Huge temp difference, input needed

[Big Steve]

OK Guru's here what I got. We bought plans from Frank and built a 24" by 67" RFS to his plans. It's all 1/4 steel (firebox, cook chamber, baffle plate). We built it to plan. Did I say that? Yes, because I don't think his plans are the problem so that means it's our technique or just weird juju. The cook grate is essentially 22.5" by like 48" wide.
Today we cooked a boat load of chicken halves. I was shooting for a hot/fast cook at 300 degrees and sort of got that worked out pretty good. The big issue is a we have a huge temperature difference (70 degrees) across the cook grate. Firebox side if hotter (obvious right) than the "reverse flow path opening at the end of the baffle plate". 70 degrees seem excessive to me. The BP is 1/4" thick. I'm considering adding another 1/4" plate over the top of the firebox and extend that to under the cook grate for the first foot or so. The idea being that the additional 1/4" will help insulate the hot zone more and push the heat further down the BP, thereby equalizing temperatures. Anyone else experience this and are we on target for "levelizing" the temp? Really looking for some input on this cause the delta seems excessive. 25 degrees from one side to other, OK I can work with that, but 70 seems large to me.

Thanks,
Big Steve

[Gizmo]

2 ways to look at this:
1) 70 degrees means you can cook low and slow on one end and hot and fast on the other end.
2) If all you want is a bunch of chicken all done at the same time then "by golly" it had better be an even temp.

Unfortunately both are correct most of the time. So maybe your solution should be removable…..?

Short answer:
Your "shadow plate" idea is spot on - as long as there is a breathable inch of gap at least between the BP and the Shadow plate. Hopefully you have the height in your rig to allow you to install these 2 plates.

I think you'll be pleased with that solution to "levelizing" (really?) the grate temp…..

Long answer:
For the sake of argument - radiant heat transfer is instant and convective heat transfer takes a little while.

A shadow plate forces the radiant heat to pass through a piece of steel on it's way to the BP which slows it down and gives the convective heat a chance to catch up - sort of like starting the runners out in staggered start lines on a round track so they all have the exact same distance to travel to a non-staggered finish line.

[Big Steve]

Thanks Gizmo for you insight. Do you how others solved the same problem?

[Gizmo]

What does the space measure between the top of the BP and bottom of the grate…. ?

[Big Steve]

3 to 4 inches (from memory).

[Gizmo]

Would it be reasonably easy to slip a shadow plate in there against the firewall and about half way up that gap and say about half way across the CC? You'll just need a reasonably good fit against the walls for now - you can "deal with the seal" later for better results. If you don't get a good seal you can run a small bead of high-temp silicone along that joint for trial purposes to stop hot air leakage for testing. It's not too hard to xacto cut that bead later and remove it.

Normally the shadow plate is the FIRST thing that the fire's radiant heat hits. In your case it will be the SECOND. I'm thinking you'll want it that large to delay the heat build up a very long time…. You might consider tack-welding tabs to the walls to set the shadow plate on for now - so you can remove/resize/remount to be removable, etc. Then re-burn it to see what effect you're having.

On the re-burn it would be nice to have 3 temp probes in it:

FB end - the temp with the 2-stage effect of the shadow plate, (compared to the other 2)
middle - how much heat is it dumping out the end of the shadow plate, (compared to the 3rd one)
far end - how much heat your main BP is dumping out the original BP gap (which should be nearly unaffected by any of this)

[Big Steve]

We have two thermomenters in the cook chamber door about 3" above the cook rack. I'll use a digital to verify each for accuracy and then place it to get some readings. We also have access to a Ifrared camera (posted pics in another thread) which we can use to get a picture of the thermodynamics and what's going on regarding heat distribution throughout the cook chamber. Once that's all done I'll load the grill up with some bread and try the toast method. I find the learning and tweaking phase the rewarding. How boring would it be to build a huge smoker and not have to learn how it behaves and to nail every cook?
Gizmo so you're saying eventually the dead air space between the baffle plate and the shadow plate should be sealed 100%?

[capt j-rod]

Couple of tricks... 1. Smaller pieces of wood, and move the fire away from the throat, closer to the door.
2. Raise and lower the end opposite the firebox. ( a little goes a long way)
3. Wind really screws up cc temps, look for a calm area/wind break
4. All else fails build a bigger badder BBQ and use temp fluctuations as the excuse to the wife!

[Big Steve]

Couple of tricks... 1. Smaller pieces of wood, and move the fire away from the throat, closer to the door.
2. Raise and lower the end opposite the firebox. ( a little goes a long way)
3. Wind really screws up cc temps, look for a calm area/wind break
4. All else fails build a bigger badder BBQ and use temp fluctuations as the excuse to the wife!

I had not considered items 1 or 2. THAT's why this is such a great forum. I can #1 affecting the hot spot but will #2 have much affect? The smoker is sitting on the welding table outside and it is a little low on the firebox end (maybe 1"), will raising it up have a noticable affect on reducing the hot spot/end?
As for #3 it was a fairly calm day. Finally on #4 the Wife gives me free reign on projects. Other than that she's has great judgement.

