Funnel Diameter to Grate Area ratio

A special section just for steam engines and boilers, as without these you may as well fit a sail.
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PeteThePen1
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Funnel Diameter to Grate Area ratio

Post by PeteThePen1 » Tue Jun 19, 2018 6:36 pm

Hi Folks

Please could I pick your assembled brains? I am reflecting upon the design considerations when constructing a casing for a water tube pressure vessel. I recall asking a load of questions to a load of folk and incorporating their answers into the drawings for my own casing. I should, of course, have logged and indexed the answers and I could have looked them up now...

However, the question for today is whether there is a rule of thumb out there for stack internal diameter against grate area. One would assume that our ancestors would have worked that out. There must be a sweet spot between too small a flue that will not draw and too big a flue that does not really encourage the exhaust gas to get its skates on.

For the record, Frances Ann's flue size was chosen on the grounds of aesthetics. I worked out what size funnel looked 'right' and then worked back from there. Happily it draws quite well. Being scientific the ratio of flue diameter to grate area is 10.6%.

Regards

Pete
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Re: Funnel Diameter to Grate Area ratio

Post by gondolier88 » Tue Jun 19, 2018 8:25 pm

Pete,

The flue should generally always be tapered; something that locomotive and traction engine manufacturers cottoned onto early on. With a relatively small throat at the smokebox end the gasses are accelerated and help draw the gasses- especially with a properly installed blower/blast pipe arrangement. Ask any steam engineer and the answer will be always be different- some prefer a shallow taper all the way, others a steep taper and a parallel section- whether studies exist on the differing steaming properties of either design on a given boiler I'm not sure- but I'd say both work in a similar way.

As for diameter it relies on a few factors; length of flue- a longer flue will draw better and so a narrower throat and overall diameter can be used. Fuel- solid fuel requires a larger flue than gas or oil firing as the products of combustion contain large amounts of unburnt fuel which restricts the flue physically as well as making for a denser emission.

I've just done some quick calcs based on grate area and flue diameter [at throat] of boilers I have come into contact with and they all come in between 1:15 and 1:16 based on area of grate in sq.in and area of throat in sq.in.- this appears to be about right for the diameters of flue we deal with on typical launches.

Just to confuse matters, wood burning stoves and a few woodburning boats have flue dampers fitted to manipulate the flue gasses to suit the particularly fussy demands of burning wood in a large combustion chamber...

Greg
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Re: Funnel Diameter to Grate Area ratio

Post by barts » Wed Jun 20, 2018 5:22 am

One thing to remember is that any 'rule of thumb' should likely distinguish between boilers that force the draft w the engine exhaust, and those that rely on the density difference to provide draft.

- Bart
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Re: Funnel Diameter to Grate Area ratio

Post by DetroiTug » Wed Jun 20, 2018 1:33 pm

For the pounds per solid fuel per hour we run which is very similar to a wood heating stove for a house, a six inch diameter flue is standard and adequate. The Tug's boiler has a six inch diameter flue (inside the 12" stack) and with the forced draft, the firebox looks like a forge, without the forced draft, it still works fine, no trouble breathing, if in doubt use an eight inch.

I have a woodstove for a wall tent with a good size firebox and it uses a four inch flue and has very good draft. Six inch diameter is ample for our size boilers.

Greg, I calculated the tug grate area vs flue area and it is 1:11. 20" dia grate vs 6" dia flue.

Cross sectional area of 6" flue is 28" using the 1:15 that would be 420 square inch grate, around 15 X 30 grate - about right for a Robert's style boiler or Hereshoff etc and plenty ample for round grates. I think the 1:15 is a good standard formula.

-Ron
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Re: Funnel Diameter to Grate Area ratio

Post by PeteThePen1 » Wed Jun 20, 2018 6:34 pm

Thanks Gentlemen.

It is really useful to have that sort of information especially backed up with data from known successful boiler installations.

Regards

Pete

(I am amused that my own flue is not a million miles off compared to the examples, given that it was not scientifically derived.)
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Re: Funnel Diameter to Grate Area ratio

Post by fredrosse » Thu Jun 21, 2018 10:01 am

6 inch diameter is good for most steam launches. When flue gas pressure losses are governed by the boiler internals, then stack size could be oversize with almost no effect, the boiler, grate area, and air inlet damper govern.

On my propane fired boiler I use a 6 inch stack, 36 inches high, and it is quite a bit oversized. When steaming at full fire (about 90,000 Btu/hr) the stack temperature is 410F, and with a 4 inch orifice placed in the stack, the temperature goes up to 450F. That implies that the draft system has too little resistance to flow, and hence lets in too much excess air, quenching flame temperature.

The plan is to add an economizer coil in the stack, which will provide enough restriction to flue gas flow such that excess air is reasonable, but first I need to get a "round tuit".
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Re: Funnel Diameter to Grate Area ratio

Post by Kelly Anderson » Sat Jun 23, 2018 3:05 am

PeteThePen1 wrote:However, the question for today is whether there is a rule of thumb out there for stack internal diameter against grate area. One would assume that our ancestors would have worked that out.
Indeed they had, but your question is worded to ask for a comparison between apples and oranges. The question to ask is, "Is there is a rule of thumb out there for stack internal area against grate area."

For natural draft solid fuel burning boilers, the answer is that the ratio between total grate area and stack cross section area is 4 to 1 and 6 to 1 (if your total grate area is 100 square inches, then the cross section of your stack should be between 17 and 25 square inches, and so on). If your grate is round, the answer is simple in that a stack one half the diameter of the grate always has one forth of the cross sectional area.

As naturally follows, the area of the ash pan door, and the air area through the grate are also to be 1/4 to 1/6 of the total grate area.

For induced or forced draft boilers, the above doesn't necessarily apply, likewise for oil or gas fired boilers.
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Re: Funnel Diameter to Grate Area ratio

Post by PeteThePen1 » Sat Jun 23, 2018 10:59 am

Thanks Kelly

You can no doubt feel the heat of my blushes from where you sit. Indeed that is the correct question I was intending to ask, but was too obsessed with looking at various sectional drawings where diameter is the visible measurement.

Thanks too for the added data. That is really useful.

Regards

Pete
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