Adding a grate to a block arch - need your input.

[jimmol]

I had a small steam pan block arch, 25" long and 20" wide which I modified to hold an 18" by 34" continuous flow pan from Silver creek. (Pictures below) I have two 4' stacks through the cinder blocks and plan to reduce to 2" below the pan. The stack throat narrows to 14" across before turning up to the stacks.

Currently, the firebox will be 16 by 14 after the firebricks go in and 14 deep. Does that seem adequate? Any reason to make it deeper? Maybe smaller?

Thanks to all who take a look and especially those who share their years of experience with us.

PS

This forum has too much information... This was originally a post asking about adding a grate to a block arch.

I had some questions about adding a grate since, after skimming through numerous articles about block arches, continuous flow pans, evaporator pan sizes and the like, it seemed like adding a grate and air from the bottom would be an easy way to increase boil. Then, in searching for how to add a profile picture, the search turned up this tread:http://mapletrader.com/community/showthread.php?5534-Wood-burning-efficiency&highlight. Kind of adds a different light to the whole air from the bottom idea.

So what I was going to ask, no longer needs to be asked regarding the grate. But if you have an opinion about adding a grate to a block arch, please share it.

[ecolbeck]

I wouldn't hesitate to add a grate to any arch. It will definitely increase heat output despite the known inefficiencies of the design. Air under fire (AUF) is used because its the simplest and cheapest way to get a hotter fire. The craftier members of this forum have made air over fire designs (AOF), similar in concept to modern efficient wood stoves. There is tons of information to be found on that topic in these forums. The issue I ran into with grates on a block arch was finding material that could withstand the heat, particularly with a blower. The fireplace grates that I used on my block arch melted after one season.

[TapTapTap]

Yes, you want grates to allow airflow from up below your fuel.

And, I should mention that grates can be expensive. I'm embarrassed to say how much my 30"x8" replacement grates cost. But I'm confident that just a plain piece of steel angle iron won't hold up to the intense heat.

[eustis22]

angle iron will NOT stand up to the heat, even if you do make it a V. See if you can find 2 inch square tubing from a local salvage yard or metal supply. It will last much longer and be cheaper than a sturdy enough grate would cost.

[buckeye gold]

I have had angle iron grates in my arch for 10 years and they are fine. when I extended the firebox I added 3/4" bar stock and it does fine too. Heat goes up and added air under fire should initially cool the grates some. I would not be afraid of good heavy angle iron. Now it added some 3/4 bar stock in the top of my box to support brick and they drooped from the heat.

[jpcole]

I switched to angle iron grates about three years ago and won't use anything else. The cast iron ones only lasted 1-2 years after they were flipped because they droop and burn up. The ashes fill the V and the AUF will help them last a lot longer without having to flip them. Don't go cheap, use 1.25x3/16 angle or larger. It will last.

[Cjadamec]

Angle iron grates work really well. With AUF and when they fill with ash they will last many years. You don't even need heavy steel. Mine is made from a cut up bed frame and after a lot of use it still looks like the day I made it no sagging or warping at all.

Another user made his grate out of steel wire fence posts which are very cheap and easy to get.

Either way adding the grate under the fire is definitely worth it.

[maple flats]

You can not compare heating a house with boiling sap to make maple syrup. They are a universe apart. To heat a house you try to burn the least amount of wood you can while extracting the heat needed to heat the home. When making maple syrup you try to get the boil to be a hard as you can get.
This is not so you can brag about your gallons per hour (GPH) but the faster the boil the better the syrup and the less time you need to be boiling. In heating a home you might just add wood every 6-8 hours, maybe even 12 hrs depending on the stove, but in boiling sap it is very common to add wood by using a timer, just to keep it uniform (I add wood every 8 or 9 minutes when boiling without visitors and with visitors there I change it to 10 minutes sometimes.
A grate will help you get a faster boil. Good combustion needs good air flow. Boiling faster needs good combustion.
If you want to save wood, read all of the things you can about evaporator efficiency on this site and look up "combustion efficiency " on the UVM site. Much of it will not work for a temporary block arch but some might help. A grate will help, but if you can somehow get more air in blowing down from above from the sides can improve the efficiency.

[TapTapTap]

I agree that the steel angle iron option should work in smaller arches. I have a 30"x10' Leader Vortex Arch with 2 separate blowers. It's like a blast furnace! Therefore, I chose the manufacturer's cast grates even though I have free access to very heavy angle (as big as L6x6x3/4).

[jimmol]

...A grate will help you get a faster boil. Good combustion needs good air flow. Boiling faster needs good combustion....
A grate will help, but if you can somehow get more air in blowing down from above from the sides can improve the efficiency.

