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[bewford]

Hello everybody. i have been lurking around here for a while just learning. I have been planning out a gasser build in my head and have even picked up a few materials so far. my questions are.
what would be a good diameter and  wall thickness to use for my heat transfer tubes? Second is any opinions on using a round firebox versus a square one? is one better than the other. Thank You for any input

[bullworker1]

Hi Bewford, square is always better, sharp corners ,lapped seams, any thing that can obstruct the air flow helps with heat transfer.
a round cylider is way to easy for air flow.

[Scott7m]

Google scotch marine heat exchanger, its been around forever and is extremely efficient.  Empyre and portage and main use them on there gassifiers and they are very very efficient round tubes.

[bullworker1]

Scott7m is correct, many boilers have hundreds of ft of tubes in them, makes for huge sq. ft. of heat exchange , but that's not possible in a smaller unit.
If you have ever seen in fared pictures of heat ex changers you would notice that any sharp outside corners or the nobs that protrude from the inside walls of cast iron boilers are much hotter than the surrounding surfaces, they collect heat and transfer it into the water.
If you look back at earlier posts you will see where members who have built furnaces, and Evan some who have bought furnaces realize all that air going in the front and zipping out the stack isn't doing any good if it takes all Btu's with it , the first thing they want to do is put in some type baffle, which can improve efficiency , but how does it do it ?  a piece of plate blocking the exhaust path can help by slowing down the air flow, but that same pc of plate isn't transferring
 any heat into the water jacket, its just storing heat to let it escape out the chimney later.
so with some thought you can build these restrictions into your furnace, [ i call them spoilers]  so that they are part of heat exchange system.
so getting back to square or round , i prefer square, it might not be as easy but it lets me mess with the air flow ............. just my thoughts   

[martyinmi]

The Empyres and the P&M's both have EGT's in the 250*-350* range, which is very close to "ideal". Much lower and you have to start worrying condensation issues, as well as installing condensation drains.

[bewford]

thank you for the opinions guys. I pretty sure im goin round tube soley because i got a good deal on some this weekend at an auction. 20ft. sticks for 20 bucks each. So slowing the airflow thru the heat exchanger is desireable to increase thermal transfer???

[yoderheating]

 From what I've read I believe you simply need to disrupt airflow rather than just slow it down. Air tends to flow down the center of a tube taking the heat with it. Anything that can be done to force the air to scrub the edges of the tubes would be helpful.

[bullworker1]

Great reply Yoderheating, you hit the nail on the head........................

[Scott7m]

I know most have the tubulators in them, and I agree with the principle until I think about the empyre 100 elite, it has one set of tubes, no tubulators and exhaust right under 300, I wonder ho it draws so much heat off so fast

[jackel440]

Large amount of surface are is needed to transfer the heat.That is why you see so many tubes in the exchanger section.

[peacmar]

This has been quiet for a lil while but ill chime in....

The importance of tube diameter is as follows:

The quantitative sum of the cross-sectional areas of combined tubes of the heat exchanger must be equal to or greater than the volume of exhaust gasses exiting the combustion chamber at any given moment.

Length:

Use the maximum allowable length for given design.

Horizontal flue gas travel transmits heat to the heat exchanger tubes twice as efficiently as vertical flue gas travel.

1 boiler horsepower (measure of heat transmitted) requires 5 square feet of surface area to contact in a horizontal path, or 10 square feet of surface contact area in a vertical path to transfer the above said heat through the tubes and into the water.

Wall thickness has very little effect on performance except for the first few seconds of operation.

Turbulators in a vertical path can improve perform to like horizontal conditions.

Calculate surface area with the inside diameter of the tube, not the outside.

Use equation for circumference then multiply by length to achieve total area.