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  • Fire-box / flue design

    Does anyone have any information on how to build a fire-box and flue for a direct fired kettle?

    I have a direct fired 5BBL kettle, it is a single walled kettle and I am in the process of insulating this and adding an outer cladding. Currently looking at using Chloride-free rockwool for the insulation.

    I am not sure what the burners on this are rated to, they are able to bring 5BBL up to a boil, but it is taking way too long. Insulating the kettle should help this somewhat, but I would also like to add a fire box around the burners to help with efficiency and ventilation of the gases. No one here in the local area (Cusco, Peru) has much of an idea about fire-boxes or flue setups, however they can pretty much fabricate anything if I can show them what I need.

    What I had in mind was to enclose 3 sides of the burner with stainless steel panels and then insulate those with rockwool. Then the 4th side would be the flue. Questions I have at the moment are how big should the flue be? I understand it needs to be insulated, but does it also require an extractor fan or similar? How low should the side panels go? The burner is approx 60 cm off the floor and the frame around it is approx 80 cm square.

    Any suggestions or advice would be appreciated.

  • #2
    I am assuming you will be using butane or propane? I would start at 8" double wall type B vent pipe. Try to stay away from 90 degree angles so run two 45s separated by straight pipe to get the angle to follow your wall. Depending upon the building material, cement or wood, you will need a triple wall pipe for through wood (double wall would work but I like overkill), a wall thimble if through the wood frame wall and a single wall roof flashing if through a wood roof. Cut as much plywood clearance away from the pipe so the flashing can be still nailed and solid to the plywood and underneath roofing material. Rock wool here would be a good idea if you can't get asbestos piping (still available in Peru as far as I know. I can get it here in Belize but getting scarce). Make sure you have proper hangers for the pipe and run two sheet metal screws through the joint where two pipes meet (just through the outer wall of the pipe, not the inner).
    Does the kettle currently have a heat apron? If yes, this will work and just weld in brackets to hold the burners a minimum of 12 inches from bottom of kettle. You really need to determine the BTUs of your burners, I would guess you will need a minimum of 380,000 btu to get a decent boil in your lifetime. If you go with a fire box, then go with a forced air unit. Depending upon the distance to your vent cap (make sure you have a storm collar where the vent exits the roof through your flashing) you may need a booster fan to vent the CO2 out just as added safety.
    Side panels I would take to just above the floor so you can still get underneath for cleaning and mopping. Heat rises as you know (except for the radiant) so insulating around the apron to the upper lip of the kettle with flexible fire proof insulation would be fine (you will need to remove at times to clean the exterior of your kettle). Another option would be to build a free-standing enclosure around your kettle but not attached to it. Insulate that, run your SS or galvanized sheet metal over the insulation so the kettle sits in the cubicle. I did this because we had low ceilings and had to place a condensate hood over the kettle. With access only from the front, it keeps me safe when my mind is on a thousand other things but still allows me to slide our 3bbl out for cleaning and access to fittings/sensors. It also acts like a secondary flue directing the heat up through the condensate hood and fan.


    Garrick
    Island Time Brewing & Distribution Co. Ltd.
    Belize

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    • #3
      The burners are a pipe style burner running on propane. They were made here locally, hence why I am not sure of the rating. You can see them here in this picture. The kettle it's self is on hinge at the front so I can tip it for draining and cleaning.

      What I had in mind was to enclose it on 3 sides, and there at the back where you can see the pipe burners going in to build the flue. From there I was thinking of taking it up to a side wall and out, or going through the ceiling if need be, but that will be much harder as it's concrete. How high does a flue need to go?

      Garrick by "forced air unit" are you referring to burners like this? Or fitting a fan or similar to the firebox to push air in?

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      • #4
        Different kind of set-up than mine. Mine is mounted to a concrete tiled floor with cinder block walls all around. The kettle is about two feet from the block wall and I built two wood frame walls out from the cinder block. Insulated them, screwed on 3/4" Plycem board and then sheets of SS on both sides and the back. My flue goes through the cement wall in back and rises three feet above the roof line. The natural stack effect does the rest so no need for a booster fan.
        From the pic, it looks like you have a circular heat shield in place. My personal opinion? Bolt a square tube frame to the back and sides (to the front rail as well), rock wool inside the 2x2 sq tubing frame then attach flat stock on the inside and outside. The metal sheeting should help reflect some of the heat back into the kettle and the heat rising should help with the kettle temps fluctuating less. This will leave your front open for tilting and cleaning the kettle. The flue, you will need to make a rectangle to round duct to fit over your burner tubes (unless you light the tubes from the draft holes). Just make sure the flue is separate from the enclosure you build around the burners in back, otherwise the enclosure will act like the flue instead of the flue. To clarify, this means you will need to build something to fit the bottom of the kettle bottom so when it is seated the gases cannot escape into the enclosure but must exit through the flue in back. However, from the pic the place looks open enough to allow some CO2 to leak in and not cause any issues but for safety sake best to do it right.
        For height? Usually 15 feet is the minimum so as to produce natural ventilation with three feet of that being above your highest roof point (this whether through the wall or through the roof). Theoretically, the higher you go the better the draft though, as there is a delta equation for this.
        Yes, that is what I was talking about regarding forced air.

        Garrick
        Island Time Brewing & Distribution Co. Ltd.
        Belize

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