Leaving sensitive electronic equipment out in the weather is asking for trouble, and the solution is not as simple as building sheds and outdoor cabinets. Historically, telecom equipment has been housed in such structures, and the cooling of these facilities has been satisfactorily implemented using traditional methods. How-ever, in many newsystems being developed and deployed today, such as broadband, ISDN, cellular/wireless, and cable Internet services, heat dissipation densities have been increasing considerably, raising the possibility of heat-related failure.
Serious constraints
The introduction of electronics into the outside plant has imposed serious constraints on enclosure design, since temperature and humidity are the two major causes of
electronics failure in the telecommunication industry. Because systems may not include environmentally resistant designs, enclosures must provide an environment in which
these systems can survive.
To meet the demands of protecting equipment in indoor and outdoor environments, the industry has been investing time and effort into thermal management. Thermal
management of the outdoor enclosure has become an issue of paramount importance. It is only necessary to remove the internal heat generation from the electronics due to the
effects of solar loading which can be substantial depending upon the size of the enclosure and its orientation towards the sun.
Thermal management of telecom equipment needs to be tailored to the type of environmental conditions the equipment will encounter. Equipment is usually classified as indoor
or outdoor equipment. Indoor applications include switching equipment (cards for different applications), servers, routers, modems, and batteries that will be housed either
in central office (CO) installations or at the customer's premises, depending on the end use. Outdoor applications include all the functions normally found in COs but housed in
environmentally controlled enclosures, sheds, and shelters.
In addition to ensuring that equipment remains at optimal temperature and humidity conditions, the telecom engineer must ensure that the equipment remains at reasonable
thermal conditions for short periods of time during power outages or cooling equipment failures until main cooling is restored. Thus, the designer must consider appropriate
thermal management systems for normal conditions (steady state) and for emergency conditions (transient).
These guidelines apply to both CO and customer premises equipment (CPE) indoor installations as well as outside plant equipment. Aside from normal switching and processing
equipment, it is vital to pay attention to the battery equipment that supplies power backup during power outages (battery equipment also requires thermal management and
has particular housing requirements that require proper venting).
Indoor enclosures
Let's consider a typical system in which ISDN/HDSL cards (possibly including fiber-optic components) are housed in shelves that generally need to comply with Network
Equipment Building System (NEBS) standards. The normal approach focuses on the thermal performance of individual, fully-populated, fully-powered ISDN/HDSL cards, and
the thermal performance of fully-populated shelves of ISDN/HDSL cards.
The shelves were studied in particular to ascertain whether the shelves would be NEBS-compliant. Figure 1 shows a typical shelf distribution for this system type.
Thermal management analysis and design work needs to include board- and system-level considerations, heat flow should follow the least-resistance path from the boards
(single vertical boards or boards inside discrete pieces of equipment such as modems, and routers) to the ultimate heat sink (the surrounding air or the air-conditioning
vent).
For the case of individual cards such as ISDN or HDSL cards, the goal is to determine if the board's thermal performance is within the design parameters and to find ways to
reduce board temperatures. Computational fluid dynamics (CFD) simulations may be used to determine that the cards do not present problems with respect to high
temperatures except under the blockage points such as the ribs in shelf cage.
Tests should always be conducted to obtain typical temperatures found in the system during normal and full-power operation.
By doing so, we can corroborate the CFD simulations and assure that the system will be operating at relatively low temperatures.
After card thermal analysis is conducted, system level analysis using banks or shelves of these boards are normally conducted.
The above systems are typically encountered in CO settings, where NEBS requirements must be met. Typical inlet conditions for the equipment is 50 degrees C, and
temperatures of surrounding air inside the shelves must not exceed 85 degrees C (this is set by the component manufacturers). In addition to thermal conditions, the
manufacturer of the equipment must meet Under-writer's Laboratories (UL) requirements as well as other international regulating bodies and agencies that prescribe
dimensional restrictions for such considerations as fire propagation and seismic structural integrity.
