The Steamboating Forum

Howdy All,

I have yet to cover the outside of my boiler. The outside gets hot enough that one would prefer not to touch it. I am planning on putting 1/4 inch hardy backer board against the metal and covering that in 3/8 inch thick mahogany slats. (I have a good amount of mahogany that I bought on a whim).

What are your thoughts on finishing the mahogany ? Should I oil it ? Stain and urethane ?

Thanks in advance for the help.

Jon

Need more details here. Is this a covering for a firetube boiler, where the surfaces to be covered will only see saturated steam temperature (around 330F - 370F), or surfaces typical to a watertube boiler casing, which see considerably higher temperature?

The Hardy backer board offers very little thermal insulating value, it is essentially just cement board, and comes in flat sheets, would not bend around a cylindrical shell. Why have you suggested this?.

For the firetube boiler shell, you could just use the wood strips, they might char slightly, but be OK. Better to install at least an inch of fiberglass insulation between the boiler shell and the mahogany.

For the casing of a watertube boiler, at least a couple of inches of hi temp insulation between the mahogany and the metal boiler casing sheets. Mineral Wool, Kaowool, or other hi-temp insulation.

If the final temperature at the mahogany surface is less than 125F, any normal finish is OK. In our industries 125F is the limit before insulation or personnel guards are required.

Hi,

I have a watertube boiler. It has two layers of insulation material inside the boiler assembly. One that is a refractory board that sees the inside of the fire box on the sides and top, and another layer behind it that is under the metal casing on the boiler. The floor and sides of the firebox under the grate are lined with refractory brick.

The hardybacker is easy to work with and I thought it would be better than just having a 1/4 air space between the metal and the lagging material. (It is also inexpensive)

jon

Jon,

To really know what temperatures are what and where, invest in an infrared thermometer. There are some inexpensive ones around.

http://www.amazon.com/HDE-Non-Contact-I ... B002YE3FS4

I have a fluke which is an expensive unit and an inexpensive one I got somewhere - can't remember, but they are about the same level of accuracy.

They are really handy for checking all sorts of things on a steamplant such as feedwater temp etc.

-Ron

Well, now I am pondering..................... Should I just go ahead and spend the extra money and put a 1" ceramic blanket under the lagging ?

I assume that if I compress it I will not get the performance out of it right ? I have things cut right now so that I have 1/4" between the slats and the metal wall, I will have to re-cut some pieces if I am going to add 3/4 ".

Opinions ?

jon

Having used ceramic fibre blanket previously, it is fine if lightly crushed. Obviously crushing it flat isn't really an option, but even then it must have some insulation benefit. I have it under the cladding sheets on a 7 1/4" gauge locomotive, and you can place a hand on the sheets and not get burnt. You might not want to hold on to it for too long though.

you can get 1/2' ceramic wool blanket designed for wood stove use.

Ron,

to follow up on what you said about point and shoot thermometers, Horrible Fright has a good cheap one that you will not feel the least bit of sadness should it get drowned or run over. I have a Fluke, as well, that I got originally for fire watch after applying torch-on modified asphalt roofing. One handy use for the thermometer is when trailering the boat. When stopping for fuel or other scheduled station stop, make your approach as fast as is safe for the conditions and then apply the brakes quite hard to heat up the trailer brakes. Immediately shoot the drums or discs and note temperature differences. You can easily find any lazy, slacker brakes that are not pulling their weight. If one is significantly hotter than the others you may have a sticking or dragging brake.

While down there with your head under the boat shoot the inboard wheel bearing. Follow this with the outside bearing then the tires all around on trailer and truck. The tires on the sunny side of the rig always run about ten degrees hotter. Look for high temperatures and any large differences between various check points. This will give you an early warning of possible trouble down the road. It will also keep your fingers clean and unburned when things get hot.

Happy motoring and steaming, best regards Steamboat Mike

Compressing any insulating material lowers its insulating value, there is a double impact here, as insulation value is inherently the reduction of heat transfer, and involves the thickness of the insulation (LESS THICKNESS = HIGHER HEAT FLOW), and the material thermal conductivity (COMPRESSED INSULATION HAS HIGHER THERMAL CONDUCTIVITY = HIGHER HEAT FLOW).

When you have an insulated boiler casing, say 2 inches of hi-temp wool insulation, with steel sheet outer covering. Assuming 1000F inside the boiler flue gas space, and the outer sheet metal covering at 150F, that is approx. your present condition.

If you add an inch wood insulation placed directly on the sheet metal, then the outer surface temp will drop to about 135F, but the sheet metal temp will rise to about 390F, too high for wood in contact with the sheet metal. A layer of 1/4 inch cement backerboard would not change this condition significantly.

Now add a 1/2 inch air gap between the sheet metal and the wood, then the outer surface temp will drop to about 120F, but the sheet metal temp will rise to about 540F, too high for wood in contact with the sheet metal, but the inner wood surface temp calculates to 260F, this is OK. However these calculations assume ideal conditions, and the hangers that traverse from the boiler sheet metal to the wood can bring some extra heat, and higher local temperatures, which might char the wood in some places.

John, for your case, I think the best workable solution is to use an air gap between the boiler casing sheet metal and the wood, AND to allow plenty of air circulation thru the air gap, air entry near the bottom of the casing all around, allowing air flow into the bottom, and holes in the wood at the top of the casing (or possibly just an annular gap about 1 inch wide between the wood and the stack connection, or something similar).