Hopper tubes effect on surface area

I did some math during the last hangout, and I wanted to post the results here.

You might have seen Wayne’s hopper tubes and wondered if there really is much surface area gained by that. After all, they’re only 2" tubes. Wouldn’t a slightly bigger hopper “outer skin” make up the difference?

First off, it’s made simpler by the fact that all our dimensions are across one plane - the hopper tubes basically extend the full height of the hopper, and so all we need to calculate is the increased cross section of the hopper.

Actually, if we do the math (C = pi x 2 x R) you can find an interesting fact - every time you add a 2" pipe it’s adding a surface area equivalent to a 2" larger drum. No fancy math needed - two inches really is two inches, whether in hopper diameter or in a hopper tube.

Starting with a 22.5" drum, Wayne originally had 4, 2" tubes. This adds 8" and makes the drum effectively 30" diameter, a 36% increase.

Lately Wayne’s used 6 tubes for a 54% increase, and 9 tubes on the 94 Ram, an 81% increase.

If you go hog wild like Peter did, you might get 19 tubes on your hopper. This is close to the practical maximum that can fit around the drum. 19 x 2 = 38". Peter has nearly tripled his hopper surface area (168% increase), which is now equivalent to a 60" drum.

Note to the math wizzes. The theoretical maximum that would fit (jammed together with no spaces) is 35 2" tubes. What’s interesting is that no matter what diameter you make the tubes, the maximum added surface area is always the same, pi times the barrel’s diameter. This means you can theoretically only quadruple the surface area, and you can do it using any size tubes you wish.

Howdy Chris,
That sure was an eye opener.
I’ll likely go that route on the next hopper.
I wonder how this will compare with a genuine Monorator… I’ve never seen anyone build a accurate version of one. Most are way to tall… nothing like the one pictured here.http://www.hotel.ymex.net/~s-20222/gengas/monorator-eng.html
It looks like Pepe is doing it now?
I wonder how the math would work out on a Genuine Monorator? You would have to take into account the bottom of the hopper as well as the cross section.
Great post, thanks Chris.

And all the added “wall” area is surely at a lower temperature. O.K. Chris, you’ve convinced me. I’ll cut the old tubes off the old hopper, add a couple more, and go to mod 3 on my hopper.

I have an idea that having that much cooling surface available to Sub-Zero air would cause serious problems. As it is now (on my rig) without any external cooling tubes, I get tons of water and tar condensing out and flowing (as liquids) into the hot-water-heated condensate tank. So much, in fact, that I only get a dribble out of the drain below the Cooler rack. But in the Balmy summer temperatures I believe I could use the extra cooling power. Seasonal Hoppers?

“Seasonal hoppers?” Hey, when I was a kid, we had “seasonal tires”, (N. Indiana),and changed them accordingly.

Hi guys,
Thought I’d chime in since I just did the math on my monorator. With a 24" D top, 21" height and a 4 1/2" overhang my monorator has approx 16 sq ft of surface area available with varying degrees of relatively cool surfaces. The ledge (the part of the hopper sticking out beyond the burner shell) is not directly above the hearth heat stream so it remains much cooler. This 4 1/2" ledge alone has almost 2 sq ft of surface area. For me that would equate to 2 2" tubes 21" long. I thought about adding 4 2" tubes but I’ll wait to see how it works as is. 4 tubes would give me a 25% increase in surface area. 8 tubes a 50 % increase.
I used sq-ft for clarity. A 2" tube 21" high (my dimension) has a surprising .92 sq ft of surface area.
Find the circumference first in inches (pi times the diam), then multiply by the tube height in inches to get the surface area in sq in. Divide that by 144 sq in per sq ft to get the answer in sq ft.
The surface area of the tubes is significant as Chris points out.
Carl, we have only one seasonal tire up here, they’re called snow treads, lol.

Yes seasonable “adjusted” hoppers makes sense.
Not so sure though that if the external air temperatures got cold enough that the condensate chilling, and in tubes freezing would not self correct and revert back to internal hopper circulating. You actual long term frozen down people be the proof of that.
An outer insulation wrap with a thin metal weather shield would be an easy enough to on/off do. Fredrick Ek and VesaM both speak of on/off winter sheet metal shielding “too keep warmed”. Same as truckers do with radiator covering.

