UDS Exhaust - Help

I leave one cap off and adjust with the ball valve and that works for me. I cook a lot hotter than 225 though - more like 270 to 300. My exhaust is 2" btw.

If you had read the Mother of all UDS Threads this has been discussed at great length and you would know that the optimum exhaust size is 2". I run 3 3/4 intakes 2 caps and a Ball valve like gtr 1 cap off fine tune wit the valve and I too cook 275-350. If the exhaust is to small it damps the fire.

froggyshooter said:

GTR. Thanks.
that sounds a lot like what I do although getting to stabilize at one temp is difficult. You don't think the exhaust then is any issue?

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In all honesty I don't know. What I can say is, based on lots of time watching smoke come out of my exhaust (staring at exhaust stacks when cooking is a good thing to do IMO), I don't think I'd want less than 2" for exhaust.

As far as getting it to stabilize - are you allowing time for the temp to settle in? Some folks fiddle a lot and that can lead to fluctuating temps.

I don't know that 2" is the optimum exhaust diameter. Weber uses 4 - 3/4" that equates to an area of 7" on the 22" wsm and is fully capable of smoking hot and fast. A 2" exhaust has an area of well over 12 square inches which I believe without any damper control is overkill for the UDS.

I also have no problem with my stock exhaust vent on my Weber lid.....which is much smaller in area compared to a full 2" exhaust as mentioned above.

foam2 said:

I don't know that 2" is the optimum exhaust diameter. Weber uses 4 - 3/4" that equates to an area of 7" on the 22" wsm and is fully capable of smoking hot and fast. A 2" exhaust has an area of well over 12 square inches which I believe without any damper control is overkill for the UDS.

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Much different Thermal Dynamic flow with a domed lid as opposed to a flat drum top.....

razrbakcrzy said:

Much different Thermal Dynamic flow with a domed lid as opposed to a flat drum top.....

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Ok, I can see that. Then how about eight 1/2" holes on a flat drum top? The total area of those eight exhaust holes are significantly smaller than a single 2" exhaust.

razrbakcrzy said:

Much different Thermal Dynamic flow with a domed lid as opposed to a flat drum top.....

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Domed lid, flat lid, exhaust is exhaust. Weber uses a 3 to one ratio in intake vs exhaust. If you look at the pbc threads, the unit has a massive 3" that is perhaps 30% blocked and 4 exhaust holes where the rods go through. In the PBC videos they even go on to say while the intake is fixed, they recommend that even if you're only cooking on the grate to still insert the rods to prevent the pit from getting too hot. So in this instance the pbc probably uses a ratio higher than the weber and they have a flat lid. The idea of thermo dynamics on a domed lid vs flat with regards to the size of the exhaust is absurd.

Carbon said:

Ok, I can see that. Then how about eight 1/2" holes on a flat drum top? The total area of those eight exhaust holes are significantly smaller than a single 2" exhaust.

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foam2 said:

The idea of thermo dynamics on a domed lid vs flat with regards to the size of the exhaust is absurd.

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The whole effect of heat rising and creating a draw up through an exhaust stack is thermo dynamics and fluid dynamics at work. If you think its absurd, your absurd....

I have personally experimented with diferent length exhaust tubes from the barrel flat top and I can with the exact same inlet openings, change the temp by as much as 50degrees with a longer exhaust tube. I currently use a 6" exhaust but with a 12" stack the drum will run 50 degrees hotter than the 6" with the exact same setup....

with the normal offset location of the bung hole on a flat top drum the heat on the side oposite the exhaust will rush up against the top of the flat lid and turbulate causing a high pressure zone(turbulent flow), where the heat that rises adjacent to the exhaust opening will rocket straight up and out the 2" exhaust (laminar flow) causing a vaccum(low pressure) and thus a positive flow or a draft in the upward orientation....

With a domed lid the heat all rushes up toward the center exhaust and out, thus the webber lids usually can get by with a much smaller exhaust because of this effect. (the eight circumferential holes would eliminate the turbulence since the hole are all equidistance from the center thus requireing less voulmetric flow)

If you notice the exhuast on a weber usually comes out with a fairly high flow rate and a nice amount of force. Conversely try looking at a drum with an offset exhaust, where the exhaust comes out a failry low flow rate.... but with a higher volume. It's all about volumetric displacement, how much air can you move thru the inlets up through the coals across the food grates and out through the exhaust..

Now, Im done with this. This place was not meant to argue or point finger at each other and call each others ideas absurd, Instead we should be considering all the things each other have to say in the intrest of our mutually shared hobby and passion... This was not meant for the person who simply wanted to discuss this topic, have a nice day..

Heres a little light reading for you...
http://francesa.phy.cmich.edu/people//andy/physics110/book/chapters/chapter9.htm

Could be your fuel.

razrbakcrzy said:

The whole effect of heat rising and creating a draw up through an exhaust stack is thermo dynamics and fluid dynamics at work. If you think its absurd, your absurd....

