Problem with small boilers is surface to volume ration is bad. A lot of radiating surface with a small volume.
Making some big assumptions I've made a very simple mathematical model of the boiler
Then using the cooling curve data I've estimated the heat loss to be 35W at the operating pressure of 40psi (140°C).
This then allowed me to look at estimating the heat input from the first design of the methylated spirit burner. It is a bit variable, but probably expected as it uses a chicken feeder and there is some air flow.
The heat input for this first design is around 54W. This is why I was seeing a very marginal capability for this boiler.
I've now run a few experiments to understand the optimum distance between the spirit burner and the boiler. Normal practice is that the hottest point is at the tip of the inner blue flame, however, there is a depth to the boiler surfaces as shown by this diagram.
So, using the simple assumptions, a new firebox and some more measurements on the instrumented boiler I get the following curve:
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.
I wrapped the boiler and electrically heated it so I could estimate the heat loss. This way I could then compare unclad to maximum cladding.
At the designed operating point of 40psi and 140°C bulk water temperature the heat loss unclad is ~35W and with maximum cladding it is around 12W.
The cladding is a lightweight fleece, then a aluminium coated bubblewrap.
This has made me realise that I could make a low voltage miniature immersion heater - I will post something on this.
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