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Dutch hobby build. my first reverse flow project.

post #1 of 12
Thread Starter 

Hello all!


I'm Dennie from The Netherlands. 26 y/o and working as a car mechanic

My english isn't that good, but i'm learning.

In my free time i like to build stuff out of junk.

I'm cooking on a el-cheapo offset smoker. 13 inch. Too small...but it's working great after i updated it with some extra metal.


So now i want to build something bigger, and better. So i picked up a old watertank. 300 Liters (80 gallons??)


In Holland we calculate in cm's. I don't understand how i should calculate in inches. It's a shame i know!

I hope someone can help me out with calculating the firebox.


My tank measurements Diameter 630 mm Capacity 300 liters Length 1245 mm







Edited by Smulder NL - 7/5/14 at 8:37am
post #2 of 12
welcome1.gif 25.4 mm = 1 inch.... 1000 cc = 61 cubic inches...

post #3 of 12
Thread Starter 

Thanks Dave.

Is there a smoker calculator on the internet that calculates in cm's? I only found Feldon's.


My tank diameter is around 25 inch, and lenght 50 inch. Feldon's calculator says


Firebox Size
8177.08 cubic in.


Anyhow, this is my tank:


Processed By eBay with ImageMagick, z1.1.0. ||B2

Edited by Smulder NL - 7/5/14 at 5:09am
post #4 of 12
Feldon's needs some help when making a reverse flow.... this calculator is based on Feldon's and makes some adjustments for better air/heat flow.... Try it out and if you need help, I'm here...... Your numbers show the tank about 100 gallons..

...click on pics to enlarge...

This calculator I use for the FB/CC opening.... It lowers the opening to make more room for the RF plate and cooking grates....


Measuring for the FB/CC opening, take into account the metal thickness, to get to correct size opening....

post #5 of 12
Thread Starter 

Is it a 100 gallon tank?? I see!

there was a rubber membrane inside ....i think that thing holds 80 gallons...(http://www.deltaline.nl/deltaline/pompen--ketels--onderdelen/drukketels-wellmate-en-gegalvaniseerd/hydrofoorketel-horizontaal-varem/c-24/c-1380)


i was planning on making a firebox out of thick steel, Just a simple design with a simple door and a ashtray underneath it, behind the door.

But now i'm looking here, checking out what everyone is building....and i see some insulated fireboxes.


Can i do without? 

Edited by Smulder NL - 7/5/14 at 8:40am
post #6 of 12
Insulated FB..... Considering the surface are of the FB and CC.... more heat will be lost in the CC.... If wood is really expensive, you could consider insulating it... weigh the expense... probably $200 to insulate it.... and 20 hours labor... you can insulate it later....

The important part of the FB is great air flow to move the heat from the FB to the CC.... less wasted heat..... the upper air inlet plays a big part in moving heat..... a properly operating smoker doesn't use much wood after the initial "heat up" of the unit......
post #7 of 12

Here is a very handy program for converting metric to other units. I have used it for years and keep it on my desktop computer and also available for smart phones in the applications store, it has volume, distance, speed and a large variety of engineering unit conversions. It will help you when you read the forum and wish to get a feel for what is being discussed in terms of units you are used to seeing.




Good luck with your build and keep us posted on your progress!

post #8 of 12
Thread Starter 

Thank you LenDecaturAL, very useful!


I did some measuring today... i messed up at the start.
The correct length is 42 inch and 24 inch diameter (so how many gallons is it???)
The fabricators measurements where measured including the water outlet.

a buddy of mine will bend a box for me.
24x20x20 inch.


this is what i get on feldons calculator....



Is my box going to be too big? I am going to use an ash pan and a coal basket. 

post #9 of 12

It is approximately 75 gallons according to those measurements. You will want your RF plate mounted inside the smoker 5 1/2 inches from the bottom of the cylinder. This will work out well since the tank cut when measured up 5 1/2" will result in a cord width of approximately 20 inches which will match up well with the width of your firebox. That plate height will extend across the cook chamber and stop approximately 4" from where the tank dome begins. This will result in over 75 square inches for the critical areas required for good air flow in the smoker. Feldon's calculator needs 1.5 times the recommended area for a reverse flow smoker to assure good air flow, the numbers are correct for a standard offset configuration smoker. Since it gets cold in the winter in your area, it may be good to insulate your firebox if you plan on cooking much in the winter, it is something to consider.


Here is Feldon's calculator with your numbers.





The firebox will not be too big; however, you must use a false firebox size that equals close to 100% to obtain the correct numbers for the openings into the cook chamber, you can always build a firebox of any size you wish once the openings are calculated based on the cook chamber volume.

Edited by LenDecaturAL - 7/8/14 at 10:25am
post #10 of 12
Thread Starter 

Thanks for your reply, I will take a good hard look at it again. Because i don't understand the whole story (language & math issue)
Dutch winters are not all that cold, and i'm not planning on using it on very cold days.



I think i understand, you are calculating with a small FB so you get the measurements for the opening in the CC. 


If i make that horizontal cut in the tank, at 5 1/2 inch, how "deep" should i go then? Is that something important?  

Edited by Smulder NL - 7/8/14 at 11:57am
post #11 of 12

The idea for how "deep" to make the cut is to cut through the dome area and into the cylindrical section of the tank. There is an area where the pieces are doubled at the weld that is much more difficult to cut so it is best to cut on the other side (deeper) of the double thickness, it will be easier.


With your edit, you are correct and understand the reason a "fake" firebox size is used for CC opening, air intakes, and exhaust pipe sizes. Do not worry about your language skills, they are vastly better at a second language than mine! When I was in the Netherlands, I met several people that spoke 4 to 5 languages. I can speak English but butcher Spanish!

post #12 of 12
Thread Starter 

Alright then, back again.
Did not do anything on my smoker for a couple of months.
This weekend i decided that i should take a look, and do some quick measurements, to see howmuch steel i need.


My tank is not thick it is a few mm's....i can lift it on my own.

The firebox and rf plate i'm building will be thicker. 

All and all This will not be a thick smoker, i'm hoping this will not be such a big problem on how the thing will work

It is only welded in the middle, no caps are welded on, so no 


I did the cut for the fb in the cc.

came out not straight....
I need my dads laser, and need to put the tank level on the ground so i can make some straight cuts.

will do that next week.


For the door i'm thinking to do the horizontal cuts first.

And the vertical cuts not al the way.

That way, i can weld on the horizontal flanges, and weld on hinges. Flanges will be blind welded, and i will use those bullet style weld-on hinges at first.

When that is done, i can cut the vertical lines and the thing will come loose. 


I am hoping that i reduce the warping of the door like that.

The horizontal flanges will be welded on with the door off.




I really like the endcap on this thing, gives it a nice touch with the bolts and all.

I'm going to weld it shut on the inside....and cut the pipe of.

Will do something with it for the look...not sure what.


Next thing to do is making the firebox and order some steel.

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