How to Design Proper Thermal Management into Custom A/V Racks

More complex home theaters are adding more BTUs to the contemporary residence, increasing the need to better regulate rising temperatures in A/V component racks.
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Houses are getting smarter. Dave Farber, futurist and computer science professor at the University of Pennsylvania, noted in a recent New York Newsday article that the familiar names in home appliances like Maytag and GE are now joined by other equally familiar names, though ones not usually associated with refrigerators and washing machines. Behemoths such as IBM, Intel and Microsoft are now contenders in the home appliance arena. Systems designers/installers for residential environments will need to learn a wide range of new technologies and integration strategies to create these smart homes.

As cool as the future might be, however, it won't change the laws of physics. The need to manage heating and cooling in these more complex homes remains as critical as ever-and even more so when you consider how many heat-generating electronics will be part of them. The home theater trend alone is adding both dBs and BTUs to the contemporary residence, between speakers, amplifiers, video projectors and digital signal processors. The bottom line is that as homes become more systems-oriented, your approach to their heat management has to be systematic as well; the future of media in the home is a system of distributed audio and video.

Establish Relationships. The best place to start is in the planning stages of a new construction or renovation, such as coordinating architectural plans, building contractors, HVAC plans and millwork specialists. Unlike industrial applications, thermal management of electronics in the home has to take into account that these management strategies need to be transparent to users. The systems designer can take the lead in establishing a relationship between these key players in assuring that heat management is an integral part of the house. For instance, informing the cabinetry maker that a particular piece will need to have its heat vented from the top while drawing cool air from below is not enough if the builder doesn't provide some place to vent that heated air. The earliest stages of construction are the best times to note critical details, such as which way studs are running, to see where best to place flexible hoses and blowers, and to see how HVAC is running through the structure.

oManagement. As noted earlier, complex homes won't change the fact that heat and air still operate by certain basic principles. The way to best understand thermal management is to think of it not as introducing cool air but rather as removing heated air. Heated air behaves in a predictable way in enclosed environments. One of those predictable characteristics is that it does not simply blend with newly introduced cooler air; instead, pockets of warmer and cooler air can develop. This can have the effect of keeping the cooler air from reaching the places it's needed most and concentrating warmer air where it can cause problems. For instance, putting an air conditioning vent into an electronics cabinet with the intent of cooling it will cause formation of what are known as thermal gradients. The preferred approach to this situation is to install an air conditioning return vent, which will pull heated air out of the space.

While there are two distinct approaches to thermal management-active and passive-it's the active approach that best lends itself to residential environments, and this involves integrating fans to move air in a predetermined path at a predetermined rate.

Much of the heat generated by residential entertainment and home office electronics will be contained in cabinetry and closets as well as built-ins throughout the house. These are the locations where so-called "hot spots"-pockets of heat-can crop up. With the use of effective thermal management techniques, the integrator (and homeowner) can avoid these hot spots in the first place. Heat should be exhausted out if the ambient air inside the closet exceeds 75 degrees F, which provides a sufficient buffer before reaching the maximum recommended constant operating temperature for most equipment, which is 85 degrees F. (Studies have shown that for every 10 degrees F rise over 85 degrees F, digital equipment life is reduced by approximately 40 percent or more.)

Experience provides plenty of evidence that moving air from the bottom of a cabinet toward a vent at the top is optimal. This tends to result in the lowest internal cabinet temperature. The most common flow configuration, however, is flow from the front of the cabinet to the rear or sides, particularly with amplifiers, which is also manageable. Natural convection-the tendency of warm air to rise-can be used to the installation's benefit. The more heat, the more cubic feet of air per minute (CFM) flow occurs. Just be aware that vents must be carefully located, since vents also create friction; the more open area, in the form of slots or perforations, the better. For multiple convection-cooled amplifiers, put vents in between, unless the amplifier manufacturer states otherwise. Most other non-amplifier equipment that has internal fans will draw air in through the rear (or sides) and exhaust out the sides (or rear). This re-circulates the cabinet air and care should be taken as to its placement so the natural convective rise of heat is not disturbed. Downward airflows, however, are almost always a bad idea, creating "mixed convection" (mixture of forced air and convection) during operation and in the event of fan failure.

