Server racks are everywhere in manufacturing today, no longer confined to the ideal environmental conditions of climate-controlled IT clean rooms. As these servers, routers, PLCs and other mission critical components are distributed to the shop floor, the sensitive electronics must be enclosed in cabinets that protect them from airborne particulates, humidity, and other hazards present on the shop floor.

Dangers of Overheating

Paradoxically, the enclosures that isolate the electronics from harmful substances also contribute to a different hazard — overheating. Thermal management is critically important to extend the service life of components, prevent catastrophic failures, and avoid the costs of downtime and repairs.


Pfannenberg Inc
This chart shows the difference between interally and externally mounted forced-convection and closed-loop systems.
Photos courtesy of Pfannenberg Inc.


Electronics are typically most efficient in an environment where the humidity is low and the temperature is approximately 95°F. As the temperature in an enclosure rises, it can have a lasting effect on the electronics. Tests have shown that an increase in temperature of 18° shortens the life expectancy of electrical components by more than 50%.

Compared to variable frequency drives, which are typically rated for a maximum temperature of 122°, computer components are especially sensitive to heat and should be kept below 104°.

A variety of cooling methods are available. Two of the most common are forced-convection and closed-loop cooling.

Forced-Convection Filter Fans

Convection methods take advantage of ambient air when the temperature surrounding the enclosure is moderate and consistent. Natural convection is the process of hotter, less dense air rising and colder, denser air sinking under the influence of gravity, which consequently results in heat transfer. With the use of louvers or grilles with filters, cooler ambient air can enter the enclosure near the bottom, and hotter air can escape near the top of the enclosure. However, this natural convection method alone usually provides less cooling effect than is necessary for today‘s components.

Forced-convection systems, commonly known as filter fans, employ fans to augment and boost the effect of natural convection. The cooler ambient air is drawn into the enclosure through a filter in the lower part of the enclosure, and warmer air is exhausted above, also propelled through a filter. If the fans are properly sized, a slight positive pressure builds up inside the cabinet in comparison to the ambience, so that only filtered air flows into the enclosure. Forced-convection systems can also be set up with a single fan, either pressurizing the system by propelling air into the system or drawing air at the exhaust.


open-loop system
An open-loop system
Photos courtesy of Pfannenberg Inc.


Filter fans have an additional cooling benefit in addition to removing heat from the enclosure — they also cause air circulation inside the enclosure, which mitigates the occurrence of hot spots that are particularly hazardous for sensitive electronics.

Such devices generally meet the heat removal needs of typical electronic equipment in environments where the cooling load of the electrical components and the ambient temperature are within the acceptable range (the temperature rise inside the enclosure would be limited to roughly a 10°K rise above ambient).

Therefore, filter fans are widely deployed in climate-controlled manufacturing environments, where the ambient temperature is moderate (65°-80°F) and consistent.

However, because ambient air flows through filters into the enclosure, these forced-convection systems are classified as open-loop cooling and do not provide adequate protection in especially harsh environments involving washdown requirements, heavy particulate matter, or the presence of chemicals capable of damaging electrical components

Closed-Loop Cooling

In environments where ambient air and moisture must be kept out of the cabinets (NEMA-4 environments), closed-loop cooling is required. These systems consist of two separate circulatory systems that keep the ambient air from mixing with the air in the enclosure — one system seals out the ambient air, cooling and recirculating clean, cool air throughout the enclosure, and the second system uses ambient air or water to remove and discharge the heat.


closed-loop system
A closed-loop system
Photos courtesy of Pfannenberg Inc.


Typically, these systems employ an air conditioning system with a compressor and refrigerant to cool an evaporator coil that's placed inside the compartment that circulates air from within the enclosure. In the separate compartment that is exposed to the ambient air, the refrigerant flows through a condenser to discharge heat.

This approach is considered a closed-loop system, which means it isolates the electronic components from the outside air. But, because of the compressor, it is more expensive to purchase, consumes considerably more energy, and has higher maintenance and repair costs.

Maximize Rack Space

Filter fans offer a simple, effective, and economical solution for environments with moderate, consistent ambient temperatures that are free of corrosive, oily contaminants or other hazards requiring special protection.

Filter fans have been widely installed in electronics enclosures for decades, but a problem has emerged, as the enclosures are increasingly used for distributed IT systems. Cabinets are being filled to capacity with racks of IT components, leaving little or no room for cooling systems. Often, the cabinets have been designed and filled without thermal management in mind.

Unfortunately, the traditional design of all filter fans placed the fan in a housing that protrudes into the interior of cabinets to achieve a minimal profile on the exterior where the filter is accessed. This design limits the options of where the filter fan can be placed depending on the location of the rails in the cabinet. Sometimes, the arrangement of the rails makes installation of a filter fan impossible. Even when a traditional  filter fan could be installed, it significantly limits the usable space within the cabinet. To take advantage of the benefits of filter fans, valuable rack space is sacrificed.

In response, a new category has been created, specifically designed for electronics enclosures used for IT components — externally mounted filter fans. These so-called “zero protrusion” filter fans house the fans and filters on the outside of the cabinet walls. These systems use the same filters and have the same cooling capacity as traditional filter fans in a product specifically designed for space-restricted locations. Installing the new systems is easy, and new installations and retrofits use the same standard cutouts in the cabinet walls.

Fortunately, with this new option, it is now possible for IT cabinets to fully utilize their rack space — on the manufacturing floor and in other space-restricted locations — while keeping their cool.