Kiln venting

[moonzie]

Hi folks,

First time posting.  I'm pretty new to ceramics but my wife has been doing it for a few years now.

We built a tiny studio in a 12x12 room in our garage. In it, we have a tiny L&L E18S-3 kiln we bought from L&L new. I built a vent system collection box and piped it outside of our garage. It's 4" metal pipe, 3 90 bends on the suction end, total run length is about 25 or 30 feet. The fan is located in another part of the garage in my attempt to minimize any chances of fumes leaking into the room via the positive pressure end of the fan.  This is the fan I used:

https://www.amazon.com/Hydro-Crunch-Inline-Booster-Grounded/dp/B072N332WT/

Problem is, there are lots of fumes in the room (esp. early into the firing) and some corrosion/discoloration around the lid began shortly after we started using it. When the fan is running, I tried holding a lighter up to the single 1/4" hole we drilled in the center of the floor of the kiln and it gently pulls the flame down into the hole. I have a video of this in my review of the fan on the page I mentioned above. I feel no movement of air coming out the exhaust end of the pipe. We tried the smoke test around the lid of the kiln - nothing was being drawn in around the lid (with the peep holes pluged). 

I did not cut any bypass holes/slots into the collector box.

I installed a room vent and it helps, but I wasn't expecting as much fumes as we are. Do I need to install a beefier vent fan? Everywhere, I read about CFM and various tests, but nobody mentions static pressure. I'm thinking I need to make a manometer to get this sorted out right. I know the motor should be outside the vent, but an inline duct fan was easier to install.

 

Help!

Edited December 14, 2021 by moonzie

[neilestrick]

18 minutes ago, moonzie said:

I did not cut any bypass holes/slots into the collector box.

What did you use to connect the duct to the kiln? Whatever it is, it must have holes in it. With only the little hole in the kiln, you're starving the fan for air. Also, the fan must draw in room air through those holes to cool the air being pulled from the kiln. Once you get that sorted, you may find you also need a hole in the lid, as small kilns tend to seal tighter.

[moonzie]

I built a box out of galvanized sheet - pretty stout sheet. The box is screwed into the kiln stand and everything is sealed with metal HVAC tape and attached a 4" metal flex hose. All the piping is taped. I did drill a hole in the box for testing, but taped it back up figuring it would draw the most air from the kiln with no holes/openings. The fan motor is installed a good 20 linear feet away from the collector box and in the cooler part of the garage, so by the time the air reaches the fan, it shouldn't be too hot.

If I allow some external air into the collector box via some hole(s), will it draw more from inside the kiln? I was thinking it'd be the other way around.

Maybe having smelly fumes in the room is normal - I don't know. 

 

Edited December 14, 2021 by moonzie

[neilestrick]

It's a balance between room air and kiln air.

[Hulk]

Hi Moonzie!

I built up a diy downdraft kiln vent as well (seven cubic foot Skutt electric), using an inline fan*, four inch ducting, galvanized sheet for the box, which has two adjustable vents. I'm running it with the vents closed down to leave about 3/8" square on one side, the other almost shut.

I believe the vent does work, that is, it pulls a small stream of superheated atmosphere from the kiln (which is replaced by ambient through the various cracks, small vent hole, etc.), for the exhaust end (outside) is definitely warm! ...and there's strong wax smell outside (where it belongs) early in glaze firings...

There were significantly less fumes, however, not enough that I was comfortable hanging about in the studio, even with the rollup (it's a single car garage) door, windows, and man door full open, so, I've since set up an overhead hood with 400 cfm fan, which does a job on the heat and fumes.

I wouldn't expect a downdraft setup to ameliorate fumes entirely.
Note: any fan system will require provision for adequate make up air.

*When the inline fails (it will, a matter of time), I'll replace with somewhat more suitable.

[moonzie]

The room isn't super air-tight.

There is a bit of rust that's formed on the underside of the lid handle along with the discoloration around the metal band of the lid. I guess I was just expecting the downdraft would be providing adequate ventilation.

