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Up Topic Welding Industry / Technical Discussions / TIG project: gas log heat exchanger
- - By DaveSisk (**) Date 01-21-2003 21:07
OK, I think I have a first good TIG project lined up. I've already helped a neighbor make a new grill grate out of some tubing and expanded metal, but this one will require a little more precision I believe.

We have a wood burning fireplace right now. It's a masonary fireplace with glass doors, screen, blower, and wood grate/heat exchanger. I was surprised to recently discover that it's all one piece! Anyway, after our recent power outage from freezing rain, we decided to go ahead and get the gas logs we've been talking about for a year installed (I got tired of slonkin' out into the yard at 4am with tree limbs and ice falling around me to get more wood!) I've bought the gas logs and just have to get a plumber to come install them.

The issue is that I'd also like to keep the blower and heat exchanger deal. With a wood fire, that blower was VERY effective. However, it won't work well (if at all) with the gas logs. The gas logs would have to sit on top of the grate/heat exchanger, which would not only look kinda stupid, but the air circulating through the grate wouldn't get hot (gas log heat goes up!). I've looked at a couple of blowers/heat exchangers designed to work with gas logs, and they are kinda dinky and expensive. One ($250) sucks air from the top of the firebox and blows it out the front, which I'm not sure that I like. We'll be burning ventless gas logs, but we still plan to open the flue at least one click. (The instructions with the gas logs say that, even though they are ventless, open a window about an inch if they will be burning for a long period of time.) With all this in mind, I'd really rather not mix the gas log exhaust with the air in the room to such a high degree (a little is OK). I like the idea of sucking air out of the room, circulating it through a heat exchanger, then blowing it back into the room (which is what the current wood blower/grate/heat exchanger does). There was another model that sucks air from the bottom, circulates it to the top, then blows it out the top, but this one would require the gas logs to sit on top of it and I dont' think I'd like how that looks, plus this unit is $500. Ouch! Seems like a ridiculous price for such a simple thing...

So, what I intend to do is to cut the grate/heat exchanger off of the current door/blower/heat exchanger assembly (the grate/heat exchanger tubes are merely square tubing), weld up a new heat exchanger out of square tubing that will be positioned above the gas log fire (top back of the fireplace), then re-attach it to the blower. Sounds like a good TIG project, don't ya think?

I may get started on it this weekend. Should be a fun project, and instead of costing $100+ to replace the glass doors/screens + $250-500 for a blower, it'll cost about $50 worth of tubing (if that much) plus part of a weekend.

I think the current grate/heat exchanger tubing is 1" or so square and 1/16" thick, so I'll probably use the same thing. Any suggestions on TIG technique, settings, etc? Should I just butt-weld the ends together? Or should I do something more complex like cutting small pieces of tubing slight larger, and slipping it over the joints, then lap-welding those in place? It doesn't need to be perfectly air-tight, just pretty good. I'm thinking simple butt-welds (some straight, mostly right-angle) throughout will be the best answer...any thoughts?

Thx,
Dave

Parent - - By dee (***) Date 01-22-2003 04:42
Dave,
Your final question "any thoughts" is giving me problems...
...I think so but I'm not sure.

The only issue I can think of is one of air volume/velocity and would have to do with the number of square tubes going into the manifold ends, if I have the right impression of the proposed weldment...
...I'm afraid I'm clear as mud and keep nodding off into my keyboard but hope you take the clue under advisement in your design. There is a match between the size of the common tube and the number of tubes leading into it which will change the velocity of, and heat absorbed by, the air inside the exchanger; it will also vary according to the fan you are planning to use.
Sorry I am not at my best & good luck woth the project.

d
zzzzzzzzzzzzzzzzzzzzzzzzz
Parent - - By DaveSisk (**) Date 01-22-2003 15:19
Good point. Thanks! I discovered from poking around last night that the assembly with the glass doors and screens actually is a seperate piece...it's just mounted 2" above the hearth so the front of the blower assembly can fit flush underneath it. That's good!

Anyway, what I was thinking was to run perhaps 2" or so tubing horizontal at the bottom, have maybe 3 1" sections going up and over the fire, and down on the other side, then feed back into another 2" piece of tubing...want to keep it simple...I think this should allow the air to slow down in the tubes over the fire to absorb more heat, then speed up on it's way out...sound reasonable? Or am I designing in a deficiency?

