I was curious as to why the burn back and hopper smoke during shut down was so common on the 1070 (and I am told the 1250 as well). While I was able to reduce the occurrence on the 1070 by first lowering to a lower temp (250 +-), I was more interested in the "why" this problem seems to occur in the 1070, when it did not happen often in the older 590. Looking at the two pits for differences:
1. old 590 has four large slots on top for discharge, while the new 590 and 1070 have multiple slots on top but they are narrow. I think the total exhaust opening on the newer machines is larger.
2. Fire pot and auger assemblies on both old and new 590s are inside the chamber, while the whole assembly is below the floor of the 1070 with the top of fire pot flush with the floor.
3. The 1070 has a front loading hopper with a very short auger tube, compared to the 590's which have long auger/pot assembly.
4. The new 590 and 1070 both have 3rpm auger motor while old 590 had I think a 1.5rpm motor
5. The 1070 has a large opening at the top of the hopper and the cover seems to just rest on it when in place, I'm not sure it touches it even. The 590 has a smaller lid and is held more snugly in place with a clasp. More on this later
#1 I think small narrow slots create more friction and thus resistance to exhaust and thus reduce the volume allowed to pass. (and clog up easier). Flow is always faster the farther it is from the edges of exhaust surface, so there is little room for increase with the slotted exhaust. The old chimney exhaust follows the law of laminar flow with highest friction on the surface of interior with markedly reduced friction in the center, so the old chimney exhaust would allow less resistance to flow, and thus permit exit of more volume depending on its diameter. BOTTOM LINE: SLOTS ARE LESS EFFICIENT, SO YOU NEED A LARGER TOTAL AREA FOR THE SAME FLOW.
#2 I don't think the position of the assemblies matters as far as contributing to exhaust or pressure changes.
#3 I think the shorter auger assembly contributes to the ease of back flow of fire and exhaust into the hopper for obvious reasons.
#4 The new 3rpm motor propels twice the volume of pellets for combustion at the "high" end and thus twice?(theoretical, perhaps the auger blade is adjusted) the volume of exhaust gases is formed at the high end. And because there is less resistance to pressure in the hopper auger assembly, you are more likely to get backburn and smoke in hopper???
#5 I think this is the main reason for easy back flow of smoke. #1 may be a contributing factor, but I do not know the total area of the new exhaust. But I do know that if there was no leak in the hopper lid, there would be no flow, and no oxygen for combustion in the tube (or less of it).
REC: SEAL THE HOPPER LID! There should be no risk since the there are no alterations of the cooking chamber or exhaust system.
I had several burn backs at shutdown recently, and the pit was clean and no obstructions found.
Here is what I did and it was easy. Using Lava Loc 1/8" thick. This thickness does not interfere with opening or closing the lid.I have made 4 500* plus cooks since with all kinds of variations and had NO shutdown hopper smoke or even post shutoff smolders in pit. I am cautiously optimistic. Comments appreciated.
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Topic: New 1070 On Board (Read 13090 times)