THERMAL INSULATION

Thermal insulation is material which retards the flow of heat .It is used as a barrier between two bodies at different temperature either to reduce the heat loss from the hotter body or reduce heat entry into the cooler body .it is used to conserve energy .

Mechanism of  insulation

The insulating material reduces the heat transfer by replacing a shot heat flow path of  lower resistance by a longer path of high resistance .Most of the inorganic and all of the organic insulating material own their insulating properly to the presence of very small pockets of air or gas .These pockets are formed by flakes ,fibers or nodules of the solids or by cells in the material itself . Each pocket must be sufficiently small to cause considerable resistance to direct are flow so that little heat is transferred by convection from one side of the cell to another .

Amount of heat conducted is limited because of long circuitous of heat flow through solid matter and very low conductivity of air pockets .This heat transfer is further reduced because of generally low thermal conductivity of solid matter . The amount of heat transferred by radiation is limited because of the presence of solid matter which is opaque to radiation .Hence the amount of heat flow through insulating material is much small ,


Reflection Insulation -
Reflection insulation is insulation which obtains its major resistance to heat transfer by sheet material of low emittance and absorptance . These sheet barriers must be so located and spaced as to minimise convention by the air and must be supported in such a manner that the supports provide little heat transmission by direct conduction .Because of its necessary construction , reflective insulation is a system of material assembly rather than homogeneous mass. However the passage of heat through a reflective system is identical to that described for mass insulation in that all modes of heat transfer combine to transmit heat from higher temperature surface to lower temperature surface .

Vacuum Insulation
Variation of both the mass insulation and the reflective insulation are vacuum insulation .This type of insulation is so constructed that either the mass insulation or the reflective shields are located in space which is sufficiently tight and strong so that the air in the space can be evacuated .By this means the transfer of that part of heat which normally is transmitted through air is reduced to a very low level. As the conductivity of air 20°C is approximately 10KJ/hr-m°C the reduction of this conducted heat to almost 0 by air is a major factor in the control of heat flow .Although the principle is only recently being adopted into industrial insulation its basis is as long and well known as Dewar flask named after Sir James Dewar who developed  it in early part of this century .To reduce the weight of construction of sheets which can withstand the pressures caused by an internal vacuum another development has been to replace air in the space by a gas of lower conductivity that air .In most instance this gas is inert and is cycled and direct to keep the internal insulation free of moisture and to control the rusting of steel sheets .

Types of  Insulation Material 

The characteristics of thermal insulation  are mainly determined by its composition .Thus thermal insulation are classified into five major types .
1. Flakes
2. Fibrous
3. Granular
4. Cellular
5. Reflective

1. Flake Insulation - It is composed of small particles of flakes which finely divide the air space .These flakes may or may not be bonded together . Expanded mica is popular as flake insulation .

2. Fibrous Insulation  - It is made of small diameter fibres  which minutely divide the air space .These fibres may be inorganic or organic fibres commonly used are wood or cane , hair , synthetics etc . Inorganic fibres comprise , glass , rockwool , slag wool , asbestos etc .
3. Granular Insulation  - It comprises small nodules containing voids or hollow spaces . It is not considered a true cellular material since gas can be passed between the individual spaces . Vegetable cork , silicate of calcium , magnesia etc , fall in this type .

4. Cellular Insulation - It is made of small individual cells sealed from each other  Rubber , plastic etc . fall in this type .

5. Reflective Insulation - It is composed of parallel thin sheets of foil of high thermal reflectance and spaced to reflect radiant heat back towards its source .The spacing is also designed to provide restricted air or gas spaces .The restricted air space reduces heat transfer which would be caused by convection and conduction .In most instance the thin reflective sheets are made of aluminium or stainless steel .

Properties of Insulating Materials 

 1. Physical properties -
2. Chemical properties
3 .Mechanical properties
4. Thermal properties











This artical was taken by P.L. Ballaney