Insulation
Purpose:
● According to law of thermodynamics, heat flows from a higher temperature region to one at a lower temperature. Hence there is a continuous flow of heat.
(1) From the suRRounding to the refrigerated or air-conditioned space, so increase the space temperature.
(2) From the heated space to the outside thereby lowering the inside temperature.
● Any material that prevent the transfer of heat or offers resistance to flow of heat through it is known as "Thermal Insulator".
● The heat flow is either by conduction, convection or radiation.
● The thermal insulator cannot stop the heat flow completely but it can reduce the rate of heat flow through it.
● By insulating the wall, floor of a refrigerator, cabinets, brine pipe line, refrigerant pipe line, the heat flow from the surroundings can be retarded.
● Insulating materials are also used for duct, steam carrying pipe, gap between door and windows.
● Insulation becomes necessary to achieve and maintain the desired temperature where low relative humidity has to be maintained.
Types of Insulating Materials used in Refrigeration or Air Conditioning System:
● There are different type of thermal insulating materials, the selection of this is depend upon their use.
● The basic five types of insulating materials are explained below:
(1) Flake insulation :
It is composed of small particles of flake that are finely divided the air space. Expanded mica is a flake insulation.
(2) Granular insulation :
It is composed of small nodules that contain void or hollow space. e.g. vegetables, cork, calcium silicate, magnesia.
(3) Fibrous insulation :
It is composed of small diameter fibres that are finely divide the space. They may be organic or inorganic e.g. glass, rockwool.
(4) Cellular insulation :
It is composed of small individual cells that finely divide the air space. It produced from glass, rubber or plastic.
(5) Reflective insulation :
It is composed of parallel thin sheet of foil having high thermal reflectance, spaced to restrict radiant heat transfer across the space. Spacing is designed to reduce the heat transfer by conduction and convection e.g. Aluminium and stainless steel foils are used.
Insulating Materials:
The insulating materials are of two types:
(1) Natural insulating materials.
(2) Artificial special insulating materials.
Natural Insulating Materials:
●The natural insulating materials include cork, hair, grass, sawdust, red wood back.
● Proprietary materials incorporated a mimber of natural material to form a heterogeneous mass with ach binder to produce wall board, bats, blankets .
(1) Cork:
The bark from the trees is dried and pressed and hacked at moderate temperature. During this process, the natural gum melts and spread, binding together the entire mass It is able to resist the flow of heat. It is light in weight.
(2) Wool :
The wools such as rock, mineral wool. glass and alag and certain type of minerals are melted and blown by air into fibrous form.
Special Insulating Materials :
(1) Foam glass:
It is a trade name for a porous glass block insulation with scaled pores. It is suitable for the exterior wall and the floor or low temperature room.
(2) Fibreglass :
● Fibre glass server a dual purpose of providing thermal as well as sound insulation when applied to ceiling and wall.
● Fibrous insulation is composed of millians of air cell formed by laying of fibres having lower thermal conductivity. .
● These are of low cost and reduce the operating cost of the conditioned plant due to thermal conductivity and less wear and tear, light in weight.
● It is non-hygroscopic.
● Fibre glass permits flow of moisture vapour essential for the efficient working of the cold storage.
(3) Polystyrene (Thermocole):
This can be supplied nearly any desired shape for large production available as a rigid board, beads, moulded into shape for pipe/curved surface covering and can be cut easily with saw, light weight allow relatively low vapour transmission.
(4) Plastic foams :
● Main reason in selecting a plastic foum as insulating material are low thermal conductivity compressive strength, water vapour transmission, and dimensional stability and method of installation.
● The property of low permeability to water vapour is essential to prevent condensation or freezing of water inside the cellular structure.
(5) Urethane :
● It is known as super insulation in refrigeration and air conditioning.
● Rigid urethane foam is increasingly specified for insulating pipe, ducts, walls, roofs slah perimeter, basement and curtain walls.
● It is more expensive.
● Rigid urethane foam in a cellular plastie form material when two liquids, isocyanates and polyols are mixed in presence of certain additives and catalytic agents.
● The mixture foam after 10 to 30 sec expand 30 times of original volume.
● I feet the cavity to be insulated.
(6) Polyurethane foam (PUF):
(a) Type of Insulation : Organic fibrous insulation.
(b) Temperature range: 220℃ to 110℃
(c) Thermal conductivity : It has low thermal conductivity value of maximum 0.021 W/mK at 10°C
(d) Vapour permeability : It is not easily ignitable and has negligible water permeability.
Properties of Ideal Insulating Materials used in Refrigeration and Air-Conditioning System
● Jacketing of Insulation (Cladding, cementing, etc).
● The properties of thermal insulating material as listed below
(1) Low thermal conductivity : The thermal conductivity must be low as possible which reduces the required thickness of the insulating material, it should put a high thermal resistance in the path of heat.
(2) The material should have low heat absorption capacity.
(3) Light weight: This is necessary to avoid the use of heavy structural members. This became more important for refrigeration and air-conditioning system of railway, aeroplane, automobile.
(4) Resistance to fire : The material should be non-combustible ie. it should not catch fire. It should not support and spread fire.
(5) It should have high corrosive resistance. Resistance to chemical and fire. Extent of shrinkage or cracking during use, ability to withstand vibration, noise, mechanical damage.
(6) Strength : The insulating material used must withstand the pressure coming on them.
(7) It should be water-repellent, i.e. moisture absorbed by the insulating material increases conductivity and reduce strength.(Resistance to heat weather and adverse atmosphere condition)
(8) Odourless: It should not repell any type of objectionable odour when either wet or dry.
(9) Low cost: It should be easily available and u of low cost.
(10) Resistance to fungus and vermin.
Properties of Insulating materials
Properties of insulating material can be described as
1. Thermal properties.
3. Physical properties
1. Thermal Properties :
Thermal conductivity : The most important thermal property of insulating material is thermal conductivity. Thermal conductivity is defined as ability of material te transfer heat or material to be insulator it should have low thermal conductivity. Thermal conductivity changes with material and density of material. Compressible substance such as wool, if loosely packed is better insulator than if closely packed.
1. Coefficient of expansion
2. Specific beat capacity.
2. Mechanical Properties :-
A Mechanical property of insulating material is important when insulating material provided has to carry some load.Important mechanical properties are strength in compression, tension shear and impact.
