GEK Wiki / v3 Downdraft Reactor
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v3 Downdraft Reactor

Page history last edited by jim mason 10 years, 11 months ago

2) Building the Downdraft Reactor for GEK v3.0


back to Fabrication Instructions for v3.0 kit





 Parts needed:    

     - Reactor outside vessel tube

     - Insulation inside vessel tube

     - Bottom end plate

     - Middle flange ring

     - Top flange ring

     - 5/16" x 5/8" bolts (5 count)

     - 5/16" nuts and washers (5 count)

     - 1/2" 90deg plumbing elbows (5 count)

     - 1/2 npt to 15/16 gas flare adapters (5 count)

     - Reduction bell mounting ring

     - Reduction bell

     - 1/2" x 6" pipe nipples (5 count)

     - 1" x 6.125" nozzle tubes (5 count)

     - 3/8" street elbows

     - Clay gasket material

     - 1' of extra 1" pipe for nozzle adjustment spacers

     - Not pictured: Lid plate and parts (full pictures and assembly below), inner reactor vessel individual parts


Note: This could be said to be the core element of the GEK, and involves the most complicated fabrication of the various GEK parts. Pay careful attention to all instructions, and make sure you complete each step before doing the next one- there are many build order dependencies.


1. Inner Vessel: Weld inner insulation vessel tube onto bottom flange plate.

  • Orientation: Slag side of flange plate is facing down. 
  • Insert (5) 5/16" bolts into small holes, fastening them to the flange plate with nuts on the other side. This will hold them in place as you weld the bolt heads to the plate. Make welds gas tight.
  • Pull the two tabs on the vertical seam of the vessel into the center of the widest slot in the flange plate. One slot in the plate is larger- look closely. The two tabs should slightly overlap, probably not more than 1/4".
  • Fit all tabs on top edge of inner vessel into slots in flange plate.
  • Alignment: The inside edge of vessel tube needs to be EXACTLY aligned with inside of holes for air intakes.  This is what determines plumb for the nozzles.
  • Weld up bottom flange completely
  • Weld up side seam.
  • Finish all welding on this inner insulation tube as you will not be able to get back to it once the outer tube is welded on.



2.  Weld 1/2" 90 deg elbows into bottom flange end plate.

  • You might need to grind off casting debris on the elbows to get them to fit through the holes in the bottom end plate.  If you need to, do NOT grind off the collar as this collar is used for offset spacing during the fabrication below.
  • Align elbows and weld them into bottom flange end plate
    • On Bottom (bottom view): Elbows should radiate outward to the left.  The air lines spiral outwards clockwise, passing as close as possible to the next air inlet tube as they radiate outward.  Thus the elbow should be aligned so that the air line will pass just to the side of the next elbow. See pictures at for clarification.  The collar on the elbow should sit on the bottom end plate.  Do NOT grind off the collar, as this spacing is needed so there is room to screw in the SS tubes.  Do NOT position the elbow further out or it will obstruct the grate when inserted into the gas cowling
    • Inside Reactor.  The elbows need to be aligned so the air inlet nipples rise straight upwards and parallel with the insulation vessel tube wall.  You can use a right angle to draw straight vertical lines up the inside for alignment.  Then use a 12" or so 1/2" pipe nipple with a coupler on the end to sign to this line.  Push the elbow and the coupler up against the wall to ensure parallel orientation to the wall.  A combination square will assist with vertical alignment. The line and the coupler/elbow collars are your jig alignment tools. Use a clamp to hold the jig in place.
    • The elbows need to be welded in gas tight.  It is easy to miss spots around the neck of the elbow. 




 3. Outer Vessel: Weld mid flange ring (the one with air holes) onto outer reactor vessel tube

  • Orientation: Use any tab and slot position.  Center your starting tab in the middle of the corresponding hole.  Make sure tabs are pulled all the way into slots with C clamps.  Use C clamps as you weld around circle. We've had the best results with extra deep throat C clamps and a small homemade "pi symbol" shaped clamp helper, seen in many of these photos, the intervention of which provides a flat surface on flange edges for the clamp foot to sit. See photo below.
  • Weld over the tab and slot joint on the INSIDE of the vessel to ensure gas tightness. You will not be able to access this weld point after step 4! See photo on right




4. Outer Vessel: Tack, but DO NOT WELD top flange ring onto outer reactor vessel tube.

  • Orientation: Use any tab and slot position, but make sure flange is flush with top of vessel tube
  • Pull vessel tube and flange tight together with C clamps as you tack around the circle
  • Make unusually small but sturdy tacks, as we need to fit the "Air Neck" part up very close to this joint later
  • Clean up flange holes with a 11/32" drill


(photo shows GEK v2.0, but top flange attachment is exactly the same on the v3, using very small tacks)




 5.  Join and weld outer reactor vessel assembly to inner insulation vessel assembly

  • Slip outer reactor vessel assembly over inner insulation vessel.
  • Align holes in outer vessel with "mousehole" cutout slots in inner vessel near the top edge- these will allow instrumentation and ignition access. Align the two holes and cutout slots by eye as closely as possible, while also fitting the tabs and slots together on the bottom.
  •  Pull outer vessel tube and inner vessel end plate with elbows tight together with C clamps as you tack and weld around the circle.




