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Condensation control

A better understanding of condensation and advances in insulation performance can prevent condensation forming on refrigeration pipe work systems, says Michaela Störkmann

The challenge faced by maintenance engineers on refrigerated systems, where the line temperature is lower than the ambient temperature, is how to prevent condensation forming when water vapour comes into contact with colder surfaces of the pipework and equipment.

If this issue isn’t addressed, it can result in corrosion and failure of equipment, plus additional repair and maintenance costs resulting from wet ceilings, spoilt goods or disruption to production processes.

Another factor that needs to be taken into account is that the insulation effect of a material deteriorates greatly when it becomes damp. There is direct correlation between accumulation of moisture from condensation and increased energy use.

The best way of tackling these issues is by correctly specifying high-performance insulation. Aside from improved energy efficiency, it can help make significant savings on maintenance budgets.

Temperature and humidity

At a given temperature and with a given relative humidity, air contains a defined amount of water vapour.

If the air is cooled down, it reaches the dew point, the stage where it is 100 per cent saturated. 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, which we all know as condensation.

This is most visible on colder surfaces because of the difference in temperature between it and the surrounding air, for example, pipework in a refrigerated system.

Warm air is able to absorb more water vapour than cold air.

Warm air, when it is cooled below the dew point, results in the formation of elevated levels of moisture on pipes and equipment. This is a common issue in refrigerated processes where there are ‘hot’ and ‘cold’ areas within a production environment, combined with background heating systems that raise the air temperature, allowing it to hold more water vapour.

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 per cent saturation being exceeded – and condensation forming.

If the temperature is kept above this level it will avoid the formation of condensation.

So, applying this physical law to refrigeration applications, the insulation thickness must therefore be designed so the temperature never falls lower than the dew point anywhere on the surface of the insulation material.

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 achieve this using our ArmWin Thermal Insulation Thickness Programme.

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.

Calculating the insulation thickness needed to prevent condensation depends on a number of variables including the object to be insulated, for example, a flat surface or cylindrical object, ie a 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, meaning thinner insulation thicknesses are required on pipes compared to flat surfaces.

Flexible insulation foams tend to be the most effective method of insulating the wide range of pipes and equipment in a refrigerated process. These kinds of foams can be used to effectively insulate assets against condensation down to minus 200 deg C.

Conclusion

Preventing condensation on the surface of pipework and equipment is a vital requirement in all refrigeration systems with the line temperature 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 has a dramatic effect on performance.

For cold applications, insulation requirements should always be assessed, specified and installed taking into account all these factors.

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, too, that the minimum insulation thicknesses to prevent condensation are different to insulation designed to prevent energy losses. Much higher energy and carbon dioxide savings are possible if greater insulation thicknesses are specified.

Michaela Störkmann is Armacell technical department manager EMEA

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