt from sparks.

You would have to do your own airflow engineering.

I'd start with one of the cheap axial workshop blowers to prove the idea.
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05 Nov 2020 01:14 #188358 by txtrone
Replied by txtrone on topic Downdraft design questions
I have a cheap tube axial fan now. It might be worth a try, I would have to price the air rams and associated actuators. If it worked I would still be facing the software challenge. How challenging would the software be to configure to operate 8 devices based on the location of the torch?

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05 Nov 2020 03:13 #188365 by rodw
Replied by rodw on topic Downdraft design questions

I have a cheap tube axial fan now. It might be worth a try, I would have to price the air rams and associated actuators. If it worked I would still be facing the software challenge. How challenging would the software be to configure to operate 8 devices based on the location of the torch?


The software would be easy. The software for my example was 4 lines of code but it could be done in 1 line :)

I'll have a look at it.
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05 Nov 2020 03:39 #188367 by txtrone
Replied by txtrone on topic Downdraft design questions
Thanks Rod, much appreciated. What model Airtac is that you are using? Is it connected to a Mesa card?

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05 Nov 2020 11:33 - 05 Nov 2020 11:35 #188384 by rodw
Replied by rodw on topic Downdraft design questions

Thanks Rod, much appreciated. What model Airtac is that you are using? Is it connected to a Mesa card?


Sorry not sure what you mean about the Airtec.

So just doing some early thinking about this. First convert to inches being your machine units. You said you have 8 vents and the table is 144 inches long and 72 inches wide.

So let there be 8 zones numbered 0 to 7 with 0 in the front left and 7 at the rear right.
We can make an array of hal pins to agree with this structure
The zone boundaries on the Y axis will be at 36", 72" and 108"
The zone boundary on the X axis will be at 36"
We can also set an array of pins to set there pins in you hal files using setp.
We need to convert a position to a zone.
The X axis is easy
int xzone = (axis.x.pos-cmd < 36) ? 0:1;
The Y axis is a bit trickier
int yzone;
if(axis.y.pos-cmd < 36)
   yzone = 0;
else if(axis.y.pos-cmd < 72)
  yzone = 2;
else if(axis.y.pos-cmd < 108)
   yzone = 4;
else 
   yzone = 6;

so now we need to resolve the zone number
int thezone = xzone+ yzone;

So now its time to set the pins
int i;
for( i = 0, i < 7, i++){
   pin[i] = (i == thezone)? 1:0;

So there are probably a few typos above
I think the logic is right, I just have to flesh it out into a component another time.
Last edit: 05 Nov 2020 11:35 by rodw.
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05 Nov 2020 14:02 #188392 by txtrone
Replied by txtrone on topic Downdraft design questions
Thanks!

The solenoid valve in your video, I believe it's an 'AirTac' brand. Just curious which model number it is and how it connects to your controller.

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05 Nov 2020 17:39 #188415 by rodw
Replied by rodw on topic Downdraft design questions
Its just a 24 volt solenoid from a random ebay supplier directly controlled by a 7i76e output. You'd just need to add a flyback diode to protect the output. This would be connected accross the terminals with the band on the realy pointing towards the output. (IN400X series diode is all thats needed.)
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