‘Thermal insulator’ is defined as, “any material, which retards or offers resistance to flow of heat through it”. Thus, insulations are used to prevent heat transfer between ducts and surrounding. Thermal insulation however cannot stop the heat flow completely, but it can only retard or reduce the rate of heat flow through it. Insulating materials have extremely low thermal conductivities. By using insulating materials, the heat flow rates from the air conditioned buildings (from building to outside for winter air conditioning), and from outside to the buildings to be air conditioned (for summer air conditioning) can be considerably reduced. Properly applied insulation helps in economic running of refrigerating plants as well as heating plants. The selection of insulating materials for domestic refrigerators, cabinets, brine pipe lines, refrigerant pipe lines, cold storage rooms, steam carrying pipes, ducts carrying the hot air etc. is based on their Ébility to retard the heat now rate’. The selection is also done on the basis of economic and structural consideration.
Purpose of Insulation
Primary Function/purpose:
- “To isolate the warm surroundings from the refrigerated region”.
- Insulating materials are mostly non-metallic and have structure, in which, there are numerous cells containing air or gases.
Secondary Functions
- To retard the flow of heat from surroundings.
- To make possible better control of temperature in the refrigerated space.
- To minimise temperature fluctuations.
- To prevent water vapour condensation.
- To minimise water vapour transmission.
- To give better surface finish.
- To add to the strength of walls and ceilings etc.
- To absorb vibrations and reduce noise.
- Air Conditioning Systems
- To reduce the spreading of fire and flames in case of fire hazard.
Advantages of Providing Insulation
The thermal insulation prevents the heat loss – in or out from the system and Prof des the following advantages:
- It reduces overall operating cost by reducing the fuel consumption.
- It reduces heating or cooling capacity of the required system by cutting the heat loss or heat gain.
- Better process control can be achieved by reducing the temperature drop of a fluid in heated system and the temperature gain of fluid in a refrigerated system.
- Controlled temperature keeps the workers comfortable in a heated system and prevents surface condensation of a refrigerated system above dew point.
- It provides fire protection to the plant and piping.
- It helps to reduce the noise level of the system and absorbs vibration also.
Types of Insulating Materials
There are five basic types of insulations, which may be used alone or in combination.
Flake insulation:
It is composed of small particles, which finely divide the air space. Examples: Vermiculite or expanded mica.
Fibrous insulation:
It is composed of small diameter fibres, which finely divide the air space. They may be organic or inorganic and may or may not be bonded. Examples: Inorganic fibres such as glass or rock-wool.
Granular insulation:
It is composed of small nodules, which contain voids. It is not considered as a true cellular material, because gas can be transferred between the individual spaces. Examples: Magnesia, calcium silicate and cork.
Cellular insulation:
It is composed of small individual cells, that finely divide the air space. It is produced from glass, rubber or plastic.
Reflective insulation:
It is composed of parallel thin sheets of foil having high thermal reflectance spaced to restrict radiant heat transfer across the space. Spacing is designed to reduce heat transfer by conduction and convection. Examples: Aluminium and stainless steel foils.