Enclosure fans and filters - a few considerations for use

In recent years the use of fans and filters as a cooling method for electrical enclosures has dramatically increased. This demand has been very good for the industry since it has given manufacturers a reason to develop better fan and filter products at competitive prices. For users, this has provided higher IP ratings while keeping maximum airflow to protect critical electrical equipment from high temperatures. With the new developments it is easy for users to lose sight of the important issues involved when choosing a fan and filter unit. The following article (see downloads for the full article and figures) discusses these issues including; their suitability; the physical characteristics and arrangement of fans and filters for use with enclosures; the implications of using filters; benefits of using hose proof hoods.

Fan and filter units, when supplied, in general consist of four parts; the fan motor, the filter housing, the filter mat, and the grill. In the past the units needed to have a gasket which would create a seal between the unit and the enclosure, today, using a foam seal attached to the filter housing, the units are supplied ready for installation. A feature of most designs is the quick push-fit mechanism that allows speedy installation. This mechanism does not require screws but involves the user simply pushing the unit into the cutout until a 'click' is heard meaning that the filter is locked into place. If the units are UL approved, the fan and filter units require a locking mechanism which secures the grill onto the filter housing. For example, all Rittal fans and filter units are UL approved and have a clearly visible fastening mechanism which ensures that the grill is secured, maintaining high protection levels. As well as the physical product, users should expect relevant certification to be provided as part of the supply.

The standard arrangement for installing fans and filters on enclosure systems follows the well-known fact that hot air rises. Also, most large heat generating equipment directs air from top to bottom, i.e. they suck cool air from the bottom of the enclosure and blow energised air towards the top. With this in mind, it makes sense to direct cool air in at the bottom of the enclosure, and allow heated air to escape the enclosure at the top. In essence, this means that it is standard practice to install the fan and filter unit at the bottom of the enclosure and install an outlet filter at the top. Because of this arrangement of air being forced into the enclosure, slight pressurisation of the enclosure is achieved which aids keeping dust out of the enclosure.

Airflow v filtration:

Designing a solution that prevents dust and water from entering electrical enclosures is accepted as a key issue when designing electrical control panels for use in industry. In the UK and Europe, fans and filters are classified on how well they perform this prevention. Each system is given a rating that is split into two measures; the first measure illustrates a level of dust protection; the second figure demonstrates a level of water protection. The rating is known in the electrical engineering industry as the International Protection rating or IP rating (see chart 1 download). The standard industry protection rating for fans and filters is IP54 because they use only standard filters, for IP55, fine filters are often thicker and denser than standard filters, this prevents dust particles, down to several microns, from entering the enclosure. Despite filters being extremely efficient for preventing dust they invoke a serious side effect that not only increases the cost of a solution in dusty environments but also can result, if unchecked, in serious consequences.

Filters reduce airflow because they restrict the air from entering the enclosure; this effectively means that larger fans need to be used to compensate for the loss of airflow. Consider Rittal's SK3326.100 fan using a standard filter mat along with an appropriate outlet filter. The airflow with this set up would be 360m3/hr though when using a fine filter mat in place of the standard filter mat, the flow reduces to 275m3/hr (See chart 2 download). In this circumstance, it may be possible for the fan and filter to be undersized where the reduction in flow has been overlooked. An oversight of this kind could cause downtime and ultimately lost production. Another important issue when dealing with fans and filters is the issue of service i.e. regular changing of filter mats to ensure sufficient airflow through the electrical enclosures. In very dusty conditions it would not take very long for the filter mat to become clogged and effectively be a barrier stopping air flow altogether. It is recommended that regular service be exercised when using fans and filters in arduous conditions and depending on the severity, the use of fans and filters could even be called into question.

Preventing water ingress:

The prevention of water ingress in some applications, including the food, beverage, and chemical industries it is of the utmost importance. For this reason IP56 can be achieved using a hose proof hood in conjunction with fine filters, this arrangement also meets the requirements for NEMA 3R + 12 (See chart 1.). Hose proof hoods tend to be of stainless steel since in damp conditions oxidation is a risk and stainless steel is already used in the construction of enclosures for these environments. The hoods usually come in two parts; the first is a stainless steel frame, the second the hood itself. To install the frame, which holds the filter and hood in place, the grill of the fan and filter needs to be removed. By removing the grill, the frame can be installed over the opening of the air inlet/outlet. Once the frame has been screwed into place, the hood in most cases simply slides over the frame to create a tight seal and IP56 protection.

Points to remember:

Recently fan and filter units have become popular as a cost effective weapon against heat build up within enclosures. As a result, manufacturers have developed better products that integrate features enabling UL and other certifications. Fan and filters are arranged so that cool air enters the bottom of enclosures; they should only be used where sufficient airflow is possible with the current enclosure surface area and when ambient temperature is lower than the required internal temperature. Leading manufacturers of fans and filters supply 'click' fit systems that integrate the gasket seal so that separate gaskets are not required.

The industry standard for rating fans and filters is through the IP rating; fans and filters offer IP54 with standard filters and IP55 with fine filters, though users should be aware of airflow issues when using filters and particularly fine filters. Fans and filters offer great protection against water with the use of hose proof hoods that are able to protect against heavy water jets. Fans and filters are a cost effective method of cooling in suitable applications although when used in inappropriate situations, can cause catastrophic failure.

About Rittal:

Note to Editor: Rittal is one of the largest and most widely known manufacturers of enclosures and accessories for the electrical, electronic and datacommunications industries throughout the world. The Thermal Management Systems have full compatibility and integration capability with Rittal's Power Distribution components and Enclosure Systems.

Biography:
Jonathan Brindley joined Rittal as Product Manager for Systems Climate Control Solutions over two years ago and is currently based at Rittal's UK headquarters in Hellaby, South Yorkshire. Previously, Jonathan worked for Alstom Power in their gas turbine division, where he spent three years in Marketing.

The picture shows a Rittal enclosure heat exchanger.

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