Understanding the pit calc

[Mississippi smokin]

Hey guys hows everyone doing on this frigid morning? I've been plugging some numbers in the pit calc and I Ind myself with more questions than answers I'm afraid. Not being the most mathematical person you can imagine my surprise when I really got into building this smoker learning how much math it actually entails. So hopefully yal can help a boy out . Ok so here are a few questions to start with ; 1) I have a 25 gallon propane tank I'm using for my RF smoker. I've used the tank calculator and it says my volume is 29.1 gallons, can that be right? 2) my firebox is 156% which I know now is too big so can I convert some space to catch ash and raise the fire grate up to reduce the size? Would that work? Ahh so many questions... 3) Ive planned to use a 10×2 square air inlet for my fb , pit calc says I need 0.52 inlets so does that mean my inlet is too large? 4) I planned on using a 3" I.D. pipe for my chimney is that a good size? Thanks in advance guys I know I'm a pain in the ass

[Kcd2016]

I'm not a pro, I haven't even built my smoker yet, but I have some thoughts.

1) I have a 25 gallon propane tank I'm using for my RF smoker. I've used the tank calculator and it says my volume is 29.1 gallons, can that be right?
I DON'T KNOW, but have you cut the tank open yet? Are you able to fill it's entire volume with water? If so you could measure the volume exactly. (Assuming you've already safely purged the tank) You could weigh your empty tank on a standard bathroom scale, write that empty weight down. Then fill the bottle up entirely with water. Then weigh the tank again. Now subtract the empty weight from the full weight and then divide that number by 8.3. (1 gallon of water weights about 8.3 pounds) That will tell you exactly how many gallons the tank holds assuming there were no pockets of air in the tank when you filled it. FYI 30 gallons of water is 250lb so make sure your scale can handle that plus the tare weight of the tank. Come to think of it propane tanks usually have the empty (tare) weight stamped on the side somewhere don't they? Maybe that's just the little 20lb tanks...

2) my firebox is 156% which I know now is too big so can I convert some space to catch ash and raise the fire grate up to reduce the size? Would that work? Ahh so many questions...
What about filling the extra space with firebricks? I don't know that fire grate elevation effects the firebox calcs, also I know I've read that you don't need to consider things like fire basket and ash pans when figuring the firebox volume

3) Ive planned to use a 10×2 square air inlet for my fb , pit calc says I need 0.52 inlets so does that mean my inlet is too large?
It means it's bigger than what's required but you can always adjust your vent doors to close down the opening - you just don't want too small of a hole.

4) I planned on using a 3" I.D. pipe for my chimney is that a good size? Thanks in advance guys I know I'm a pain in the ass
I'd like to know this answer too. I think I've read that 4" is a good minimum size

Again, I'm a newby too so take my input a big 'ol grain of salt.

[Mississippi smokin]

Thanks for the tips kc . I've been told about the firebricks before but trying to figure out how high to place my damper if I use them has me all messed up. For that matter I really don't know where my damper goes in relation to the actual fire in the fb. I'd assumed it would be below the fire but that may be wrong. Should have got plans for this sucker but I'd already started it before I realized I didn't know what I was doing

[Kcd2016]

Putting the intake below the fire is standard operation.

You don't have to put the bricks in the bottom either, I'm assuming you could stick them in the back or run them up the sides, or just anywhere they won't interfere with airflow. If you could line the top plate that would be most efficient if that's what you want to go for.

It looks like you have 1086 cuin extra volume to fill up, divide that by the volume of 1 brick to get the number of bricks you need. Wikipedia says that a standard firbrick is 9" x4.5" x3" so 1086/(9x4.5x3)=8.9 bricks. Call it 8.

Again, I'm a noob.

[temurf]

Kcd is right. The air intake needs to be below the fire for propper combustion

... I'd already started it before I realized I didn't know what I was doing

I know the feeling. If I waited till I knew what I was doing, I wouldn't get any thing done.

[Pete Mazz]

1) I have a 25 gallon propane tank I'm using for my RF smoker. I've used the tank calculator and it says my volume is 29.1 gallons, can that be right?
2) my firebox is 156% which I know now is too big so can I convert some space to catch ash and raise the fire grate up to reduce the size? Would that work? Ahh so many questions...
3) Ive planned to use a 10×2 square air inlet for my fb , pit calc says I need 0.52 inlets so does that mean my inlet is too large?
4) I planned on using a 3" I.D. pipe for my chimney is that a good size?

1) The calc is correct as long as you used exact inside dimensions. Propane tanks leave room for expansion.

2) Make a V shaped firebox. It'll help keeping a small fire going.

3) Multiple intakes are best but making it too big really doesn't hurt since you can just shut it down.

4) For that sized tank 3" should be fine.

[Mississippi smokin]

Thanks Pete , I haven't got the pipe for the stack yet I just wanted to have an ideal size , as far as the fab goes does anyone have a few pics of the inside of theirs I can see? Not exactly sure how I'm going to lay it out as far as the ash pan and wood rack . A few more questions while I'm on a roll . 1) I keep seeing a reference to a shadow plate and while it's been explained in other threads I'm still struggling to grasp exactly where it goes and if I need one. 2) do I need to paint the inside of the cook chamber?

[Mississippi smokin]

Pete could you clarify about the v shaped fire basket or show me a picture possibly?

[Pete Mazz]

A shadow plate is used when your FB is under your BP. It prevents a hot spot on the BP.

A normal fire basket but has the sides tapered to keep the coals from spreading out. Maybe someone who built one can post a pic.

CC is just seasoned with oil. Never paint.

[Squiggle]

I think forty_creek could post a pic of his here, it's probably the best example of a tapered fire basket going round.

[Rodcrafter]

This is my trailer rig basket before it is finished.




This is my backyard one