Flame to Boiler Distance

One thing that has concerned me is what is the optimal Flame to Boiler Distance?

The original firebox and methylated spirit burner was generating approximately 54W. At the operating point of the boiler at 40psi and 140°C the losses are 35W. So, there was not that much headroom for adding more water or taking a bit more steam.

burner to boiler distance

This small vertical boiler was designed to be the test bed for my miniature traction engine boiler. I must say that it has been a steep learning curve.

The simple mathematical model of the boiler gives estimates of the burner output. Therefore I could vary the distance and plot heat output versus distance.

K. N. Harris’ book he states that 1″ should be regarded as the minimum distance between the top of the wick and the boiler barrel. He states that 1.5″ is better (~38mm). My original boiler design was significantly less than this.

What I immediately noticed when doing this experiment was that the boiler heated up very rapidly. This was probably a mixture of air supply and burner to boiler distance. Hopefully this data set will help me explore this.

Test Firebox

test firebox for the vertical boiler

I needed a tall simple firebox that would allow me to vary the distance between the burner and the boiler.

My solution was to make this out of stainless steel sheet. This was quite simple to mark out and fold up.

There is a hole in the top with a raised lip onto which the boiler cylinder fits.

At maximum height this gives me a burner to base of boiler distance of 120mm, a significant increase over what Harris describes as the optimum. This means I should be able to change the distance and explore what is the optimum with data.

I described the instrumentation setup in an earlier post and I have kept the same setup here.

What is the burner to boiler Distance?

boiler and burner cross-section diagram

As a friend pointed out to me this should be rather simple as the hottest part of the flame is the blue inner cone tip.

Hopefully this rather simple diagram of the cross-section through the boiler will help to picture the problem.

As you can see the boiler and inner flue extend over quite a distance and so there might be a better optimum. I agree that this still might be the base of the boiler as that then means the flue removes all of the heat from the waste gas as it exits.

The distance I’ve used as a reference is the distance between the top of the burner case and the bottom of the boiler inner bulkhead.

The flame itself changes with air flow and surrounding conditions and so is not such a great reference to use.

Changing the Burner to Boiler Distance

I changed the burner to boiler distance by using a piece of 2″ wide steel plate that fits perfectly in the firebox width and then supports the burner and the feed tray. A series of pieces of wood were then used under the steel plate to change the height.

Naturally the air opening changes with the burner height.

Data and Results

Using the instrumented boiler I heated it up for each test distance from around 50°C water temperature to >120°C. Thus giving me enough data to estimate the heat input power. I also measured a new cooling curve for this firebox and boiler setup so that I got an accurate heat loss estimate versus temperature.

Once I had processed the data I could then plot the burner heat input to the boiler versus 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.

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