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This column outlines the factors to be considered when considering outdoor enclosures (or cabinets) to house wireless electronic equipment. This will review typical environmental conditions and affiliated thermal management recommendations. Typical equipment housed in the enclosures include base transceiver station (BTS), E-911 location measurement unit (LMU), wireless backhaul, fixed microwave transceivers, and power and battery support.
As wireless operators continue to build out their infrastructure to support ever-increasing bandwidth requirements, one commonly overlooked aspect is enclosures and cabinets needed to house these electronics. Enclosures come in a multitude of configurations, and operators need to understand the long term operating costs affiliated with their selection of features for enclosures and cabinets. In many cases, the installation of a thermally managed outdoor enclosure will be more cost effective and practical than a new building or shelter.
Thermal management systems
There are a number of standards and specifications for the thermal performance of an outdoor cabinet. Although written for wireline carriers, Telcordia GR-487-CORE is used by wireless industry and is based on the foundation that the thermal management system must properly maintain equipment with an outdoor ambient temperature range between -40°C and +46°C. The design of the thermal management system must take into account the operating temperature range of the enclosed electronic equipment, and address the heat dissipation from that equipment plus the total heat load contribution of solar radiation from exposure to direct sunlight.
Outdoor enclosures and cabinets commonly deploy four main types of thermal management technologies – direct air cooling, air conditioning, air-to-air heat exchange and thermoelectric cooling.
Direct air cooling (DAC): A combination of vents, fans and filters, DAC systems are open-loop systems that bring outdoor ambient air into the interior of the enclosure for cooling purposes. Fan capacity can range from 50 to 1000 cubic feet per minute for enclosures. Relative to actively cooled systems, DAC systems are highly reliable and have long life expectancy because the only moving parts are the fans to circulate air. DAC systems typically employ inexpensive mesh filters to prevent particulate contamination from entering the enclosure, or alternatively can also be equipped with high-performance hydrophobic filters that prevent moisture entry into the enclosure.
Heat exchanger (HEX): HEX’s provide superior above-ambient temperature management. HEX units are closed-loop systems, meaning they isolate the outdoor ambient air from the interior air of the enclosure. Closed-loop systems prevent intrusion of any particulate contaminates, such as dust, moisture and humidity from entering the enclosure. HEX’s are highly reliable and have long life expectancy because the only moving parts are the fans to circulate air.
Air conditioner (A/C): A/C offers the highest performance thermal management technology, able to support very high heat loads and cool the interior of the enclosure far below ambient air temperatures. Most A/C units deployed with outdoor enclosures are closed-loop systems, and are based on a vapor compression cycle where a refrigerant undergoes a change of state (from a liquid to a gas). The refrigerant absorbs thermal energy from within the enclosure and transfers it to the outdoor air. Additionally, this process also removes humidity from within the enclosure.
Thermoelectric cooler (TEC): TEC’s use the Peltier-Effect, where current is applied across two dissimilar materials and creates a temperature differential. TEC’s offer variable and scalable incremental cooling or heating in a compact form factor. TEC’s have very high reliability and long life expectancy because the only moving parts are fans to circulate air. TEC’s lack any moving parts or circulating liquid, require no maintenance and consume far less energy to operate than A/C.
Many external factors affect the thermal performance of an outdoor enclosure – primarily the range of seasonal temperatures and humidity, the thermal load produced by the sun, and the thermal load produced by the equipment. More than any other feature, the thermal management system could have the largest impact on the operational expenses.
Thermal management features
Using the parameters of the enclosure’s intended environment, enclosure manufacturers can use off-the-shelf systems, or design and build a custom thermal management system to accommodate your enclosure configuration. Other features affecting the design of the thermal management system includes:
Zone cooling: The enclosure, and all of its affiliated accessories, should accommodate multiple temperature zones. This design methodology enables the enclosure to keep one zone at a different temperature than another. By understanding the operating ranges of the groups of equipment in the enclosure, the cooling system can be designed to maintain each group of equipment within the recommended temperature ranges. This capability is especially useful for enclosures that house both batteries and electronics.
Door supporting multiple systems: The enclosure door should support up to three different cooling systems, thus creating three different cooling zones. The manufacturer can analyze and model the thermal environment of each zone, and can recommend the optimal cooling system that complies with the equipment specifications while minimizing operational cost. Multiple intelligent controllers should be supported to manage up to three zones as well.
Intelligent controllers: A critical consideration when selecting an enclosure cooling system is an intelligent controller. An intelligent controller, in combination with multiple temperature sensors, can control the operating speed of the air-movers. These variable-speed controllers regulate the air flow, maintain a more even temperature within the enclosure, reduce energy consumption and reduce acoustic emissions outside the enclosure.
The selection of the optimal thermal system, and its affiliated power consumption and cooling capacity, will substantially affect the combined operating and capital costs over the life of the enclosure.
