Heat insulating materials for reducing heat losses

Heat insulating materials are products for the refractory lining of thermal industrial plants with the objective of reducing heat losses. Here the low thermal conductivity and the thermal capacity of air is used.
Heat insulating materials usually have a total porosity of at least 45 %, in practice mostly from 60 to 90 %, and in extreme cases up to 99 %. Besides low thermal conductivity, high porosity causes low mechanical strength, high gas permeability and low corrosion resistance. The thermal conductivity not only depends on the total porosity of the material, but also on the pore size and shape, the structure composition and the mineralogical composition. Depending on temperature, the factors responsible for the flow of heat – solid state conduction, convection and radiation – vary in influence. Maximum pore diameters of < 1 mm are necessary. Microporous insulating materials with pores < 0.1 µm have the lowest thermal conductivity.
The thermal shock resistance of lightweight construction materials has a large influence on applications. Ceramic fibre products usually resist severe thermal shock. Other lightweight construction materials are sensitive to thermal shock.
Table 1 provides an overview of the most important groups of heat insulating materials.
In graph 1 the thermal conductivity of various heat insulating materials as a function of the temperature is shown. Several processes can be used to make the pores of the insulating materials, for example burnout, foam, bloating (swelling) and gas propellant processes, the use of evaporating liquids or solids and the utilisation of fibre structures and natural or synthetic lightweight additives.
In furnaces and plants with low mechanical load and without corrosion stress, a design with lightweight heat insulating materials has almost completely eliminated heavy designs with dense, refractory materials.

Table 1: Overview of ceramic and mineral Heat Insulating Materials
Material Type	Bulk Density	Thermal Conductivity (W/mK)			Application
	kg/m³	400 °C	800 °C	1200 °C	(°C)
Heat insulating bricks (kieselguhr, vermiculite, perlite)	350 - 700	0.12 - 0.23	0.19 - 0.30	-	750 - 1000
Lightweight refractory bricks	500 - 1400	0.13 - 1.30	0.17 - 1.20	0.23 - 1.10	1000 - 1800
Lightweight refractory concretes	400 - 1400	0.13 - 0.90	0.17 - 0.95	0.45 - 1.00	900 - 1400
Ceramic fibre materials preferred	64 - 1500 120 - 460	0.08 - 0.45	0.15 - 0.45	0.29 - 0.72	600 - 1800
Calcium silicate materials	200 - 300	0.10	0.17	-	1000
Microporous materials	150 - 350	0.03	0.06	-	900
Mineral foams	200 - 800	0.12 - 0.30	-	-	650
Mineral wool materials	100 - 400	0.06 - 0.10	0.20 - 0.25	-	500 - 700

General bases