Mailbox Mod Failure

[Winterrider]

believe the elbow and a no greater than 4 " exhaust stack will improve your flow .

Edit , 4 " in length on the exhaust .

Just curious why only 4" length on stack recommended ? I did 24" thinking better draw. Right or wrong, not sure. Seems to work in my case.

 

[chopsaw]

Condensation at the point where the stack cools . Drips back into the smoker .

 

[Winterrider]

Gotcha, mine is on side so I use elbow.

 

[daveomak]

If the stack cools too much, it will stop the air flow...

 

[pinjim]

The heat being generated by the flame is creating pressure by the gas and expanding air... Try a 90deg. elbow on the outlet of the MB mod..... That should work...
"Maybe" you will have to close off other inlets at the bottom of the smoke chamber.. but I doubt it... I would use a refer magnet and close only 1 hole at a time so you don't starve the burner for air... That could be dangerous... If the flame goes out, you will have a bomb waiting to explode....

I ordered the elbow. Of course, with Amazon being in their current state, they said it'll be 9 days before it gets here. I've stopped using Amazon as much just for that reason.

I'm not sure if it'd be practical to close off the bottom airways as it's basically a giant circular hole around the burner.

If all of this fails, I may punt and go back to old methods. I never thought I'd say it, but it may encourage me to move on from the old Smoke Vault. It's been my tried and true smoker for many years. She's built like a sherman tank. But, it seems to have its shortcomings too. It's hard to hold a steady temp. Mine leaks like a pig around the door (I know I can seal it). It's not insulated. My flame burns yellow and their customer service couldn't figure it out (I replaced the entire burner assembly). And now this.

In my ideal world, I'd get a smoker that is electric, insulated, 24" wide and works with the mailbox mod. Maybe that's a pipe dream, dunno. I haven't looked for 5 years or so as the old Smoke Vault just kept trucking along....

 

[pinjim]

Still a failure.

Amazon has sucked lately. After waiting two weeks for my 3” aluminum elbow, they sent an email saying I could get a refund since it never came. So off to Lowe’s I went. Of course, all they had was galvanized but I bought it just for proof of concept. I took it back out after the experiment.

And the result was the same. See the video below. I’ve played the the vent dampers, no difference.

I guess I’m out on the mailbox mod.

 

[Lonzinomaker]

Check out the electric Smokin-IT #3, racks are 14.5 X 21.5 inches. You won't need a mailbox mod with it.

 

[pinjim]

Another try. This actually works. Of course, I need a better setup, but it’s forcing the smoke into the smoker.

 

[chopsaw]

Did you try the elbow on the outside of the smoker ? So you get the smoke rising as it goes in .

 

[pinjim]

I did not try the elbow on the outside, but there’s vertical lift with the current setup. I’m convinced it’s as people suggested, the hot expanding gas creates pressure, not a vacuum.

 

[daveomak]

Have you tried the MB mod with the burner off ???
Have you tried it with the pan removed from the smoker ???
That might work for cold smoking...
A smoking tube would work inside the smoker with the gas on...

 

[pinjim]

And here’s my current solution. This seems to work, I got tbs the whole smoke and no smoke coming out the front of the mailbox. A little smoke did some out the mailbox mounting holes, but I sealed them yesterday. I’m happy with this, albeit less than ideal. Not sure what I’ll do if it rains, I may splice in a long low voltage cable so at least the water won’t get into the 110vac circuit.

 

[pinjim]

Yeah, I can run it with the burner off and the smoke goes right into the smoker, no fan needed. Agreed, it should work for cold smoking.

 

[tridiot]

What's the size of the exhaust on the Smoke Vault ?? There should be about 2X the exhaust as there is for the inlet to the MB mod.... The openings on both ends of the flex duct should be 3" to fit the flex correctly... and provide adequate air flow....

Bottom line, you are going to need more airflow.

Cut out the restrictor plate.. About the exhaust.... it needs to be about 2X more area than the holes you drill in the MB door....

I’ve tried every combination possible for the various vents. I did find that opening the vent on the other side made the problem worse.

