New Turbulators

[Nofossil]

My new turbulators came in. I have 6 HX tubes in my EKO 25, each 21" long. I installed a set of six turbulators before last night's fire. It's too early to make a definitive statement about the effects, but the temperature drop between combustion and flue was the highest I've seen - about 110 degrees higher than the previous week's average.

There are a lot of variables, and I haven't had a chance to do a scientific efficiency analysis yet. Still, looks pretty promising based on the one data point.

 

[Sting]

May I ask what source you used?

on line?

Local?

I like this spiral design better than mine but in the short time I have been lurking I understand a simple length of chain hanging in my vertical fire tubes could accomplish a similar effect!

 

[wdc1160]

May I ask who you bought these from?

 

[Eric Johnson]

I really like the chain idea, too. Those look good, nofossil. I wonder if you'll have to clean your hx tubes more or less often with these things installed.

Is that mild steel, or something more exotic?

Do you have a good way of pulling them out of the tubes?

(I'm in interrogation mode today, apparently)

 

[Nofossil]

I had a local metal shop make them. They're mild steel. It takes a somewhat special rig to twist them. The shop had done ornamental ironwork and had a twisting rig that they built for that. Cost me $120 for the six of them. Because they're not perfectly straight and they're a very close fit, they take a little pressure to push into the tubes, and they stay in place (so far). I have a very sophisticated removal tool - says 'Vise Grip' on the side of it.

I'm thinking that a bit more twist might be better.

I don't know what diameter and length other models would need, but the shop is happy to make them if anyone else wants some.

Zenon also thought he might be able to get them, but never responded to my email.

 

[webbie]

The older Tarm turbulators (for the oil gas side and/or the wood side) were basic steel. The temps reached in the water tube area are generally low enough so that a bent piece like this is not affected....the water around the tubes also tends to keep these areas relatively cool.

I never remember replacing even a single turbulator under warranty.

 

[Eric Johnson]

I wouldn't worry about the heat, either. Mine still slide easily up and down in the tubes.

 

[barnartist]

They look nice NoFo. Seems pricy though, but probably not with steel and labor of twist. I paid like $30 for steel and for them to slice it for me. Twisted them myself, and I have 16 of them suckers. Two big table vises and a big adjustable wrench. It was almost fun.
I think more twist would be OK, I dont get much buildup and I dont burn as well as you. I too have to pull them out with Grips.
Im looking forward to your results.

 

[sled_mack]

Do they need to fit that tightly? Are you trying to get extra heat transfer by having them tight against the side of the hx tubes? If not, having them slightly loose makes cleaning easier.

 

[jebatty]

NoFo -- looks great, but since you're the data guy, try the chain idea and see what the difference is. I'd be willing to chip in a few $ for some chain. Let us know. BTW, the chain I used in my Tarm was 1/2" barn cleaner chain, which has links a little longer and wider than standard 1/2" chain. My chain still fit cleanly into the hx tubes, no side contact.

 

[Nofossil]

I am trying to get some steel-to-steel contact for additional heat transfer. I might decide to go looser next time.

I'd be happy to try chain, but I don't have any spare chain that's anywhere near the right size. If I come across any I'll try it - it would be easy enough.

I'd have built my own, but I don't have a shear, and my attempt at getting a clean spiral wasn't pretty.

I know a shop that does a lot of ornamental work - might be cheaper, especially if I don't specify a non-standard width.

I'm really looking forward to doing another burn with calibrated wood. Right now, I'm in a section of the pile that has a lot of variability and a lot of junk.

 

[rsnider]

not to sound stupid but what are turbulators for anyway?
just to slow down the exhaust in the heat exchanger?
I'm not real familiar with this design yet. but interested and now looking hard into the eko and biomax.

 

[Eric Johnson]

On the new EKOs and the BioMax, the turbulators double as hx cleaners. The levers on the side of the boilers move them up and down in the tubes when you yank on the handle. Saves you having to do it by hand.

Barnartist: You have 16 tubes in your 60? I only have 10, last time I checked.

 

[jebatty]

NoFo -- I'm not sure that steel to steel contact should be the goal. The heat transfer area between the edge of the turbulator and the wall of the hx is miniscule. Would it not be better to seek to achieve a distance between these that maximizes flue gas contact with the walls of the hx? Too close and it would impede flue gas contact. Too far and it would allow flue gases to escape by staying internal rather than external to the flow. The purpose of the turbulator is to cause turbulence in the flue stream so that as many molecules of hot flue gas as possible come in contact with the hx walls.

