Let's Exchange Some Heat

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Passive cooling devices like air-to-air heat exchangers effectively cool electronic enclosures to within a few degrees of ambient temperatures, thereby protecting sensitive electronics from overheating.
By Gregg J. Baldassarre and W. John Bilski

Enclosure thermal management in today’s high power applications has proven to be a challenge for the system-level engineer. Heat loads generated by electronics inside cabinets — also known as enclosures — are increasing. In the past, many applications could be cooled by allowing the surface area of the cabinet to dissipate the internal heat load by radiation and natural convection. However, with the internal heat loads in communications and other electronics cabinets reaching 8 kW and higher, the system-level engineer must develop a cooling strategy that meets the thermal and overall packaging requirements of the application.

Most engineers favor passive cabinet cooling solutions such as air-to-air heat exchangers as opposed to active solutions such as liquid-to-air heat exchangers or air-conditioners (compressor or Peltier-device based).

• Passive air-to-air heat exchangers utilize the internal cabinet air and external ambient air circulated by fans to cool the cabinet interior.

• Active liquid-to-air heat exchangers utilize a mechanical pump to circulate a single-phase fluid such as water, and air-conditioners use liquid refrigerants and a compressor to move the coolant, or Peltier devices.

Of course, the use of a passive cabinet cooling solution implies that the application can tolerate an air-to-air heat exchanger solution driven simply by the difference in temperature (ΔT) between the external ambient air temperature and the maximum allowable internal cabinet air temperature condition. Heat exchangers are inherently simple devices that do not require complex mechanical parts such as compressors. Thus, the air-to-air heat exchanger offers a balance of reliability and low cost.

This article focuses on the operation and advantages of various passive air-to-air heat exchanger enclosure cooling solutions. Passive air-to-air heat exchanger styles include:

• Heat pipe heat exchangers.

• Air-impingement heat exchangers.

• Crossflow heat exchangers.

In addition, the article will briefly discuss augmenting air-to-air heat exchangers with liquid-cooled cold plates and active liquid-to-air heat exchangers for electronics enclosure cooling.


Cabinet Cooling Considerations

Cabinets, or enclosures as they often are called, are engineered to provide a protective environment for the equipment they contain. The system-level engineer will review the various cabinet and environmental specifications published by organizations such as the National Electrical Manufacturers Association (NEMA), International Electrotechnical Commission (IEC) and Telcordia (formerly Bellcore).

Outdoor telecommunications cabinets commonly will follow NEMA 4 guidelines for enclosure integrity. NEMA 4 implies that the cabinet will provide “some protection from windblown dust, rain, splashing water, hose-directed water and ice damage.” These cabinet specifications also would be applicable to any air-to-air heat exchanger that is integral with the enclosure. If the cabinet must conform to NEMA 4 then so must the cooling system. It must provide cooling without compromising the cabinet integrity. A NEMA 12 cabinet also is sealed; however, a NEMA 12 sealed cabinet only is required for indoor applications, so the seal is more dust resistant than water resistant (no hose-down testing). Hose-down testing is shown in figure 1.

In addition to reviewing industry standards, the system-level engineer must carefully consider the effects of solar loading before designing a cooling strategy for an outdoor cabinet. Insulation (double walls) and solar shields often are employed to reduce the solar effect. Without proper consideration, the thermal management of a cabinet can become out of control.

Solar loading is highly seasonal and is dependent upon the latitude where the cabinet is located. For example, a cabinet that is 5 by 3 by 4' typically would be subjected to a solar heat load of approximately 100 W/ft2 (325 W/m2) in the United States. Much of this solar heat is reflected, so a more reasonable number for heat absorption into the cabinet is usually about 25 W/ft2 (82 W/m2) applied to the southern side or cabinet roof. Thus, the solar heat load would correspond to a total (worst-case) heat load of about 438 W on the roof of the cabinet. Additional heat would be absorbed by the cabinet sides; however, the amount would depend on the cabinet’s orientation to the sun. A worst-case scenario often is derived by assuming solar load on two adjacent sides as well as the roof. Insulation and shielding on all sides could reduce the load by as much as 90 percent.


Internal Air Handling

An effective enclosure or cabinet system-level thermal-management strategy always has an efficient air-handling plan as its backbone. Excessive turns, tortuous pathways and air short-circuits can lead to reduced performance at the heat exchanger. Software programs such as FloTherm and Icepak can be used to model airflow patterns. Some companies have conducted smoke testing to help visualize airflow patterns within a cabinet. Airflow through the board rack area is particularly challenging due to the current dense packaging demand. It often takes trial-and-error testing to optimize performance.

