# First Build Design Questions

Discussion in 'Reverse Flow' started by malin, Jan 14, 2015.

1. ### malinNewbie

I have decided to build my first reverse flow smoker.  I have a 24 inch diameter piece of 1/4 inch wall pipe that is 24 inches long.  I have used the calculator and have to sizes for the openings and fire box but have the following questions:

1 Where is the best place to put the intake holes for the firebox?

2 Does it matter where I place the reverse flow plate in relationship to the grill and the firebox to cook chamber opening. That is does it    have to be within so many inches of the grill to work properly?

This is my concept.  I am not married to it and would really appreciate any constructive criticism that anyone could offer me.

Thanks, Tom

2. ### daveomakSmoking GuruOTBS MemberSMF Premier Member

Last edited: Jan 16, 2015
3. ### daveomakSmoking GuruOTBS MemberSMF Premier Member

Calculations for a standard design, reverse flow smoker..

Volume of the Cook Chamber.... Use the Inside Diameter of the tank...

Diameter X Diameter X 0.7854 X Length = Volume in cubic inches / 231 = Volume in gallons

24x24x0.7854x24 = 10,857 cu. in.

Volume in cubic inches X 0.004 = FB/CC opening in square inches..

10,857x0.004 = 43.4 sq. in.

Volume in cubic inches X 0.004 = Area under the RF plate in square inches...  43.4

Volume in cubic inches X 0.004 = Area required at the end of the RF plate in square inches ...   43.4

The above areas are necessary for great air/heat/smoke flow.... using less may cause an overheated FB.... Narrow / Long CC may require an increase in those numbers due to increased surface area friction to volume... Wide/Fat Short CC may use smaller numbers due to reduced surface area friction to volume...

Volume in cubic inches X 0.001 = FB air inlets in square inches...

10,867x0.001 = 11 sq. in.

Recommended upper and lower air inlets... Upper air inlet directly across from the FB/CC opening to facilitate moving heat from the FB to the CC, and insuring good air flow through the CC.. The lower air inlet should be situated at or below the fuel grate.... The two air inlets can share the designated square inches of opening....

I would go 10" below the grate and 2-3" above..

Dave

Last edited: Jan 16, 2015
4. ### malinNewbie

Thank you very much Dave.

I had run the numbers using the link you provided.  It found your calculator very clear and easy to use.  I thank you for providing it.  I was just a bit confused about where to place the RF Plate.  From what I read it appeared that the RF plate would be placed at the same level as the top of the FB to CC opening since both numbers were the same.  I had initially planned on extending the RF plate out past the end of the CC and letting  it be the top of my firebox.  Apparently I misinterpreted what I read.  I now see that number is more of a minimum so I will raise my RF plate a couple of inches above the FB to CC opening and call it good.

Again thanks for your input.  This is a very expensive project for me and I want to get it right the first time.  Feedback from someone with your experience is greatly appreciated.

Tom

5. ### daveomakSmoking GuruOTBS MemberSMF Premier Member

Tom, morning......   Very glad all went well....    The numbers are the easy part.....   Thinking about putting the smoker parts together....  and how they fit together...  The details....  that's the hard part....   which way should the door open....  what design of air intake...  drain valves and hinges...

The folks here have been through it all and can provide some good advice.....     We are all here to help .....

Dave

6. ### malinNewbie

You're right.  Thanks to your calculator the numbers were easy part.  I have been looking at pictures of other builds and am amazed at the skill and ingenuity of the people on this site.  I have picked up lots of great ideas now I just have to choose what will work for me and get started.  I will post some pictures as I go and if anyone sees something that looks questionable I hope they will point it out.  I am sure I will have questions as I progress and I am thankful for the body of knowledge contained in this forum.  Hopefully I will be looking for recipe ideas instead of build ideas by spring.

7. ### malinNewbie

Just 2 quick questions Dave.

1)   I am using a plenum off the side of the cook chamber.  The opening to the CC is 5.5 inches wide and 6 inches high.  The top of the plenum is 6 x 6 square.  The chimney will come out of the top of the plenum.  I want to know how high up on the side of the cook chamber to place the plenum.  I was thinking of centering it between the grill and the top of the CC.  By the way I have not mounted it yet so if it is too large please let me know as now would be the time to correct it.

2) While looking at the calculator to determine chimney height above the CC it looked like 36" is recommended and then adjust the id of the chimney to achieve the 36" chimney.  Did I read that correctly?  If so I will wind up with a 2.5" inside diameter for the chimney.

8. ### malinNewbie

This is what it looks like now

9. ### daveomakSmoking GuruOTBS MemberSMF Premier Member

10,857 x 0.017 = 184 cu. in.  =  26" tall x 3" ID...   which I would go with 36" tall x 3" ID.......   that calculated stack of 2.5" ID gets back to the volume vs surface area problem and too much friction......

Placement of the exh. plenum sounds good...

10. ### malinNewbie

Thanks so much Dave that is the way I will build it.