Help with ideas on governing duct pressure

exhaust
pressure
duct

#1

In a brainstorming mood? I’m trying to come up with a clever mechanical way to regulate the pressure in the exhaust duct coming from the GF. Here’s my setup:

I have a roof-mounted industrial exhaust fan with a variable speed control (up to 1000 cfm). I plan to adjust the speed control so that there is a tiny (small fraction of a psi) negative pressure at the GF outlet while the machine is operating and the fans are on. (This may require using a bleeder opening upstream if the lowest fan setting is too large.)

The tricky part is that I want to design some mechanical apparatus that will adjust the suction at the GF outlet when it is done and its fans go off. Without this, there would be a huge increase in suction and it would likely be bad for the internal GF fans. I want just enough post-cutting suction to pull out the lingering fumes without spinning the GF fans excessively, again, a small fraction of a psi.

Of course, I could just get up and adjust the external fan or the bleeder opening every time the GF finishes, but I’d prefer an automatic solution. Any ideas? Maybe some sort of a rocking, weighted, bleeder opening?


#2

Would something like a damper work?

http://m.homedepot.com/b/Heating-Venting-Cooling-HVAC-Parts-Accessories-Ducting-Venting-Duct-Accessories-Dampers/N-5yc1vZc5ho

They have a few different kinds. One idea would be if you used the free floating version, you could add a weight at the bottom. If you calibrate it right, it could be just barely open when your roof fan is on, but fully open when the Glowforge kicks in.


#3

is it going to be used to vent anything else, or will it be dedicated to your glowforge? if it’s the latter, you could probably just attach something to a current sensing outlet that you plug the glowforge into - gf spins up, so does the fan, gf turns off, so does the fan, etc.

if it’s going to be used for multiple devices you could have a louvered grate inside of the venting if you wanted to block off the glowforge, and with another relief for the pressure, it should remain unaffected?


#4

A simple flow meter (or acoustic meter for that matter) on the outlet of the GF in the hose with an arduino would suffice. I wouldn’t use current sensing as that is more complex than simply sensing airflow. Like this:

https://playground.arduino.cc/Main/ThermalAnemometer

I’d use an Adafruit Feather M0 personally because you get a lot of power for the M0 based on with built in WiFi making it way easier to talk to your fan controller (I’ve used the built in web server to control/acquire data)


#5

depending on how the fan is set up, current sensing doesn’t need to be complex at all, with a bunch of off the shelf parts. i know it’s a maker community in part but i don’t think you can say “oh current sensing so complex, just build an arduino-based flow sensor”.


#6

Wait, don’t you mean current sensing on the GF? Why would you want to sense on your fan? I assume you were doing AC sensing on the draw of the GF to figure out if it is “lasing” right now… That would be harder…


#7

i am, but they make purpose-built circuits for it that make it as easy as plugging the gf in


#8

But the goal here is governing pressure in the duct, using airflow sensing is just that. If the wind is blowing hard onto the outlet and there isn’t sufficient forward flow you can set up a PID loop to keep a constant flow. Knowing the current draw of the GF isn’t solving that problem. And you can sense at the beginning and end and even compensate for loss through the ducting.


#9

my whole point was prefaced with ‘depending on how the fan is set up.’ there’s a million ways to do what he asks. maybe the glowforge is right next to the fan controls, and if you can easily tell the fan when the laser powers up it’d be quick to set up.

but whatever.


#10

Thanks for all the good ideas. I still would like to make it a mechanical solution for robustness’ sake. I guess it comes down to regulating a constant small negative pressure, independent of flow velocity. Seems like it could be done with a simple pressure-regulated bleed valve.


#11

I think I’d want to find out if drawing extra air through the forge is enough of a problem to worry about before getting too tricky.


#12

There have been several reports of lingering fumes in the GF after the fans turn off. They are apparently very noticeable when the lid is lifted. I was hoping to remove those. And besides, I enjoy designing gizmos like this!


#13

oh I’m not saying don’t keep the venting going. I’m just questioning the premise that constant 1000CFM pull through the glowforge would cause any issue to the glowforge.


#14

I will say that is WAAAAAAY more than I think it needs. I am betting 50cfm is enough to prevent the tiny puffs of smoke from coming out after.


#15

Interesting. I was always taught to immobilize fans in a computer when using a vacuum cleaner to pull out the dust. It’s stuck in my noggin that over-revving them is bad for them. Is that true, anyone?


#16

i can’t comment to that effect but what i can say is that i have always blown out computer fans with compressed air, which gets them spinning like one of those old siren whistles. never had one go bad but anecdata isn’t necessarily that useful.


#17

It will generate a high voltage in the coils. Whether that damages the driver chip depends if it has protection diodes and if not which direction you spin it probably.


#18

During Makerfaire I asked about this exact thing. The Purex filter provided was pulling ~117 CFM (it has a display that shows filter status and live CFM gauge) which then cause the Glowforge’s exhaust fan to free spin I’d guess 800~1000 RPM. I was told that was okay and not to worry. Which means they have protection diodes on leads to the fan (Or this protection is built-in the fan - I’d have to see if I can see what model fan they are using) to prevent backflow of current. (Maybe?)


About the filter
#19

thanks for that! it’s a relief to know that this is fine should i add some extra tubing and a booster. i mean, i figured it would be, but…