Objectives for Delta T

Determining the optimal temperature differential — often called delta T and abbreviated as ΔT — generally is a function of two factors:

• The maximum ambient temperature condition.

• The maximum operating temperature of the componentry inside the cabinet.

For telecommunications applications, most engineers use Telcordia’s -40 to 115°F (-40 to 46°C) external temperature as a starting point. The capacity of heat exchangers is based upon a balance of surface area and airflow rate (figure 2).

For comparison’s sake, assume that a heat exchanger of a given configuration — with an air supply of 500 cfm per side and a 2,000 W heat load — requires 19 m2 of surface area. With the same fin design and 250 cfm per side air supply, the same load might require 54 m2 of surface area. It is important to note that the relationship between surface area and airflow is not linear but is affected by a number of factors. As the airflow rate increases, the amount of fin area is reduced, but with diminishing returns.

The system-level engineer must balance the ΔT needs of the system with the available mounting space and tolerance for acoustic noise generated by fans. A larger ΔT or a larger heat exchanger means that the design engineer can use smaller, quieter fans. Typically, a ΔT of less than 18°F (10°C) will result in an unacceptably large and noisy heat exchanger solution.


Heat Pipe Heat Exchangers

Heat pipes have been known for many years as an effective solution for component- and board-level cooling. An array of heat pipes easily can be incorporated into air-to-air heat exchanger core assemblies. The array of heat pipes is used as a heat transfer mechanism for moving heat from within the cabinet to the external environment. The heat pipe working fluid absorbs the heat inside the cabinet, resulting in evaporation of the working fluid. The vapor then carries the heat from within the cabinet to the cooler portion of the heat exchanger residing external to the cabinet, where the vapor returns to a liquid state, giving up its latent heat. Like all heat pipes, the angle of operation must be considered. A heat pipe typically depends upon a wick structure to return the working fluid to the evaporator. Heat pipes in heat exchangers typically use screen or grooves as a wick structure, meaning that they must operate in a gravity-aided orientation (installed in the roof of the enclosure) or horizontally (installed in a side wall).

As shown in figure 3, the array of heat pipes in the heat exchanger core moves heat from within the cabinet to the external environment. Half of the heat pipe array core is contained within the cabinet and the other half occupies space external to the cabinet. The heat pipe array core allows internal and external isolation via a sealed manifold or separator plate.

Similar in appearance to radiators, these units use forced convection to dissipate up to multi-kilowatt loads. Cabinet integrity is dependent upon the fabrication method of the core assembly and a solid divider plate that separates the internal airstream from the external ambient stream.

An important feature of the heat pipe heat exchanger core is its ability to be adapted into various mounting strategies. They include:

• Inside ductwork.
• External or internal side mount.
• Roof mount.
These packaging methods can allow the heat pipe core assemblies to be integrated into traditional mounting configurations seen in today’s enclosure cooling applications.

Heat pipe heat exchangers also offer the ability to tailor the fin pitch — number of fins/inch — to accommodate unique airflow rates as system pressure drops in both the ambient and internal airstreams.


Impingement Designs

Impingement air-to-air heat exchangers use a thin diaphragm of material to separate the internal and external airflows. The diaphragm also allows the exchange of heat from one side to the other. Air is centrally directed into a metal “folded fin” diaphragm (typically aluminum) and exhausted from either side of the fan (figure 4). The folded fin allows a large surface area for heat transfer in a small package. One of the advantages of an impingement-style air-to-air heat exchanger is it typically is less intrusive on the internal cabinet volume than a heat pipe heat exchanger.

For cabinets with horizontal components, the flow of heat to the top of the cabinet is blocked by the components above. In a scenario such as this, a front-to-side flow pattern may be more effective. Likewise, in cabinets with dense vertical components, fan trays often are used to overcome the pressure drop from the bottom of the cabinet to the top. The airflow pattern of an impingement-style heat exchanger sometimes can eliminate the need for the fan tray because the pressure drop front to back may be 25 percent or less than the pressure drop from the bottom of the cabinet to the top.


Crossflow Air-to-Air

For cabinets with high heat loads (typically greater than 1,500 W), the cross-flow air-to-air heat exchanger tends to be most effective. While many varieties of air-to-air crossflow heat exchangers exist, their operating principles are similar. Typically, the hot air from within the cabinet is circulated through the crossflow heat exchanger and cooled, then returned to the bottom of the cabinet. Likewise, the cool, external air is taken into the bottom of the heat exchanger and the warmed air is exhausted back to the outdoors at the top of the heat exchanger.

