Condensation control – The need for high performance insulation
Correctly specified insulation can prevent condensation forming on refrigeration pipe work systems. Here, Michaela Störkmann, Armacell Technical Department Manager EMEA (Europe, Middle East & Africa), looks at how a better understanding of condensation and advances in insulation performance is leading to improved specifications.
“Whilst hot installations (heating and hot-water pipes) are predominantly insulated to save energy, cold systems (such as the chilled-water pipes of air-conditioning systems or the suction lines of commercial freezers) need protection against condensation. On refrigeration systems, where the line temperature is lower than the ambient temperature, condensation is formed when water vapour comes into contact with the colder surfaces of the pipework and equipment.
Eventually, this condensation can result in considerable damage. Apart from the expense of repairing, there may be further maintenance costs resulting from wet ceilings, spoilt goods or disruption to production processes. 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 the moisture in the insulation.
Condensation control is therefore the primary aim of any low-temperature insulation.
Why condensation occurs
Condensation occurs simply because there is only so much water vapour that air can absorb. At 100% saturation, known as the ‘dew point’, the air is completely saturated causing moisture to be released in the form of droplets on cold surfaces.
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. Warm air is able to absorb more water vapour than cold air and so air that is cooled below the dew point will result in the formation of moisture on pipes and equipment. In the example below, cooling the air from +22°C to its dew point at 18.4°C, results in condensation (Figure 1).
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.
Maintaining temperature above the dew point to avoid condensation
Applying this physical law to refrigeration applications means that the insulation thickness must be designed so the temperature never falls lower than the dew point anywhere on the surface of the insulation material. In Figure 2, the insulation thickness must be at least 11 mm in order to prevent condensation forming (ambient temperature 22° C, line temperature 6° C, relative humidity 80%, pipe outer diameter 35mm). In practice, it is seldom possible to obtain a product with exactly the insulation thickness calculated. Therefore, the next largest manufactured insulation thickness is selected.
Copper pipe outer diameter: 35mm
ϑi = + 6 °C
ϑa = + 22 °C
ϕ = 80 %
Dew point =18.4 °C
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.
The ambient conditions
In order to determine the minimum thicknesses for low-temperature insulation, assumptions must be made about typical ambient conditions. The maximum values listed in Table 1 are used by insulators, specifiers and plant operators to reflect typical conditions.
A common mistake is to underestimate the impact of the relative humidity on the insulation thickness required to prevent condensation. For example, in some areas a 10 per cent increase in humidity can mean the insulation needs to be twice as thick.
The 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.
The 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. In such areas, the air circulation (convection) required to create a sufficiently high surface temperature above the dew point is prevented. In these build-up zones, the heat transfer coefficient is lower (Figure 3). As a result, the risk of condensation and frosting increases significantly.
A build-up zone clearance of 100mm is recommended between insulated pipes as well as between the pipes and the nearest wall or ceiling. In the case of vessels and other equipment, the distance can be increased up to 1000mm.
Thermal radiation is a type of heat transfer that occurs through electromagnetic waves. The transfer of energy through radiation is not restricted to one transfer medium. For example, unlike thermal conduction or convection (heat flow), thermal radiation can 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 radiation energy.
- Absorption: the radiation which strikes the surface of a body with a lower temperature is transformed into heat.
Dark-coloured bodies emit more radiation energy than light-coloured ones and inversely, dark-coloured bodies absorb more thermal energy than light-coloured ones.
The measure for the emissive power of a material is the emission coefficient, whilst the measure for the absorptive power is the absorption coefficient. Interestingly, the emissive power of a body of a certain colour is as great as its absorptive power, for instance, a vessel that is completely black has the greatest absorptive or emissive power. These factors need to be taken into account when specifying insulation on a refrigeration system, as can be seen in Table 2.
Table 2 shows the emission and absorption coefficients of some typical insulation system surfaces. As the table shows, it is largely the nature of the surface of the insulation material or its jacket (apart from the influence of other radiating bodies) which determines the contribution of radiation aS to the heat transfer coefficient. A synthetic-rubber-based insulation material absorbs much more thermal energy than, for example, an aluminium foil. This has an extremely positive effect on the insulation thickness required to prevent condensation, so the higher the absorptive power, the lower the insulation thickness.
Determining insulation thicknesses
Another crucial factor when calculating the insulation thickness required to prevent condensation is whether the object to be insulated is a flat surface or cylindrical object (pipe). 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. One of the consequences of this is that thinner insulation thicknesses are required on pipes compared to flat surfaces.
To avoid having to carry out such complex calculations, the ArmWin Thermal Insulation Thickness Program can provide all the typical calculations required for the refrigeration, air-conditioning, heating and plumbing sectors.
The tool has been updated for use online, offline and as an app, allowing technical calculations to be carried out far more easily and quickly. New features include the possibility of entering individual data on the project in question and storing calculations as a pdf. The program is also linked to product information on our website with a glossary to explain key terms.
Preventing condensation on the surface of pipework and equipment is a vital requirement in all refrigeration systems where the line temperature is lower than the ambient temperature.
To achieve this, low-temperature insulation must be correctly specified and able to 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 especially insulation requirements should be assessed, specified and installed by specialist contractors. If unsuitable materials, inadequate insulation thicknesses or poor installation practices are used the refrigeration system becomes vulnerable to condensation and corrosion.
It is worth remembering that the minimum insulation thicknesses to prevent condensation are different to insulation designed to prevent energy losses.
As the results of a study carried out by Armacell show, much higher energy and carbon dioxidesavings are possible if greater insulation thicknesses are specified. Greater levels of insulation, which means thicknesses exceeding those required for condensation control, require slightly higher investments, however the pay back can be substantial if the savings are projected over the lifetime of the system.”
Further information on the company can be found at: www.armacell.com