Reverse flow problems

Discussion in 'Reverse Flow' started by bhawkins, May 17, 2013.

  1. I just recently turned my grill into a reverse flow and am having problems with it. First off the   CC is a 16" diameter and 34" long. That gives 6832.64" cube of air space. Feldon's recommends 2277.55" cube for the fire box. Mine is 18"x15.5"x9" giving a total of 2511.00" cube. That is 233.45" cube oversize. The exhaust is 3" ID and is 16" above CC. Recommended was 17.77". It was over 20" and I cut it with a reciprocating saw and got it just a little short. The FB to CC opening follows the contour of the CC so it is a 8" radius at 2.5" tall, giving 20.064" square of area. Recommended was 20.09" square. The RF plate is 13.5" wide and sits 3.75 inches from the bottom giving an area of 35.287" square. The only thing I am not sure of is the opening around the end of the RF plate, however after not being able to gain temps I did take it back to work and shortened it by 1.5" to make the opening bigger.As you can see the end of the tank is rounded and after shortening the plate it comes back into the straight sides, so I am not sure how to calculate that. Air intake is 3.25" diameter only one needed. I did go back and add another intake higher up to see if that would help it to draw the heat better. Even with RF plate installed the smoke poured out like crazy, however I was lucky to get 190* inside. I can take the RF plate out and open the old exhaust like a straight flow and get over 400*. Install RF plate, temp drops. With RF plate in place I can even lay my hand on the underside of the CC and leave it without burning myself. This is my first attempt at a reverse flow so I would appreciate any input you guys have. If I had access to what I needed here at home I would try to raise the area under the RF plate just to see what would happen. if I cannot get this right I will just move my better exhaust and use it as a straight flow. Here are a couple of pics.

    .

     
    Last edited: May 17, 2013
  2. Open the cooking chamber door and get a pic of the reverse flow plate, and open the firebox and get a pic looking in from that direction at the reverse flow plate, well get you fixed up!!!
     
  3. Ok Mr. Wizzard, maybe this will give you the view you need.

    Looking into the CC from FB underneath RF plate


    RF plate at an angle to show gap


    RF Plate straight on from a distance


    Edge of RF plate with tape to show width of gap


    From inside CC back toward FB to show opening height


    Plate inserted level with top of FB opening to diffuse heat and cool down any hot spot coming out of FB. Is 8" into CC 1" below RF plate.


    I did not weld down these plates down(thank goodness). They can be altered, replaced, or removed as needed.
     
  4. I dont think the problem is at the reverse flow end, ...Id work on the firebox to cook chamber opening. Maybe raise the plate a lil too.  If you close that top vent and open the bottom more, does it make any difference?  Does the door seal fairly good?

    You got everything there to make this a great lil smoker, just got to get it to breath right.
     
  5. When I close the top vent and open the bottom vent all the way it will flame up real big, like a fireplace fire. Even at opening the bottom just a little bit and opening the top all the way it seems to draw the smoke good but not heat up. The way it acts I would say that either the opening needs to be bigger, or the RF plate needs to be raised. The Firebox seals off decently, however I do have some kind of big leaks on the CC door.I am not sure how much is too much leak, I do want to seal it up though. I don't know where I can locally get stove rope. I do know there are some appliance stores in B'Ham. Looks like rain today so I may try to run some down.I can make the opening up to 1" taller by cutting out along the top edge. That would put the opening right up to the RF plate as it is. I would be able to get more plate Monday from work to raise the RF plate. Like I said this is my first time and it seems to work fine as  a straight flow, so I would say it has to be the plate not high enough. Do you think a piece of plywood would hold up well enough to test that theory?? or is the plate being steel what brings it to temp?? I know the plate acts as radiant heat, but the heat still has to come around the plate and into the top don't it?? 
     
  6. Almost double posted!!
     
    Last edited: May 18, 2013
  7. I give up. I cannot get this thing above 175* with RF plate in place. You mentioned getting it to breathe right. If the firebox was even bigger, how much would that help?? 
     
  8. 2 things come to mind, make sure lower vent is only open to give fire plenty of air, get rope and seal her up, can be found at any wood stove outlet.  Could be exhaust is too small to allow air to move out thus warmer air into tank, or exhaust is allowing too much to escape.  Trial and error my man, but I agree, air flow is the problem, especially if fire is blazing and tank is cold.
     
  9. I would for sure work on closing up the leaks in the cook chamber door since that should be a relatively easy fix with a gasket material.  You can have a lot of heat escape from a poor fitting door and also create problems with having enough draft.  You also have some air restriction created by the turns in your exhaust pipe that could be choking it down.  I really suspect that it is not just one problem but probably the cumulative effective of several things.  Start with the easiest fixes first and move to the more difficult as you go.  Don't give up on it.  You will get the help you need from this site. 
     
  10. I know that the volume of the exhaust is the most important, but is it possible that even with correct volume the diameter could be too big and allowing the heat out too fast?? If I move the exhaust to be able to go straight down into the CC I will have to put it on the back side so I was thinking a slightly smaller dia. to make it easier to cut the hole, and not take up so much room.
     