The parallel air paths are typically separated by thin layers of aluminum that make up the crossflow heat exchanger core, similar to the impingement-style heat exchanger. The key difference is that the internal cabinet and external ambient airstreams are in crossflow vs. being impinged.

The edges of the thin aluminum core typically are sealed with a room temperature vulcanizing (RTV) compound. Care should be taken when specifying crossflow heat exchangers as some are made with RTVs that evolve acetic acid as they cure. The acidic vapors can cause corrosion of sensitive components when trapped in sealed cabinets. Thicker sections of RTV can take weeks or months to fully cure, meaning that acidic vapors may still be generated after the heat exchanger is installed in the sealed cabinet.

While a primary advantage of crossflow heat exchangers is the ability to dissipate higher cabinet heat loads, a potential disadvantage is the greater airflow rates associated with crossflow heat exchangers generate relatively high noise levels.


When Air Is Not Enough

In some electronic cabinet cooling applications, the cabinets contain high power components that cannot be cooled by circulating air alone, or the external ambient air temperature is not cool enough to allow an air-to-air heat exchanger to solve the problem unaided. In these applications, cooling devices such as liquid-cooled cold plates and liquid-to-air heat exchangers are added to the cooling solution to maintain proper cabinet temperatures.

Air-to-Air Heat Exchangers Augmented with Liquid-Cooled Cold Plates. For high power components that cannot be cooled by the internal cabinet air, the heat load or watt density of the electronics is high enough that they require pumping a single-phase liquid to provide direct component- or device-level cooling. Traditionally, a glycol-based coolant such as ethylene or propylene (preferred due to its lower toxicity) glycol, mixed with water, would be pumped through a cold plate directly attached to the high power component under these high load or high watt density applications. Because air has limited capacity as a heat transfer medium and is a poor conductor of heat, removing heat from an electronic component via pumped liquid circulation is more effective for high heat load or high watt density applications.

Liquid coolants are capable of removing more heat with less temperature difference than air would in the same application. By taking the heat directly from high power-dissipating components, you reduce the heat load being dissipated into the internal cabinet air. As a result, less heat load is transferred to the air-to-air heat exchanger being used to cool the enclosure.

Liquid-to-Air Heat Exchangers. In some applications, even though device-level heat is being collected through a liquid-cooled cold plate attached directly to the high power components, the ambient air external to the cabinet still is not cool enough to maintain the internal cabinet temperature at an acceptable or required level. As a result, when a liquid coolant such as water is available, an active liquid-to-air heat exchanger can be used by the system-level engineer to cool the enclosure.

In this case, the hot air is circulated through the liquid-to-air heat exchanger mounted in the bottom of the cabinet. The cold air exiting the liquid-to-air core then passes up through the cabinet and — after being warmed by the electronics — is returned to the bottom of the cabinet to be cooled again.

Liquid-to-air heat exchangers are mounted in the bottom of the cabinet to minimize moisture concerns within the cabinet. For instance, if a leak occurs, it will not drip on the electronics if the liquid-to-air heat exchanger is installed in the bottom of the cabinet. Likewise, the cold liquid-to-air heat exchanger can accumulate condensation. If this is possible, a drip pan and drain should be incorporated into the cabinet design below the heat exchanger.

In summary, a variety of heat exchanger thermal solutions exist for enclosure or cabinet-level cooling. Only sealed cabinets must use heat exchangers because nonsealed cabinets can circulate room or outside air through the cabinet to keep the components and the internal cabinet temperature within acceptable levels.

However, even cabinets that do not need to be sealed sometimes use heat exchangers because sealing the cabinet keeps out dirt and insects that can make cabinet maintenance troublesome. The heat exchanger must maintain at least the same level of integrity as that of the cabinet (NEMA 4, NEMA 12, etc.).

The largest air-to-air heat exchanger imaginable will only bring the temperature of a cabinet close to ambient. Air-to-air heat exchangers can never bring the internal cabinet temperature below the external ambient temperature (i.e., refrigerated). To get internal cabinet temperatures below ambient requires a liquid-to-air heat exchanger and a source of sub-ambient coolant, or an air-conditioner.

In many applications, heat exchangers are preferred over air conditioners because they are cost effective, reliable and use less power for a given amount of cooling, which reduces operating costs. Power usage is important not only because of the cost of electricity but also because the system may be forced to run off battery backup during power outages.


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