A grate it is, now how large a firebox?

Two things stood out as I read though the Thread referenced in the first post. First, the talk was about adding blowers to introduce air into the arch. A friend once had a wood stove, 16 x 30 free standing. It had one tube along the length. After the fire was going, if you dampered down the air intake, this tube would supply air and you would see flames during out side each hole. The heat increased about 25%. No blowers needed.
Second, my block arch, due to the limitation of the twin 4" stacks, would start smoking around the pans and upper block joints if I opened the front air wide. As soon as I closed it up the smoke stopped, if I closed it more, the boil kept going. As I look back, was it because of the air bring sucked in where the smoke was going out - above along the top. Maybe that's why it got some good boiling going.

I am going to run a 1/2" ceramic blanket around the inside and the firebrick (stacked for now) over it where I can. This should cut heat loss into the block and reduce the air coming in between the blocks. I am also going to run an angle iron frame around the pan and set it atop one ceramic blanket or flat fire gasket, to help seal it. I may leave some gaps up at top to draw some air in as it runs. I will work on it in the summer and change what is needed. I need to finish it up before the next cold snap.

So what would be the best firebox size?
1931219313

This is current size, 11 x 14. Grate is cast iron, 7 inches by 16. The grate is touch the front inside of the firebox (bottom). At the top is a 4" block I can remove to make it larger. 15 x 14" I have a second grate to add next to the first.

[Cjadamec]

I don't think you need a bigger fire box if you are going to insulate the block. All a bigger fire will do is use more wood. As you said you are also draft limited by your stack.

On the insulation note, you should put the insulation between the fire brick and the cinder blocks. It will protect the insulation and perform better.

Make the area under the pans and leading to the stack as air tight as possible. It will force air through the fire and give you a better burn. Air being drawn in around the pans will cool the pans down.

A solid improvement would be to transition to a round metal stack. The cheap round metal ducting either 6 or 8 inch is cheap and effective and it comes in 5 foot lengths. You can make a square opening at the back of your arch with brick and use insulation to seal up the corners around the stack. It will make a huge draft improvement. I used a stacked cinder block flue for a season and I know it's a really poor draft even when stacked up 6 feet.

[jimmol]

19325
I don't think you need a bigger fire box if you are going to insulate the block. All a bigger fire will do is use more wood. As you said you are also draft limited by your stack.
So you feel the 11x 14 would be fine.


On the insulation note, you should put the insulation between the fire brick and the cinder blocks. It will protect the insulation and perform better.
I am assuming that if it gets compressed by the firebrick - the weight of the brick against the insulation - it will be fine?


Make the area under the pans and leading to the stack as air tight as possible. It will force air through the fire and give you a better burn. Air being drawn in around the pans will cool the pans down.
I was toying with putting insulation inside the 3 sided of the arch, then putting the cinder/solid cement blocks that build up the vertical arch back in - 15 5/8 + 1/2 +1/2= 16 5/8 and the pan is 18 wide. I may be able to do that in summer. For now I will insulate exposed sides. The firebrick will stack just about to the top, though somewhere in a firebrick installation, it said something about leaving and inch or two, which I am thinking would allow the heat to get to the pan at the edge.


A solid improvement would be to transition to a round metal stack. The cheap round metal ducting either 6 or 8 inch is cheap and effective and it comes in 5 foot lengths. You can make a square opening at the back of your arch with brick and use insulation to seal up the corners around the stack. It will make a huge draft improvement. I used a stacked cinder block flue for a season and I know it's a really poor draft even when stacked up 6 feet.
In 2012, the arch had just the cinder/cement block stack. Several blocks cracked - and now that I think of it, spending 3 dollars to replace 3 of 5 blocks is way cheaper than running a stack - So in 2013 I added 4" stacks inside - pictured below. In either 2014 or 15 I added an additional 4 foot. I put a piece of galvanized mental across the bottom, cut out a 4" hole wth a hole saw for metal, put the stacks in them and out and folded the stack below the metal to be flush with the metal. Across the arch side I put a piece of 3/4 L frame to hold up that side of the metal.
I will have to give thought to the 6/8 inch with brick base. I will see how this year runs.

19325

[jimmol]

I don't think you need a bigger fire box if you are going to insulate the block....

After reading around some more, I checked my wood and all mine are cut to 16 to 20 inches so I choose to make it 4 inches bigger so I do not have to recut the wood I want to use for my arch.

Picture shows insulation cut to size.

I am putting together the firebricks in my basement and will set them in when the mortar is dry.

19396

[Cjadamec]

Not having to re-cut wood is plenty of reason to make the box a little bit bigger.

I'm finishing up a 200 gallon run from this past week today. The insulation is worth the effort.