Outdoor enclosures
Outdoor cabinets are normally designed to house various equipment configurations with dissipating heat rates raging from 500 to 10,000 W, depending on the size and type of
equipment. These cabinets are installed in various environmental conditions and should be fitted with either air conditioning or air-to-air heat exchangers as needed.
The goal of the designer is to maintain the peak temperatures in the cabinets below a certain level, which is normally prescribed by the electronic equipment manufacturer.
Humidity levels are of concern as well, but since most cabinets are sealed and their temperatures are much higher that the air's dewpoints, humidity is generally not a
problem (after the transient effect of opening and closing the enclosure is eliminated).
The designer should be aware that the air temperatures within the cabinets will be a function of heat generated by the electronic and cooling equipment. Other ambient
conditions include: temperature, solar radiation, wind speeds, objects surrounding the cabinet (shading, ground reflections, buildings, and trees), cabinet design (surface
area, shape, paint), and air infiltration.
Figure 2 presents a comprehensive sketch of a typical outdoor cabinet. Often battery back-up units are stored within the cabinets in separate compartments. The battery
compartments must be vented that harmful fumes can escape. Cabinet design must ensure an evenly distributed battery temperature and the batteries must be kept at 25
degrees C when at all possible. Batteries are not actively cooled (they are only heated), however, there are some cabinet makers that have begun installing active cooling
systems for battery trays.
Load calculations
We are now ready to begin the design of the thermal management system for a typical cabinet. The first step is to realize that the design temperature is the temperature the
cabinet air will reach when there is heat balance. In equation form, this looks like:
Qbalance = 0 = Qequipment+
Q solar_load + Qcooling_system
where, Qequipment is the electronics heat dissipation, Qsolar load is the solar heat load, and Qcooling-system is the amount of heat removed by the cooling system. The solar
load is a complicated term because it includes contributions from all modes or heat transfer. To illustrate, consider this equation:
Qsolar_load = Qradiated +
Qconvected + Qconducted
Normally, the value of Qradiated will always be positive (towards cabinet), but the other two can be either positive or negative, depending on the cabinet's temperature. Thus,
if Qbalance is not zero, the temperature inside the cabinet is either higher or lower than the set temperature and the cabinet is losing or gaining heat by convection and
conduction.
Two techniques, Sol-air and modified Sol-air, are normally used for cooling load calculation methods, as stipulated by the American Society of Heating, Refrigerating, and
Air-Conditioning Engineers (ASHRAE). The Sol-air method involves calculating heat loads using an external temperature that lumps radiation effects and sensible air
temperature. It is important to understand that the cabinet's solar load is calculated for the worst conditions in these calculations; solar load values can range from 50/100 W
up to 3000/4000 W, depending on the size, internal, and external conditions.
Calculations would also include instantaneous solar radiation effects and delayed effects. The delayed effects include the slow build-up of energy that the external walls
accumulate as they absorb solar radiation. There are several approaches to the cooling of enclosures, involving both active and passive techniques.
Fully active: air-conditioning
Once the heat rate to be removed has been calculated, a cooling system must be matched to the outdoor cabinet. If, for example, cabinet air temperatures must be kept below the
maximum ambient (outside) conditions, the preferred method is the installation of air-conditioning units. Typically, capacity does not include capacity for cooldown, since the
calculations were carried out for steady-state operation.
A system of this nature does not include cooling capacity to bring the system design inside temperature from 55 degrees C or above starting conditions. Thus, transient effects
are not included. Typically, air conditioners have outlet air temperatures of around 15 degrees C (or below) to achieve the cooling required.
Similarly to fully assisted cooling systems, once the heat rate to be removed has been calculated, a cooling system must be matched to the outdoor cabinet. If, for example,
cabinet air temperatures are kept above the maximum ambient conditions and the load is not too high, an air-to-air heat exchanger is suitable.
Heat exchangers allow for sealed electronics compartments, but have much lower operating and maintenance costs (along with battery back-up service) for short downtime
periods. Heat exchangers' heat removal capabilities, unlike air conditioners, change as a function of cooling air and cabinet air values; in fact the heat removal rate is a
function of the differential (Tcab - Tamb). Unfortunately, if off-design temperatures are encountered, either the cabinet overheats or overcools.