  11. I dont think your exhaust is too big, looks about average for that size smoker.  Still, looking at the pics again, Im not really seeing anything that is sticking out that is way off.  It appears your getting flow because I can see the smoke coming from the stack, and its not pouring out real bad everywhere else.  Id like to see the firebox to cooking chamber opening a little bigger, and a little bigger firebox, but.....???

    Try putting something on top of the firebox, like some scrap plywood and see if it makes a difference, might be loosing too much heat through the top. Get your air running through the fire grate, not over it, and let the fire burn good and hot ( small hot being better than large cold) and lets see what happens.

    Wish I could just be there to help.
     
  12. And your temp guage is working good,    right??
     
  13. The last time I cooked on it I put the fire inside the CC off to one side. The dial on the lid showed 250* all day long. It took about 12-13 hours to do two butts. I don't really remember exactly how long, but from what I have read here it was close to the times that everyone else said that it took for theirs to cook. Right now I only have the one probe therm., but it does seem to be close to it.When I fired it up for the first test I put the probe on the FB end of the grill to watch the temps there. I didn't have the small plate to diffuse the heat off the RF plate in place and it read about 10* hotter there than my dial did so  I figured that was about right. I don't understand it either, I get good smoke out the stack, just don't seem to move the heat. I even closed off the big exhaust and opened the little one on the other end,and removed the RF plate, and did good to hit 250* as a direct flow. You may be, at least partly right, about raising the fire grate so that the air flows through it better. I will try that ASAP and see. I do know that even putting the fire in the CC like I was I had to raise it off the bottom a bit. I do know this, if i ever try to build another another one I want a round fire chamber so that the air will flow through the fire from the end better. I did put a false top into the FB and sloped it toward the CC a bit in hopes that not having a lip at the top of the opening would help. No luck. I did build a fire basket so I will put legs on it to raise it and see what I get. If that works then I can always change out the door and put several smaller vents as close to the bottom as I can get them.
     
  14. daveomak

    daveomak Smoking Guru OTBS Member SMF Premier Member

    B Hawkins, morning.....  The only thing I see, like others, is the CC / FB opening.... maybe opening it up would help .....  But the exhaust could be the problem where it takes a 90% turn.......   I would suggest install a plenum at the exhaust..... Here's a drawing others have used....   Also, on the length of the stack,  the length need to be above the cook chamber....  I would cut the stack at the elbow and weld the entire length on top of the plenum....  even if it's too long.... install the entire length on the plenum....  This design allows for a smooth transition of heat and smoke from the cook chamber out the exhaust....   Ignore the numbers..... this drawing is from another build......

    Dave

     
  15. Thanks Dave, I think I can handle that.I was wondering what the best way to do that was. Any specs on how big this needs to be or do I just cut the stack off the chamber and make sure it covers the existing hole, or do I make the hole in the tank a little bigger, change it to square....?

    I did raise my coal basket above the intake and it was still running cold. I took some bar clamps and stretched them across to pull in the FB door tighter and it got up to about 275*ish. I think I am going to have to re build the FB door and make it stiffer, so I can seal it better. I was also thinking about putting in slot vents close to the bottom of the FB. Do you think I should put one on one side, one on each side, or one in the door?? My way of thinking is that if I put one on each side then they don't have to be as tall and I will have more head room for piling on the coals for long burns.

    I don't know if I will have it by memorial day or not, but will be cooking somehow. Got two butts and three rack pack of ribs, and corn on the cob!!
     
    Last edited: May 22, 2013
  16. sqwib

    sqwib Smoking Guru OTBS Member

    Ditto Dave  the only thing I could see being the problem from the pictures, however you may be chocking the fire, from the looks of the smoke.

    But before making any permanent mods,

    please take pictures of inside the firebox, I want to see what type of airflow you have under the fire. You pretty much want the lower vent under the fire.

    Also for sh**s and giggles fully open the lower vent on the firebox and close the upper vent on the firebox, raise your basket if it is not raised.

    Raise the pit 2" higher on the opposite end of the firebox.

    Place a thermo inside the stack just at the top and take a reading of that as well.
     
  17. sqwib

    sqwib Smoking Guru OTBS Member

    Does your fire struggle when everything is closed up?
     
  18. Thanks SQWIB, I can already tell you that with the bottom vent where it is, it is not really below the fire. It actually comes about halfway up my fire basket. Just playing I inserted a bolt into the bottom of my basket to raise it above the lower vent. It did do better, as well as clamping the FB door tighter. I have sealed the CC door with stove rope. I will probly make a new door and add the plenum as Dave suggested.Certainly makes sense. Like Weedeater said, it may be several problems, however there may also be one that is really killing me. The 90* stack may be it. Also I will put smaller rectangle vents on the sides of the FB as low as I can get them so I don't have to raise the basket so high. I really do appreciate all the help you guys have given. If you think of, or see anything else please let me know. I really would like for this to work. I really don't have a lot I can work with here at home, so it will be a little while before I can make all this happen.
     
  19. sqwib

    sqwib Smoking Guru OTBS Member

    Yes I think this is a good start
     
  20. I would almost bet you have two big problems your vent needs to be underneath your fire and you need some way to shut your stack down so not so much heat is going out of it
     

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