The properties of insulating materials can be broadly divided into following four categories.
- Physical properties
- Chemical properties
- Physical Properties
- Mechanica properties
- Thermal properties.
Physical properties
These properties include density, porosity, moisture resistance, coefficient of thermal expansion, low odour level, in-flammability, compactness and surface tension. The desired physical properties for an insulating materia are,
- Low density (to decrease its weight).
- It should not possess its own odour nor should it pick up odour of other particles present in the refrigerated space.
- It should resist moisture penetration and should not deteriorate, when the moisture collects on it.
- It should not be flammable; otherwise it may be dangerous, in the presence of electricity etc.
Chemical Properties
Chemical properties enable insulation to retain its own structure in hostile environment and also prevent t from being corrosive to substances. An insulation material, which is to be used in chemical industry, must possess adequate resistance to acids and corrosive gases.
Mechanical Properties
The important mechanical properties required are high abrasion resistance and hardness, good compressive strength etc.
Thermal Properties
Thermal properties of insulating material are most important. They are,
- Low thermal conductivity:
The ability of a material to retard the flow of heat is thermal conductivity.
It should be low in case of a thermal ‘insulator.
- Low heat or thermal capacity:
It is essentially measure of the heat absorbed for a certain temperature rise. Furnace insulations must have low thermal capacity for quick response to temperature changes required.
- Low thermal diffusivity:
It is measure of the rate of progression of temperature rise in an insulating when certain heat energy is available at one of its surfaces.
- Low combustibility:
It is characterised by flash point, smoke index, fire point etc. To reduce fire hazard, a low combustibility is necessary material,
Desirable Properties of Insulating Materials
The desirable properties of a good heat insulating material are:
- Low heat conductivity: The thermal conductivity must be as IOW as possible, which will reduce required thickness of the insulating material.
- Permanence: Materials may disintegrate as a result of Internal chemical activty or as a result of exposure to surrounding conditions. The insulating materials must have high resistance to the above mentioned activities.
- Strength: The insulating materials used must withstand the pressures coming on them. Some materials tend themselves readily to various types of constructions. Structural strength is generally obtained by the use of wood or framework.
- Light weight: This is necessary to avoid the use of heavy structural members. This property becomes more important for refrigeration and air conditioning systems used for moving vehicles such as automobiles, railways, marine and aeroplane.
- Water-repellent: Moisture absorbed by the insulating material increases conductivity and reduces strength. Material, which resists absorption ether as free water or water vapour should be selected. Moisture absorbing insulation deteriorates rapidly due to growth of mould.
- Odourless: It should not possess any type of objectionable odour when either wet or dry. Also, they should not pick up odour of other particles present in the refrigerated space.
- Fire-proof: This is an important factor, when insulation is used for fire-proof warehouses.
- Low-cost: The insulating material should have low cost and must be easily available.
- Thermal stability.
- Physical stability.
- Low specific gravity.
- Chemical stability.
- Low specific heat.
- Resistance to vibration and shock.
- Porous and fibrous texture.
FACTORS TO BE CONSIDERED FOR SELECTION OF INSULATING MATERIALS
- Low thermal conductivity.
- Resistance to fire.
- Resistance to vermin and fungus.
- Ease of application.
- Vapour permeability.
- Mechanical strength and rigidity.
- Less moisture absorption.
- Low cost.