We agree that heat transfer is always from hot to cold , right ?
Now let s try to clean something about ovens and insulation .
Lets start with Newton ....................
The rate of heat loss of a body is proportional to the temperature difference between the body and its surroundings.
Heat transfer................we are dealing with this three ways of heat transfer
Conduction .......
heat is transferred by conduction when adjacent atoms vibrate against one another, or as electrons move from one atom to another. Conduction is the most significant means of heat transfer within a solid or between solid objects in thermal contact
I can imagine how hard it will be to vibrate atom to jump from one fiber to other fiber in kaowool .........in bricks ?
Convection.....
is the transfer of heat from one place to another by the movement of fluids, a process that is essentially the transfer of heat via mass transfer. Bulk motion of fluid enhances heat transfer in many physical situations, such as (for example) between a solid surface and the fluid.
There are lot of air inside 128 kg m/3 kaowool , don t you think ?
Radiation.........
Thermal radiation occurs through a vacuum or any transparent medium (solid or fluid or gas). It is the transfer of energy by means of photons in electromagnetic waves governed by the same laws.
Thermal radiation is energy emitted by matter as electromagnetic waves, due to the pool of thermal energy in all matter with a temperature above absolute zero . Thermal radiation propagates without the presence of matter through the
vacuum of space.
Thermal radiation is a direct result of the random movements of atoms and molecules in matter. Since these atoms and molecules are composed of charged particles (protons and electrons), their movement results in the emission of electromagnetic radiation, which carries energy away from the surface.
Then we have this ...............
Insulation, radiance and resistance
Thermal insulators are materials specifically designed to reduce the flow of heat by limiting conduction, convection, or both. Thermal resistance is a heat property and the measurement by which an object or material resists to heat flow (heat per time unit or thermal resistance) to temperature difference.
Radiance or spectral radiance are measures of the quantity of radiation that passes through or is emitted. Radiant barriers are materials that reflect radiation, and therefore reduce the flow of heat from radiation sources. Good insulators are not necessarily good radiant barriers, and vice versa. Metal, for instance, is an excellent reflector and a poor insulator.
The effectiveness of a radiant barrier is indicated by its
reflectivity, which is the fraction of radiation reflected. A material with a high reflectivity (at a given wavelength) has a low emissivity (at that same wavelength), and vice versa. At any specific wavelength, reflectivity=1 - emissivity. An ideal radiant barrier would have a reflectivity of 1, and would therefore reflect 100 percent of incoming radiation. Vacuum flasks, or Dewars, are silvered to approach this ideal. In the vacuum of space, satellites use multi-layer insulation, which consists of many layers of aluminized (shiny) Mylar to greatly reduce radiation heat transfer and control satellite temperature.
Now everyone can connect the dots .........in what bricks are best, in what ceramic fiber board are best , in what kaowool is best ..........What we want is to slow down as much we can heat transfer , so it is good to use different insulation materials .
Back to your oven ............no one says she's not good .
This is not proof of good insulation, by any means...............it's just proof of how much heat/energy your oven emits/loses in the environment , nothing else .The fact that the surface temperature is not higher is just because the total area of your furnace is large enough to transfer the temperature in environment , temperature which come from elements inside of oven /where temperature means something to us/ through the bricks .
There are many different insulation material to reduce the flow of heat .None of them / commercially available to us / individually is best/good at successfully preventing all three modes of heat transfer ....... conduction, convection and radiation .
Good isolation means .........energy save , faster heating and better heat uniformity in oven .
With only bricks , that light bulb on PID that tells us when the elements are on , will turn on much more often then on oven with more insulation and that is fact