6. Insert & Weld One Access Tube to Outer Vessel

  • Insert one of the provided 1" x 2 1/2" tubes into the hole directly to the right of the vertical weld seam. Not the other hole! See photo
  • Allow it to protrude 3/8" inside the reactor
  • Keeping it squared up and parallel to the mid-flange (this is very important!), tack and then make a full weld around to seal it




7. Fit & Tack the "Air Neck" Over top of Reactor

  • Start as photo shows, fitting 1st hole in air neck over inlet tube
  • Push and fit it into place, keeping top and bottom edges of air neck pushed fully tight up against mid flange and outer reactor wall
  • Tack as you go around, again making sure air neck is tight to mid flange and wall of outer vessel. There will be a 3/32" to 1/8" space between top edge of air neck and top flange. Your tack welds made previously on this joint should not interfere with the air neck being pushed tight to the wall






8. Fit and Weld 2nd Access Tube Through Air Neck

  • Fit 2nd access tube through corresponding hole in air neck and just through the hole in reactor wall. This tube should not protrude through, but be flush with reactor wall. This tube also needs to be parrallel with the plane of the mid flange. Use of a die grinder or a round file on the holes may be required to ensure a good parallel fit. Make a full weld around the hole on the inside of the reactor, also where it comes out of the airneck on the outside



9. Weld 1" Threaded Air Intake to Air Neck

  • Fit over the larger, remaining hole on the airneck. Again, make sure it is oriented parallel and in plane with the mid flange.
  • Tack and then make a full sealing weld around the intake



10. Bolt Reactor into Cowling

  • Insert reactor into gas cowling, and align the bolt holes in the flange plates
  • A bolt hole directly below the 1" threaded air intake should be oriented 2 holes off, counterclockwise from the gas inlet on the cowling. Please see photo on the right for the proper orientation
  • Bolt down tightly! This step is to keep the mid flange from warping during welding, which comes next




11. Stitch Weld Around the Air Neck, Alternating Top and Bottom Edges

  • We've used 2"-3" welds at a cooler setting, making a weld on the mid flange/airneck seam, and then alternating to the mid flange/top flange seam, leaving 2"-3" unwelded spaces between them as we go. This distributes heat and allows some cooling as you go around. See photos.
  • The 3/32"-1/8" gap above the airneck and beneath the top flange should become fully filled with weld bead as you go. Both of these seams need to be good and airtight for proper functioning of your GEK!
  • Keep rotating the vessel, filling and alternating stitches until both of these seams are fully welded
  • Weld vertical airneck seam
  • When fully welded, set aside and let cool. Do not remove bolts until it has had a chance to cool off!




12. Unbolt Vessels and Weld in 1/2" NPT to 15/16" Flare Fittings

  • Before unbolting you may have to straighten/flatten some portions of the mid flange and cowling top flange with a hammer
  • Unbolt vessels and pull out reactor, then flip it upside down
  • Weld in 1/2" NPT to 15/16" gas adapters for air inlet tubes down through the mid flange plate. Fit the narrower threaded ends through the holes in the midflange. Make sure the adapters are centered in each hole, and aligned accurately with side of vessel tube. Also, one of the corners of the hexagon of each fitting needs to be pointed directly toward the reactor wall for best welding. Weld all the way around, making sure they are air tight.
  • Finish welding the seam all the way around bottom of the midflange





6.  Reduction Bell

  • Weld reduction bell onto reduction plate.  The reduction bell is made from stainless steel.  Ideally you weld this with a stainless steel welding rig.  If you do not have this, you can also weld it with a standard mild steel mig.  The weld is on the outside of the bell, thus it will tend to not erode in combustion. 
  • Install reduction tube in bottom of reactor. 
    • Put ceramic tape on the bell flange.  Coat ceramic tape with fire mortar before installation. 
    • Tighten nuts with washers onto bolts in bottom to hold in reduction tube



7. Install Nozzles 

  • Weld 3/8 street elbows onto ends of 1" air nozzle tubes.  Make sure the elbows are angled at 90 degree from the nozzle tubes
  • Screw in 1/2" X 6" pipe nipples into bottom of the reactor.  Tighten modestly with pliers
  • Put clay gasket material around bottom of nipples, and a thin coating around the threads at the top of the nipples
  • Push the nozzles down over the nipples, into the clay.  Take care to not get clay sealant into the nozzle tube passage.
  • Depending on your nozzle preference, and desired final height, you may want to install some spacers on the 1/2" nipple before pushing on nozzle tubes.  In general, you should start with the nozzles at a height above the reduction cone, equal to the diameter of the reduction cone entrance.
  • The nozzles should point inwards towards the center of the reactor, and be firmly positioned in the clay.  They should be difficult to turn.
  • The large photo below is from an earlier version, but when installed your v3.0 nozzles should end up oriented like these






8. Construct Reactor Lid

  • Clamp lid plate to a fabrication table or thicker plate of steel, all the way around with at least 6 clamps. Or tack weld the plate flat. This plate in particular loves to warp from welding heat.
  • Fit (2) 2" halved couplers over appropriate holes, tack in place
  • Make 1/2" stitch welds around the couplers, alternating sides around the couplers and allowing plenty of cooling time between welds. Take your time with this part. Again, it just loves to warp into a potato chip shape
  • Remove clamps and fit 5/16 bolts through, place cover plate over bolts and tighten down with nuts
  • Weld bolt heads all the way around





back to Fabrication Instructions for v3.0 kit












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