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The importance of effective enclosure climate control

The importance of effective enclosure climate control Thermal cycling, condensate formation and dust are the three biggest causes of failure in electrical enclosures. That makes climate control a key factor in enclosure design. Christian Westwood of Rittal explains.

An electrical enclosure is a closed housing with access to create a protective environment for installed equipment at a specified location. It is when the protective environment component of this definition is considered that the requirement for cooling and heating equipment must be assessed. Climate control may be employed to maintain electrical components at their optimum operating temperature and ambient conditions within an enclosed space, offering protection against high and low temperatures, condensate, dust, dirt and corrosive substances.

Should the heat dissipated by the installed electrical and electronic equipment not be removed, causing the temperature inside the enclosure to increase, the behaviour and longevity of components may be adversely affected.

Although low temperatures may also affect the functionality of components, when they are considered in conjunction with high temperatures the phenomenon known as thermal cycling may become apparent. Thermal cycling may be defined as repetitive changes in operating temperature such that thermally-induced mechanical stresses are produced in components. There are many examples of circuits in everyday applications that are subjected to thermal cycling to varying degrees of severity, including personal computers, televisions and vehicles. It is typically soldered joints that are at risk of failure due to thermal cycling and although some tolerance to this environmental anomaly may be factored into the design of a product, it may also be prudent to minimise the extremes of temperature to which sensitive control components are subjected.

There is a possibility of condensate formation should the moisture content of the air inside an enclosure be high. The obvious danger posed by the presence of condensate is the catastrophic failure of components due to short circuits. Air that has a high relative humidity not only risks the production of condensate, it may also reduce the life expectancy of components due to corrosion. Leakage currents, surface contamination and corrosion are not only exacerbated by moisture, they are also worsened by the presence of dust in the air.

Reasons for electrical equipment failure can be broadly sorted into four categories, namely temperature, vibration, condensation or oxidation and dust. The proportions with which failures may be ascribed to each of these classifications are 57%, 21%, 16% and 6% respectively. Vibration is the only one of these four groupings that may not be addressed using climate control.

Having defined the causes of electrical equipment failure, it is now opportune to assess the different forms of climate control that may minimise the risk of their occurrence. Should the ambient air external to a control enclosure be clean, dry and consistently at a temperature lower than that required inside it, then arguably the simplest form of cooling, the fan-and-filter unit, may be used to maintain the temperature inside the enclosure. Do not be deceived by the humble nature of this product; contemporary versions may incorporate innovative blade technology and electronically commutated (EC) motors. Diagonal fans, which are a hybrid of conventional axial and radial impellers, provide improved performance against increased static pressure difference which, in turn, results in greater energy efficiency and a more constant air throughput.

Air-to-air heat exchangers
Air-to-air heat exchangers provide a simple, stand-alone, low maintenance and energy efficient means of cooling electrical enclosures. The effectiveness of this type of cooling product is fundamentally dependent upon the ambient air temperature and the required internal air temperature; not only must the former be lower than the latter but the available cooling output varies with the difference between them. Air-to-air heat exchangers have only two moving parts, an internal fan and an external fan, each of which draw air through separate circuits of a heat exchanger so that, unlike the fan-and-filter unit, potentially aggressive ambient air, or any debris or contaminants present in it, do not ingress the enclosure.

Typically, ambient temperatures in excess of the maximum temperature permitted inside an enclosure result in cooling units being utilised. Not only are they capable of cooling enclosures to a temperature lower than that of the ambient air, they, similar to the air-to-air heat exchanger, also have separate circuits so that the air inside the enclosure is simply recirculated and contaminated ambient air remains outside the enclosure. An inherent side effect of refrigerant cooling is that the cooled air is dehumidified as it is blown over the evaporator coil should it contain sufficient moisture. The resultant condensate may be converted back to water vapour by an integrated electric condensate evaporator and allowed to pass through to the external air path where it may be harmlessly emitted into the atmosphere external to the enclosure. Hence, the relative humidity of the air inside a sealed enclosure may be kept relatively low using this form of cooling.

Should the ambient temperature be too high to use a refrigerant based cooling unit, then cooling may be provided using an air-to-water heat exchanger. Again, air is recirculated inside the enclosure so that no contaminants are allowed to ingress but, in this instance, heat is not removed by the ambient air, it is transferred to a water circuit and hence may be conveyed to a remote location before being dissipated. The air inside the enclosure is dehumidified should it contain enough moisture and the temperature of the water being supplied to the air-to-water heat exchanger is sufficiently low.

In conclusion, then, whatever the application, be there a risk of high temperatures, low temperatures, high relative humidity or a contaminated environment, there is a climate control solution suitable for optimising the service life of the equipment installed in an electrical enclosure.
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