I have personally experimented with diferent length exhaust tubes from the barrel flat top and I can with the exact same inlet openings, change the temp by as much as 50degrees with a longer exhaust tube. I currently use a 6" exhaust but with a 12" stack the drum will run 50 degrees hotter than the 6" with the exact same setup....

with the normal offset location of the bung hole on a flat top drum the heat on the side oposite the exhaust will rush up against the top of the flat lid and turbulate causing a high pressure zone(turbulent flow), where the heat that rises adjacent to the exhaust opening will rocket straight up and out the 2" exhaust (laminar flow) causing a vaccum(low pressure) and thus a positive flow or a draft in the upward orientation....

With a domed lid the heat all rushes up toward the center exhaust and out, thus the webber lids usually can get by with a much smaller exhaust because of this effect. (the eight circumferential holes would eliminate the turbulence since the hole are all equidistance from the center thus requireing less voulmetric flow)

If you notice the exhuast on a weber usually comes out with a fairly high flow rate and a nice amount of force. Conversely try looking at a drum with an offset exhaust, where the exhaust comes out a failry low flow rate.... but with a higher volume. It's all about volumetric displacement, how much air can you move thru the inlets up through the coals across the food grates and out through the exhaust..

Now, Im done with this. This place was not meant to argue or point finger at each other and call each others ideas absurd, Instead we should be considering all the things each other have to say in the intrest of our mutually shared hobby and passion... This was not meant for the person who simply wanted to discuss this topic, have a nice day..

Heres a little light reading for you...
http://francesa.phy.cmich.edu/people//andy/physics110/book/chapters/chapter9.htm

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I'm not going to drag this out and argue over a 2" exhaust but I will make some points. I am an engineer both by education and profession, please don't throw around thermo and fluid dynamics to justify your point. In this case with a vertical smoker, you are building heat and exhausting it in the same concentrated area and getting some benefit of convection. You are not trying to draw it to one side like you would an offset smoker which would require a larger volume of air moving through the cook chamer to accomodate. In addition to your comments regarding the longer pipe cooking at hotter temperatures, that is absolutely correct. The length of your pipe serves to add restriction to the exhaust, effectively reducing the air going through the system. In the end you can adjust the flow of exhaust like weber does or any other pits that have variable openings in the exhaust or you can add or remove length to the exhaust. The length of pipe is the same thing as how much vertical head a pump can pull - it is distance dependant when horsepower remains constant.
In the end if we are talking about smoking temperatures without any type of stoker system four 3/4" exhaust holes are enough. If you might want to cook hotter than 350 then I would say you would need to add additional intake and exhaust.

A 2 inch hole has an area of 3.14 square inches
8-1/2" holes have an area of 1.57 square inches
4-3/4" holes have an area of 1.76 square inches
3-3/4 holes have an area of 1.32 square inches

tsanchez said:

A 2 inch hole has an area of 3.14 square inches
8-1/2" holes have an area of 1.57 square inches
4-3/4" holes have an area of 1.76 square inches
3-3/4 holes have an area of 1.32 square inches

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You're right - thx.

I think the bottom line is most any reasonably sized exhaust, whether adjustable or not, will work well with the UDS.

Carbon said:

I think the bottom line is most any reasonably sized exhaust, whether adjustable or not, will work well with the UDS.

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Agreed.

To the OP. No two UDS's are the same. Keep cooking u.til you figure out how yours work. You will find that after about six cooks it will stabilize a lot more.

Lastly, I like a bigger exhaust, but its just a preference. I find I have more flexability and my food taste better.

Look the area of the exhaust will not affect how much air flow gets in or out that is controlled by the inlets.

if you have 3 3/4" inlets and you have 1 open and a ball valve partially closed you will only flow as much air as will flow thru those openings.... you will not draw a vaccum on a self powered air pump which is essentially what a UDS or any smoker for that matter is. The engine and fuel are the heat and the coals/air, the throttle is the intakes and the exhaust is the smoke stack. You will not draw a vaccum (you can not exhaust more exhaust than the air you draw in) but you can cause a restriction by under sizing your exhaust..... which will result in stagnat air and rancid acrid smoke or an inability to reach temps...If you can't get the air out you can't get the air in... the longer stack creates a restriction making the exhaust speed up on it's way out of the longer stack and the effect is intensified if the stack is tapered from bottom to top.....


If you don't believe this just please stoke you up a really hot fire put in about $100.00 worth of meat and slap that domed lid on your WSM and close the exhaust and wait about 5 hours when you take the beautiful meat out of the pit it will be nice and foul tasting with about a 1/4" of soot and raw right in the middle...

Thats why most never choke a fire with the exhaust, they starve it with the inlets....Basic BBQ knowledge.....

I may be wrong but I think, in general, a smaller exhaust port would maintain a moister environment in a UDS as long as it's not restricting air flow.