Passive means:
natural convection
Smaller enclosures (in which relatively high temperatures can be tolerated) can be cooled by passive means. Passive methods include primarily natural (free) convection and
phase-changing materials (PCMs). Natural convection is the transport of heat by buoyancy-induced fluid flows. Hotter fluid heated, for example, by a hot wall (exposed to the
sun) rises and displaces colder fluid.
The situation becomes more complex as power electronics are added, but there are ways to let the heat generated by the equipment be carried away by natural convection.
However, the designer must always keep in mind that the overall goal is to transfer heat outside by natural convection in order to keep internal temperatures low.
PCMs are substances that change phase, most often from solid to liquid, as they absorb heat. Typical PCMs include waxes, salts, and paraffin compounds for high temperature
applications, and water (ice) for low temperature applications. These materials are kept inside the cabinets in appropriately sealed enclosures, and take advantage of thermal
inertia and phase change effects.
Figure 3 shows an enclosure with PCMs during the daylight hours when solar heat will be absorbed and not allowed to heat up the cabinet air. At night, the heat absorbed during
the day will be released to the outside. While this takes place, heat will continue to be transferred in and out through the cabinet walls.
Battery compartments
Batteries for backup service are normally stored in compartments attached to or inside outdoor cabinets. These compartments are exposed to solar loads and must be kept at
optimum temperatures as prescribed by the manufacturers. These compartments must allow for the proper venting of fumes that may be given off by the batteries in the
course of their operating life. There are several thermal-management scenarios, involving battery compartments, including air conditioners/refrigerators, thermo-electric
coolers, and air-to-air heat.
If battery compartment air temperatures must be kept below the maximum ambient (outside) conditions, the preferred method is the installation of air-conditioning units.
This is typical for most systems, especi-ally those designed to be installed in all-weather conditions. The only problem with typical air conditioners is their size, since they
come in capacities much larger than needed for small compartments.
Thermo-electric cooler
Since the cooling load of most typical battery compartments is not high, thermo-electric coolers are a possibility. These are systems where cooling is achieved electrically
using the Peltier effect (of reversible electromagnetic thermodynamics). Although reliable, thermo-electric coolers are inefficient and not well-suited for remote outdoor
cabinets.
In fully-assisted cooling systems, once the heat rate to be removed has been calculated, a cooling system must be matched to the battery compartment. If battery compartment
air temperatures need not be kept below the maximum ambient (outside) conditions and the load is not too high, (which is the case for battery compartments), an air-to-air
heat exchanger is the preferred system.
In many cases, since cooling loads are not very high, flow-through fans can be used to remove excessive heat and moisture build-up in the battery compartments. Fans can also
be used for the thermal management of the compartment using thermal inertia.
Passive cooling for battery compartments
Battery compartments can be cooled by passive means; these methods include primarily natural (free) convection and the PCMs discussed earlier. The designer must test
thermal simulations with different likely scenarios such as battery placement and external temperatures.
PCMs are also being used for battery cooling. These materials are kept inside the compartment in appropriately sealed enclosures and take advantage of thermal inertia and
phase change effects. For example, an enclosure with PCMs during the daylight hours will absorb solar heat, and this energy will not heat up the cabinet air. At night, then, the
heat absorbed during the day will be released to the outside. While this occurs, heat continues to be transferred through the cabinet walls.
Whether a designer chooses active or passive means, and fol-lows any of the specific considerations we've outlined, it should be apparent that there is a systematic and
intuitive way to go about designing enclosures for efficient and safe thermal management.
By carefully considering the environmental conditions and manufacturer's specifications for the equipment you'll be housing, along with these guidelines, your equipment
should remain safe under any condition.
Maurice J. Marongiu is president and co-owner of MJM Engineering Co. Inc. He received a BS, MS, and Ph.D. in mechanical engineering from the University of Illinois at Urbana Champaign and has over 20 years experience in the thermal management fields. He may be reached at maurice@mjm-engineering.com.
