Increasing a VFT boilers output by increasing the firebox size

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Increasing a VFT boilers output by increasing the firebox size

Post by Adamsmi1 » Thu Jun 06, 2019 8:55 am

Hey everyone,

I'm looking to get a few more pounds out of my boiler and I had the idea of making a larger firebox to sit under the pressure vessel.
I run on saltwater a lot so a stack blast of exhaust or boiler steam isn't feasible



Would there be any negative knock on effects of this that i cant see?
Would the "gas passage" through the tubes restrict the flow and not allow any significant advantage?

Has anyone ever run the economizer around the outside of the firebox with any luck? you could hide all the pipework behind insulation and lagging, although having it in the stack has the same advantage :lol:

Its a 16in dia by 16in tall pressure vessel with 120 3/4 copper tubes
I usually fire with hardwood so a larger box would be good I feel

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

Post by fredrosse » Thu Jun 06, 2019 3:12 pm

In boiler jargon, an economizer is a feedwater heating device that gets its heat source from the flue gas that has already passed through the main boiler evaporating (boiling) section, and captures heat that would otherwise pass up the stack and be wasted. Hence the term "economizer".

Putting a coil around the furnace in a VFT is providing, again in boiler jargon, a "Waterwall", commonly used on all large utility type boilers. Entering feedwater can pick up significant heat in this coil, especially with solid fuels or oil firing, where the fire radiant heat is most prevalent.

This arrangement, with pumped feedwater flowing through the coil surrounding the furnace has been done many times, as a matter of fact my Domestic Heat-Power Module used just such an arrangement, the tubes were 3/4 inch OD x 0.065 inch wall steel tubing.

A few technical points need to be considered:

The intensity of the fire will burn out this coil if it is allowed to boil away all its water and become dry. Having the burner coupled to the feed pump, with both operating in unison works well, but with a hand fired furnace this would be risky business.

This type waterwall arrangement will most probably result in significant steam generation within these coils, and a two phase mixture (steam and water) will issue from this coil at relatively high velocity into the boiler drum. This is generally OK, but fluid flow transients such as "chugging" and waterhammer will be present. The severity of this issue is hard to predict, but I had no problems with it on my boiler.

If your new coil is placed in the way of flue gas flow (as opposed to a coil just surrounding the furnace), then the potential exists that your new tubing will interfere with proper flue gas flow. Of course this arrangement can be made so that it restricts proper flue gas flow, or arranged such that its restriction is acceptable. Addressing this issue would require a drawing to allow evaluation of the flow restriction.
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Re: Increasing a VFT boilers output by increasing the firebox size

Post by Adamsmi1 » Fri Jun 07, 2019 12:34 am

Cheers Fred,

Sounds like a tube water wall could be too risky..

Could you counter act the flue restrictions by increasing the funnel dia and/or smokebox height?
Is there a rule of thumb for determining how many ft2 the economizer needs to be?

Ill have a sketch and post some photos of the ideas soon..


So far on my list of mods to gain a little more performance are:

1. economizer
2. skin the smoke box & funnel
3. vacuum pump
4. turbulators
5. larger furnace

Are there any other mods you have on your steamer?

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

Post by DetroiTug » Fri Jun 07, 2019 3:18 pm

Not sure if it was intentional or not, but I calculated the heating surface of your tubes and it works out to 31.416 sq ft which is 10 X pi. Weird coincidence.


Quoted:

1. economizer (Yes the more the merrier, as long as it isn't interfering with steam generation)
2. skin the smoke box & funnel (Absolutely, a boiler without insulation is like us having no coat in the winter)
3. vacuum pump (On a compound LP, yes)
4. turbulators (With that size tube, you may not need them, and may serve to only restrict, that size tube is for oil firing, maybe that is your intention)
5. larger furnace (depends, a bigger fire is always better, but it depends on whether there is adequate draft and draw to accommodate)

On our liquid fueled steam cars if we push the burner too hard, the flame will come back out the bottom side of the burner through the secondary air holes or they go to yellow flame and soot, inadequate primary air. Everything has a limit.

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

Post by Adamsmi1 » Sat Jun 08, 2019 7:52 am

Cheers Ron,

Odd coincidence indeed haha

My tube size is 3/4"
Ive read of people using tubulators only to the water level

I believe the boiler was designed for hard firing, I guess oil could be used too tho.

Is there a way to work out how much draft you can get through the tube passage and proportion the ash pan door, grate and funnel accordingly?

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

Post by fredrosse » Sat Jun 08, 2019 10:51 am

This is a rather long and boring presentation, so most steamboaters should skip to the next entry on this thread.

In terms of thermodynamics, the low end temperature of the cycle is very relevant to plant efficiency. Fundamentally steam plants should have their low end temperature (exhaust condensing temperature) as low as practical. Large steam power plants try to run at a condensing temperature of, say 100F (corresponding to about 1 PSI absolute pressure, or 28 inches mercury vacuum). Properly designed utility condensers hopefully will give condensate water to the condenser hotwell at this temperature, but in reality the water exiting the hotwell is somewhat cooler, say 99F. This is called "condensate depression", and it is bad from a thermodynamics standpoint, as whatever amount of condensate depression is present, then the feedwater heating and boiler system must somehow supply the heat to compensate for this theoretically unnecessary condensate depression.

