Insulating Materials

Insulating materials greatly reduce the heat losses through walls. Insulation is effected by providing a layer of material having a low heat conductivity between the internal hot surface of a furnace and the external surface, thus causing the temperature of the external surface reduced.

The insulating materials may be classified into the following groups:

· Insulating bricks

· Insulating Castables

· Ceramic fibre

· Calcium silicate

· Ceramic coating

Insulating materials owe their low conductivity to their pores while their heat capacity depends on the bulk density and specific heat. Structure of air insulating material consists of minute pores filled with air which have in themselves very low thermal conductivity, excessive heat affects all insulation material adversely, but the temperatures to which the various materials can be heated before this adverse effect occurs differ widely. Clearly, therefore, the choice of an insulating material must depend upon its effectiveness to resist heat conductivity and upon the temperature that it will withstand.

One of the most widely used insulating materials is diatomite, also known as kiesel guhr which is made up of a mass of skeletons of minute aquatic plants deposited thousands of years ago on the beds of seas and lakes. Chemically this consists of silica contaminated with clay and organic matter. A wide range of insulating refractories with wide combinations of properties are now available.

The important physical properties of some insulating refractories are shown in the Table 5.7.

TABLE 5.7 PHYSICAL PROPERTIES OF INSULATING REFRACTORIES
Type	Thermal conductivity at 400 °C	Max. safe temperature °C	Cold Crushing Strength Kg/cm2	Porosity %		Bulk density Kg/m3
Diatomite Solid Grade	0.025	1000	270	52		1090
Diatomite Porous Grade	0.014	800	110	77		540
Clay	0.030	1500	260	68		560
High Aluminia	0.028	1500–1600	300	66		910
Silica	0.040	1400	400	65		830