Increasing a VFT boilers output by increasing the firebox size

A special section just for steam engines and boilers, as without these you may as well fit a sail.
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DetroiTug
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Re: Increasing a VFT boilers output by increasing the firebox size

Post by DetroiTug » Tue Jun 11, 2019 1:21 pm

flared funnels (tapering from small to large - base to top) seems alot of traction engines, wagons etc seem to have them..?
I think the reason they did that is they were puffers exhausting up the stack. The engine exhaust went through a smaller nozzle aimed upwards, the flared stack created like a venturi effect progressively increasing in volume upwards.

On natural draft, the higher the better, I don't think tapering has much effect on natural draft.

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Re: Increasing a VFT boilers output by increasing the firebox size

Post by PeteThePen1 » Tue Jun 11, 2019 9:10 pm

Bear in mind that my skill is pushing paper not engineering, but Frances Ann has a tapered inner funnel.

What I have noticed is that the smoke does seem to 'rush' out of the top rather than drift when one is lighting up. The logic for the taper is that the steam dome intrudes into the funnel base. In order to maintain cross sectional area of the base of the funnel, the first 4 inches is single skinned 200mm diameter, then there is an inner funnel in the form of a cone that narrows from 200mm diameter to 150mm. The original 200mm diameter tube continues to the top meaning that most of the funnel is double skinned except for the bit at the bottom. This gives the required 'look' whilst making for a mostly cool outer skin.

Not sure if that will add anything useful to the discussion!
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Re: Increasing a VFT boilers output by increasing the firebox size

Post by fredrosse » Wed Jun 12, 2019 1:55 am

"Condensate temp should be as low as possible to provide maxiumum vaccum, so adequate cooling water should flow in the condenser and a vac pump to allow this."
ANS: A better way to express this: "Engine exhaust temp/pressure should be as low as possible to provide maximum engine efficiency, so adequate cooling water should flow in the condenser and a vac pump to allow this. Further lowering of the condensate temperature, condensate depression, should be minimized, from a thermodynamics standpoint.

Another issue, the fluid flow transport part of the design, is pumps having trouble with condensate having little to no condensate depression, very hot condensate, near the boiling point. In the utility industry, centrifugal condensate pumps are designed specifically to work with very low "Net Positive Suction Head", and typically the pump impeller is mounted at a low elevation, maybe 15 feet below the condenser hotwell water level. This is not an option for a small steamboat! Having some condensate depression to allow pumping water out of a steamboat keel condenser typically requires some condensate depression, typically only a few degrees is necessary, so the hotwell pump suction does not burst (flash) into steam when the reciprocating pump takes its suction stroke.

"Feed water should be preheated to (temp x) before entring the boiler wether this is done via exhaust heat exchanger or an econimiser or both depends on the systems figures."
ANS: That is correct

"What is the ideal "temp x" aka feed water delivery temp?
As hot as possible? Just before boiling? "
ANS: This is generally correct, from a thermodynamics standpoint. Again, from a fluid flow standpoint, getting feedwater very close to the boiler saturation temperature invites the occasional generation of steam bubbles in the economizer, with potential water hammer problems. Early in the electric power industry (more than 100 years ago) there were several instances of "steaming economizers", but presently the accepted economizer designs for large utility boilers do not heat the economizer outlet so very close to the boiling temperature. The utility industry has other means of squeezing maximum economy out of a boiler system, but that is another story.
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Re: Increasing a VFT boilers output by increasing the firebox size

Post by Ramón » Wed Jun 12, 2019 12:02 pm

Hey OP,

Excuse my late reply. Yes, it was my thought to use the coil you thought about as a water wall - as like a first part of the evaporating surface before going into the boiler vessel, which has the smoke tubes as the second stage of your evap surface. It would be a compromise for not needing to completely redesign the pressure vessel for a bigger fire box. But yes, you could say that coil would be an evap surface and you could still coil up some tubes in the flue gas stream as an economizer. Especially since it is not all too complicated to accomplish.
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Re: Increasing a VFT boilers output by increasing the firebox size

Post by DetroiTug » Thu Jun 13, 2019 1:35 pm

The utility industry has other means of squeezing maximum economy out of a boiler system, but that is another story.
Fred, When you have time I would be curious as to what that method is beyond feedheaters.

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Re: Increasing a VFT boilers output by increasing the firebox size

Post by Adamsmi1 » Fri Jun 14, 2019 7:26 am

Ron,

Sounds like a straight funnel is the way to go, I just need to work out the proportions so to maximize the high without it looking funny.

Fred,

I've been thinking of a condenser idea. The one i have at the moment is a keel condenser. My idea was to have an inboard condenser like a shell and tube heat exchanger with cooling water running through the tube and the condensate external to that. what you were saying about the condensate depression and needing a small amount to allow out piston pumps to pick up, this could be adjusted by creating a larger then needed condenser and plugging some tubes to restrict cooling water allowing the ideal condensate return temp. Thoughts??

Ramon,

I might experiment with this idea while im working on this.


Here are some specs on the boiler at the moment, Ill get some temps tomorrow.

funnel dia: 6" 28in^2
funnel height: 40"
base of funnel to top tube plate: 11"
Ash pan door x section area: 22.5in^2
grate dia: 13.5" (1/3 ratio gap to bar) 48in^2
grate to bottom tube plate: 12.5"

looks like ash pan door and funnel are the restricting places..
I know you want to keep funnel gas speed high and that needs a smaller dia but at what point does it start restricting??
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Re: Increasing a VFT boilers output by increasing the firebox size

Post by fredrosse » Fri Jun 14, 2019 12:10 pm

"......creating a larger then needed condenser and plugging some tubes to restrict cooling water allowing the ideal condensate return temp. Thoughts??"

ANS: Plugging some tubes will only reduce the intended function of the condenser, and result in higher engine exhaust pressure. If the condenser produces very little condensate depression, then the best option here to create some condensate depression is to have a very small heat exchanger, sea water cooled, thru which the hot condensate must pass on its way to the reciprocating pump.

For a typical launch engine, this heat exchanger could be about 12 inches long x 1 inch diameter copper sea water tube, with a 1/2 inch diameter copper tube mounted inside, carrying the condensate. This can easily be made with common copper plumbing fittings and soldered together. There are several other arrangements that can be made here, but far less complicated than a typical shell and tube HX. Sea water flow is parallel to the main condenser sea water flow, and can be throttled to limit the amount of condensate depression as needed.
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Re: Increasing a VFT boilers output by increasing the firebox size

Post by Adamsmi1 » Sun Jun 16, 2019 5:09 am

Fred, I realised when I did the Celsius to Fahrenheit conversion for these numbers i misread what the ideal condensate temp was (40°f) not celsius so i can see why plugging a shell and tube HX has no merit..

Heres some figures from a run i went on today,

Feedwater 108°f
Steam inlet 262°f
Exhaust 131°f
Condensate return 54°f
Funnel base 221°f
Funnel top 169°f

How do they look
I can see feedwater is a bit to be desired

Cheers, Adam
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