Firebox and airflow for a 200 lb cabinet smoker

Posted: Tue Aug 23, 2011 9:29 am

How much heat is required to cook 200lbs of meat ?
More to the point, how would you solve the problem of getting the heat to the right places in the cook chamber to evenly cook this much meat ?

This is my second year with a vertical cabinet smoker. I call it the "Fridge" because it was made from an industrial cooler - it has 8 2'x2' racks for 32 sq.ft. of cooking area. The design is not a direct clone, because I wanted to have multi fuel capability (pellets, charcoal or propane) and some flexibility to change things around - like to make the heat flow differently in the cook chamber. It has 3 * 3" exhaust outlets and I am looking for ideas of how to route the heat transfer tube to have multiple heat inlets or to add a liner to get the cooker walls to radiate heat and make the air reverse flow.

The firebox I made last year was large enough for logs, and was fully insulated, but there were safety concerns with the door and I could not get the door gasket to stay on - so I decided to make it pellet fired. The exploration of electronics and mechanical doodads required to move pellets to make fire are covered elsewhere on the forum, but in summary, I was not cooking with pellets by the 4th of July, so I decided to build another GF charcoal firebox.

Here is a photo of the new firebox with the heat transfer tube that brings the heat to the center of the smoker. There is a thermocouple mounted in the center of the heat transfer tube on the bottom. I use a 5.6CFM fan that fits inside the 2x4 opening of the air inlet.

I decided to go with a more standard approach that is common to most GF charcoal burners - the inlet below the grate and the outlet above the grate and a much smaller fan run by a PID controller. I made the firebox from the charcoal chute off my old firebox. The dimensions are 8" x 9" x 16" and it uses 2"x4" inlet and outlet tubes and a 1" stair tread grate that can be flipped to make the charcoal bed 1" thicker for large cooks. The chute for this fire box is an 8" round stainless steel tube that I got at the surplus store. Total cost is for this firebox was less than 100 bucks and the weight is less than 40 pounds. It mounts in place of my pellet feeder using two nuts and a collar.

Mounted on the back of my fully insulated cooker cabinet. This firebox has no insulation at all.

I am making the silicone bead from last year wider in this photo. I used high temp silicone RTV and plastic wrap to keep it from sticking when I shut the door. After a few hours, just pull off the plastic wrap and I had a perfectly formed gasket.

This photo shows the firebox after a few burns. The 2000 degree header pain has burned off due to rapid cooling during a rainy cook. In this photo, I am bringing the meat up to temperature with flash <above> and without the flash <below> The heat transfer tube is at 850 degrees F

For this 211lb cook, (lots of brisket and butts in there), I cooked in pans, and kept the temperature in the heat transfer tube to less than 1000 degrees. I noticed last year that the grease smells funny when the drip pan is really hot - I never measured it, but guess it may get to 600 or 700 degrees.

If I use the thermocouple in the cook chamber as a control point, the controller will drive the fire to the full CFM of the fan, which will easily take the fire over 1,000 degrees. I want to keep the drip pan less than 500 degrees (to prevent char like buildup, but still make smoke).

For now all of the heat is coming in below the drip pan and it goes up the walls of the smoker to the meat in the chamber.

Here are my ideas: I am thinking of using one of my exhaust vent locations as an alternate heat inlet... think that will help ?

Here is a photo showing the 3 exhaust holes. I could send a heat transfer tube up the back of the cooker to the top hole, making the heat converge in the middle of the cooker for exhaust.

Also notice the 1/2" space on the left and right of each rack where the angle iron rails are. That could be used to make a hot air channel that would release the hot air near the top of the cooker. Just need to rivet some 22"x60" pieces of sheet metal in there (?)

Posted: Wed Aug 24, 2011 8:48 pm

More info:

I am hoping that someone with HVAC skills may read this and help me understand the heat load in BTU for all that cold meat, and how many BTU needs to go into the cooker and how many times the air needs to change in the cooker to keep the hot air circulating for maximum heat transfer.