So there are several problems at work here. We have to look at the physics of the whole thing. Heat rises, yes. But air pressure causes movement and it wants to find the path of least resistance, right?

So... your smoker wants to draw air in the bottom from whatever path is easiest. Right now, that port on the left side and the port on the top are easy to get air IN. Your mailbox is fighting you... that long corrugated tube has resistance. Your mailbox is heating up and trying to push air (and smoke) out any way it can - through the exhaust port, through the intake port, etc. Right now, when your smoker is all closed up, it is exhausting through the intake port and pulling in from the smoker, why? Because the smoker is heating up, burning fuel, pulling air through the left lower intake port, and possibly through the roof intake, as well, building pressure inside the box and pushing it out through the tube to the mailbox.

Open the door on the smoker, and all the pressure inside plummets, removing some of the restriction against the mailbox's exhaust tube and allowing smoke to flow, just like your video shows. So... your simple solution is to figure out how to encourage air to flow out the top instead of building pressure in the box and pushing it out the largest hole (and the one most difficult to pull FRESH air in through - remember, the oxygen needed to combust is being consumed in your mailbox).

I look at it like the ports in a ported subwoofer box. It isn't just about port DIAMETER... it's also about port LENGTH. When you're tuning a subwoofer port, you have to take into account the resistance offered by the walls along the length of the port. Think of it this way.... if you have a drinking straw that is 3" long, you can blow through that and inhale through that all you want, it doesn't bother you at all. If you make that straw 10" long, you CAN blow through it, but trying to inhale through it is going to leave you gasping for air.... right? Your smoker is EXHALING through your 'straw' which is the long tube coming in from the mailbox.

If you open the door to the smoker, that open doorway then becomes the easiest exit for everything, allowing the pressure built up in the mailbox to push into the smoker, carrying the smoke with it.

I think you said cold smoking, without any heat source running, things flow the way you want them to, right? This is because your large chamber is totally passive at this point, and the only source of pressure is your mailbox... the heat and pressure inside can overcome the resistance of the exhaust, while drawing IN through the intended intake (on the mailbox). As soon as you turn on a heat source in your large chamber, the dynamics of pressure (and heat) change all of the factors, reversing the flow. The pressure and heat from your large chamber override the smaller amount of heat in the mailbox and push it out, while drawing its combustion gasses (inhaling!) from the easiest/closest ports with the least resistance.

Remember... inhaling through a straw is alot more difficult than exhaling through a straw... the length of the straw simply adds resistance and exaggerates that effect.

Here's what I think... you need a SHORT run exhaust from your mailbox to your cooking chamber, with little resistance. You want your heat source in the cooking chamber to see this as the BEST place to INHALE from - it just happens to have smoke in it, and some of the oxygen has already been consumed... this works against you, slightly.

You want more port space up top, to encourage the heat to flow outward and carry air with it. Yes, this will change your cooking dynamics slightly, but it really doesn't cost you as much heat as you would think. This is why 99% of all commercial smokers over the last 50 years have exhaust vents on the top. And usually MORE exhaust than intake. All other things considered, those exhausts would be used by your heat source for oxygen and combustible gas, but when heat enters the equation, it causes changes in pressure in the cooking chamber and heat and air flow upward.

I'm not a physicist, though I do have a lot of physics and science training and experience. These are my thoughts, if you can follow them. And in actuality, if you use a square port instead of round, you'll get a LOT more airflow through it.

I think if you cut a hole in the top of your smoker and shorten up the length of the run on your mailbox-to-cooking chamber tube, you'll see an improvement. Cutting a hole in the top seems like a point of no return thing, but remember, you can easily cover this up later with a piece of sheet metal, or even make a larger flap you can open and close just like a regular damper.


So does this logic make sense to anyone else?

<edit> This is why your fan mod works... it increases pressure in the mailbox, also burning faster and creating more heat, but additionally I'd bet the oxygen levels in the mailbox's "exhaust" are higher with the increased flow, thus helping supply your propane burner's oxygen needs. The combination of more pressure and more oxygen help overcome the factors against you in the cooking chamber and give you the flow you want.