When insulation is correctly specified it can prevent condensation forming on refrigeration pipework systems.
Here, Michaela Störkmann, Armacell Technical Department Manager EMEA (Europe, Middle East & Africa), discusses how a better understanding of condensation and advances in insulation performance are leading to improved specifications.
“The single greatest issue with refrigeration and air-conditioning pipe and ductwork is that surface condensation often occurs. This condensation not only accelerates the rate of pipe corrosion but can severely impact on the heath of building occupants because it results in mould growth.
"Insulation used on refrigeration and air-conditioning pipe and ductwork must therefore prevent condensation if it is to extend the working life of pipework. Aside from this, the role of the insulation is to maximise energy efficiency of the system."
Hot and cold
Eventually, this condensation can result in considerable damage. Apart from the expense of repair, there may be further maintenance costs resulting from wet ceilings, spoilt goods or disruption to production process. In a food or drinks company, condensation raises serious concerns around product quality and contamination.
Moreover, the insulation effect of a material deteriorates greatly when it becomes damp, resulting in large increases in energy losses and corrosion of the equipment from moisture. For all these reasons, condensation control is therefore the primary aim of any low-temperature insulation.
Why condensation occurs
At a given temperature and with a given relative humidity, the air contains a defined amount of water vapour. If air is cooled down, it reaches this 100% saturation at a specific temperature. If the air is cooled further, some of the water can no longer be held in the form of invisible water vapour and begins to form liquid droplets (dew point). The respective water vapour content of air at a given temperature can be calculated to work out the extent to which air of a certain relative humidity can cool without 100% saturation being exceeded and therefore condensation forming.
In order to prevent condensation, the surface temperature of the insulation must be as high as, or higher than the dew point temperature under defined ambient conditions.
There is a relatively straightforward way of calculating the insulation thickness required to ensure that the surface temperature of the insulation is at least as high as this dew point. This involves knowing the line temperature and the ambient conditions (ambient temperature and relative humidity), defined as expected maximum values.
In addition, it is necessary to determine the thermal conductivity of the insulation material, the object being insulated (pipe, duct and equipment) and the heat transfer coefficient of the surface of the insulation.
Although these variables should be calculated by the insulation specialist or installer, it is useful for the specifier to know how these individual factors influence the insulation choice and its maintenance
Thermal conductivity of the insulation material
The thermal conductivity value of materials typically used for technical insulation range from 0.030 to 0.060 W/(m²K). One parameter which influences the thermal conductivity is the mean temperature. In the case of elastomeric insulation materials, such as AF/Armaflex Class O, the thermal conductivity increases as the temperature rises. This has a decisive influence on the insulation thickness, because the lower the thermal conductivity, the thinner the insulation thickness. The thermal conductivity of materials should therefore be shown in combination with the mean temperature.
Heat transfer coefficient
The heat transfer coefficient depends on the type of flowing medium, the flow speed, the character of the wall surface (rough or smooth, shiny or dark) and other parameters. The heat transfer coefficient usually consists of heat transfer through convection and heat transfer through radiation.
Convection makes a substantial contribution towards improving the heat transfer coefficient. The faster the ambient air flows, the more heat is transported. Therefore, in practice and when designing plant, it is essential to ensure that pipes and ducts have sufficient clearance to each other, walls and other installations. If this isn’t done it will prove difficult to install insulation materials correctly and there is also the danger of a build-up zone being created.
Shape of the object
Another crucial factor when calculating insulation thicknesses required to prevent condensation is whether the object to be insulated is a flat surface or cylindrical object (pipe). One of the consequences of this is that thinner insulation thicknesses are required on pipes compared to flat surfaces.
In the case of cylindrical objects the logarithmic ratio of the diameter of the insulated pipe to that of the un-insulated pipe must be included in the calculation. To avoid having to carry out such complex calculations, we have developed our ArmWin Thermal Insulation Thickness Program, which provides all the typical calculations required for refrigeration, air-conditioning, heating and plumbing applications.
To achieve this, low-temperature insulation must be correctly specified and installed, meaning it will perform over the long term, even under critical conditions. A key element of this is ensuring that the correct insulation thickness has been used. Another crucial factor is the quality of both the material and the installation, since this can have a dramatic effect on performance.
For cold applications, insulation requirements should be assessed, specified and installed by qualified contractors. If unsuitable materials, inadequate insulation thicknesses or poor installation practices are used the refrigeration system becomes vulnerable to condensation and corrosion.