line lm u03b4t is approximately 90 u00b0 f and the gap has been diminished tp 0 002 inches this description of interface structure is only true when the smaller and internal hemisphere of the concentric walls is the hotter one bounding the heat source since it is highly desirable that the gap be maintained and that the walls do not touch even at high temperatures the design of the structure can be calculated to avoid contact it is also a design feature that the gap be substantial at room temperature to facilitate assembly and disassembly and that this gap diminish during high temperature operation to reduce u03b4t thereacross while ensuring that the gap walls never contact each other in this particular apparatus the reactor wall 12a should not exceed a temperature of 1500 u00b0 f and the tube temperature of wall 14a should be maintained at 1437 u00b0 f since the temperature drops across each wall each shown to be 24 u00b0 f for a heat transfer of 190 watts per square inch that leaves a u03b4t across the gap of a maximum of 61 u00b0 f if the u03b4t is greater then the tube temperature would be too low and if the u03b4t is less then the reactor wall might become too hot if the gap became closed then there could be thermal interface loading on the walls which is undesirable and furthermore there would be possible difficulties in disassembling the.