Say I am using a 10 CFM fan on my stoker. This would move 600 cubic feet of air per hour through the smoke chamber. If the fan is run at 100% for the entire hour, my empty cook chamber at 19.3 CuFt. (26"x26"x58") would have 31 air changes per hour...(even more changes per hour with the volume reduced after adding the meat) thats a lot of fresh air for a room, but consider this a small room with a very high heat load with all that cold meat to warm up.

The meat will absorb the heat from the air based on the temperature of the air and the time it takes for that air to move across the meat. As the air moves from the bottom of the cooker to the top, it encounters cold meat and the heat exchange is made, the air heats the meat (or the meat cools the air) until the air is exhausted out of the cook chamber.

I found this neat calculator that makes it easy to see how duct size and chamber sizes will speed up or slow down the air flow in the cooker:
http://www.comairrotron.com/airflow_calc.shtml

Using my 10CFM fan and a 2x4 inlet, we see a 2MPH breeze, which really blows the coals... that same air speed in the 26x26 cook chamber would take almost 3 minutes to go from the grease pan to the top of the cook chamber... lots of time to transfer the BTU of the air to the cold meat.

With the smoker fully loaded with cold meat, and the stoker fan running at full blast to make the heat transfer tube temp 750F, I measured the exhaust temp to be at 152 degrees. I did not keep track of this measurement over time, but would guess that as the meat gets up to temperature there is less of a difference between the desired cook chamber temp of 225 and the exhaust temp. (if the meat is at 180, I would expect the exhaust to be near 180)

I have measured my heat transfer tube temp to be around 450F when my cooker is holding 225 during the last few hours of a cook. Meat would be in the 160-170F range.

Has anyone looked at exhaust temp compared to meat temp during a cook ?

Posted: Thu Aug 25, 2011 7:57 am

Here is a cook log from the week before...
The data logging was written on the back of an envelope as I went out to check on the cooker every 30-45 minutes (or hour?) I did not record the time.

I was attempting to tell what the difference in temperature is between different areas in the cooker. The meat load was light - less than 40 lbs, so this does not show the worse case pictured above. It does show kind of a baseline for my cooker though... the relationship of fire temp and chamber temp as the meat is cooking. In this case the difference between racks was less than 10 degrees, but there was no meat load in the upper half of the cooker.

I am using a thermocouple in the center rack that is attached to a PID controller for the stoker fan. That is represented as the "Rack4" temperature.

I loaded 22lbs total of brisket on Rack 2 and 3, and used a meat thermometer to measure the temperature in the center of the bottom rack - Rack1... then after a couple of hours, I added 17lbs total of pork butts on Rack2 and Rack3 and moved the meat thermometer up to Rack6.

The highest temperatures are for the xfer tube, where I have a thermocouple to monitor the fire temperature. This thermocouple is mounted in the bottom center of the heat xfer tube. To hold the empty cooker at 225, the xfer temp is around 450. When adding more meat, the PID controller will push the fan harder, and that raises the xfer temp... I have seen it spike to over 1000 degrees. If you are cooking more than 50lbs of meat and using a stoker, your cooker is probably doing it too.

... so the net of all of this is that if the firebox is kept at a certain temperature (like 700 degrees), the cook time will vary depending on the amount of meat that is in the cooker. 30 pounds may take 1Hr/Lb, and 80 pounds may take 1.25 Hr/Lb while 200 pounds may take 1.5 Hr/Lb (or longer). I am trying to tune my cooker so that the large cooks will come in as planned using the standard time like 1.25 Hr/Lb while keeping the fire temperature from being too crazy hot.

Posted: Wed Sep 07, 2011 8:11 am

I did a 60 lb cook using logs on Labor Day.
I was not using oak or hickory or mesquite logs... I used temperature logs.