[jimmol]

Good to know. Still below freezing here so will not tap till March.

[Tater]

The purpose of firebrick is to protect the insulation from being damaged by the wood. Thus, it's fine to leave your brick well below the top of your arch if the ceramic blanket will hold itself in place and you won't be stacking wood all the way to the top against the side. I have AOF in my arch, and the 2" square tubing (which is wrapped in 1/4" ceramic blanket) sits on the top row of firebrick with about 4" of exposed blanket extending up to the top rail the pan sits on, meaning my firebrick stops like 6.5" below the pan.

The material you use for your grate is less important than the design of your grate. As you can see from reading this thread, some have problems with angle and cast, while others believe one or the other is better. If you think about this from a physics perspective, heat rises. The fire may move the heat around a bit (especially if you are injecting high pressure air), but if you have a proper amount of draft, the chimney will literally be sucking the heat up and away from your grates. I have been around a campfire on 4" of ice on a lake at a skating party, and the ice under the fire barely gets wet, much less melting away. Steel takes a bit more than ice to melt/warp, so obviously something else is going on. If your grate has 1" wide slots for large chunks to fall down through, you will get a lot of heat under your grates. If you add AUF to that, and the airflow hits those large coals that drop though the grate, you have just made a forge, and no grate will handle that well. Thus, you need to minimize the size of chunks that fall through the grate. Angle layed side by side (vvvvvv) with a 1/8"-1/4" gap will keep the heat up away from your grate (the v fills up with ashes plus "rounds over," which keeps the coals well above the grate. Air under fire, whether natural draft or forced, will help to cool the bottom side of the grate, and the narrow gap is plenty to allow ashes to go down and air to come up.

As Cjadamec said, increasing the pipe for the chimney will help. As we all know, heat rises, and the warmer the air is the better it rises. A larger pipe has a larger volume to surface area ratio, which means the exhaust gases (aka smoke) gets cooled less than it will in a smaller pipe. Thus, it will rise faster. The hottest and fastest moving part of the draft is in the very center of the pipe, and it slows as you get closer to the cooler outside of the pipe.

AUF (air under fire) is a great tool, but without enough oxygen above the fire, you are literally blowing your wood out the chimney. I finished my steel arch 2 years ago a bit after the season had already started (yeah, planning ahead is good, and doing ahead is even better, [and hindsight is always easier than foresight]), and only had time for AUF. I used 2 high CFM computer fans and blew a lot of coal black smoke out the chimney. That black stuff is unburned fuel (ie wasted). I had an airtight arch (planning for AOF), and I had to run with the door open because, as you have read in that thread linked in post 1, my AUF was burning the wood, but it wasn't burning the gases generated as there was a severe lack of oxygen in the actual heat-producing part of the fire. I added AOF (air over fire) plumbing the following year, and switched from the computer fans to a bounce house blower I bought from fleabay for around $45. It was a fair amount of work, but the fire burns hotter, my boil is better, my stack temps went from 1200++ to 750-900, the jet black column of smoke that used to pour out of the stack is gone (it is actually unusual to see anything but heat waves coming out the stack), and of course the best benefit is that I use less wood. The only downside is listening to that raging inferno inside my homemade arch and wondering whether it might suddenly break out, but so far so good! :lol:

If you have an unlimited supply of wood (ie split and stacked by someone else at no charge to you), AUF will get you a hot fire (hotter than natural draft). AOF is generally more work (maybe more work than is worth it for a block arch), but the results are absolutely fantastic. I started on a block arch with the previously mentioned computer fans blowing from the front of the arch on to the base of the fire. It worked, and I made syrup. Gotta start somewhere!


PS

This forum has too much information... This was originally a post asking about adding a grate to a block arch.

We've all been down that rabbit hole. And it's doubtful we'll ever get out! Be careful about spending too much time on here, or you and your money will soon be parted. :mrgreen:

[ecolbeck]

If you think about this from a physics perspective, heat rises.:

Just to be picky:
:) heat doesn’t rise. Warm air rises because it is more buoyant than the surrounding air. Heat actually flows from areas of greater heat to areas of lesser heat (cold).

[Tater]

Just to be picky:
:) heat doesn’t rise. Warm air rises because it is more buoyant than the surrounding air. Heat actually flows from areas of greater heat to areas of lesser heat (cold).

I knew that line was gonna get me in trouble! Almost reworded it, but very little of the heat "flows" from the firebox to the ashbox. Certainly not enough to ruin my post with advanced thermodynamic theory. But yes, I did oversimplify things, maybe too much.

[jimmol]

Thanks for the information and the stories. I need to get to work as this weekend could be in the 40s.