 

 

Edited December 14, 2021 by moonzie

[neilestrick]

1 hour ago, moonzie said:

The room isn't super air-tight.

There is a bit of rust that's formed on the underside of the lid handle along with the discoloration around the metal band of the lid. I guess I was just expecting the downdraft would be providing adequate ventilation.

 

 

If working properly the downdraft should provide pretty good ventilation. Not necessarily 100% though. You may smell fumes during wax burnout, but for the bulk of the firing you shouldn't smell much. You should still be able to be in the room without discomfort.

2 hours ago, moonzie said:

I feel no movement of air coming out the exhaust end of the pipe.

It's probably starved for air since it's only drawing through the small hole in the kiln. Open up the holes in the connection box.

[moonzie]

If using a lighter up against the vent hole isn't a good test, what is? Smoke against the lid cracked open with a dime? I guess I just don't know how people accurately test their vent systems except for with their noses lol.

[neilestrick]

49 minutes ago, moonzie said:

If using a lighter up against the vent hole isn't a good test, what is? Smoke against the lid cracked open with a dime? I guess I just don't know how people accurately test their vent systems except for with their noses lol.

The match test is difficult, especially since the draw changes as the kiln heats up, and as the lid gaps open as the firing increases. I'd trust your nose more than anything. But the vent needs a source of air in order to work properly, so open up that hole in the collection box.

[moonzie]

I started a bisque fire last night and did some testing with a stick of incense. 

With the lid closed, bottom peep hole plugged, stick of incense inserted fully into the top peep hole and kiln vent on, you could see it drawing the smoke down except for when the elements kicked on it would burp some smoke out. This was while the kiln was at 100*F.

I then opened up two 5/16" holes drilled in the collector box and found it did not change the behavior described above. I drilled a third 5/16" hole. No change. Tried plugging the collector box holes with my fingers - no change. 

If I moved the burning part of the incense into the firebrick area of the hole, the smoke would escape the kiln.

Later on, when the kiln was at 670*F I tested again with an incense stick and only the top plug removed. This time, no smoke stayed in the hole whether the collector box holes were plugged or not. There was a smell I'd associate with firing ceramics in the room.

I'm fairly certain at this point, I need a better fan motor setup. More researching...

[Dick White]

Something that I think gets lost when a non-science person looks at these downdraft kiln vents is that the how venting occurs. The fan does not directly suck fumes out of the kiln. Rather, the fan at the other end of the vent pipe is pulling a significant air flow through the collection box which causes a slight suction through the small holes in the bottom of the kiln via the venturi effect.  If the intake holes in the collection box are blocked, there is no flow through the pipe to the fan and the fan just spins in static air. With static air in the vent pipe and collection box, there is no venturi suction down through the holes in the kiln floor. One difference between the Skutt and L&L versions is the Skutt collection box has an intake hole that was designed for a reasonably appropriate airflow through the box, but is otherwise not adjustable. The L&L collection box has an adjustable shutter that you can tweak the amount of air flow through the box to get just the right amount of suction through the holes in the kiln floor as determined with the match flame test. Note that the system only works if the piping from the collection box to the fan is in good airtight condition. If there are unsealed joints or corroded holes in the pipe, the fan will pull air through those closer holes before pulling air through the collection box at the other end, reducing the venturi through the holes in the kiln floor. Gaps around the lid and between sections do not change the venturi through the holes in the kiln floor. Those gaps simply provide the make-up air going into the kiln as fumes are gently sucked out the bottom. There are enough such gaps spread throughout the kiln body that watching for changes in smoke intake at a peep isn't meaningful.

Edited December 16, 2021 by Dick White
Added a bit more...

[moonzie]

Thanks! I didn't realize that it worked from the venturi effect.  I thought it just worked by negative air pressure pulling a suction. 

Maybe I was mislead by L&L's own documentation on this... 

https://hotkilns.com/sites/default/files/2021-11/ventsure-instruct-print-11-1-2021.pdf

On page 5 under The Smoke Method, Step 2 reads "Start with the bypass valve in the fully closed position. This will give it the maximum suction in the kiln." 

