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Discussion Starter #1
Anyone any thoughts on how to design a better alcohol burner.The usual Aster designs are a series of vertical tubes which are filled with ceramic wicks. They work well but don't make optimum use of the space in the rectangular firebox.

Reason for asking is that I changed the design of the burner on my Aster Challenger a few years ago. I replaced the tube burner with a rectangular box in which I inserted strips of ceramic cloth. It worked really well on rollers and came up to steam faster than the burner that was supplied with the machine. So back on the shelf it went......

But....

When I came to run the Challenger yesterday, I realised the new burner was fouling one of the axles on the rear bogie. This messed up the weight distribution and caused the center engine to slip badly. So I now need to build a new burner as I can't find the original which I stored 'somewhere safe'. I think another rectangular box design with an accommodation for the axle is the way to go but wondered if anyone had any other ideas.
 

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I would try the type that is in the Aster Daylight. It is the one of the best if not the best Alcohol burner I have seen. It could be built to clear the axle easily.
 

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Whatever you design, it must allow for the 'correct' mixture of air to match the 'wicks'.
Now what is that ratio?
I'm not sure, but there must be information on it somewhere.
The Accucraft Royal Hudson has practically the complete underside under the wick tubes is filled in, so one would assume very little air is needed as it burns very well.
Looking forward to hear how you get on, although sometimes it may not be worth trying to re-invent the wheel!
Cheers,
David Leech, Delta, Canada
 

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Robert,

Since the height of the wicks is a fixed ratio to the bottom of the boiler (with some small variance) for optimal combustion, and the original challenger burner was the right height without fouling the trailing truck axles, would it not make sense to raise your existing new burner to provide the needed clearance?

I believe I have a spare Challenger burner here that I can measure for you for dimensions if you want to get back to baseline on the way to mounts and the heights of things. For the sake of clarity, can you provide a few photos of your new design?

The original burner sat pretty much flush to the bottom of the rear frame area and the feed tube went into the burner at a downward angle from the feed pipe connection between engine and tender. The wick tubes roughly protrude into the firebox about 1/4" and the wicks an additional 10mm above that.

The ratios David discusses are two fold. Air entrainment and wick area. You need a lot less air than most think (30% is a good base reference) to achieve optimal combustion and keep the thermal efficiency of the fire up. Adding air is as simple as opening out the firebox or adding air entrainment holes into the fire area. Reducing air is accomplished by the plate at the bottom of the burner or adding a "choke" path to reduce the air velocity that comes up around the fire.

Wick ratio is the surface area of the wicks vs the surface area of the tubes. That is a crucial index ratio and ideally you want the wick index number to be slightly higher than the tube index number, especially on an engine with an axle pump or additional cylinders, like the challenger.
 

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Discussion Starter #5
Ryan/David, thanks for the input. It would be helpful if you could send me the dimensions of the original burner. I've built a five tube burner from 3/4 inch brass tubing. I raised the center tube slightly to ensure clearance to the rear bogies. I did a quick calculation - a rectangular burner presents about 25% more area than tubes so I am also going to build a burner Allegnehy style and compare the two.
 

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Discussion Starter #6
I found to original burner and now understand why I replaced it - it consists of 15 small tubes of 7.5mm internal diameter mounted on a 20 x 93 mm rectangular base. It may be very simplistic, but I would think that the burn is directly proportional to the area exposed. 15 x 7.5mm tubes - 663 sq mm area. If I can expose the entire rectangular area, I get 20 x 93 (minus wall thickness) = 1638 sq mm. In other words an open rectangular burner exposes over twice as much area to burn so should put out twice as much heat assuming it gets enough air.

Not sure why the burner was designed this way.....

60414
 

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Not too sure if the wicks as seen in the photo will burn very well.
I would think that the concept of 'many burner tubes' is to allow air to flow between them so that each individual wick gets sufficient air.
I would think that with the wicks interfering with each other you are blocking any air from passing.
Is that how the instructions told you to make the wicks?
Regards,
David Leech, Delta, Canada
 

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Interesting, first I would think that the "burner area" is the surface area of the wicks.

But you apparently calculated the 663 sq mm area based on the tube diameter? Maybe you were equating the surface area of the wick to the surface area of the tube?

Just curious. I think I would have estimated conservatively 8 mm diameter and 10 mm tall wick area times 15, which would be 8 times pi times 10 times 15, where I get about 3,800 sq mm of surface area... yes I do see that the wicks can touch each other, but I think that may be offset by the increased area of a wick over the smooth surface of a tube.

Again, just curious, and I may be way off in my reasoning.

Greg
 

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Discussion Starter #9
Not too sure if the wicks as seen in the photo will burn very well.
I would think that the concept of 'many burner tubes' is to allow air to flow between them so that each individual wick gets sufficient air.
I would think that with the wicks interfering with each other you are blocking any air from passing.
Is that how the instructions told you to make the wicks?
Regards,
David Leech, Delta, Canada
I don't think the current state reflects the way I set up the wicks in the first place - they were neatly bunched into each of the tubes and about 1cm high.

Robert
 
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