I plan to reuse the front vent assembly, fan, switch, etc. from the current blower since it all fits right and looks pretty nice. I don't know what the specs on the blower are, but I'll take a look when I tear into it...

Thx,
Dave
Parent - - By pipewelder_1999 (****) Date 01-22-2003 15:35
The smaller the size of tube, the better exchange of energy takes place. If you get too efficient, you may have draft problems. But thats only theory on my part.

I'm contemplating doing a similar project with my Wood Burning Ashley Stove. It just seems an awful large amount of heat is wasted up the stack. I hobe to find some discarded/scrapped baffles for a shell and tube heat exchanger and make a removable insert that will go in the chimney.

Have a nice day

G Austin



Parent - - By jwright650 (*****) Date 01-22-2003 16:57
Gerald,
I was thinking about your post. I wonder what is going to happen to the chimney when it runs cooler than before. Creasote(sp) may condense and stick to the sides of the chimney if you cool the mixture too much. I've seen people put dampers in the flue pipe and they need to clean thier chimney alot more often than I do. Yes, it keeps alot more heat in the stove, but the chimney will be cooler. Maybe you could design it where you can open it up every so often and run the stove real hard and clean the chimney back out before it becomes a problem.
Just a thought,
John Wright
Parent - By pipewelder_1999 (****) Date 01-22-2003 18:32
I was going to make mine removable for cleaning. I have a 6" Schedule 40 chimney.

G Austin
Parent - By DaveSisk (**) Date 01-23-2003 05:30
So, sounds like I should target a larger number of small tubes rather than a small number of large tubes, correct? Again, what metal would you recommend? Mild steel? Stainless? Aluminum? (Chrome Moly? <g> just kidding!) Also, do you think thinner tubing (like 1/16") would be better or would thicker tubing (like in 1/8") be a better choice?

Thx,
Dave
Parent - - By dee (***) Date 01-22-2003 20:59
Dave,
for some reason I simply cannot get my mind to switch over to the mode of thinking we need for this. Perhaps the impression the keyboard has left on my face while sleeping has something to do with it?

Anyway, if you maintain a similar balance between the total (open) cross-section area of the exchanger tubes compared to the area of the manifold tube(s) you should be pretty close to OK . Pureists will correctly argue that length has something to so with it, but I am pretty sure the fan can handle it. If it OK before it should be OK now.
The worst that can happen is the air pass too slowly through the exchanger (and get proportionately hotter) for it to circulate as you would prefer it to in the room. You will probably be trying to work against natural convection with the air naturally warming the ceiling rather than the floor and may need the velocity to impose the circulation you desire. I am certain you have my drift.

Forgive me for picking on you here but I had a little chuckle over your first post... something about "pioneer spirit" and America... (and they say we're getting soft)... oh well, there's a lot to be said for our ingenuity anyway, but wouldnt it be easier to pile the wood closer to the door... and stuff like that. I don't generally like to make fun at others' expense, it's just that I had my little tinfoil cap folded with the wrong side out last night and it must have done something to my brain waves.
Thanks for taking it in the spirit of fun- I'm sure you'll be looking for your chance (it should be easy to find) to return the courtesy ;-)

Regards
d
Parent - - By DaveSisk (**) Date 01-22-2003 21:33
Dee:

I'd post a smiley face if I knew how! (Meaning I do appreciate your humor.) Yeah, yeah, I could stack the wood closer to the door...but hey, that wouldn't give me a welding project to do, now would it? <g>.

I had to get up about every 1-2 hours to toss more wood on the fire from the stack I had inside. I would much rather not even have to do that, to be honest! Call it laziness, but these conveniences (like gas logs instead of wood) are readily available to us and in the long run about the same price...I can't see a good reason not to take advantage of them! Between a challenging job and a 5 month old daughter, life gets pretty busy, so I'm all about making things easier...

