Fire Grate & Coals

[pyro]

I'm looking for the correct design for a fire grate. I have an old wrought iron grate from a fireplace. The kind with just a few square bars spaced a good 4" apart. I have a hard time getting past 600 degrees on the flu for the first couple hours until a nice bed of coals really get going. By the end of my last 8 hour session the grate was completely buried in coals. I must have had 6" of coals that just sat there glowing. In fact it was very difficult to stop boiling as I was running out of sap. However at that time, I was able to load up wood and briefly get 1000+ flue temp. I feel my fire grate may be too far away from the pan. Evaporator plans are attached. I'm guessing the top of the grate is roughly 20" from the pan. I'll have to go out and measure.

I also found a new grate that is more like a solid flat piece with some slots cut out for airflow. But I have to cut it to fit into my evaporator and haven't tried it yet.

What is the correct fire grate, and where should the coals be?

[ecolbeck]

What are you using for an arch? Is there good airflow under the grate? For a small evaporator a 20” gap between grate and pans sounds like a lot but there are quite a few different designs out there. In my rig there are virtually no coals that build up and shutdown happens quickly when I stop adding wood.

[eustis22]

What kind of wood are you burning? Coals are bad. Pine is good because it burns hot (if fast) and leaves no coals

[Ed R]

Coals should fall through your grates into an ash pit below. Your grates should form a flat platform for your wood to sit on. Every time I fire I run a black iron fire poker under my wood to make sure my grates are not plugging up and get natural air flow under the fire. I like having a deep ash pit and try to keep up on my ashes so I have a lot of depth under my grates. I do not run a blower, just natural draft.

[Big_Eddy]

To me - almost any coals are an indicator of too much wood, too large wood, or not enough air.
Try using smaller wood, or less wood more frequently, and each time you add wood, use your poker to make sure there is plenty of space under the grate for airflow.

Sometimes if I'm only burning hardwood (oak or beech especially), I get coals starting to form. I only add half as much wood each firing for 1 or 2 cycles to let them burn down, or change over to pine from a different end of the pile.
I've been known to scoop a shovel full of coals out of the ash pit just to open up the airflow again. No coals at all is what I strive for.

[pyro]

Forgot to add pics/plans of the arch.
21107
21106

I am burning mostly oak & maple. I don't have any soft wood. I also intentionally fire slower than most 15-20 min on avg as I don't sit around all day.
There starts out with excellent airflow under the grate. This is at the beginning of the fire when its generally not very hot. But as coals build it plugs up all area below the grate. This is also when I get the hottest fires.