Optimising the Firebox and Burner

This won’t be the last, but optimising the firebox and burner has moved the boiler to a place where it can deliver steam to a steam engine. This has involved a number of changes to the system. Some backed up with data, some just good engineering.

Burner Wick

flame interaction with the boiler

The wick on the methylated spirit burner was just too long. This shows the flame produced when I was optimising the distance. I should probably go and re-run that experiment with a better flame.

At this point the wick protruded above the top of the burner.

The wick was cut flush with the top of the burner. This reduced the area and amount of fuel being fed to the flame. Immediately the flame burnt with less orange and more blue.

optimised burner

This now uses a ceramic matting for the wick. This works very well.

Fuel Feeder

The fuel feed is done with a chicken feeder. So a tank of fuel that drips fuel into a tray. The issue with the initial design was that the tray was very small.

This made it difficult to get the fuel feed rate correct. Essentially this tray is the reservoir for the wick and needs to be at a consistent level if it is to maintain a consistent flame.

The estimated heat into the boiler swings between 40W and 85W with the original design.

boiler estimated heat input
chicken feeder and burner

The new tray looks like an animal feed trough, but most importantly it is significantly larger than the original round tray.

In the photo you can see the original tray cut off and the new tray fitted.

Optimal Flame to Boiler Distance

power output versus burner to boiler distance

This plot shows that the peak in heat output is when the burner is around 85mm from the base of the boiler. Also, it shows that this firebox and burner is producing nearly double the amount of heat that the original design generated.

I checked this calculation by looking at the boiler temperature versus time data. The original firebox and burner took just under 8 minutes to take the water from 60°C to 100°C. The new firebox and burner took just over 2 mins 30s for this same temperature change. You have to consider heat losses and heat input, but this makes sense.

Data collection for this experiment was quite fun and the firebox I used for the experiment was modified to produce this optimised firebox design.

I settled on 60mm distance from burner to bottom of the boiler for this optimised design. A good balance between most heat output and looks.

Optimised Firebox

new and old fireboxes

On the left and behind is the firebox I started making to burn coal. The firebox on the right is a modified version of the original methylated spirit firebox. The one in the centre I made by modifying the the experimental one.

heat output versus time for the new firebox and burner vs original design
Heat input to the boiler plotted versus time for the original firebox and burner versus the optimised design
optimised firebox

I probably haven’t finished optimising the firebox and burner. However, this is a good design that is a huge step on from the original design.

This optimisation and design of the small vertical boiler is taking a lot of time. But I’m learning a huge amount about boiler design along the way.

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