The Steamboating Forum

I plan to use a stainless steel liner with an enamel outer for my funnel.
Ideally A4/316 would be my preference, but could I use a lower grade of stainless?
I know that “you get what you pay for“, but .......
TIA
Ian

Virtually any grade of stainless would be more than adequate for stack service. For stack gasses below about 700F, ordinary steels are fine, and much less costly. The ASME Boiler and Pressure Vessel Code allows virtually all common steels up to 650F, and only temperatures higher than that result in a reduction of allowable stress. With a reasonably efficient boiler, stack gas should be below 650F.

For the last several years I have been using an ordinary galvanized stove pipe section, which takes high temperature paint very well after the shine of the galvanized surface is heated with a big torch, to simulate a couple of years of weathering. Without this heating the galvanized metal surface will refuse to allow good paint adherence. My stack temperature is only about 410F, but the paint I use claims service to 1200F.

Remember you can buy these ready made up from places like Machine Mart. Also the inner does not have to go all of the way to the top.

Excellent, I wasn’t aware and will investigate.
Thanks
Ian

Mike Cole wrote:Remember you can buy these ready made up from places like Machine Mart. Also the inner does not have to go all of the way to the top.

Side question for Fred: (or anyone else)

How dependent on diameter is natural draft? My stack is 8" diameter and around 5' tall and single wall. I would love to have a liner to reduce the chances of someone touching a hot stack but I don't want to reduce the boiler output.

Do you think reducing the I.D. to 7" would choke off the draft?

This is a VFT-30 during wood.

fredrosse wrote:For the last several years I have been using an ordinary galvanized stove pipe section, which takes high temperature paint very well after the shine of the galvanized surface is heated with a big torch, to simulate a couple of years of weathering. Without this heating the galvanized metal surface will refuse to allow good paint adherence. My stack temperature is only about 410F, but the paint I use claims service to 1200F.

This is interesting, because I had trouble finding a fit with the black stove pipe in that would properly fit Nyitra's boiler bonnet. ...
I ended up getting new-old-stock galvanized stove pipe that I painted black with "Rust-Oleum® specialty high-heat spray paint retains its color and finish in temps. up to 1200°F." Can this weathering be achieved by sitting a few decades in a family hardware store that doesn't get much business?

Great minds think alike?

-CB

A trick I use for painting galvanize is paint it with vinegar first. There is also a paint sold by Cabela's called "Duck boat paint" that has good adhesion on slick surfaces.

To answer your question, yes galvanize oxidizes with age and becomes very rough over time.

-Ron

"Do you think reducing the I.D. to 7" would choke off the draft?

This is a VFT-30 during wood."

ANS: A simple thumb-rule is appropriate for a VFT: Make make the stack area at least 50% of the total fire-tube area. This refers to the flow area of the stack = (PI/4) * DiaStack^2, and the flow area inside the firetubes = (PI/4) * IDtube^2 * Number of tubes.

A tall stack can allow smaller stack area, high stack temperature also, but this thumb-rule given above is generally adequate.

Obviously there are conditions that can change this, for example on my boiler I plan to fit an economizer coil inside the stack, so it is larger than the thumb-rule calculation, and my boiler suffers from too much air flow (and low firebox temperature) because I have not yet fitted the economizer coil. If only I could find a round tuit....

One of my acquaintances does a lot of research into combustion science, or at least did, until he retired a couple of years ago.

He made a comment which mildly surprised me about domestic flues above both open fires and closed stoves. That was, given a fixed length flue in a normal house, the area was often too big for generating a good draught and hence minimal smoke into the room. I have no proof, but his argument was that the smaller area flue implied faster flue gas flow, less adiabatic cooling from expansion, and less heat loss on account of both the faster transit and the reduced cold wall area. Hotter flue gas means lower density, and hence better draw (if you believe him). I should perhaps also point out (to put this in context) is that the flue area in my own house starts at 30 square feet at my chest height, and then tapers steadily until finishing in a 10" internal diameter pot at the top some 25' above. It doesn't need a small boy to go up it as chimney sweep: I'm a little larger than the average man, and can get up it (long-handled brush in hand) easily.

We are not in the same regime as mentioned above with most of our steam boats, but the concepts are worth bearing in mind. Furthermore, if the flue is being given a liner, the insulation from an air-gap should keep the inner surface of the flue hotter, and so the "draw" may actually improve, or perhaps stay much the same, and either way there should be less soot deposited on the inner surface.

Standard woodstove flue size is 6", and they are larger and burn very hard by comparison to most small fireboxes. The 24" diameter tug funnel drops down to 6" and it will burn very hard. For most small boilers, 4 or 5 inches is probably ample.

-Ron