Gravel or rock? Backup my theory.

[satamax]

Hi everybody.

Well, i started few threads here and at permies, and mentioned gravel in several others. Ernie said to Jeff Lawton to use pea gravel in his portable RMH. But i've had a message from à guy, saying that pea gravel didn't work in his bench, and he went monolithic.

That was with tube, and not half barrels.

In roofing, i've learnt that, up to 3cm gap, there's no convection. So, i don't need pea gravel or slightly bigger, but rather, fist sized rocks. May be a smidge smaller, so there's convection into the bench and all the rocks get heated. Even better, closing the bench on top, making it airtight, could prove intresting. Or even, may be, do this with two openings at each ends and a fan circulating the air inside the mass, when needed, with a thermostat.

What's your opinions guys?

[Donkey]

Seems that the simplest, most direct solution is best.
Space between elements within your mass will MOST likely act as insulation. The amount of fiddling and energy intensive moving parts to mitigate seem hardly worth it.

Make mine monolithic.

[satamax]

I know Donkey.

But, cos, there is a but, i will need to move the mass. The lighter i can do is 600kg elements. I can carry that with a pallet truck. I have a crane to move things outside. But, inside it's a chore.

[grizbach]

satamax,
You could make a wooden frame that would hold slices of mass.

For the slices, make a mold about 8cm thick that you could put your fist size rocks into. Use cement to bind them together.
The duct passage through it can be a lot flatter of a shape to give more surface area contact.

Enough heat should be scrubbed off by the barrel as to not affect the cement in the bench. If the mold was cut in half vertically, there would be less cracking from heat stress. Also it's symmetrical, so L and R are the same.

[grizbach]

You could also make a bench like mine.
donkey32.proboards.com/index.cgi?board=discuss&action=display&thread=197

A few drums could be bolted together to make it more modular.

I plan on taking out the ductwork someday and make it like a small 2 stage bell.

[matthewwalker]

Griz, awesome build. I'd missed that thread.

Max, I've seen that picture of the grass in your yard. I'm not buying that it needs to be so dang difficult. Monolithic mud mass, easy peasy. Comes out as easy as it goes in. You could demo a big mass with a shovel and a wheelbarrow in an hour or two.