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Heat Transfer Coefficient

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The term “heat transfer” means the transfer of heat between a fluid and a solid wall (e.g. a wall of a pipe or vessel). A distinction is made between the inner heat transfer – i.e. the transfer of heat between vessel or pipe medium and the pipe or vessel wall – and the outer heat transfer – i.e. the transfer of heat between the vessel/pipe wall or its insulation material and the ambient medium (Figure 1). For the calculation of the insulation thickness needed to prevent condensation, the influence of the inner heat transfer is negligibly small and will not, therefore, be taken into account in the following.

When heat is transferred, the heat flow is proportional to the wall surface and the difference in temperature at the wall of the object. The proportionality factor a, in our case aoutside (aa), is also known as the heat transfer coefficient and has the unit W/(m²•K). It depends on the type of flowing medium, the flow speed, the character of the wall surface (rough or smooth, shiny or dark) and further parameters. The heat transfer coefficient usually consists of heat transfer through convection and heat transfer through radiation.

Convection

Convection makes a considerable contribution towards improving the heat transfer coefficient. The faster the ambient air flows, the more heat is transported. In practice, it is therefore essential to ensure that pipes and ducts do not lie too close to each other or at an insufficient distance from walls and other installations. Apart from the difficulties of installing insulation material professionally if this is the case, there is also the danger of a build-up zone being created. In this area, the circulation of air (convection) which is needed for a sufficiently high surface temperature is stopped, i.e. in such build-up zones the heat transfer coefficient is lower because the contribution of convection decreases (Figure 2). As a result of this, the risk of condensation forming increases significantly.

In DIN 4140, therefore, a distance of 100mm between the insulated pipes and from the pipes to the wall or ceiling is required. In the case of vessels, apparatus etc. the distance should even be 1000mm.

Thermal Radiation

Thermal radiation is a type of heat transfer where the heat is transferred by electromagnetic waves. The transfer of energy through radiation is not restricted to one transfer medium. Unlike thermal conduction or convection (heat flow), thermal radiation can also spread in a vacuum. In the case of thermal radiation, the mechanism of heat transfer consists of two sub-processes:

  • Emission: on the surface of a body with a high temperature heat is transformed into radiated energy.
  • Absorption: the radiation which strikes the surface of a body with a lower temperature is transformed into heat.

Dark-coloured bodies emit more radiated energy than light-coloured ones; on the other hand, dark-coloured bodies also absorb more thermal energy than light-coloured ones

The measure for the emissive power of a material is the emission coefficient e. The measure for the absorptive power is the absorption coefficient a. The emissive power of a body of a certain colour is exactly as great as its absorptive power. A vessel which is completely black has the greatest absorptive or emissive power. Table 1 shows the emission and absorption coefficients of some surfaces of insulation systems. As can be seen from the table, it is to a large extent the nature of the surface of the insulation material or its jacket – apart from the influence of other shining bodies – which determines the contribution of radiation S to the heat transfer coefficient. An insulation material on the basis of synthetic rubber absorbs considerably more thermal energy than, for example, an aluminium foil. This has an extremely positive effect on the insulation thickness required to prevent condensation, i.e. the higher the absorptive power is, the smaller the insulation thickness becomes.

From the explanations given above it has become clear that the heat transfer coefficient is influenced by many factors which cannot, as a rule, be determined exactly and clearly. However, it is important to define a value for the heat transfer coefficient which is as close to reality as possible. Formulae for approximate calculations of the heat transfer coefficient can be found in the appropriate standards.

By way of simplification: where the conditions as far as space is concerned are normal, the following assured empirical values can be expected for the aa-value for installations insulated with Class O Armaflex.

Dipl.-Ing. Hubert Helms, Armacell GmbH
“adapted from article published under series of
“Key Terms in low-temperature insulation”

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