[capt j-rod]

Block up the firebox end and use a floor jack on the opposite end. The short answer is yes changing the height evens the temps end to end. I have an adjustable baffle plate using an acme lead screw. (Idea stolen from "the original" frank cox) changing the angle with the trailer jack affects temps more than closing the gap on the baffle plate. When ever you make a change, wait a MINIMUM of 20 minutes to see how it responds. I guarantee that after you are done cooking if you go out 45 mins later it will be close in temp. We generally only add wood every hour. We also learned that all wood is not created equal. 2yr seasoned dry red oak is how we roll. We made wood last year that we will burn next year! A little hickory never hurt anybody either. Season your wood out of the sun with either a shed or tarp. Don't ask, we don't know why, but it is bad ass!

[Big Steve]

Awesome, can't wait to give it a try. We are using orange wood and some oak and typically have to add new wood every 30 to 45 minutes. We can get orange wood for free around here, so that's what we use. It smell nice when burning and is not overpowering. I'd like to get a source for hickory but honestly haven't put any time into finding one, just too busy getting the smoker to behave as expected. With Gizmo's and your tips I have plenty to work with on the next cook. Thanks for taking the time to write.

[capt j-rod]

One last thing, my baffle plate actually sits on top of my firebox. I left the bell on both ends of my tank, so my cooking grate is not directly over the firebox. It is awesome for baking potatoes (12+) if you throw them in foil. I'll dig out some pics of the rig. I'll warn you in advance, you will want a trailer after seeing this!

[SmokinFF]

[SoloQue]

Here is a late entry. I have 3 options for you to try
1 Rig a heat sink in the FB end of your cooker by making a raised shelf (2 inch max) that extends 1/3 down the body of the BP and snuggly fits from back wall to front wall of the CC. This reduces the radiant heat quite a bit on the FB end of the CC

2 At the begining of your cook take your rig up in temp until the nose is at least 10F over where you want the cooking temp to be for your whole rig. Now slowly close the exhaust using 10 minute increments and watch the FB end temps begin to drop. Keep doing this until the nose has fallen down into your target cooking range or you begin to see too much white smoke. hopefully you will find that the variance has greatly decreased before you are starving the FB.

2 Go buy a welding blanket and fold it over double. Lay that across the nose of your cooker as an insulator and repeat step 2. My wife did some stitching for me and ole Pops has a nice nose cover now, looks like a hat

I struggled with as much as a 30 - 40 F swing depending on the temp outside during a cook but now I literally run flat #'s between nose and FB even on the windiest of days after implementing these 3 steps

Hope this helps

[Big Steve]

We're going to focus on the "shadow plate" with an air gap, sealed at all seams on our next effort.

To date I've cooked everything with the exhaust stack wide open. Are you guys pinching back on the exhaust along with the inlet vents for normal smoking?

[Big Steve]

So we're going to build a "shadow plate" to help equalize the temps across the cooking grate more. It's taking longer than expected so I may use some tin foil pans with sand in them on the next cook as a quick fix substitute for the shadow plate. Still should be able to learn something from this and see how differently it behaves.

[SoloQue]

I'd go ahead and cover those sand pans with aluminum foil. Easy clean up to reuse the heat sink pans and also prevents anything from "dusting" up from the sand itself.

[Big Steve]

I'd go ahead and cover those sand pans with aluminum foil. Easy clean up to reuse the heat sink pans and also prevents anything from "dusting" up from the sand itself.

Might I say "GREAT Advise".

[Big Steve]

Here's a follow up post on solving the high temperature difference across the cooking grate. Today I smoked up 5 racks of spare ribs (3-2-1 method) at 225. Fire was the normal charcoal bed with one chimney of lit coals, followed by sticks of orange wood for smoke and flavor. I had the inlet vents open about 2" on each side. I also used some foil pans and filled them with sand and aluminum foil and placed them over the firebox/cook chamber overlap and another one under the grate on the extra hot side of the cooking grate. While the pans o'sand helped they did not completely cover or insulate the hot zone but it did help. Last cook we had a 70 degree difference across the cooking grate. This time I pinched back the stack damper and in doing so that slowed the draft and we saw immediate results, the "hot end" dropped down and the "cold end" picked up temp to the point I was able to realize a 25 degree difference. The stack is 6" wide and I had the damper about 3/4 closed. I was cautious and concerned but the smoke was running clear and the meat had a mild smoke flavor which we're finding is typical of orange wood. No acidic taste at all.
As soon as I'd open the stack damper the temps would start to spread apart on our two thermometers. Close it down and pinch it back to 3/4 closed and the temps fell into line. I can live and work with a 25 degree difference but we're moving forward with welding a "shadow plate" over the firebox hot end and will build into it a 1" sealed air gap. I'm hoping that this will result in a 5 degree temp difference. Anyway thanks for the suggestions on solving this problem. Now how far is too far to pinch back the stack damper? Do any of you have this issue and how do you run your smokers vents and dampers for a 225 low and slow cook?

[Rodcrafter]

I find some smokers adjust better with air coming in and some adjust better with air going out. The main thing is not to cause it to smoke white. I try to use a bit of both to get just what I want, it is all in the learning of the cooker.

jm2cw

[SoloQue]

Sounds like some great adjustments.