Wood Stove Design & Heat Transfer

[Roospike]

This is the point i was trying to make on "mass' and "thickness" of a stove on how it puts off heat and also holds heat longer and more even over time as like the "idea" of soapstone.
Anybody that thinks "all steel stoves are the same are wrong. Thanks for the post BB of the PDF file as show below of the area of interest.
Also i had noted in a different thread that 2 steel stoves with the same CF fire box and the mass is different with heat , burn and have different BTU ratings.
I noticed Craig was saying in a post of two stoves with the same fire box size that had different BTU ratings would be about the same.

[quote author="Webmaster" date="1162590148"]If the fireboxes are the same size, I would in general consider them similar in heating capacity. .[/quote]

Thread: https://www.hearth.com/talk/threads/4165/

note: not picking on Craig here , just using the thread as an example. ;-)

This is not so with stoves and mass size even tho the fire box size is the same.
I pointed out my brothers steel plate stove is a 3.0 cf fire box rated at 65,000 BTU's and weighs 345 lbs (3/16" body and 1/4" top )
My stove has a 3.0 cf fire box rated at 97,000 BTU's and weights 475 lbs.
(1/4" body and 3/8" top )
These two stove burn at different temperature (his being hotter per average hourly burn at stove top temperature) burn at a different rate and have different burn times. The larger of the two stoves can and will but off a higher BTU over an 8 hour burn if run this way.

I also posted information of my stove top burn temperature at starting a new fire and running up to 475° and then pulling back to 375° - 425° for the long hull as were al lot of other owners of other brands of stoves running up to 600° and pulling back to 525°-550° for optimum burning temperature.
Now when you compare two different stoves and different masses of stoves ....... well there going to run different.
Posted by another was i was running my stove at too low of a stove top temperature to be properly efficient. Again , untrue with a different size/mass of stove.

Two steel stoves running at the same top temperature and the stove being 150 lbs different in weight the large mass stove is going to put out heat differently and hold heat longer. This is why the thicker bigger mass stove is going to run at a cooler/lesser of a stove top temperature and put out the same heat a smaller mass , thinner steel stove will put out at the higher temperature.

So when some one ask what is the ideal stove top tempture to run there stove at to be efficient .....its going to depend on the size / mass of the stove.

I think the old general rule of if its a steel stove it gets hot fast , blast you out with heat and cools down quick and dont have very long burn times is "old information" and "bogus information" to put all steel stove in the same group.

You can't just gauge a stove off the fire box size and compare this one to that one. It does not work that way. Heat transfer works different with different thicknesses and the mass of steel.

 

[Roospike]

http://pods.dasnr.okstate.edu/docushare/dsweb/Get/Document-2509/F-9432web.pdf

Wood Stove Design
Most wood stoves on the market today are made of either
cast iron or sheet steel. Neither material is inherently more
efficient than the other. Both materials have nearly identical
heat transfer characteristics when stoves of the same wall
thickness and weight are compared. The main differences
between the two types are the materials used in construction
and the method of construction.
Sheet Steel Stoves. Sheet steel stoves fall into two main
categories. Stoves with walls less than 3/16 inch thick are
generally referred to as sheet metal stoves. Stoves with walls
3/16 inch or thicker are called plate steel stoves.
The major problem with sheet steel stoves is the potential
for warping. This is most pronounced in stoves with thin
walls—1/16 inch or less. When warping occurs, air leaks can
develop around doors and other joints and these can drastically
affect the ability to control the stove’s air intake.
With thinner stoves, there is also the possibility of burnout.
As steel is heated, it oxidizes, which causes it to literally
corrode away. This is a slow process and under normal
operating conditions a sheet steel stove will last many years,
depending on the thickness of the material. However, the rate
of oxidation, or burn-out, is much higher when a stove is
operated at a very high temperature (“red hot”) for a sustained
period of time. Thin-walled stoves continuously operated at
high temperatures have been known to burn out in a single
season.
When considering the purchase of a stove manufactured
from sheet steel, there are a number of things one needs to
consider. First of all, check the thickness (or gauge) of the
material used. Table 1 lists some of the more common
thicknesses and their corresponding gauge number. Generally
speaking, the heavier, thicker-walled stoves will provide
more even heat for a longer period of time with a greatly
reduced potential for burn-out and warping. Heavy, thickwalled
stoves do take longer to heat up, which can be a
disadvantage in situations where quick warm-up is desired.
Another area of concern is workmanship. Be certain that
all joints are properly welded together with continuous welds.
Non-continuous or spot welds can result in open joints and air
leakage if the panels begin to warp. Make sure doors and door
frames fit tightly. On thinner-walled stoves, doors and door
frames should be made of cast iron to prevent warping and air
leakage. On heavy plate steel stoves, doors are generally
made of even heavier gauge metal and will many times be
warranted by the manufacturer against warping.
When considering a thin-walled steel stove, look for one
with a metal or firebrick liner. These liners serve several
functions. They help protect against burn-out, and they also
increase the mass of the stove, which will help provide more
even heat over a longer period of time. In addition, they also
help maintain a higher temperature in the fire area, resulting in
more complete burning of the volatile gases.

Cast Iron. Cast iron is alloy made from iron ore, carbon
and silicon. Cast iron stoves are made from sections formed
in molds. These sections are fastened together with bolts and
furnace cement. The seams of cast iron stoves are generally
manufactured in such a manner that they overlap. However,
air leakage still is possible if the joints are not tightly sealed with
furnace cement.
When compared to plate steel stoves, cast iron stoves
tend to be heavier and more massive. This is probably the
basis for the idea that cast iron provides more heat than steel.
A major advantage of cast iron is that it is highly resistant to
warping. Also, because most cast iron stoves tend to have
relatively thick wall sections, they have a good reputation
against burn-out.
A major disadvantage of cast iron is that it is brittle and will
crack quite easily if not handled with care. Cast iron stoves
also tend to be quite a bit heavier than steel stoves, which
makes them more difficult to move around.
To prevent cracking, certain precautions must be taken.
With a new stove, begin with small fires for the first few days
to season it. A roaring hot fire the first day can result in
extensive damage. Cast iron stoves can be cracked if they are
dropped even a short distance, so care must be taken during
installation to prevent damage. Cast iron stoves are also
particularly sensitive to sudden changes in temperature and
severe blows. For example, throwing a large ice-covered log
against the back or side wall of a hot stove can cause severe
damage.
When selecting a cast iron stove, make a careful check
of all seams and joints to be sure they are sealed tightly. The
best way to do this is to shine a flashlight inside the stove and
look for places where light leaks out. Be especially careful
when checking doors and lids. Any air leaks in these locations
can make draft control very difficult. Look over side panels
carefully to be sure they are of uniform thickness.

 

[wg_bent]

Yeah I was reading all that the other day too Spike, glad you pointed it out. What it made me wonder was that since cast iron is generally thicker thus has a longer heat life, and soap stone longer still, if you made a steel stove thick enough, how thick would it take to be same heat life as the soap stone?

 

[begreen]

The article is somewhat dated. I think the thin steel stoves being referred to are the basic Ashleys. I think I bought the one I had for about $60. Look at the number of Fishers and Bear family stoves around and it seems that a well built, 5/16 ot 1/4" steel stove with normal maintenance and proper burning will outlast most of us.