"Q" Train

[temurf]

A longer 3" stack with the same volume of a shorter 4" stack has 2-3 times less flow rate.

The way I understand it, a higher flow rate would equate to a hotter fire with all other factors being the same.

Do I have that right, or am I way off on that thinking?

[Pete Mazz]

You're right but you don't want to be limited by a smaller diameter stack.

Velocity has to increase to maintain the same flow rate in a smaller stack. Let's say you want a flow rate of 2 cu ft/min to maintain temp and you have a 4" stack. The air velocity thru the stack is 4.5 in/sec. Make that a 3" stack and the velocity has to jump to 8.5 in/sec to maintain the same flow rate. The same amount of air is moving thru the smoker but the speed of the air moving thru the stack has changed. With a 4" stack, a damper can effectively choke it down if desired, giving you another level of control. What you don't want to happen is adjusting your intakes to control temps only to find your stack is limiting you. Friction plays a role here but the numbers don't take that into account. Since a smoker is basically a natural convection system any obstacle in the airflow affects it.

Using RC's favorite analogy of having the smoker upside down and pouring water in the FB, too small of a stack limits how much water can move thru the smoker.

The part of the equation I'm guessing at is what IS the optimal flow rate thru a RF smoker? I used 2 cu ft/min but it's probably more. Where's Frank?

[temurf]

I am going to go with the 4 incher regardless but now you have got me curious:

The part of the equation I'm guessing at is what IS the optimal flow rate thru a RF smoker?

I understand that you are THE man when it comes all this mathemagician stuff. If you have to guess, then my feeble little mind doesn't stand a chance.

[Rodcrafter]

What????
Pete are asking to be BTUed? You are in big trouble.


Making memories.

[Pete Mazz]

I don't know if there is an answer. It may all be based on the individual smoker. I know they make flow rate sensors but I've never seen one used on a smoker. I'd like to know tho.

[Squiggle]

Me too, would be very interesting!

[mp4]

I was thinking the velocity would be higher in the chimney, but it should be the same through the firebox, throat and cook chamber, right?

[Squiggle]

That's what I thought, but it would be interesting to get a reading & know for sure for sure.

[Pete Mazz]

I was thinking the velocity would be higher in the chimney, but it should be the same through the firebox, throat and cook chamber, right?

To have the same flow rate, velocity has to increase. The velocity thru the intakes would be high, less thru the throat and BP gap, less again thru the CC and finally higher thru the stack. If you're using an adjustable BP gap, you're changing the airs velocity.

[mp4]

I was thinking the velocity would be higher in the chimney, but it should be the same through the firebox, throat and cook chamber, right?

To have the same flow rate, velocity has to increase. The velocity thru the intakes would be high, less thru the throat and BP gap, less again thru the CC and finally higher thru the stack. If you're using an adjustable BP gap, you're changing the airs velocity.

So not exactly the balanced approach that we are striving for then, right? That helps make sense of it all...we want stead and even flow and when velocities change we end up with a herky, jerky air flow.

[temurf]

Finally got some more work done on this project.

Here is the fire basket. I found ascrap of smaller expanded metal to keep the coals from falling through.

[temurf]

We put these little pieces of angle under the back end so the front doesn't fall as you slide it out.

[temurf]

Mounting damper on outside of FB door.



Rod got stuck. I'm told that's why they call it "stick welding"

[temurf]

I still need to install the gasket but she should seal up nice and tight with these adjustable latches from http://bbqsmokersupply.com/

[temurf]

We have been working on this build for a month now and so far all we have completed is the fire box.

It would be easy to get frustrated at the slow progress but I am not going to. When I add up all the time we have been able to invest in her, it only comes out to about 4 or 5 full days of actual work.

The FB is done and I am really excited about that.

[Rodcrafter]

It will be worth the trouble. I noticed you have the toggle latches that have the rubber tipped bolt, you can just put an carriage bolt in there and it wont be melting the end. Just trying to help.

[temurf]

You are correct Rod, the latches are rubber tipped. I have been wondering how well that rubber would hold up with all the heat, and I have already considered changing them out. I do appreciate the help. Thanks.

[temurf]

Okay.

We have gotten some more work done on her and I finally have time to post some more pics.

Its not easy finding the time to work. Its even harder finding the desire to do hot work such as welding and grinding and cutting when the outside temp approaches and passes the 100 degree mark.

But I choose to live in the desert so I can't really complain too much. I just suck it up and deal with it (I can handle a couple hours at a time).

[temurf]

The vertical cabinet from the donor store bought offset needed some modification.



In this pic I have the vert cabinet sitting on the FB so I could check the fit.

The box was very flimsy and even if the weld holding the lower door hinge wasn't busted off, It still would leak like a sieve.

[temurf]

I wanted to reuse the same grates that came with it so we put the stiffening angles on the outside like so:



The angle all the way around the inside of the door stiffened it up nicely and with some gasket material it should seal quite well.



The piece of angle at bottom right in the pic is running horizontally across the front of the box to keep it square while welding the angles all around. I haven't decided yet if I will remove it or leave it there.

[temurf]

Here is the almost complete modified warming cabinet. The can on top kept the rain out when I wasn't using the donor smoker.



Here is a close up of the latch mounted on the side.

We put a length of strap over the legs of the angle to form a triangle and then capped it. I think it will give it a cleaner look after we are all done



And yes RC those are the same rubber tipped bolts that I have not changed out yet.

I am just excited to have made some more progress on this project.

[Pete Mazz]

[Rodcrafter]

[temurf]

We had a productive weekend and I finally have some time to post some more pics.

We put a piece of plate on the bottom of the WC. I'm thinking it will help with regulating the temps, but its really there as a spacer so the doors don't bind on each other.



Mounting the FB on the bottom of the WC

[temurf]

Here is the cut out between the WC and the CC. I cut a round piece slightly larger than the white circle in the pic. It will have a similar cut out and I am going to bolt it on the inside of WC for a damper.



Lining up the CC.