And further up on Page 5 under Operation Step 5: "Use the flow control on the Bypass Box to modify the flow of exhaust - a larger flow control opening reduces the flow of exhaust fumes from the kiln, and a smaller flow control opening increases the exhaust."

 

So this is why I've been running with no inlet vents on my collection box. I figured I'd start with max suction and see how it performed. And I was just going by L&L's own documentation. Maybe this logic applies to larger kilns with multiple downdraft holes? I only drilled one 1/4" hole in this e18S.

Edited December 16, 2021 by moonzie

[Bill Kielb]

2 hours ago, moonzie said:

I then opened up two 5/16" holes drilled in the collector box and found it did not change the behavior described above. I drilled a third 5/16" hole. No change. Tried plugging the collector box holes with my fingers - no change. 

If I moved the burning part of the incense into the firebrick area of the hole, the smoke would escape the kiln.

Later on, when the kiln was at 670*F I tested again with an incense stick and only the top plug removed. This time, no smoke stayed in the hole whether the collector box holes were plugged or not. There was a smell I'd associate with firing ceramics in the room.

I'm fairly certain at this point, I need a better fan motor setup. More researching...

Downdrafts rely on really small pressure differences. From a science standpoint we don’t suck things out, we lower the pressure in the kiln a tiny bit and the pressure outside the kiln forces it’s way in and creates this tiny bit of counter flow. In theory we need to offset the buoyant pressure in the kiln with this counterflow so to speak. Opening something as large as a sight port likely instantly zeroes out any negative pressure within portions of or the whole kiln so now locally any buoyant force can exit everywhere.

I think the manufactures suggest smoke testing with everything closed, not sure have not read one for some time but I would advocate for it. Down drafts work pretty well, not perfect but definitely are effective. If you make your downdraft draw too much kiln air, you will burn out the fan and your kiln will struggle to make temperature. Most downdrafts I have seen are not strong enough to eliminate the smell of wax burnout, it’s just what I have observed. So any fumes that small and that buoyant are likely not fully exhausted either, just the wax smell is easy to notice at about 700 degrees.

The only approved way to remove fumes as practical is with a properly designed hood. Still downdrafts do a decent job and actually as the kiln moves to red heat the atmosphere inside becomes extremely thin so a little negative pressure goes a long way.

Follow the fan manufactures setup, over drafting has other real issues. If you have a need for near 100% certainty then a designed solution (hood) is likely necessary.

Here is an old video of a design to replace a downdraft https://youtu.be/etpa2Pc9Hug It might give you some ideas of the fan power necessary.

 

Edited December 16, 2021 by Bill Kielb

[neilestrick]

If you completely open a peep hole, that will be too large an opening and spoil the draft. It's only pulling through a 1/4" hole in the bottom, which is much smaller than the opening of a peep hole. It may also take a minute or two before you see a noticeable change in the draft after making adjustments. You have to build the negative pressure before it will do anything. There is also a chance that your fan simply isn't powerful enough, but I would try working with what you've got before investing in a new fan. The fan used in the Skutt and L&L systems is a 146CFM squirrel cage model, and it can handle up to 20 cubic feet of kiln volume, whether all from one kiln or split among two kilns.

[moonzie]

Thanks, guys. I'll try popping out a few more holes in the collection box and see if that helps. I can always seal them up with HVAC metal tape if it's too much. If after I've got 8 hole drilled and there's still no change, I'll look into a different fan motor.  The unit's only 15 watts. With no ducting attached, it moves quite a bit.

I just wish this stuff was documented better!

[Hulk]

I'm good with flame and/or smoke tests on a cold kiln for proof of concept - that the fan system does indeed pull from the kiln, also for getting an idea how/if adjusting the shutters influences the pull.

In use, however, the warm and smelly outflow provides a positive indication - it's working*.

As for hanging about the studio whilst firing, I'm ok with only very minimal exposure to vapors, fuming, etc., hence, if the overhead system wasn't in place and working, I wouldn't be in there - even with the rollup door, windows, and man door open - except for checking pyrometer readout (whilst holding my breath). Yep.