Details of how I collected temperature logs for about 3.5 hrs of a very wet and stormy cook are found in this thread http://www.phpbbserver.com/phpbb/viewtopic.php?t=3946&mforum=smokinjim on this forum.

This test was during a really bad rain storm, and my project box eventually took on enough water that the Ardunio stopped working. I let it dry off the rest of the day and was able to power it on again the next day... will have to re-install the SD card and see if it is still working OK, but seeing the blue LED on the Mega main board was comforting...

Notes:
A) Noticed the temperature on my PID controller was set to 200 degrees, so I ramped it up to 250 and went inside to dry off and take a nap.
B) The PID controller is reading 250 at this point. I knocked the temp back down to 225 for a while.
C) Look at the blue temperature trace. Something happened about the same time I had lowered the temperature, and I think it was me tilting the enclosure to let water out of it. Apparently one of the wires may have gotten wet and I think that affected the resistance measurement for a while.
D) Logging stopped when too much water got in the enclosure.

After an exciting day of BBQ, I took the SD card out of the logger and opened the log file with my computer and did the formatting and other observations about the cook and some of the questions I had about my cooker.

I made the graph using the spreadsheet application in OpenOffice, and added the text baloons and colored lines using another image editing tool.

Observations:
1) Meats on the lower racks cook faster (already knew that.... just could not visualize or compare with other racks) I still do not understand why.

2) The temperature in the middle rack of my cooker is virtually the same temperature as the bottom rack of the cooker (I thought I would see a temp difference with the meat cooling the air... but had only 60 lbs of meat in there this time and was not using aluminum pans)

3) It took several minutes of open door time to get all of the probes in the meat and on the racks while standing in the rain with 40mph wind gusts and 8 tangled meat probes... temp dropped below 160, but you can see the recovery time was about 25 minutes on the chart. The temperature climb from 225 to 250 was at a rate of about 1 degree every 2 minutes. I am sure this is where I will see differences in the slope of this line if making a change to any of these:
* depth of the coal bed
* grate design for more air
* increase the CFM of the fan
* load it up with a larger meat load

4) The cook temperatures on the graph are about 7 degrees lower than my PID thermocouple was reading. I am sure I can tighten this up with some programming tweaks before the next cook.

5) Notice how the Brisket2 temperature (rack2) increases faster and crosses over the Brisket3 temperature (on rack 3) and rack3 stays below rack4. This follows along with observation #1 that the meats closer to the bottom of the cooker get more heat and cook faster, but I still do not understand why.

6) The cooker temperature taken below the meat compared to above the meat was virtually the same for 96% of the 200 measurements that were recorded, and I did not see more than 1.5 degrees difference in the measurements that were different.

7) After dropping the temperature from 250 to 225, you see the rate of cooking slows a bit, but the meat is continuing to cook although the stoker fan is not running.

8 ) I need some longer temperature probes to be able to measure the stack temperature... looking at that next.

Other humorous story:
I was working on wrapping ribs later in the day, and I bumped my trusty Taylor digital meat thermometer and it came unplugged and it fell right into my grease bucket ! Very bad since I was not using catch pans under the meat and there was enough black, tarry grease and rain water in there that it was floating, and it only got worse when I tried to take it out - so it is a goner.

Posted: Mon Sep 12, 2011 9:40 pm

Cool when go over the electronics further. Do you have plans?? Everything look great.

Posted: Tue Sep 13, 2011 1:11 am

Will post something this week on the electronics thread. There is some minor calibration work to be done, but I am getting some good infor from the device.

Here is another temperature log from one of my best cooks to date. This one shows the warmup, loading up the meat up and the overnight portion of a 100 lb cook in my smoker. There is 50 lbs of Brisket and 60 lbs of butts, I trimmed most of the fat off the briskets and cooked all of the meat with no pans. I collected about 4 cups of grease in the drip bucket. All of the meat was done when I pulled it at Noon the next day.

