Sizing the safety valve

[fredrosse]

Before building the boiler for my boat, a test setup was arranged to see just how much steam evaporation could be expected from a single firetube.

On e-bay I got a 20 jet propane WOK burner, with individual brass jet burners screwed into a cast iron ring. At 4 PSIG gas pressure this assembly would put out 220,000 BTU/hr, about 11,000 BTU per jet. The brass burners resemble the tip of an ordinary propane soldering torch. The assembly would stay lit, because the burners pointed in all directions, and kept each other lit. Individual burners would not allow such high gas pressure, and the flame would blow off the burner tip. Sawing a small prong (similar to the side electrode of an ordinary spark plug would allow reasonable gas pressure (10- 15 inches water column) without blowing off the flame, but still could not go to 4 PSIG gas pressure.

I made a single vertical tube boiler, firetube was 1-1/4 in OD x 0.095 wall (1.06 in ID), 18 inches long, steel. The shell was a piece of 3 inch diameter conduit, which could be filled with water, and the single firetube would evaporate the water. A single jet burner was mounted on a jig that allows an adjustable burner distance to the firetube. Tests were conducted with the jet flame entirely inside the firetube, not impinging on the tubesheet or tube to tubesheet joint.

I ran the jet at 4.2 inches water column pressure, giving about 2150 BTU/hr propane burn rate. With this setup the temperature at the outlet of the firetube was about 1100F, with poor steam generating capability.

Making a stainless steel coil spring, inserted into the firetube from the top improved performance considerably. This coil spring glows red hot at the bottom, improving radiant heat transfer to the tube, while the spring also breaks up the flow inside the tube, creating turbulence and mixing of the hot gasses, which also improves heat transfer. The coil spring is about 3/ 4 inch diameter, coil pitch about 3/ 4 inch, and 15 inches overall length, made with 1/ 8 inch stainless steel wire. Outlet temperature went down to 360-380F, and the heat absorption in the single firetube was 1800 BTU/hr, producing about 1.86 pounds per hour (PPH) of steam. This setup would give about 90 PPH steaming capacity for my boiler, having 48 firetubes. With an economizer coil above the boiler, even better economy (and more steam generation) could be had.

As it turns out, in my rush to get the boat underway last month, I just used a big propane weed burner under the boiler, but more tests will be made on the individual jet burners, they are much more quiet than the roar from the weed burner.

[fredrosse]

Picture of WOK Burner ring and individual burner jets

[87gn@tahoe]

My father made "Turbulators" from strips of stainless steel which were twisted (forget the rate of twist), inserted in the firetubes, and hung in groups of 4 or 5 by a steel rod running through a hole drilled in the top of each. They do an outstanding job of breaking up the laminar flow and aiding in providing more output from the Semple VFT40 in my father's boat.

Before installing them, the boiler was woefully inadequate for our engine.

[Wearyman]

Fred,

I don't know where you live, but if you are in the States you might want to try this http://www.tejassmokers.com. The owner manufactures some pretty impressive burner heads meant to run both propane, and Nat Gas for heavy-duty barbecue and smoke pits. He also sells all the accessories and parts., Apparently he's done some fabrication work for guys making Steam boat boilers before. (He sent me photos of a really nice all-copper water-tube boiler that he fitted a special propane burner for.)

Here are the Cast Iron Brass tipped Jet Burners he makes for Propane use: http://www.tejassmokers.com/newproducts_page6.htm

I love the quote at the top of that page:

These high heat, cast iron jet burners can produce up to 432,000 BTU of heat per square foot of burner area, require no adjustments, and develop temperatures of up to 1900 degrees F. Roofing contractors use the big cast iron jet burner as a roofing tar pot heater and asphalt heaters ! These burners are also used on live steam trains, back yard trains, maple syrup evaporators, evaporators, and professional steam cleaning machines.

So I'd image one of them would work for you.

