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© Eamonn Ryan | RACA Journal
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and the important influence of surfaces and shape on thermal calculations . Heat is transferred via three mechanisms :
• Conduction to other bodies that are in contact
• Convection to the surrounding fluid or gasses
• Radiation ( infra-red )
HEAT TRANSFER BY CONDUCTION Heat passes through solid materials that are in contact by means of conduction . The rate at which this happens depends on the thermal conductivity of the material in question . Metals generally have high thermal conductivities , while those of non-metals are considerably lower . The lowest thermal conductivity is shown by materials where the amount of solid matter is quite small , but where there is a considerable proportion of voids that are not big
enough to carry heat by means of convection . The Coefficient of Thermal Conductivity ( k-value ): The thermal conductivity ( k-value ) is defined as the heat flow in watts ( joules / second ), which takes place across a cube 1m 2 in cross section and 1 metre thick , if the difference between the two faces is 1 K ( 1 Celsius degree ) – Units W / mK .
The Thermal Resistance ( R- value ): The R-value is calculated by dividing the thickness of insulation materials ( m ) by the coefficient of thermal conductivity – Units m2K / W . The thermal resistance is defined as the opposition to the passage of heat through the material . R-value = thickness ( m ) k-value ( W / mK )
Thermal resistances of components can be added together and thus the total thermal resistance of combined structures can be obtained .
HEAT TRANSFER BY CONVECTION Convection is the bodily transfer of heat by fluid molecules . Liquid or gas is warmed up by contact with a surface and moves away carrying the heat with it . It is called forced convection if the movement is caused by an external agent , for example stirrer , pump , fans or wind . Alternately , if the movement is simply due to a change in density of the fluid-due to the temperature , we talk of free convection .
Convection is important in considering the transfer of heat to surrounding air , and the influence of wind .
HEAT TRANSFER BY RADIATION Whenever there are two surfaces at different temperatures facing each other , or there is a single surface surrounding or facing an open space , there is a heat exchange by radiation . It occurs at the speed of light by emitting or absorbing infrared radiation .
EMISSIVITY AND REFLECTIVITY Emissivity , ( symbol ‘ e ’), is a dimensionless number that states which fraction of heat supplied to a surface is emitted in the form of radiant heat , at a specified temperature . For example , 18 % of radiation from the sun onto a surface of polished aluminium will be absorbed , while 82 % will be reflected . Only 3 % of the lowgrade heat that passes to the sheet will be further transmitted .
TYPES OF INSULATION When looking at other insulation materials , there is a tendency to look upon thermal conductivity as the sole factor , but this is not always the most important factor . Other considerations are : mechanical strength , physical and chemical resistance , temperature resistance , fire resistance , chemical resistance , hygroscopy , protection against animal and fungal attack . When looking at different thermal insulation materials , one must consider thermal performance in the main , but all systems and applications must be seen holistically .
The best thermal insulation materials have a very high void content , such as small bubbles of air / gas , which are stopped from moving around by solid materials , which surround them . In practice a k-value of about 0.030W / mK is as low as one can achieve with insulation based on air encapsulation such as high void open cell foams and mineral wools . In reality the k-value is rarely below 0.035W / mK .
Looking at the types and forms of insulation , we find that there are four main types of insulation :
• Fibrous insulation is composed of small diameter fibres that divide the air space finely . The fibres may be parallel or perpendicular to the surface being insulated and they may be separated or bonded together . Mineral wools are typical examples of this type of insulation . The occupational health and safety aspects of these products have been extensively studied and they are considered worldwide to be safe .
• Cellular insulation is composed of small individual cells of gas separated from each other by a polymer / plastic or rubber . The cellular material may be glass or foamed plastic such as polystyrene , polyurethane or rubber .
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RACA Journal I July 2023 www . refrigerationandaircon . co . za