*I've little doubt that it is possible to overdo downdraft venting.
How much is too much, just right, not enough air movement through the kiln? idk

[neilestrick]

1 hour ago, moonzie said:

I just wish this stuff was documented better!

They're trying to sell vents, not teach you how to make your own.

[Bill Kielb]

2 hours ago, moonzie said:

I just wish this stuff was documented better!

That in-line fan in the top thread very likely is not capable of developing sufficient draw or suction if you will. Fans rated 100-150 cfm with approximately 1” of pressure differential are more appropriate. That fan is designed to operate in free air or even with helping pressure so to speak. A small prop fan like that is really not likely to work well for this type of use.

[Bill Kielb]

1 hour ago, Hulk said:

I've little doubt that it is possible to overdo downdraft venting.
How much is too much, just right, not enough air movement through the kiln? idk

I measured it in the video and got 7 cfm per kiln gross which was mostly room air. Very likely much less than 1/4 cfm  of kiln air per kiln. (Don’t make me calculate it, it’s tiny)

[moonzie]

Yeah those squirrel cage fans are going to be a lot more capable of 'work' in terms of creating a pressure difference. I was reading some ancient mailing list threads from 1998 where somebody was complaining that a Bailey vent system with no bypass componentry didn't work at all but a L&L system that had a bypass worked great.

That they tell you your vent system works great when you put a flame up to the 1/4" hole as a test is why I've been confused. It's much more complex than that. And most people probably just buy the kit, plumb it all in and call it done.

There's a level of science to this that I suppose was taught to me at some point but have since forgotten. My dad was a HVAC business owner and I worked with him but I never sat down and went into ventilation design theory. I'm sure he'd school me if he were still around. He passed on in 2013. Thanks for the help guys! I'll post an update after I've modified a few things.

[moonzie]

2 hours ago, Bill Kielb said:

That in-line fan in the top thread very likely is not capable of developing sufficient draw or suction if you will. Fans rated 100-150 cfm with approximately 1” of pressure differential are more appropriate. That fan is designed to operate in free air or even with helping pressure so to speak. A small prop fan like that is really not likely to work well for this type of use.

Would this be an appropriate motor?

https://www.grainger.com/product/DAYTON-OEM-Blower-1TDP7

On the specs it lists 110cfm @ .5" sp but I'm not sure if that's differential or.. well however else it's measured. overall?

[neilestrick]

24 minutes ago, moonzie said:

Would this be an appropriate motor?

https://www.grainger.com/product/DAYTON-OEM-Blower-1TDP7

Yep, that's the one that L&L and Skutt use.

[Bill Kielb]

16 hours ago, moonzie said:

Would this be an appropriate motor?

https://www.grainger.com/product/DAYTON-OEM-Blower-1TDP7

On the specs it lists 110cfm @ .5" sp but I'm not sure if that's differential or.. well however else it's measured. overall?

As Neil confirmed, that should be fine ……. And it is externally cooled so less worry about designing around that issue. If you made it through the video these things need to develop about -0.1” W.C. At the pickup manifold and from measurement flow is likely less than 20 cfm depending how much area is provided to draw room air at your mixing manifold. The starved nature of operation of these things tends to make them a bit noisy BTW. Notice the inlet limit temperature is 104 degrees as well as the ambient operation temperature. This is fairly low so more room air than kiln air - always.

Edited December 17, 2021 by Bill Kielb

[moonzie]

I was recently told by wife and father in law that if we opened up the hole from 1/4" to maybe 3/8 or 5/16 it would draw better. I'm concerned that it'd draw too much.

[neilestrick]

25 minutes ago, moonzie said:

I was recently told by wife and father in law that if we opened up the hole from 1/4" to maybe 3/8 or 5/16 it would draw better. I'm concerned that it'd draw too much.

The L&L and Skutt vents use a single 1/4" hole for that size kiln. However you're dealing with a different size vent motor so those numbers won't necessarily be the same for you. I would try drilling a second 1/4" hole, rather than enlarging the hole you have, and see if that makes any difference. You can always plug it up if you end up not needing it.