New firebox liner for Otter

[barts]

Otter has an oil burner, and the firebox has been .022 321 stainless sheet bent into a circle for the last 15 years, surrounded by Fiberfrax... well, a couple of years ago the stainless started to warp and twist, and it's gotten bad enough to seriously interfere with combustion. I hauled the boiler out of the boat, and am now almost done stripping the boiler and removing the pressure vessel so I can get at the firebox underneath.

I've decided to replace the Fiberfrax (wearing a respirator, natch) that formed the firebox insulation, and apply a couple of coats of refractory slurry made from 'Satanite', an AP Green refractory. This should give me a lightweight firebox (important to reduce heat up time and stop safety blowing after arriving in port) while protecting the Fiberfrax from contact with the oil flame. The 'Satanite' is good for 3200 F - rather more than the Fiberfrax - so it should make a good hot-face coating. I'll take pictures and report the results. This sort of setup has been used for propane fired forges and oil fired casting furnaces - we'll see how it works here. There's a lid on the firebox with a large center hole that is used to retain the flame in the firebox until it has finished combustion; this will now be made of Fiberfrax, well coated with 'Satanite', rather than formed out of stainless. I hope this will reduce the soot formation.

This has been a busy year - Otter has a new 12 gallon integral fuel tank, Racor fuel filter and all the bearings adjusted on the engine/

- Bart

[Steamboat Mike]

Hi Bart, sounds like a very workable plan. Two thoughts for your consideration based on some very long ago experience I had with another A.P. Green product that I believe was called "Miszou Castable Refractory". The stuff was good for about 2800 degrees F.

One suggestion is to place chicken wire formed to fit the firebox and the lid/baffle so that it will be in the center of the wall thickness to act as reinforcement for the refractory. The best material is stainless chicken wire or square mesh hardware cloth. The mesh should be fairly small gauge wire and large openings like chicken wire so it acts as a reinforcement and not a a parting line for the refractory to split upon later. The wire does a great job of keeping the refractory intact when it develops cracks in service. Stainless chicken wire is not easy to find but in this internet age should be available somewhere. Ordinary hardware store variety works ok too if the stainless turns out to be made of unobtanium or some other exotic alloy.

Another idea is to form up the firebox liner with concentric Sonotube material that gives the desired wall thickness. If exact tube sizes are not available you can take the nearest larger size and slit it lengthwise and remove an appropriate amount of material then Gorilla Glue the cut back together with hose clamps or wire. After the glue cures arrange spacers to keep the tubes concentric and pour your castable refractory. The trick is to design it so that the entire forming apparatus is combustible. After the refractory has taken its set and air cured per manufacturer's directions simply install the whole thing in the firebox and burn out all the form material and let the refractory take its ceramic set from the heat of the oil fire. It will make a very durable liner. Just be sure to follow directions for the refractory to allow proper drying so the stuff won't steam and spall on first heating. I think I left an electric heater in the firebox for quite a long time to thoroughly dry the refractory. It lasted many years with coal fired in direct contact with it.

Good luck with the refit, best regards, Steamboat Mike

[barts]

Steamboat Mike -

Thanks for the comments! I ended up going with Mizzou as well since it's available locally, and I'm in a time crunch. I've setup the kaowool floor and have a thin cast Missou skin on the second layer of kaowool. We'll see how that hardens over the next couple of days; I wet downt the wool to prevent too much drying. I'm now working on the forms for the castable vertical walls and the lid. I want to avoid too much mass in the firebox to keep the safety from blowing when I shut down the fire, so I'm trying to keep the thickness of the castable sections reasonably low.

i like the idea of some reinforcing in the castable; the kaowool does this to some extent but it's not particularly strong. Hmmmm.

- Bart

[barts]

Some photos of progress:

- Bart

[barts]

More photos:

[barts]

last pics. I used the propane feed through the oil burner's steamport (as I do when warming up) to start heating the Mizzou lining.

The lining came out pretty well, despite some problems being off center. There's some scantily covered areas right around the lighting and burner access ports; I'm going to lean the casing over and cover the patchy areas with more Mizzou like I did for the floor without a form.

The Mizzou seems quite strong. I'll try an oil fire (atomized with propane) later on in the week.

- Bart

[fredrosse]

You must run with a really intense fire to distort the stainless steel so much. I used a large stainless steel cooking pot in the base of my boiler, it often ran red hot, but didn't get quite as "burned up", and basically kept its shape. With the boiler tubes sooted up in your picture, how much was the steaming rate hindered?

[barts]

You must run with a really intense fire to distort the stainless steel so much. I used a large stainless steel cooking pot in the base of my boiler, it often ran red hot, but didn't get quite as "burned up", and basically kept its shape. With the boiler tubes sooted up in your picture, how much was the steaming rate hindered?

The fire is pretty intense (forced draft, oil + steam atomizing) and impinged directly onto the stainless, which was backed by kaowool. This meant that there really isn't a lot of cooling, and the stainless ran bright red/yellow much of the time in the center section. The stainless was in the boiler since about 1998 or so; I originally fitting lightweight firebrick but it cracked, as much, I think, from the trailering up and down I5 as from the fire. I continued to use the firebrick base, which worked well since cracking didn't matter much there so long as the pieces remained in place.

The steaming rate has varied over time. The boiler definitely has a harder time producing steam when it's sooted up as shown here; however, the soot seemed to stick in the less turbulent places and flake off other places. I am making a steam lance to make it easier to clean the tubes some, but as you can tell from the pictures the boiler coils form quite a nest. If I force the draft heavily that seems to help matters for a while; the new firebox should help here by retaining the flame longer in the combustion area. This may be helped by a mistake I made; the forms shifted somewhat during casting and the center hole in the refractory through which the flames are to emerge against the coils (around the bottom of the boiler drum) is shifted towards the front of the boiler. This may help increase the combustion time as well.

I'll know more this weekend when I get the boiler reassembled and in the boat; the proof, of course, is when I take her out the following Sunday. I'm down to the wire on time, though.

- Bart

[barts]

I never had time to properly shakedown Otter's new firebox other than limited tests in the driveway to find any plumbing issues, but she performed flawlessly nonetheless last weekend during our steam out and back to McConnell Island in the San Juan Islands. I can recommend Mizzou refractory for this purpose - it is heavy and retains heat, but it's sturdy and seems to work quite well.

- Bart