Boiler sqft

[johnp]

How do i calculate the sqaure footage of my VFT boiler? it has 85 3/4" tubes 14"long, the outside dimensions are 16 1/2" diameter and the height is 14" tall. and should i have fire bricks in the bottom of the fire box? or just a 1/2" steel plate which will be bolted to the stringers. Also do i need a grate with a wood buring boiler?

[87gn@tahoe]

You take the outer circumference of the tubes, multiply by length, multiply by number of tubes, and divide by 144 to get sqft.

Your tubes heating surface is approx 19.47sqft/hs.

Now, there's additional heating surface in the tube sheets (area of tube sheets minus area of opening of tubes). If you have a water leg firebox there is a bit there, and it is some of the most effective bit.

[Lopez Mike]

To add to Wes's post:

Does your boiler have a water leg? That is, does the water area extend down on the sides of the firebox? It doesn't sound like it.

About how far up the tubes is your normal water lever likely to be? A lot of manufacturers (and most of us from laziness!) count the full area of the flues but I don't think the area of the flues that is above the water level does much for steaming capacity. Maybe a little super heat. Steam doesn't conduct heat nearly as well as liquid water.

Then it's a geometry calculation. As Wes says, your tubes have an area of about 20 sq. ft. I strongly recommend adding a water leg.

And yes, you need a grate so that the air can come up through the wood fire. You'll need two doors. One above the grate to feed the fire and one below to clean out the ashes and to regulate the air supply.

On the bottom, a layer of steel, with some insulation under it to keep heat from the fire and ashes from endangering your boat. If you don't have a water leg, you will need insulation, not fire brick, around the sides of the fire area so as not to waste heat (and to avoid setting fire to you and your boat!).

If you are designing a new boiler, I heartily recommend having a water leg. It's the most effective part of the heating surface. (harp, harp)

Fire brick is for protecting steel from intense flames like in an oil fired forced draft boiler. It is not a particularly good heat insulator.

Others will weigh in with their experiences. I've seen some poorly designed units but most steam well. It's not rocket science (unless it blows up!). I'd copy some existing design if possible. Beckman is an example of a conservative setup.

Mike

[johnp]

thanks for the answers guys, it doesn't have a water leg, the inside perimiter is lined with fire bricks and the outside of the boiler is lined with insulation. it had two doors as you said but no bottom plate. i was thinking of bolting/welding the firebox to a 1/2" plate of steel. what should i use for insulation under the plate?

The tube diameter is 3/4" i.d. i would assume the o.d. would be 1"? That would put at about 25sqft. Correct?

[farmerden]

Over here on this side of the big pond they manufacture a concrete style board to place under wood stoves.I used that .And under that is one of my copper fresh water tanks so it's going to be hard for the fire to get to the hull.When I'm running on used oil I've seen those fire bricks take on a pretty good glow so don't think of them as insulators-they're not- but their mass helps keep the heat more constant. Den

[Lopez Mike]

The thermal mass of fire bricks is amazing. My dad and I once ran a full sized Shay locomotive. He got old and didn't want to do it any more and I had other fish to fry so the museum turned over the operation to a new crew.

They did a number of fairly odd things but one was to decide to re-brick the firebox. Unfortunately, they built it up with the ends against the fire and the firebox wall. Ended up at least twice as thick! You could open up the firing valve (oil fired) and wait a half of a minute for anything to happen. And you could drop back to a spot fire minutes before parking and have lots of heat to play with. I suppose they liked to hear the safety pop. They had to tear it all out and do it right.

Mike

[fredrosse]

The boiler surface area is calculated based on the surface area receiving heat, which in the case of a VFT (firetube type boiler) this is the inside surface of the tubes. Inside diameter x Length x Number of tubes is the total tube surface area for a firetube boiler. Tubesheet area plus water leg areas are added to get the total heat transfer surface area.

[johnp]

Thanks Fred, so it looks like i have a 21.4 sqft boiler. should be ok for my little engine, i think.

[fredrosse]

That surface area sounds about right. The attached picture is my boiler going into the boat, 16 inch dia x 18 inches high, very similar to yours.

The base is a 24 inch square steel plate, 1/4 inch thick. I welded pipes to the base for water cooling, but have not needed this as the plate only gets slightly warm after several hours steaming. On the plate is 1/2 inch ceramic felt insulation, covered with standard firebrick, above that is a stainless steel fire pot, with 4 inch diameter burner port welded in (I use a big propane torch for heat). The stainless steel pot has a bottom liner with another 1/2 inch layer of ceramic felt, and a stainless steel guard plate above that to keep flame impingement off the ceramic insulation.

I think if I eliminated the stainless burner pot and had a fire directly above (or on) the firebrick, things would get considerably hotter, and the cooling tubes on the base might require water circulation. With wood firing, leaving a small amount of fine ash on the firebrick (1/2 to 1 inch thick) would provide a considerable improvement in the insulation from heat getting to the base plate.

Den's placement of a water tank directly below the boiler is the best option, assuming no wood hull frames can get overheated by proximity to the base plate.

[mtnman]

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