In our condensing steamboats, the condensing temperature (cycle low end temperature) is typically higher, say 20 inches mercury vacuum, corresponding to a condensing temperature of 161F. Most keel condensers produce much larger condensate depression, say 20F (often even more than that), so the water to the hotwell comes in at 141F.

The worst case is for a puffer, exhausting its engine steam directly to atmosphere. Here the “low end temperature” is 212F, but typically a puffer will draw feedwater from a lake, at, say 72F. In effect this is the same as a condensing plant operating with a condensate depression of 212-72 = 140F. Theoretically we could send 212F feedwater into the boiler, but the simple steam plant sends in 72F feedwater. The extra boiler heat (from the boiler/fire) needed to compensate for this relatively cold feedwater input is called “Condensate Depression Heat Loss”.

In the simple puffer steam plant, running at say 100 PSIG steam pressure, this effective condensate depression results in the boiler needing to produce almost 14% extra heat, this is significant extra load on the boiler. So here we can see the potential to recover most of the Condensate Depression Heat Loss, by passing the cold feedwater through a heat exchanger which is directly connected to the engine exhaust line. While heat exchangers need some temperature difference to produce heat flow, the condensate can be reasonably brought up somewhat close to the effective condensing temperature with a relatively small heat exchanger. This is named an “Exhaust Steam Feedwater Heater”.

At the other end of the spectrum, the large utility steam plant, the condensate depression is only 1F, the Condensate Depression Heat Loss is only a very small fraction of boiler load, and the heat exchanger needed to recover much of this condensate depression loss would need to be enormous, due to the only minuscule temperature difference needed to drive the heat transfer process. Utility plants therefore never use such feedwater heaters, they just try to make sure that condensate depression is maintained at a very low temperature difference.

In between these two extremes, the simple puffer plant vs. the large utility plant, lies the condensing steamboat plant, with its Condensate Depression Heat Loss somewhere between the very small loss that would never justify an Exhaust Steam Feedwater Heater, and the puffer that can benefit immensely from such a heater. The prudence of having an Exhaust Steam Feedwater Heater on the condensing steamboat plant depends on its numbers, so what is the condition on your steamboat?
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Re: Increasing a VFT boilers output by increasing the firebox size

Post by cyberbadger » Sat Jun 08, 2019 3:01 pm

fredrosse wrote:
Sat Jun 08, 2019 10:51 am
This is a rather long and boring presentation, so most steamboaters should skip to the next entry on this thread.
I read all of it and I appreciate you talking your time. Nyitra is a puffer, and her firebox of exactly 16" ID, so I'm watching this thread with interest. I have been considering putting in an economizer coil for sometime.

Thanks Fred!

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

Post by Ramón » Sat Jun 08, 2019 11:05 pm

Hello

I am not totally sure if your boiler has a wet or dry fire box. To vertically enlarge a dry firebox with no increased grate size is not worth the hassle.

You could do what you described and use a coil, but this would be more like an evaporating coil like in a WT. but I would NOT place a coil outside a dry fire box, between the fire box wall and the insulation as you described. Because of one point: it would work as a radiation surface only. That sounds good in theory, but the fire box wall between the radiation of the flames and the coil would effectively marginalize the radiation heating effect. Instead, you could remove the dry fire box wall and use a coil as a water wall as mentioned above.

If you plan on having a bigger fire, you have to redesign the smoke tube layout in any case. Using a wet fire box would be a much, much better condition to increase the output by raising the roof of the fire box.
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Re: Increasing a VFT boilers output by increasing the firebox size

Post by DetroiTug » Mon Jun 10, 2019 9:29 pm

Is there a way to work out how much draft you can get through the tube passage and proportion the ash pan door, grate and funnel accordingly?
Yes, there are formulae for just about anything on a small steam plant, but my experience has been, experimentation is the only method that provides a truly accurate result. The cross section of your 120 3/4 inch tubes is 53 square inches. That is similar to an 8 inch diameter stack of 50 square inches. However there are different levels of restriction in different size round tubing. Forced draft or even atmospheric draft and velocity can have an effect on the result. I'd guess around 7 inch diameter is equal to your firetube flow. Refer to back to my initial suggestion on experimentation. If I had a nickel for every time I thought a change would work and it didn't, I'd have lots of nickels. Mostly in the area of vaporizing liquid burners, the arch nemesis of the steam hobbyist. ;)

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

Post by Adamsmi1 » Tue Jun 11, 2019 8:26 am

Fred thanks again! I too appreciate you sharing your knowledge.

Okay so let me try my best to explain back to see if i understand.

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.

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.

Ill be taking the boat for a run with some new coal i have on the weekend and ill get some numbers with an infared temp gun.

What is the idea "temp x" aka feed water delivery temp?
As hot as possible? Just before boiling?



Ramon,

The boiler is a dry leg, im aware water legs can generate alot more steam.

Do I understand correctly your idea for creating a "water leg" by coiling tube in the firebox area and using is as a steam generator and then plumb into the boiler?
A sort of retrofitted water leg thats more part of the boiler then an econimiser?

Ron,

I have a healthy supply of scrap materials from work so experimentation is welcome!
The stack is 8"

Have you, or anyone for any matter had any experience with flared funnels (tapering from small to large - base to top) seems alot of traction engines, wagons etc seem to have them..?

Thanks everyone!
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