 

[drizler]

wOW I hope they work for you but for gawds sake don't go back there for any more. You are paying wayyyyy too much for something you can make yourself. For future reference don't ever buy steel at a hardware store ect. Go to the local structural steel fabricator. Easy to find they are big enough and stand out and usually in an industrial park. You can buy a stick of 1/8" mild steel for somewhere around 15 bucks and its 20 0r 22 feet long. These guys could tell you how to do it . You don't usually find much like that in a scrapyard so better to buy it new unless you really like to dig and have your tetnus shots up to date.

http://www.hobartwelders.com/weldtalk/forumdisplay.php?f=34

 

[steam man]

not to sound stupid but what are turbulators for anyway?
just to slow down the exhaust in the heat exchanger?
I'm not real familiar with this design yet. but interested and now looking hard into the eko and biomax.

Since no one has specifically addressed this I'll give it a shot. Digging deep into my industrial boiler background (turbulators are not used there) turbulators are probably meant to reduce or eliminate the "thin gas film" next to the heat exchanger tubes by increasing the flow velocity of the flue gas next to the tubes. This "thin gas film" really reduces thermal heat transfer to the metal and ultimately to the water. The metal to metal contact is probably insignificant. Keeping them slightly loose may be more prudent. Turbulators kind of reminds me of that gas saver thing that you put in the inlet of your air intake on your vehicle to turbulate the intake air for more efficiency. I think mythbusters did a show on that. As for that thing the air is already pretty "turbulent" and it isn't meant for heat exchange like the fire tube turbulators which seems like they can be effective. I am just wondering what the downside of the turbulators are: the possibilty of stack temps too low, draft problems, clogging up, or whatever. I have faith nofossil will figure it out and inform us. Any other theories on the turbulators?

As for making your own-I have used industrial suppliers for small amounts of metal. Be aware that they may charge a handling fee (say $20.00) for any size purchase plus an amount for each cut. I have been there. Smaller places should be no problem.

 

[jebatty]

I am just wondering what the downside of the turbulators are: the possibilty of stack temps too low, draft problems, clogging up, or whatever. I have faith nofossil will figure it out and inform us.

My experience has been stated in other threads. Mine are the 1/2 inch chain, homemade turbulators. My issue was stack temp too high, 600-900 on very dry pine. With draft adj I got this down to below 700 and falling as the load was consumed. With the turbulators, I now am 400-600 during the burn. No clogging, chain does pick up a little dust, but I'm not cleaning the hx tubes any more frequently than before. I clean when temps consistently start rising above 600. With everything very clean, I'm at 525 max.

 

[dadams]

So, from a newbie to this discussion who is an engineer type just starting to explore renewable source heating options:

y'all are correct that the highest resistance to heat transfer is at the gas-side boundary layer, which is in laminar flow with a very low velocity at the wall and thus a low local heat transfer coefficient. However, the theory re inducing turbulence to minimize the laminar flow boundary sufficiently to improve heat transfer would probably require an added pressure drop in the tube which I believe is pretty much required to induce a substantial amount of turbulence. I doubt this is happening from this simple spiral mechanism, and even if it were I might think you would you notice such a flow restriction in your boilers? A prior comment re metal to metal contact area being insufficient to contribute via conduction is also likely correct due to insufficient contact surface area, as backed up by the alternate chain mechanism perhaps also working. I would postulate if you are seeing a material improvement that the most likely mechanism is radial mixing of the gas in the tube. At low velocity, gases will tend to be in laminar flow and not well mixed. So in a short exchange section, the gas at the wall gets cooled off while the gas in the middle of the tube could pass quickly through the heat exhchanger and come out still hot due to not much radial mixing. In such case, anything that induced a little bit of turbuelence and radial mixing would perhaps raise the gast temperature nearer the wall and improve heat transfer. Industrially, efficient exchangers have a significant gas-side pressure drop, getting the velocities up and helping turbelence and heat transfer coefficients. If system requrements don't permit such perssure drops normally a very large amount of tube surface area is required to achieve good heat transfer (plate or Plate/fin exchangers for example) Just my two cents, but I'd bet a weeks pay on it.

