How Different Enclosure Air Flow Patterns
Affect the Choice of Air-to-Air Heat Exchangers

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By W. John Bilski and Gregg Baldassarre


Designers of enclosures for electronics continue to explore ways of increasing the density of boards and devices inside electronic enclosures while minimizing internal temperatures. Today's enclosures, cabinet sizes and designs vary substantially in width and height up to seven feet tall. But they have one thing in common: The density of the electronics inside is increasing — and so is the heat load. Properly cooling the internal electronics requires thoughtful selection of heat exchangers in relation to air flow patterns inside the cabinet.

Using natural convection in the enclosure works only if the cabinet contains just a few heat-generating components. If the cabinet doors are opened to improve air circulation — or if high-powered fans are used to move air through the cabinet — then contamination from dust, vermin and debris is likely to occur. To reduce internal temperature, using air-to-liquid heat exchangers or cold plates are not a viable alternative, because they may introduce unwelcome condensate in close proximity to the electronic components.

Fortunately, alternatives exist that are ideally suited for modern enclosure designs. To optimize air flow and heat transfer inside electronics enclosures, two air flow paths are commonly used — a vertical flow pattern and a horizontal flow pattern (see illustrations). Either a heat-pipe core or an impingement core may be selected depending on the air flow pattern. Both technologies will maintain a NEMA 4/IP 66 (water tight) or NEMA 12/IP55 (dust tight) seal of the enclosure.

 

Two-Phase Heat-Pipe Heat Exchangers for Vertical Air Flow

For tall cabinets that employ a vertical air flow pattern, a suitable heat exchanger can be made using two-phase heat-pipe technology. Heat pipes use a unique capillary action that provides extremely effective thermal conductivity. The shape of a heat pipe can be cylindrical or planar. Inside the heat pipe, the surface is lined with a capillary wicking material. The heat pipe is evacuated and back-filled with a small quantity of a working fluid, such as water, acetone or methanol. The fluid absorbs heat in the evaporator region by vaporization. Then the vapor carries the heat to the condenser region where the vapor condenses, releasing heat to the cooling media — in this case, the ambient air surrounding the heat pipes projecting from the enclosure. The condensed working fluid is pumped back to the evaporator side by gravity (or by capillary action if working against gravity).

Heat pipes are a totally passive heat transfer system. There is no pump to wear out or that requires an additional energy source. In application, the heat pipes are arrayed as a parallel-plate, top- or side-mounted “HX-style” heat exchanger. Separate internal and external energy-efficient fans draw air through the interior and exterior portions of the heat exchanger, which are separated by a flange and closed-cell neoprene gasket. This arrangement prevents the introduction of potentially contaminated exterior air into the sealed NEMA (National Electrical Manufacturers Association) enclosure.


In an "HX-style" heat exchanger, separate internal and external energy-efficient fans draw air through the interior and exterior portions of the heat exchanger, which are separated by a flange and closed-cell neoprene gasket.


In enclosures employing horizontal air circulation, double-sided impingement technology is used in front- or side-mounted "HXi-style" low-profile heat exchangers.

Versatile mounting options allow for heat exchanger placement on doors, side, top, or back of the enclosure. The light weight and compact size of a heat-pipe heat exchanger make it ideal for space-constrained applications. The drawback is that, due to the heat-pipe heat exchanger's compact and highly efficient design, it may be subject to fouling in dirty or oily industrial environments.



 

Impingement Heat Exchangers for Horizontal Air Flow

In enclosures employing horizontal air circulation, double-sided impingement technology is used in front- or side-mounted "HXi-style" low-profile heat exchangers.

The double-sided impingement system utilizes a core that separates the internal and external sides of the enclosure. Internal fans draw in the hotter, inside air and blow it toward the fin core (see horizontal illustration). By impinging against the fins, heat is transferred from the air to the fin core. Likewise, a set of outside fans draws in the cooler, ambient air and blows it toward the outer side of the fin core, removing the waste heat.

The fin core is made of thin aluminum, a material that reduces the thermal resistance (due to conduction) to negligible levels. A double-sided impingement heat exchanger can dissipate twice the heat load compared to conventional counter-flow heat convection in which air flows parallel to the fins. Thanks to its design and efficiency characteristics, an “HXi-style” heat exchanger is comparatively 50 percent smaller. The size reduction allows more packaging flexibility for enclosure manufacturers.


In enclosures employing horizontal air circulation, double-sided impingement technology utilizes a core that separates the internal and external sides of the enclosure. Internal fans draw in the hotter inside air and blow it toward the fin core.

Double-sided impingement heat exchangers also maintain the NEMA 4 and NEMA 12 integrity of the enclosures and also may meet UL and Telcordia GR-487-CORE standards.

As a closed-loop system, a double-sided impingement system employs fewer moving parts to reduce the chance of mechanical problems. Stock units are available as 2, 4, 6, or 8 fans with a low noise level of 64.5 dBA for the two-fan version. Custom designs are available to handle high wattage heat dissipation requirements. Typical installations are performed using low-profile design and modular dual-fan configuration. Depending on the cabinet size, double-sided impingement heat exchangers can be deployed singly or in combination to provide installation flexibility in any orientation. Integrated custom features such as alarms and temperature controls are also available as an option.

Additional advantages of a double-sided impingement system include a shorter air flow path through the electronics. The shorter path means less resistance to air flow and the possibility of eliminating the fan tray(s). Furthermore, due to the wide air gap between the impingement plates in the core, air can flow freely, reducing the risks of fouling in dirty or oily environments.



Ensuring the Appropriate Heat Exchanger Selection

Making the proper heat exchanger selection will ensure that the electronics cabinet can control the heat generated from the components within the enclosure. Effective heat transfer lengthens equipment life and improves reliability. Two heat exchanger technologies have emerged that provide optimum performance for two common enclosure air flow patterns: two-phase heat-pipe heat exchangers for vertical air flow and impingement heat exchangers for horizontal air flow. With proper application of advanced heat-pipe or fin-core components within the given heat exchanger type, reliable and efficient thermal protection can be achieved in a wide range of cabinet sizes and profiles.



 

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