Heat Transfer and Surface Coefficients
The purpose of thermal insulation materials is to reduce the transfer of heat. We shall, therefore, consider the heat transfer process when a hot pipe is insulated as:
- Heat transfer across the hot pipe - insulation interface.
- Heat transfer through the insulation material.
- Heat transfer across the insulation - air interface.
For insulated pipes there are two surface coefficients to consider.
The inner surface coefficient of heat transfer, hi, which is the transfer of heat between the pipe surface and insulation, a typical value would be 1000 so that it is usual to ignore the value of hi.
Since the insulation material is in close contact with the pipe so that the inner heat transfer may be assumed to be without heat loss.
The outer surface coefficient of heat transfer, he, which is the transfer of heat between the outer surface of insulation and air.
Transfer through the material is a function of the thickness and thermal conductivity.
The transfer of heat from the surface of the insulation to air is a complex process and is dependent, amongst other factors, upon:
- Shape of the body
- Material of the surface
- Surface emissivity
- Relative orientation
- Temperature difference between the surface of the insulation and air
- Air movement over the surface
- Radiation exchange with other bodies
For pipe insulation the area of surface exposed to air will increase with increasing insulation thickness.
The evaluation of surface coefficients is complex involving a study of the convective contribution hcv, and radiative contribution hr, to the overall surface coefficient. For insulated pipes and surfaces where the temperature difference is small it is a reasonable approximation to use the following values of surface coefficient, dependent upon the emissivity of the surface.
| Surfaces of low emissivity, e.g. bright metal, polished, hammered, plain and dull aluminium ( emissivity coefficient less than 0.2) h = 5.7 |
| Surfaces of medium emissivity, e.g. galvanised steel, aluminized steel, zinc-aluminium coated steel, aluminium paint and comparable surfaces (emissivity coefficient greater than 0.2 and less than 0.9) h = 8.0 |
| Surfaces of high emissivity, e.g. matt black surfaces, some unfaced insulating materials, cement, paper, cloth, finishing composition, plastic coated metals and paints other than aluminium ( emissivity greater than 0.9) h = 9.0 |
The above values are taken from BS EN ISO 12241.
It is important to realise that this approach, the use of surface coefficients, is only a convenient approximation that operates over a restricted temperature range. The surface coefficient is not constant.
For condensation control calculations it is important that the proper value for surface coefficient is used since the thickness requirement for a low emissivity surface will be about double that required for a high emissivity surface, i.e. thickness will be increased by the factor 10/5.7 = 1.8.
The overall outer surface coefficient of heat transfer may be considered as the sum of the convective and radiative contributions, he = hcv + hr
where the convective contribution is dependent on air movement, relative orientation and the type of material. The radiative contribution is dependent on the nature of the surface and its emissivity.
BS EN ISO 12241 treats the topic of surface coefficients of heat transfer in detail.
For comparative or approximate calculations the following equations for the value of he are given:
- Horizontal pipes inside buildings he = A + 0.05 x DQ
- Vertical pipes and walls he = B + 0.09 x DQ
where A and B are defined constants with values for non-metallic surfaces of A = 8.5, B = 8.7 with emissivity of 0.94 .
DQ is the temperature difference between the surface of the insulation and surrounding air. |
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