Best regards,
Dave
Parent - By jwright650 (*****) Date 01-23-2003 12:30
Dave,
Dee always is looking out for us here on this BBS. He manages to crack a smile on our faces or break us out in laughter somehow. I'm glad we have him looking out for our health, because it is good for the heart to smile once in awhile. Yeah, we pick back at him when we get the chance.(nothing mean though)
Smile Dee,
John Wright
Parent - - By DaveSisk (**) Date 01-22-2003 16:40
Any suggestion on what type of metal to use? Mild steel, stainless, aluminum? I was originally thinking mild steel, but gas logs produce some water vapor as part of the combustion process, so I would expect unfinished mild steel to rust (although the current grate appears to be unfinished mild steel, that was for a wood fire). If mild steel is the best choice, what could I finish it with that will tolerate the 600 degree or so temperature that the gas log fire will produce? Some sort of black engine paint or something?

Thx,
Dave
Parent - - By jwright650 (*****) Date 01-22-2003 17:08
Dave,
I thought about doing something similar to what you are talking about. I ended up buying a fire box with a blower built into the cabinet. Mine pulls cold air off the floor and pushes it up throught the heat exchanger and out the top of the cabinet. Mine appears to be made out of light guage galvanized sheets, bent to form a seperate channel for the air to go through. The top and back of the fire box makes up one side of this channel. I didn't take notice how it was shaped, wheather it was tapered to enhance air speed, up or down. I'll try to pay more attention to it when I get home and I'll let you know if I see something that could help with your design.
John Wright
Parent - - By jwright650 (*****) Date 01-23-2003 16:22
Dave,
I forgot to look at that heat exchanger lastnight. I'll try to do better tonight. I was getting in wood for the wood stove down stairs in the basement and forgot all about the gas log heat exchanger.(my gas logs are in the living room for knocking off that chill real quick until the fire catches up in the stove down stairs)
John Wright
Parent - By DaveSisk (**) Date 01-24-2003 02:39
No problem...thanks John! Post the info if/when you get a chance.

Regards,
Dave
Parent - By GRoberts (***) Date 01-22-2003 20:34
Header paint claims to go up to 1200F and is available at your neighborhood automotive store.
Parent - - By Niekie3 (***) Date 01-22-2003 18:24
Just reading this post made me break out into a sweat. We are having a rather hot summer in South Africa this year, so I am thinking "air conditioning".

You guys must be having a rather cold winter if there are so many of you busy with this sort of thing!

Regards
Niekie Jooste
Parent - By jwright650 (*****) Date 01-22-2003 18:43
Niekie,
It is supposed to be 5F tomorrow here with a wind chill of -15 to -20F. But it doesn't get that cold here very often. I'm not sure where all these other guys live, I'm in Virginia, USA.(about in the center of the state)
Warm weather will be a couple months away for us.
John Wright
Parent - By DaveSisk (**) Date 01-22-2003 18:56
Yup, I'm in North Carolina, USA, and we're having an unusually cold winter. Looking forward to Spring and Summer!

Dave
Parent - - By pipewelder_1999 (****) Date 01-22-2003 19:14
I worked in Richards Bay, South Africa about 12 years ago and it was a nice change to leave early fall here and arrive for the 1st part of summer there. It was beautiful there. Nothing like we expected. Other than the monkeys and sharks it was much like Florida.

I went hunting yesterday morning and it was 58 Deg F. Tonight or tomorrow it will be down near 10. Neither of those temps are normal for my area this time of year. My heater project will be a spring/summer project. For now, I'll just put another log on the fire or let more dogs in!

I am equipped for up to a "9 Dog Night" as opposed to your normal "3 Dog Night" They are even talking about snow so I expect the schools to shut down in anticipation of the flurries (blizzard).

G Austin
IUKA Mississippi.
Parent - - By jwright650 (*****) Date 01-23-2003 12:16
Gerald,
They closed the schools here today too. Only thing I wish the school officials would do is give the parents alittle more notice. They posted my kids school at 6:10am this morning and I should be leaving the house at 5:50am to get to work. So I flew like a maniac through the snow to get here as the bell was ringing.(thank goodness for 4 wheel drives)
Have a great day sledding!
John Wright
Parent - By pipewelder_1999 (****) Date 01-23-2003 14:13
We live in an area where every other family has a four wheel drive. However the 1" of snow we received overnight has shut the schools.

I was driving home from an inspection in Tuscaloosa last week and flurries were expected. At 10:30AM as was coming into town, they announced that school was shutting at 12:15. This was in anticipation of the snow and what we receeived yesterday is the only snow seen since two years ago.

G Austin
Up Topic Welding Industry / Technical Discussions / TIG project: gas log heat exchanger

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