Getting Heat into the Boiler

It’s not easy getting heat into the boiler. It might be the losses are too great. Is the wood wrap enough of an insulation? I might have to measure the cooling curve and from this establish the heat loss to ambient.

I’m heating the small vertical boiler with a methylated spirits burner. This burner is being fed by a chicken feeder.


Maybe the boiler is losing heat too fast? I wrapped the boiler in a sheet of Teflon, then a sheet of thick paper and then a layer of oak.

Surface Area versus Volume

The total volume of this small vertical boiler is small, but with this the area is quite large. The key is probably the total heating area versus volume of fluid.

Heat Input Power

spirit burner

The spirit burner is a ring that has feet to raise it off the floor and so let air into the middle. It has a chicken feeder bowl and cooling fins soldered to the supply pipe.

I originally had cotton wool and gauze in the burner, I’ve now switched that to ceramic fibre and this works much better. The flame when the burner is not under the boiler is significant, I would say quite fierce.

Air Flow

The ability to burn a fuel efficiently is down to a number of things, one big factor is getting oxygen to the flame. The oxygen is in the form of air and to get air in we need to get the burnt fuel and gases out. This means a good draft is required. When I put my hand over the top of the chimney there is just a limited flow of heat, nothing like the heat output from the burner out in the open. If lots of heat was being transferred to the water then that might be ok. But it’s not getting hot that quickly. Therefore, I believe the flame is much reduced once under the boiler.

I need more air in and more air out.

Distance from Flame

annealing copper tube

The distance from the flame to the object being heated is important. I use this parameter all of the time when I’m soldering, annealing or forming.

So, why did I not really pay much attention to this when making the firebox?

Next steps

This calls for some tests to establish the optimum setup for the boiler, burner, enclosure and air flow distribution.

  1. Instrument the boiler with a thermocouple and a data logger
  2. Measure the cooling rate of the boiler to establish the heat loss rate
  3. Measure the heating rate with the ceramic methylated spirit burner in current configuration
  4. Modify air flow
  5. Modify distance from burner to boiler
  6. Thermal insulation

1. Instrument the Boiler

thermocoupled boiler

The boiler now has two thermocouples. One at the top in the steam region and one in the lower side in the water region.

I also have an ambient air temperature probe.

All data is being logged with a Pico Technology TC-08 thermocouple data logger.

2. Cooling Curve

boiler cooling curve

The cooling curve of an object is a great first measurement.

This will allow me to build a simple mathematical model of the boiler and from this estimate the heat loss. We will actually get heat loss versus temperature.

3. A Model of the Model Boiler

model of a model boiler

This simple mathematical model has allowed me to estimate the heat loss to be 35W and the heat input to be 54W when operating at 40psi.

This has given me a lot more data for the initial design of the boiler.

5. Distance from the Flame

power output versus burner to boiler distance

I built another firebox that would allow me to change the distance between the burner and the boiler. Then with the instrumented boiler and the model I could estimate the heat input.

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.

6. Thermal Insulation

comparison of unclad versus fully insulated boiler

Electrically heating the boiler has given me control over the boiler and allowed me to use combustible insulation materials.

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 Solution: a Steam Raising Blower

steam raising blower comparison

Two different designs of steam raising blower.

Both increase the heat input into the boiler from ~50W to ~350W. A seven fold increase is not to be sniffed at.

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