THE PHILOSOPHY OF INSULATION

Application as the insulating jacket for a Steam Balloon or Steam Airship poses very demanding requirements for an insulating material, because of the extremely severe demands for lightness in weight characteristic of aeronautical applications, coupled with the very large areas to be insulated. Moreover, the material must be flexible. The idea of insulating the envelope of a hot air balloon has often been considered, but these demands are even more severe in the hot air case - so severe that satisfying them is completely out of the question. It is just not possible to provide enough insulation upon a hot air balloon envelope to be worthwhile in performance terms, without weighing it down to such an extent as to be impracticable. In the Steam Balloon case, on the other hand, it is just possible to reach a satisfactory compromise, albeit with great difficulty....

The most soundly based and philosophical source of knowledge that I have been able to find anywhere, Internet or library, upon the subject of insulation has been the website of a company that makes insulation materials for yacht refrigeration, among other applications: Glacier Bay.

This page on that site is a very scholarly discussion of the various mechanisms of heat transfer and the thermal conductivities of various materials. The author makes a very telling point: Since nearly all traditional insulation materials have a higher thermal conductivity than air, as measured in the conventional manner, one might reasonably ask, "Why use insulation at all?" The answer of course is that any volume of air larger than a very small one is extremely prone to internal convection.

In fact, all the conventional high-performance types of insulation material really use the solid material itself merely as a means to entangle as much air as possible as intricately as possible, so as to produce a sort of blanket of immobilized air which acts as a very effective barrier to heat transfer. Any insulation light enough to be of any use upon a Steam Balloon will inevitably function in the same manner; there is no other way (short of creating a vacuum).

Now the conventional methods of measuring insulation performance, which speak of "R-values" and liken the flow of heat to the flow of electricity in conductors, are not really suitable for the assessment of the Steam Balloon situation, and are somewhat suspect in any case; see this page from Glacier Bay's website which is a devastating critique of standardized insulation testing, and this page which lists "Claimed R-value" against "Actual R-value" for various materials from various manufacturers. (Particularly amusing is the passage about "sucking the air out with a straw"!)

Moreover, in conventional discussions of insulation R-values, they are assumed to be additive in the same way that resistors are additive in an electrical circuit. That may be true for the ideal case of blocks of solid material sandwiched between two test plates of infinite heat conductivity, in which case the classical heat conduction equation is valid. But the layer of air on the outside of a hot-air balloon, for example, does not have a R-value of this type - that model is too simplistic. That is to say, suppose that the outside temperature is 0oC, there is no particular reason to think that the heat loss from a hot-air balloon whose internal temperature is 120oC will be twice that from a hot-air balloon whose internal temperature is 60oC; certainly it will be greater, but the outer layers of air will behave in completely different ways in the two cases, and the ratio of the heat losses per square meter will certainly not be 2. Intuitively one thinks that it must be greater than 2, but really the only way to find out is by experiment...

And the same will be true, to a lesser degree, for any layer of insulation material in which internal air circulation (convection) is a significant factor. Consider the case of a layer of conventional plastic bubble wrap. Most of the heat conduction through this layer will be via convection. There is no a priori reason to think that the amount of heat energy per second which passes through it will be proportional to the temperature gradient between its two sides.

To revert to the electrical analogy only to dismiss it, these materials do not behave like pure "resistors" when "voltage" (temperature difference) is applied; they act more like solid-state or other devices which do not obey Ohm's Law.



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