TechArchive

Original article date: February 2000

Are you having problems with condensation? MARK JONES from OEM Automatic is your guide through the various types of condensation and how to deal with them

Careful thermal management of the internal environmental air has become an essential factor, because of increasing demands for extending the life of enclosed control systems.

One important area to consider is the elimination of condensed water vapour (dew) contained within the air molecules inside an enclosure. If unaddressed, moisture, when combined with aggressive gases or dust, can cause failures of components, such as pneumatic valves, relays, switchgear, PCBs and electromechanical assemblies.

• Dew problems manifest themselves in the following symptoms:
• Reduced electrical insulation at cable/wire joints
• Electrical creepage
• Flashovers
• Changes in contact resistance
• Corrosion of solder pads/components
• Valve gasket and seal deterioration.

There are two distinct forms of condensation: film condensation and dropwise condensation

In the former case, continuous condensate wets the surface (panel sides or component faces) and forms a continuous wet film across the entire surface, which then slides downwards under the influence of gravity. As the liquid film gets thicker, flow direction increases as mor vapour condenses on the surface. The liquid film provides a thermal barrier, so thermal energy cannot pass easily from the components/enclosure into the ambient air.

With dropwise condensation, the condensed vapour forms sporadic droplets on the surface instead of a continuous film. On inspection, many droplets of varying diameter are seen. The droplets roll down the vertical faces and clear the surface for vapour to escape, thereby aiding thermal emission by ten times that of film condensation. However, neither forms are conducive to good thermal management and should be viewed as a hazard.

As a rule of thumb, relative air humidity of 65% and upwards is considered to be problematic and therefore the risk of corrosion and other faults are more likely. Typically, if the air temperature in the cabinet is allowed to fall, the air molecules can no longer hold the water vapour in a gaseous state and hence dew is formed.

Solving the problem

By using an anti-condensation heater this problem can be avoided. Heating is initiated only when required – in daytime or peak time operation, it is usually reckoned that there is enough waste heat emitted from the processing circuits to maintain the required difference in temperature.

DBK’s range of ABC cabin heaters, which are available through the thermal division of UK and European business partner OEM Automatic, have been designed to meet market requirements of physical size, output power and method of fixing – base or DIN clip mount. This series of profiles is available in packages of 30, 45, 50, 60, 80 and 100W, all carrying UL, CSA and VDE approval.

The HP series is designed to be affixed to cabinet panels or thermally conductive surfaces. Their low profile and compact surface area makes them useful for small enclosures such as CCTV camera housings and telecoms forced convection air channels, used for pre-warming prior to PCB installation. For assemblies rated above 100W, OEM Automatic has introduced a new concept in forced convection heating called the Watt Box and Spider series.

• .OEM automatic

February 2000