Unclad versus Maximum Insulation

The cooling curve and heat output of the Unclad versus Maximum Insulation boiler are compared. Previously we looked at the repeatability of the cooling curves and the known parameters for the boiler test object. Now, using the simple model that we discussed we can estimate the heat output of the test object versus temperature.

unclad boiler test object repeat

The cooling curves for the two repeat tests on the unclad boiler showed that the test was repeatable.

Knowing the heat capacity of the test object and the change in temperature we can estimate the change in energy. With the time between temperature measurements we can then turn that energy into a power.

Heat output of the boiler test rig unclad

The plot shows heat loss from the boiler in Watts plotted versus delta temperature in Kelvin. This delta temperature is the difference between the boiler core temperature and the ambient surrounding temperature. With the boiler test object unclad we see that we are losing ~14W at 120K which for a normal ambient of 20°C this is a boiler temperature of 140°C.

140°C is a good temperature point as this is approximately the operating temperature for the boiler at 50psi.

Maximum Insulation

For the maximum insulation the boiler was wrapped in a number of layers. Starting next to the copper shell and working out we have:

  • Teflon
  • Paper
  • Aluminium foil
  • Paper
  • Aluminium foil
  • Paper
  • Felt wrap
  • Foil bubble wrap
  • Felt wrap
  • Foil bubble wrap

The overall diameter of the wrapped boiler was 65mm and so just over 10mm of insulation.

When the test object core temperature was at 140°C the outer bubble wrap surface was measured at 60°C and the wooden supports registered ~45°C. This compares very favourably with all of the other insulation tests where the outer surface wrap always exceeded 90°C when the core was at 140°C.

Therefore, this maximum insulation test is quite effective in keeping the heat in.

The cooling curve comparison between unclad and maximum insulation is interesting. The test object is kept hotter for much longer when insulated. A benign, but important check. It should be noted that the heat capacity of a water filled boiler would be much higher than the test object. Hence the cooling curve would be elongated in time for both cases. However, the impact of the heat capacity is removed when we calculate the heat lost from the boiler.

cooling curve unclad versus maximum insulation

Heat Lost with Maximum Insulation

unclad vs max insulation heat output

The heat output with the maximum insulation is still significant. However, we now have an upper and lower bound to compare our engineering insulation ideas to. If we can get down below 10W of heat lost at 120K (140°C boiler operating point) then I think we will be doing well.

The boiler barrel in the traction engine has a length of ~80mm that will be insulated and for all of these tests the object is 50mm long. Therefore, all heat outputs can be multiplied by 1.6 to estimate the losses from the Burrell traction engine.

Burrell traction engine

Miniature Traction Engine

his is a work in progress around the research, design and building of a miniature traction engine, in particular a Burrell 6NHP traction engine. My son bought me this book for my birthday as I’ve seen a number of Cherry Hill’s models and they are inspirational. This book has made me think about my approach.

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