Ramp=draft

[tonka]

I'm just curious about something and want to hear your guys take on it. Do you guys think that a ramp improves draft?

[Brian]

No it don't control draft but it does control the heat and were it goes. Heat travels the path with least resistance.

[ecolbeck]

The sole purpose of a ramp is to direct flue gasses (and the heat they contain) up against the bottom of the pan (or flues). There are recommended designs depending on your situation.

[berkshires]

I'm just curious about something and want to hear your guys take on it. Do you guys think that a ramp improves draft?

You mean like if you had an evaporator where there was no ramp, just like an even 2" all the way from the fire to the stack? Would that have worse draft? Is that your question?

No, I don't think so. If anything the ramp creates a tiny bit of back-pressure on the fire. But it should be almost none. Mostly it forces the fire up onto the pan/through the channels.

GO

[RileySugarbush]

In general, gradual changes in flow path for gases will have less pressure loss than abrupt ones. So a ramp probably has some small benefit in increasing natural draft.
My arch has no ramp at all, the firebox just jumps right up to an inch from the bottom of my drop flues. One benefit of that is it can protect the end of the flues from over enthusiastic loading. But I am sure it does decrease draft a bit. Thats not a big issue with under fire forced air, but it can make the firebox harder to seal up. More draft restriction means higher firebox pressure, and maybe a little ash will shoot out any small gaps.

One advantage of the ramp is it lets the radiative heat from the fire hit more of the pan surface, which might increase efficiency a bit.

[tonka]

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In my home built cooker I have the firebox and then straight up to the bottom of my tube pan. I'm still doing some tweaking to it. The pan size is 4x10 tube pan and 2 2x4 syrup pan. I'm currently boiling almost 300 gph. Will be making a preheater for it by this coming season. The highest stack temp I've seen is 650 degrees. So I wasn't quite sure if a ramp helped with draft or not. I currently have an old furnace flower blowing under the fire(dont know the cfms) will be looking to get a blower atleast 1,000 cfm for under the fire, I'd rather have too much then not enough. I do have air blowing over fire as well which I think I have that part good.

[RileySugarbush]

650°F is an indicator of good efficiency, especially if it is the actual internal temp measured with a probe thermometer! I wouldn't mess with more air unless you want to trade lots more wood for a faster rate. Your rate looks good for the size of your pans if I read you right that you have a 4x14 total pan area. Over 5 gph per sq feet of pan. We max out at about 4 gph per sq. foot.

Love that tube flue pan, it's like an old locomotive boiler.!! Thats a lot of heat transfer area. I aways wanted to make one. Share some more pics!

[tonka]

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A steam engine was what I built my cooker based of off. I thought its gotta be a the right way to go since steam engines were build to efficiently make steam, steam is what is needed in order for the machine to do its work.

I figured it could be anywhere from 400 to 500 gallons an hour. When I figured it out it comes out to be about 480 sq ft of heated surface going off of 1 sq ft will cook off 1 gallon water per hour.

[RileySugarbush]

That is wonderful!! Congratulations on a fine build..

[ir3333]

what are you asking when you say increase draft?
Are you wondering if it increases volume or velocity?

[tonka]

Velocity. Wouldn't volume be increased because of velocity?

[ir3333]

the volume will be determined by the size of the chimney.If the cross sectional area is less than the
chimney in any part of the arch, the velocity will increase there.This is why your draft should be small
with narrow openings under the grate.You want high velocity air blowing into the fire.So if you have no
ramp you will force the flame directly up into your pan for excellent heat transfer.If the area under your
pan is small the rest of the way, the flame will race back at high velocity along the bottom with low transfer
to your pan.I don't think one inch of airspace under your pan is a benefit.
As mentioned earlier a ramp will keep the flame hitting a larger portion of your pan straight on
before it heads back and exits. It you have high flue temperatures your inefficient and wasting wood.

[tonka]

I have a 22" stack on a 4x14. What exsactly do you mean when you say "If the cross sectional area is less than the
chimney in any part of the arch, the velocity will increase there."?

[ir3333]

Use the area of your stack in square inches.
Think of a wide lazy river that comes to a narrow section.
To move the same amount of water through the narrow section it will have to move much faster
The venturi principal..if you decrease the size of a tube you will increase velocity..or vice versa.

[ir3333]

a 22" diameter chimney would be about 380 square inches.If your arch is 48 inches wide and you had 6 inches under your pan on the way to the chimney
that would be 48 x 6 or 288 inches.The fire would move about 33% quicker in the arch until it got to the chimney where the area is wider and then the
velocity would slow down going up the pipe.An arch that size would require a bit of experimenting / calculating to find the sweet spot!

[tonka]

I've thought that since all my flues are 2 inch tubing they restrict/slow down the draft. Since my stack is almost 100 sq inches larger then all the 2 inch pipe added up, 282 sq in, could that be the cause for me not getting the ideal draft or velocity being created to pull more heat to the back of my big pan?

[ir3333]

well yes...restricting it will offer some resistance but the volume will still have to match the chimney.If you aren't
getting the heat at the back of the pan your flame may be transferring it's heat to the sap as it should and be to
cool by the time it reaches the back...what is your stack temperature?
It may be as simple as increasing chimney length or making a hotter fire.I have been adjusting my small unit for 3
years now and hope this years' changes will produce hotter and more complete boil.But it may not either?

[tonka]

The hottest I've seen my stack is 600 to 650 degrees. My fire is about as hot as I'm going to get it since I have auf and aof, from my understanding stack height dosent matter when using forced air right?

[ir3333]

with forced air everything changes.At 650 degrees you may be optimum for your arch.There are so many variables.

[tonka]

You got that right about so many variables. I also realize it's hard to understand how my cooker functions without being there and what kind of tweaks I could make to improve it if theres room to improve. I'm still hopeful that I can get that stack temp up to 800 or so degrees I could do something to create more draft to pull the heat further into my tube pan I feel like I can achieve the increase in stack temp.

Ive noticed on some of the stack bases theres an air intake to aid in draft, could that be an idea to do something like that to encourage more draft and improve the velocity? I found some fans that mount on the stack of wood stoves to improve draft but they're only rated for 8 to 10" pipe. https://www.homedepot.com/p/Auto-Draft-7-in-x-9-5-in-x-9-in-Stove-Pipe-Draft-Inducer-with-Speed-Control-AD-1/302203167