UDS Exhaust - Help

[foam2]

Originally Posted by tsanchez

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

You're right - thx.

[Carbon]

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

[Just BS]

Originally Posted by Carbon

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

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.

[razrbakcrzy]

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.....

[Carbon]

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.

[Toronto]

Originally Posted by froggyshooter

Question and Request:
Built (building) first UDS: second on the way when this one complete. Just cooked pork butt and it came out great, but had to mess with intake quite a bit.
There are 3 3/4" intakes; 2 with caps one with ball valve. The exhaust is a 1 1/2 metal stack. Got it going with minion method and closed all intakes but 100% on ball valve. When I got home 8 hours later internal (Maverick) smoker temp was 175 F and meat was 150F. This has occurred every time I close off with caps and leave ball valve open 100%. Played with intakes (cap on / off) and it is touchy. The unit is air tight when everything is closed and vent stack is closed.
My thought, but let me know if I am wrong; the 1 1/2" exhaust is causing problem.
Love the way it cooks but cannot get it to hover at 225 like you guys.
Thanks

If its not going as hot as you like then try opening up your intakes more. Logic dictates that your fire is not getting enough oxygen to idle at the temps you want.

Use everyone else's experience as a rough guideline, no two UDS behave exactly the same since not all are created the same nor do they run on the same fuel.

You didnt mention the ambient environment, how was the weather that day? If it was rainy, windy or cold this may have contributed to your low temps. Also, placement, is your UDS by a wall or out in the open?

[Forrestbro]

I have no clue as to what all these calculations are all about. I just cook food that me and my folks like to eat. With each cook, I learn a little more about my UDS - what works, what doesn't work so well. Over the past year, I've learned different heat setting adjustments for different cuts and types of meat, poultry and fish. I can now make adjustments for the weather too, because I cook year round. So, as has often been said before, "YMMV" ... and keep the Thin Blue Smoke comin'!!

[foam2]

If the area of the exhaust doesn't have any effect than why have one at all? Just because a 2" hole came on the drum that was designed to store and transfer fluids does not mean that by coincidence that it is the optimal size to control exhaust on a UDS at any temperature.
The longer pipe only serves to create restriction and restrict air leaving the smoker plain and simple - for all intents and purposes air restriction inside the smoker is the same and any pick up in velocity as compared between 6" and 12" is negligable. By your argument, any restriction on your exhaust will create rancid tasting food no matter what size the opening.
None of the things you mention at such low relative pressures and velocities contributes at all to your explaination of how the thermodynamics of a domed lid vs a flat lid requires different consideration for exhaust sizing.
The results are 4 - 3/4" holes have adequate area to control the exhaust on a UDS. A 2" exhaust will also work although it will require different intake settings as well how much charcoal you initially light up.
To the OP - this is much simpler than is being made out to be. Start out with a 2" hole or 4 - 3/4" holes and see what works for you. Drum lids are cheap.

[razrbakcrzy]

I concede, I have been beaten by the superior intellect...... I guess it has something to do with being from chitcago, there seems to be alot of them that know everything....


I have built and used 5 different UDS, with flat and domed lids in the last 2 years.... I know how they run and how the react... OP you can choose to believe somebodys opinion or sombodys experience. it's your decision....... Good luck with your changes....

[foam2]

I won't comment on where you're from either...

[hmbrewr]

So!!!! could any of you cerebral guys hazard an educated guess as to what would be the ideal length 2" exhaust pipe with a 45 deg angle cut on the top. I currently am running 12" and think that the temps are a little hotter than I would like. Not looking to start an argument, just looking for knowledge from people who are a little wiser than me.

[foam2]

Check what lengths of threadded pipe are available at your local hardware store and don't be afraid to experiment. If you like to cook at 225, you may find a shorter pipe works better, and if you like to cook hotter maybe a longer pipe. Try a few pre-cut lengths and give it a shot. I run a weber lid on my uds with no pipe and while I typically run the vent wide open there have been times when it was really cold and windy that I had to close them off by about 30% when the temp started to fall from too much drafting. So in short, there is no ideal length that covers every situation - try a few out and see what works best with your uds.

[Napper]

I have been building drum smokers for many years.(I think I am on the 4th page of the massive UDS thread) So I do have some experience. I think I have built around 25 smoker so far for myself and friends.
The best design I have to date is 4 -7/8 inch intake holes and 6 equally spaced 7/8 inch exhaust holes on the top. Just seems to work the best. Why 7/8 inch.....size of a 1/2 inch NPT slug cutter.
I will say that I built 4 IDENTICAL drums three years ago for a chicken wing cookoff and all four of them cooked differently for some reason. Not that much different but I had to adjust each one differently during the cookoff.

Just my $0.02. There is no BEST design....just find what works for you and cook enough to "learn about" your drum. The ones I equipped with a fan and PID controller could control temperature on the dot but what fun is that!

[Hawg Father of Seoul]

Originally Posted by foam2

.... Just because a 2" hole came on the drum that was designed to store and transfer fluids does not mean that by coincidence that it is the optimal size to control exhaust on a UDS at any temperature.
......

No, but it DID dictate what size most experimenting was done on.

Personally, I just drill 8 3/4 or so sized holes in the perimeter of the lid.

My new drums use a single 2 inch intake, but my old ones had the smaller intakes. My temperature was chiefly dictated by charcoal basket. I had a small basket and never could get above 250. With the bigger basket.... 350.

Lots of variables at play, but I am just a hick from the sticks.

[Daggs]

I love how people always want to reinvent the wheel. Why do you mock the ones who have brought you something so simple. Practice makes perfect!

For the record I use a 2" PVC exhaust with a ball valve along with 2, 3/4" trade size intakes both with ball valves. Mine holds temps very well.

I appreciate all the time put in by others who have shared their knowledge.