I should note that I have no connection whatsoever to this company, I just happened to find them while Googling around for something else, and ended up in an e-mail conversation with the owner regarding the use of his equipment for steam. As far as I know he's not a "steam man" himself, he's just had steam enthusiasts as customers before.

[fredrosse]

I am in the USA, and have looked at the Tejas Smokers Burners. These jet burners are the same as the ones on the WOK burner pictured previously, but the 100,000 BTU Tejas, with 10 jet nozzle, is $95. The 20 jet WOK burner is $40, quite a price reduction, double the number of burner jets for less than half the price.

[mtnman]

I am in the USA, and have looked at the Tejas Smokers Burners. These jet burners are the same as the ones on the WOK burner pictured previously, but the 100,000 BTU Tejas, with 10 jet nozzle, is $95. The 20 jet WOK burner is $40, quite a price reduction, double the number of burner jets for less than half the price.

Check your local Restaurant Supply House or used restaurant equipment dealer. This is a common accessory in a Chinese Restaurant kitchen.

[Wearyman]

I am in the USA, and have looked at the Tejas Smokers Burners. These jet burners are the same as the ones on the WOK burner pictured previously, but the 100,000 BTU Tejas, with 10 jet nozzle, is $95. The 20 jet WOK burner is $40, quite a price reduction, double the number of burner jets for less than half the price.

Well, granted, the price is cheaper. But if you compare the units side by side, you can see that quite a bit more engineering went into the tejas burners than the wok burner, thus the price difference.

Of course, I have no idea if that even matters in a steam boiler application. For all I know the simple ring is the best design. However I would think that if one is using this in a firetube boiler, then having a more even heat coverage would be preferable. I would guess that you end up with some "cold" spots using the simple wok burner, whereas a more complex burner would spread the fire out a bit more and heat more evenly.

I'm just trying to imagine the insane column of flame that the 19 inch 88 jet unit puts out. That's an INDUSTRIAL size burner right there.

[Wearyman]

On a less burner related topic, I noticed your note about adding a large "spring" coil to the inside of the firetubes. I have been thinking a bit about firetubes, and I've wondered why the tubes always seem to be straight. It would seem to me that adding a bit of a spiral to the tubes would slow down the exhaust gasses a bit and allow for more effecient heat transfer.

To get an idea of what I'm talking about, imagine your basic cylindrical firetube boiler. Now remove the firebox and stack, and slide off the outer casing so that you only have the top and bottom plates and the tubes welded to them. Now imagine two giant hands taking the bottom side in one hand, and the top in the other, and giving a slight twist to them, like twisting a pepper mill or a bottle cap. Nothing extreme, just a slight twist, resulting in an elongated spiral pattern to the tubes.

Wouldn't that pattern give much better heat absorption than the traditional straight tubes? While certainly more difficult to manufacture, if done at the optimal bend rate (whatever that is) it should dramatically increase steam output, shouldn't it?

[farmerden]

Maybe just the thought of trying to retube the boiler stopped anyone from trying that! Den

[dhic001]

My father made "Turbulators" from strips of stainless steel which were twisted (forget the rate of twist), inserted in the firetubes, and hung in groups of 4 or 5 by a steel rod running through a hole drilled in the top of each. They do an outstanding job of breaking up the laminar flow and aiding in providing more output from the Semple VFT40 in my father's boat.

Before installing them, the boiler was woefully inadequate for our engine.

We call them retarders. In the boiler that used to be in the steam launch Puke, there were twisted strips of flat steel inserted in each tube with the t piece at the top to prevent them falling through. Without them, she wouldn't steam anywhere near as well as she would with them. However, she had 2 inch OD tubes, Zeltic at 1 1/2" OD tubes doesn't have them, and in fact having them reduced performance, causing greater use of the blower etc. So our conclusion was that larger tubes should be fitted with retarders, smaller ones don't need them. It may be a different story if using forced draft, or with smaller funnels, both Puke and Zeltic have 6 foot high funnels or there abouts.

Daniel