Sand, vermiculite, or just a blanket?

[SyrupHog]

I purchased a used 2x8 traditional CDL evaporator late in December last year, but never got around to getting the sugar shack built in time to set it up, so was forced to use the old homemade evaporator for one more season. I'm getting around to bricking and setting up the CDL now and am certainly looking forward to using it this season. In the instructions for setup they recommend using sand or vermiculite to build up the rear after the belly ramp. I know that both are able to absorb moisture, which is capable of shortening the life of the arch, so I wonder if it's even necessary. Has anybody else run this style of evaporator without building up the rear? The flue pan I have has raised flues, so I understand the preference of forcing the fire through the flues, but there are other evaporators out there that don't build up the rear (the Lapierre Vision for one) and just put down a blanket. Thoughts?

[BAP]

In my Algier raised flu, I use vermiculite covered with a layer of blanket insulation to keep the vermiculite from blowing around. The vermiculite is lightweight and cheaper than insulation. I have have never had issues with it becoming wet.

[SyrupHog]

I'm definitely inclined to go the route of using vermiculite too, if for no other reason that to follow the CDL installation instructions. BUT... I just wonder though, if by not using anything to build up the rear, am I sacrificing efficiency and is it really a noticeable factor? My thinking was along the lines of this...by not having a built up rear would it help create more turbulence under the raised flue pan and promote a kind of extended/secondary like burn as the fire comes over the belly riser and then drops back down again? Or maybe this would only help speed up the draft and I'd be losing more heat? hmm...

[maple flats]

In my 3x8 raised flue I had similar questions. I asked around, talked to my dealers and then decided to use vermiculite. My first look into using vermiculite. led me into Lowes garden dept. There they had small bags and based on how much of a cubic foot they said on the package I calculated the cost. It came to just under $500. I had to find a cheaper vermiculite. I ended up going to a greenhouse supply (catering to professional greenhouses). There I bought 2 large bales that more than filled the void, all for $60.00 total (this was back in 2007, I'll bet prices are now much higher, but i'll also bet the cost compared to the big box stores is similar).
I filled the void, covered it with half bricks and it worked beautifully. If you don't cover it, the draft, especially if you add high pressure AOF will blow much of the vermiculite out the stack.
Find a greenhouse supplier and get your vermiculite there. Order the BIG bags for the best price per cubic foot, I used course rather than fine.

[30AcreWoods]

I can't see where moisture will be an issue for you if you follow Dave's suggestions. If you are wondering about temperatures; add a stack thermometer. It will show you really quickly if you are running too hot or too cold. I have the Vision and the whole premise is that the drop flues touch the blanket so the hot (burning) exhaust gasses are forced to be in contact with the flues (this is due to the air over the fire coming from intake vents around the door). There is no room in the arch for vermiculite, etc., by design. It just needs one layer of ceramic blanket. I am not familiar with other systems, or hybrids of mixed arch/pan setups.

[BAP]

Buy your vermiculite from a maple supply dealer, will be the cheapest place. Not building up under your flu pan will allow heat to go straight out the stack, versus up into the flues where you need it for maximum heat transfer.

[SyrupHog]

Ok, interesting. I was under the impression the Vision pans were raised flue, so now this makes more sense to me.

[maple flats]

Those who say there's not room for vermiculite are not talking about raised flue evaporators, but rather a drop flue.
Don't fill the space with sand, it could actually cause the arch to collapse and buckle in the middle. The arch is not designed to hold that much weight in sand, The arch is to contain the fire and support the pans which have plenty of weight in them without adding hundreds of pounds of sand.
On a drop flue evaporator, a layer of blanket insulation just below the flues is mainly how they are set up.