PDA

View Full Version : Building new roaster-interior cubic feet/vent ratios?


Pigdude
05-17-2013, 01:03 PM
Hey all. I am a noob as of last night to BBQ Brethern. Tomorrow I start a build of my first roaster. Nothin like the promise of flat steel and 2 tanks of welding gas. I have plans to build with 7 GAUGE! Stainless steel 304, double wall insulated. Box to be 4x6x30" tall (60 cubic feet). Domed top to be 4x6x24" tall (35 cubic feet). 2 charcoal trays at front/back, 16 inches from grill grate. Trays will be 8-12 inches wide, 3 inches deep, 6' long. Weight estimate is 1500 lbs. Code named: FrankenRoaster. http://www.bbq-brethren.com/forum/images/icons/icon7.gif

My question is: Does anyone have advice on the ratio of interior cubic feet of roaster and air intake vents and chimney diameter for exhaust... in say square inches?

Thanks for sharing the Tribal Knowledge!!!!

-PigDude

HankB
05-17-2013, 03:03 PM
I would go to a local store and measure the vents on a Weber cooker and calculate the inlet and exhaust vent area for the given volume. That should give you a starting point.

IMO, better to have a little too much than not enough. You did say roaster vs. smoker.

Also be aware that:

A stack will give you increased draft and air flow.
A large hole will flow more air than several small holes of the same area.


Also consider the mass of metal you are heating. 7 Gauge is pretty thick so you will have a lot of metal to heat to get it to temperature. Insulation will help keep the heat up so you will not need that much to keep it going. You might consider using thinner material on the inside to facilitate quicker heat up.

HTH

pbj
05-17-2013, 03:05 PM
http://www.feldoncentral.com/bbqcalculator.html

Pigdude
05-17-2013, 04:57 PM
PBJ- This is so close to what I am looking for. HOWEVER, this link is to calculate the cooker box to Firebox ratio.
I was looking to determine the Roaster box ratio to vent size(s) (and chimney diameter) I would need in square inches.
Any other ideas?

-PigDude

dwfisk
05-17-2013, 05:11 PM
Do you have a sketch or design, picture og something similar? having a gard time visualizing.

Pigdude
05-17-2013, 06:46 PM
Anyone else with info on how to figure this out? I like the idea of pro-rating weber grill specs.......I can't be the first one with this problem....?

pbj
05-17-2013, 07:00 PM
PBJ- This is so close to what I am looking for. HOWEVER, this link is to calculate the cooker box to Firebox ratio.
I was looking to determine the Roaster box ratio to vent size(s) (and chimney diameter) I would need in square inches.
Any other ideas?

-PigDude
Scroll down. Unless I'm completely misunderstanding, all the calculations are there

Pigdude
05-17-2013, 07:13 PM
Nope. I just have one big roaster box. Not a roaster box with an additional firebox. This is for a pig whole pig roaster.

pbj
05-17-2013, 07:40 PM
So I was completely misunderstanding. Ain't the first time.

Pigdude
05-17-2013, 07:59 PM
Hey no prob. Looked perfect at first. If only I was smart enough to make a formula...then again how would I know if it's correct. Lol. Thx.

Pigdude
05-18-2013, 06:38 AM
Anyone got any ideas?

dwfisk
05-18-2013, 07:18 AM
PS: Welcome to the forum. Please share some build pics and your first hog cook!
Just imperical, if it helps at all. My RF offset stickburner has about 1/2 the 60 cu ft volume of your roaster. I have 2@4 inch X 3 foot tall exhaust but I really keep one closed down for RF, I'm thinking of upgrading to a single 6 inch. My inlets are 2@5 inch daisy wheels and I'll be adding a third if I upgrade to a 6 inch stack. My current configuration sits at the minimums for most calculators, the upgrades I'm considering get me to better than 125% and should let me get much better fire/draft control.

So, for 1/2 your volume I'm looking at about 30 square inches of inlet and exhaust, with at least 2-3 ft stack height to get proper draft. I should think a combination that doubles that would be plenty; i.e., 2@6inch x 3 ft stacks and 6@5 inch daisy wheels spread around the base for even inflow, or just a couple of bigger ones on the ends. You can always close down the daisy wheels to control draft. Likewise you could put dampers on your exhausts, but I never use mine - I just run the pit with the exhaust wide open and control at the intakes. Again, I'm running a stick burner and need these volumes for clean smoke; should be (way?) more than enough for a charcoal cooker.

Hope this helps.

Pigdude
05-18-2013, 03:30 PM
Dwfisk-many thanks for your help. So I read your reply that i should be about 60 square inches of venting, and 2 6" x 3' stacks. This morning was a great start to my build. Serious grillage with a 6'x4 grill...24 square feet of hog heaven.
I hope to post build pictures soon. Stay tuned. Anyone else with venting suggestions?

Pigdude
05-18-2013, 03:33 PM
Anyone else with advice on how far grill surface should be from front/back coal trays below?
So far we are thinking 16".

dwfisk
05-18-2013, 03:40 PM
If you were building an offset stick burner with your cook chamber dimensions I would be real comfortable with the 60 inches square intake and exhaust calcs with the 24"-36" stack height for draft. My hesitation is you are building a charcoal "direct" unit. My gut tells me my calcs are too generous and you could probably get by just fine with less if your build needs to reduce my numbers. Maybe someone else will chime in here.
Dwfisk-many thanks for your help. So I read your reply that i should be about 60 square inches of venting, and 2 6" x 3' stacks. This morning was a great start to my build. Serious grillage with a 6'x4 grill...24 square feet of hog heaven.
I hope to post build pictures soon. Stay tuned. Anyone else with venting suggestions?

Pigdude
05-18-2013, 06:38 PM
Its a one box roaster with a 6 foot tray that is about 8 inches wide along the back/front of the box. Whole pig goes on the middle of the grate, where pig grease drops through grate onto our slanted drip bottom that has a spigot for drain on lower end. So it is indirect heat comming from the trays. The good news is that it will be double walled, 2" insulated, stainless steel 304, 7 gauge. This baby should really hold the heat. I suppose we can have the air intake bigger than likely needed, as long as we have slider or daisy wheel control over opening. We also plan to have damper at chimney for control there. I have designed the box to accept another 8" tray between the back and front tray incase we decide to insert for use with say a direct steak fry, burgers, etc. i will then just make a plug to fill the middle hole on each end when not in use. Kinda hard to visulaize. Are you with me?

dwfisk
05-18-2013, 06:41 PM
I got it, can't wait for the build pics and even better the first pig or two!

Pigdude
05-26-2013, 10:14 AM
So, now we are debating....air intakes below or above charcoal trays?
Right now the charcoal trays are along the front and back of the roaster box.
Biggest concern is ash on food.....

boiler93
05-26-2013, 10:29 AM
I would say the air intakes need to be below the charcoal trays in order to get proper amount of oxygen to the fire.