Ha! CarlZ even here in the “mild” climate PNW the actual aggressive soft rubber winter tires family go on here November thorough March every year. And Oregon/Washington these WILL be metal studded in my families. And this in and of it’s self forces you to slow safer down now driving soft and less responsive; and not wanting rip and spin out the expensive studs.
The the only actual problems even 2wd drive are 12" to 18" plus deep NEW snow falls then vehicle belly dragging. Stay home and wait for the plows to work it. Then hand shoveling the roadside plow berms to get onto the road becomes another morning and coming home “aerobic” chore in addition to woodstove 24 hour restocking back up.
Real problems is road traffic hang ups and having to turn around to bypass bowling-pin stack ups from those NOT preparing at all trying to cheap-out/luck out squeeze by with performance street tires or all-season tires that ain’t.
Wife has been home late too many times this winter having to do once, twice back-ups, long way by-passing the “optimistic” ditched, curbed fools. Always strange to me how most here cannot live without their min $100 a month high-speed always in contact connects but the cheap out on a $400-$600 set of winter tires and wheels. Annualized tire wear costs per mile here come out about the same once the initial investment is made. And that is about the cost of two tow bills; or one tow and a body/alignment shop run-off curbing or ditching repair. Hit someone; or be hit will really racks up the costs.
Lots of Urban’s whining this winter here about why doesn’t the governments do not do more about the wintered roads.
Ha! I look at their stupid choices in wheels and tires!
Only those who have at least tried to help theirself’s get a pull out tow strap jerk from me.

Steve Unruh

this may sound funny coming from me but i’ve never owned a set of winter tires… I run fairly worn out all season tires year round, only replace them when i can’t get in and out of my brothers drive way… his driveway is my tires test… If it’s that bad out, i just don’t go out… and when i do, I drive according to road conditions. only once have i not been able to make it up the hill to go to the shop… a couple other times this year I just didn’t bother trying…

as with wood gas, driving in winter safely has a lot to do with experience… something we get a fair amount of up here…

Thanks John Stout for bringing that up. I inquired about this a little while back because that thought crossed my mind as I was doing my build. It was suggested to insulate it in the winter which make sense on my stationary unit but maybe not so practical for a mobile unit. The seasonal hopper makes perfect sense to me.

Steve, I like your selection process of who gets pulled out of the ditch haha. I too always carry a tow strap in my truck. I will pull anyone out of the ditch to save them $150, unless of course they are clearly drunk, then I leave them sit.
I figure someday it may be me in there, I hope someone will also stop.

Bill, I see you are in Minnesota. Yes, there is a tendency for wood in the hopper to freeze to the cold, gooy walls. And I have not installed proper cooling tubes. But, contemplating Steve’s suggestion that if the tubes froze up the hopper would simply function as tube-less, I think he’s right. But if an un-insulated tube doubles as the drain, not good. I have wrapped fiberglass around the hopper before, but now I only insulate the drain line.
I’ve got some plowing still to do today, and then woodgassing.

Hypothetically, couldn’t one squish the tubes slightly (say 25-50%) so they were more oval to fit more tubes onto the hopper? I mean, if one squished 2" tubes by 50% the resulting ovals would be roughly 1"x3". One could then fit as many “2 inch” pipes (by surface area, unchanged) as they could 1 inch pipes (by new width).

I’m not sure where the point of diminishing returns is, but we could probaby find it with that method. :stuck_out_tongue:

Hi Brian
Then you may as well use rectangular tube. Be far easier to cut and weld than a oval tube.
Hopper tubes defiantly work, and work well, but how many before there is too many. And the further away the better cooling.
Which option ?

Yes Brian ,

More tubes could be mounted meaning more surface area but would the free flow continue and any cleaning may be more difficult .


I thought the tubes were more for convection circulation of air than heat transfer area.

Hello Andrew

I think the cooling tubes transfers some heat off causing a temperature differential with the hopper thus causing convection circulation .

Thanks wayne that’s kinda the way I had understood it all along. I guess even a blind squirrel gets a nut once in a while.

Actualy i thought they were causeing more condenceing of the hot smoke. too increase condesate removel.

Yes Kevin ,

Increases condensate removal .