You can see how the recovery temperature is affected as the meat load is added. I learned last cook that the meats on the lower racks cook faster, so I made sure to put the largest pieces of meat on the lower racks.

To get an idea of the temperatures being reported, compare the thermocouple readings from the heat transfer tube (513 degrees) and the center of the cook chamber (232 degrees) before adding the meats for probes 5 and 6.

This photo was taken around hour 2 after adding brisket#4, while I was prepping the first load of butts. I did not see brisket number 5 waiting in the refrigerator until I moved some of those butts out of the way. Brisket# 5 had kind of a late start and was the greatest distance from the bottom, but I moved it down when I started wrapping rack1, rack2 and rack3 at 8:AM.

My cooks are showing the lower racks cook faster, but the air temps above and below the meat are within a degree of each other. I think there must be something besides hot air cooking the meat - like some infrared the heat radiating off of the drip pan (?) Ever see one of those infrared turkey fryers or that Orion cooker ? Something like that may be happening in the bottom of my cabinet cooker.

Posted: Wed Nov 02, 2011 10:00 pm

Here is another cook log for a 100 lb cook with a variety of meat.

I was able to use the Megamometer and log all of the temperature data, including the heat reansfer tube temp in relation to the drip pan and the center of the cook chamber , along with several meats as they cooked:

I had seen temps over 1000 degrees on a 200 lb cook in the first season, so this year, I used the PID controller on the xfer tube and set the temp to around 750, then bumped it up to 800 when I relaized the meat was not cooking fast enough. I am getting more comfortable with those big temperature numbers now though, and have made my firebox area a lot safer.

Let me know if any of this info is useful to you.

Here is a shot of the back of the Fridge after cooking all night . There is still snow on top, expecially over the exhaust outlet !

Posted: Mon Nov 14, 2011 1:26 pm

This is a test that I did to evaluate some changes to my new controller and a new fan combination.

For this test, I was monitoring a piece of pork loin on each of the 8 racks of my cooker. The meat weights were not the same though, so this was a variable for sure, although I did record the weights and rack positions. Here is a look inside the cabinet for the duration of the cook:

Section A represents the warm-up phase, the fan is at 100% and the temperature fluctuations in the fire transfer tube seem to be related to fuel feed/lump size... I was using Cowboy Lump (no RO available at the usual getting place). I notice that Cowboy brand has some pretty big chunks, and if these are not made smaller, they will cause the fire temp to drop because they take up space from the coal bed. When a big ol' lump drops in, only the outer edges are glowing, and that is a lot cooler than a lot of smaller glowing embers. Lok at the last part of section A where the fire temp (red trace) goes from 425 to about 575 while the fan is around 25% duty cycle. Pretty sure that is just part of cooking with natural lump.

Notice there is no meat in the cooker. It appears this fuel/fan combination on a 56 degree F day took about 2.5 hrs to reach 225 in the chamber. Controller did good here - no overshoot, but I think I like my other fan that ran the temp up to 950F : )

Section B is related to the operation under a meat load.
There was a relatively short time that the fan run at 100%, then it started dropping the duty cycle to around 60-70% for most of the cook, even though the chamber was hovering around 218. There seems to be room for improvement on the fan CFM or the tuning of the PID parameters - I would rather see the fan back off closer to the target temp, because this issue made the whole cook a bit low.

Section C is me fiddling around, I was checking the temps during the day, and decided to take another fan and "force feed" the firebox some additional air to see if I could get the cook temp up a bit. I was behind the cooker and unable to monitor anything except if the fans were running, and during that time, I did not see the fans turn off.Having the additional air flow did increase the chamber temp and the PID loop was trying to fight this by backing off the duty cycle to around 15%. (green trace) I think it is interesting to see the fire temp (red trace) is dropping even though the cabinet temp is rising. This is the power of convection cooking or possibly some radiant cooking inside the chamber.