 

[steam man]

dadams-I totally agree with you-it just makes me think about all the considerations such as tube size, i.e. heat exhange area, gas flow veloctiy and mixing of hot/cold gases. I looked at an oil fired boiler not too long ago that had turbulators. My thought on the design for the heat exchanger was that due to the design certain tubes may have not been getting good flow while others were excessive. More soot in the low flow tubes. I wondered if turbulators only in the normally high flow tubes would be a better solution. I did a study years ago on heat transfer in different sections of a large boiler and it was interesting. My mind hasn't stopped yet.

Thaks for the reply.

 

[Nofossil]

Welocome to the forum, dadams - nice to have another engineer here.

In this case, we're dealing with an existing configuration that has a number of compromises built in from the start. I'm pretty sure that the spiral 'turbulators' don't induce much actual turbulence - I suspect as you suggest that the underlying mechanism is in fact radial mixing. At the velocities involved, I doubt that there's much of a pressure drop issue. It's a pressurized combustion chamber in any event - if there were an increased pressure drop, it could easily be compensated for by raising the blower pressure.

Folks have had good results using a piece of chain, which I suspect does induce more turbulence as well as increasing the velocity due to the reduced available cross-sectional area. I'm interested in doing a comparative study. I think the spiral should work better, but in the end the data tells the story.

 

[dadams]

Hi,

I went out to th EKO site and see now this a forced draft system so perhaps already has plenty of velocity and turbulence. Hard to tell as I don't easily see where the HEX tubes are and how arranged. My comments were more based on tube HEX systems I have seen in wood stove or wood furnace, which in general seem to be induced draft and therefore low velocity and not capable of managing added pressure drop (particulalry in absence of bypass for startup). So if these tubes are not large, low velocity design but instead smaller or higher velocity, not sure what the spiral would change.

Our company does some HEX work. Typically they are about 1000 tons of solid stainless steel. Not insignficant, but I am not directly involved in the detail so only speaking from general knowledge

BTW Just having moved back from Germany and studied their construction technique, energy efficiency, and solar design techniques, I am in process of planning a new home based on a few basic concepts: compact footprint, passive and active solar, PV, exceptionally high thermal mass, an insane amount of externally applied insulation, renewable fuel in some variant as a secondary heat source, and probably a ground source heat pump for low duty summer dehumidification and tertiary heat source if I don't feel like lighting a fire. I've been thinking about it for 3-4 years, it's going to be a fun project. Looking forward to conversing with y'all.

 

[Eric Johnson]

I have some pics of the top and bottom of the hx tubes in my EKO, dadams, but they're on my other computer. I'll post them when I get the chance. My 60 has ten vertical tubes running from the bottom of the secondary combustion chamber up through the water jacket and into the stack exhaust chamber. I haven't measured them, but they're problably an inch in diameter. Most people who retrofit turbulators into this type of gasifier see their stack temps drop 100 to 200 degrees. But hang in there, and I'm sure nofossil will soon produce some detailed numbers to chew on. I have the stock turbs, but they're basically identical to the ones nofossil had made. In a tarm, they're zig-zagged pieces of strip steel, instead of the twisted design that the EKOs come with.

 

[webbie]

The TARM boilers use large vertical tubes, about 3 inches in diameter - a very different setup than the EKO. Interesting, because TARM certainly does their homework over there (Denmark) in terms of engineering.

 

[dadams]

Well, you can tell pretty much everything you need to know by stack temperature as it is the ultimate measure of efficiency. If you've burned all the combustibles and subsequently extracted all the heat, you've achieved the end goal no matter how you get there. There are myriad possible "engineered" solutions to this conceptually simple task, stemming from each design decision having hopefully positive but also some negative impacts that can themselves subsequently be compensated in other ways.

I can only presume that if some have achieved stack temperature reductions by add-on bits in the boiler HEX tubes, then either the engineering was not so well done in the first place, or the designers chose not to improve their solution for one or more reasons: cost, complexity, maintenance, hazard, or something else. Not knowing your boilers, any guesses as to why these "improvements" would not have been done at the factory?

 

[webbie]

Given 1" tubes I can guess that the concern may have been flyash and soot - such a small tube does not leave a lot of room for buildup on the sides of it. In the case of TARM, the turbulators were "stock" on the 8 tubes which exchanged the heat from the oil side, and optional (but easily available) on the side that served the wood chamber.
As you can see by the cutaway enclosed, this is a fairly simple design with the tubes running up through the water jacket in the rear.