Multiple flues vs a single flue

[cfellows]

I'm looking at SandyC's 3" vertical boiler plans and have a question. His plans call for a single, 1.075" center flue. If I were to use 3, 5/8" flues, I would get 75% more wall area with about the same overall volume of flue tube. Seems to me this would greatly increase the amount of heat transfer without taking up any extra boiler volume. Is there any downside to using more, smaller flue tubes?

Chuck

 

[Blogwitch]

Chuck,

Whereas Sandy's vertical boiler uses one large tube with lots of little cross tubes, my commercial Cheddar boiler looks almost the same on the outside, but has about ten vertical 3/8" tubes (can't remember the exact amount) with no cross tubes in them.

This is the one I said I would photograph for you if you needed me to. But you said you were going horizontal.
They are usually very fast steamers, and raise steam in at most 5 minutes from cold, or even faster if you take hot water in a thermos with you to the lake.

Basically they are very easy to make, one boiler tube and four flanged ends, all the same except for where the holes are drilled. Two are silver soldered into the boiler tube, the two end ones are fixed in position with about four small screws around the periphery at the top and bottom.

Methinks I need to get out there with my camera now.


John

 

[Jasonb]

Were you still thinking of any cross tubes or just straight flues? Without cross tubes although the area is increased in the medium sized tube the gasses will tend to want to go "straight up", this is where Bogs larger number of smaller tubes come in as the flow is slower giving more time to heat the tubes.

Don't know what you are firing it on but if the main flues get too small when using cross tubes you can get a soot buildup, not a problem with gas firing.

J

 

[Tin Falcon]

Chuck : I think you would be fine but I am no expert.
I think the KN Harris books has some of the best info on boilers.
http://ebookbrowse.com/model-boilers-and-boilermaking-1976-pdf-d90169407
tin

 

[Blogwitch]

Rather than send the pictures direct to Chuck, it might be a bit more informative if I show them here, just to show how easy a vertical boiler build can be assembled.

This is a commercial vertical boiler built by Cheddar and it is well over 20 years old and has had much over 1,000 hours steaming on it.
Sorry the writing is so small, I couldn't get it to adjust the size in my basic paint program

It utilizes four flanged plates, 16 x 3/8" firetubes and a boiler tube in it's main make-up.

This is showing where I have removed the safety valve, steam take off pipe and taken out the four self tappers that hold the end flanges on.
The ceramic gas burner is screwed onto the bottom flange to hold it in the correct position.

This shot shows the straight thru make-up of the boiler tubes.
As Jason stated about gas burning clean, this boiler has NEVER been cleaned out in the burn area during the whole of it's working life.

This is the top collection area at the top of the boiler showing the two take off's. One for the safety valve, the other for the steam to engine.

They each stand up from the top soldered in plate by about 1", so making the position of that soldered in one about 3/4" from the top, as they protrude from the top loose fitting flange plate by about 1/8".

The burner soldered in plate is 1 3/8" in from the boiler tube end.

Only four entry points into the boiler on the side. The left hand one is for a gauge, the middle is if you want to fit a clack valve for feeding in water and on the right, the two water gauge fittings.

I have made many boilers to this design from 2" diameter upwards to 4", and all have steamed very freely. This one went from full cold to ready to power the engine (45 psi on gauge) between 4 and 5 minutes, depending on how cold the weather was.

I hope this had made things a little clearer.


John

 

[shred]

Thanks Bogs. That makes it a whole lot clearer for me.

 

[JorgensenSteam]

Great pictures.
Thanks

 

[steamer]

Nice write up Bogs!

Dave

 

[cfellows]

Hey John, thanks for posting the write-up. I've received two pieces of 3.125" diameter copper pipe, each with an .080" wall from Crab. I think this will be more than adequate for my boiler. I've seen you mention a good silver solder for boilers somewhere, but can't remember where. Can you refresh my memory?

Thx...
Chuck

 

[Blogwitch]

Chuck,

IMHO, that tube you have acquired will be perfect for a vertical boiler. Have you enough to flatten out to make the four boiler flanges?


It can be very difficult giving you UK equivalents as then everyone in the US then starts singing on about percentages of silver and percentages of other things, totally confusing the issue.

We tend to make everything a lot easier to understand. Go into an engineering supply and ask for easyflo silver solder, maybe two questions will be asked, what diameter? what melting range? but invariably that is very rare, they normally just ask how many rods do you want. Unless you are doing a fairly complicated assembly, you can forget about different melting ranges as once the solder has been melted and solidified, it takes a much higher heat to remelt again.

So here goes.

Normally, for small boiler making we tend to use easyflo silver solder, plus I have found that a good stainless steel flux is perfect for keeping everthing wetted as the solder flows. So for flux, I personally would use Tenacity#5. It carries on working when standard borax fluxes have given up.

99% of my silver solder work is done using pallions (small individual pieces put around or along the joint) rather than feeding from a rod. Silver solder is getting very expensive nowadays, and I have found that hand feeding can be very wasteful. In fact for most of my work nowadays, I use 0.5mm wire or 0.003" sheet, which I cut the pallions from. I can gauge the amount of solder required very accurately indeed by using that method, but I think standard rods now come in 1.5mm diameter.

I think Stew posted the making up of his loco boiler on here, and I helped him solder that up. It shows the techniques used very well on that post, especially about putting the boiler tubes and stays in.

For a vertical boiler such as this, I always soldered up the two soldered end flanges to the tubes first, then inserted the assembly into the boiler tube, then did the flange soldering with heat from the outside of the boiler. Boiler bushes etc can go in at the end, because you only use local heat for that job and it won't remelt the main joints.

John

 

[shred]

Chuck-- US source for Easy Flow silver solder (the stuff Bogs refers to above) : http://sra-solder.com/product.php/6153/89
Don't bother trying to source it locally. The greenies have banned the sale of Cd-bearing silver solders in this town and the stuff the welding places want to sell isn't the same.

Alas, I've not located a stateside source for the Tenacity #5 flux, but the ones there do reasonably well.

 

[steamer]

Chuck

Check our the manufacturer "Harris". They have cadmium free products under the "Saf-T-Silv" product label

I believe 45 and 56 are suitable .....I think we went round-y-round with this on the forum once before....

Dave

 

[doubletop]

On the subject of flux. I had a bit of a conversation on fluxes with John recently as the UK guys recommend tenacity 5 and it isn't readily available in NZ. I've been using this with great success, Since using this I've never had a failure, balling of solder etc. It just goes on and on however much heat you use, and its great for stainless.

http://www.harrisproductsgroup.com/en/Products/Alloys/Brazing/Fluxes/Stay-Silv-Black-Flux.aspx

Its a USA product

Pete

 

[steamer]

Hey Pete,

YUP...I've used the white flux and the black flux. The black flux is great for really large parts...but for small stuff, the white is just fine.

Put the bottle in a ziploc back between uses and it won't dry out on you. I can never get the bottle to seal completely once I open it, but the ziploc bag trick works great.

Dave

 

[doubletop]

I found if it drys out at all, a drop or two of water and mix it up well and its fine

Pete