In the case of a single rack in cabinetry, it is important to use a fully louvered closet door and monitor the temperature when there is no air conditioning feeding the closet.

oThe Fan. Fans are key components of active thermal management and will substantially lower interior operating temperatures as long as the intake vent placements, size, and airflow are done properly. Beware of "short-circuiting" the airflow, which occurs from having in-take points close to the fan. Venting in the wrong locations can also cause hot spots where air does not flow. Proper fan/vent placement will force more air diffusion inside of a rack, breaking up these hot spots. Additionally, fans help reduce condensation in colder environments.

As mentioned earlier, because top-venting and leading air in a bottom-to-top flow is preferred, it follows that mounting fans at the top of enclosures is also the best way to go. It's recommended that such fans be rack mounted vertically to avoid contaminants falling into the rack from above. An important point to make here is to avoid placing side vents near fans. A top-mounted fan will suck in air from the vents, not air down inside the enclosure that needs to be vented.

Where do we put the vented hot air? In the case of vertical rack-mounted fans, air is exhausted into the room. For systems employing top-mount fans, venting to a secondary space, such as a basement or an attic, is recommended. If that's not possible, another solution to venting hot air from cabinetry that's not near the HVAC ducting is to take flexible air ducting, of the type found at your local home improvement stores, run a line of that from the cabinetry to an in-line bathroom-type exhaust fan, and then through more flexible ducting to another room, where it will mix with the ambient air and be handled routinely by the HVAC system's normal operation.

Two key points deserve further attention: don't vent this type of arrangement directly outside the house; it could set up a negative pressure environment that will draw cold air into the house in the winter. Second, staying with our earlier emphasis on establishing good communications with other contractors working on the house, inform the HVAC contractor if you're planning any such internal heat distribution. Wherever you vent to will increase the heat gain of that area.

There are other considerations when installing fans in millwork. Vented or louvered doors, which can provide both form and function solutions in a residential setting, present a particular issue. If a vented door has less than 68 percent of its area open (i.e., vented), it's recommended that a fan be employed in the enclosure. There is one exception-though it's rarely encountered when the equipment inside the enclosure has high static pressure front-intake fans built in. You want equipment and ventilation fans to work together. The way to achieve this is to use a fan in the top of the rack "in series" with the equipment's built-in fans which will increase the static pressure (decrease the air system's impedance) so air can be "pulled" through the vented door more effectively. In this "series arrangement" both the rack fan and equipment fans work together as a team, increasing the cooling effectiveness.

Fans have various levels of performance. There are two salient characteristics: airflow rate and static pressure. Airflow rate is the volume of air moved per unit of time, commonly expressed as cubic feet of air per minute (CFM). Static pressure (S.P.) is the pressure or suction the fan is capable of developing. In a rack, it is the measurement of resistance to airflow in forced-air cooling, otherwise known as system impedance (as air travels through intake vents and filters, the air pressure drops). System impedance is the sum of all drops in pressure. The system designer has to choose a fan that can operate at the appropriate level of static pressure; if it doesn't, the CFM value will decrease, diminishing efficiency. (In situations where there are inlet restrictions, a blower should be selected rather than a fan. Blowers typically are capable of a higher static pressure.)

Finally, all fans fail over time. Simply put, the faster a fan runs, the faster it wears out. Slower running fans are quieter. Variable speed fans are "self-adaptive;" they take into account changes in ambient temperature and the varying power dissipated by equipment. The ability to adapt to the requirements of the situation is critical; unnecessary air that is forced through the rack will deposit dust inside the electronics, reducing the thermal transfer process. Slowing the airflow down to the required amount will reduce the deposited dust. The most practical way to extend fan life is to use a proportional speed thermostatic fan control circuit.

This is all critical material to staying on top of the game when it comes to residential A/V installations. Houses will continue to become increasingly complex. Your job as an integrator is to make sure the house doesn't outsmart you.

Multiple patent-holder Bob Schluter is president and chief engineer at Middle Atlantic Products in Riverdale, New Jersey.

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