42 gallon reverse flow patio build calculations check, please

[LanceR]

Hello All

Now that we've sold the farm in Central NY and "retired" full time to our NC place I hope to have more time for projects like smokers. I've never built an RF smoker before and it's been a long while since I tinkered with RF smoker dimensions so I'd appreciate some input on my figures.

Among the collection of 120 gallon tanks (hopefully) awaiting smoker builds I have one that calculates out to around 85 gallons based on about 21" diameter, 56" straight section with 4" standard end flanges and walls between 1/4" and 3/8" (maybe thicker as it is a lot heavier than the 120 gallon tanks). I intend cutting the tank midway between the end seams for two approximately 42 gallon patio smokers. I'll tweak the figures as needed after I cut into the tank and see for sure how thick it is.

So, with some minor rounding of figures.....

I figure the total volume of the intact tank at between 17-18,000 cubic inches. Half that would be 8,500-9,000 cubic inches for the cooking chamber (CC) and, as it's a smallish smoker I'll round the firebox and airflow figures up some. I figure that if the airflow dimensions are a big big it won't hurt but the other way around.....


That leads me to a firebox (FB) volume of around 3,800 cubic inches and,

A minimum FB to CC opening of 38 square inches and,

A minimum space under the reverse flow plate (RFP) of 38 square inches and,

A minimum opening at the end of the RFP of 38 square inches and,

Minimum FB air intakes of 12(?) square inches and,

An exhaust stack volume above the CC of around 216 cubic inches.

With a fairly large diameter CC for the volume I expect to use a rectangular FB-CC opening which I'll make up as a damper door and that there will be more than ample space under and at the end of the RFP.

Other design thoughts are for at least one of the patio smokers to have a warming box over the FB. I suspect I will use a square or rectangular exhaust stack inside the warming box and large vents top and bottom to regulate heat. I think I have a source for enough 1/2" plate for the firebox if not both the firebox and warming box......

The firebox will have a raised fire grate with ash pan underneath and use air intakes both below the grate and across from the FB-CC opening.

Question: Is the calculated FB volume the area above the fire grate or the entire cavity?



Thanks for looking and I'm looking forward to hearing your thoughts.


Lance

 

[SonnyE]

Never worried about it. If it looked big enough, it probably was.

I'd suggest you make sure it's empty before you put a torch or a grinder to it.
Unless you like that kind of yard art.
I always purged with Nitrogen before opening.
Play safe, and best wishes for a Long and Happy retirement!

 

[daveomak]

I figure the total volume of the intact tank at between 17-18,000 (20,000+)cubic inches. Half that would be 8,500-9,000 (10,000)cubic inches for the cooking chamber (CC) and, as it's a smallish smoker I'll round the firebox and airflow figures up some. I figure that if the airflow dimensions are a big big it won't hurt but the other way around.....


That leads me to a firebox (FB) volume of around 3,800 cubic inches and,

A minimum FB to CC opening of 38 (40) square inches and,

A minimum space under the reverse flow plate (RFP) of 38(40) square inches and,

A minimum opening at the end of the RFP of 38(40) square inches and,

Minimum FB air intakes of 12(?) square inches and,

An exhaust stack volume above the CC of around 216 cubic inches.


This would be better than a rectangular opening... AND may I suggest you do not have a flapper
where this opening is..

 

[LanceR]

Thanks Dave!

I picked up a nice clean 1/2"x 18-1/2"x22' chunk of A36 hot rolled steel last week for only $125 that is earmarked for a welding table. It has primer on it but that's easy to deal with. I think the seller may have another slab and, if so, I'm going to try to get my hands on it for the fire box and the end of the tank.

Do you have any thoughts on insulating the top of the firebox so the warming cabinet doesn't get too hot? I suspect that the end abutting the cook chamber and the exhaust stack running up the back wall inside the warming cabinet will give all the heat I want. I have a fair bit of 1-1/2 and thicker firebrick and was toying with the idea of welding nuts to the top of the firebox and using angle iron to hold a layer of firebrick. That way I could take out bricks as needed if I wanted more heat in the warming box.....

Again, thanks for the input and for your willingness to help others.


Lance

 

[daveomak]

Adjustable air inlets on the sides.. near the top and bottom of the warming box...

 

[LanceR]

I figured I'd put some pretty big vents top and bottom with maybe a perforated diffusing plate in the bottom to prevent a hot spot right over the fire. Thanks for the reply.


Lance

 

[LanceR]

Well, it looks like after a lot longer pause than I'd like that I'll be able to start welding this thing up in the next month or so.

Here in NW North Carolina the best price I've found for steel looks like Race City Steel over by Lake Norman. They have 1/4" pickled and oiled hot rolled steel for around $5.75 a square foot and it looks like you only pay for the cut size and not the sheet they cut it from. They actually price by the square inch so for the plates at least you pay for only what you are going to need. And they offer

They also offer plasma, laser and water jet cutting in addition to shearing, rolling and bending. I'll be stopping in next week with some drawings and have them price out just the rectangular firebox and reverse flow plates and also have them price the air holes, door openings etc for the firebox and warming cabinet and a tapered RF plate with a bend in it for slope and drainage.

As an added plus they deliver in my area once a week and will deliver to a residence......