Audio-Video Equipment Racks - Part 2
Rack Selection, Equipment Mounting Tips,
and Thermal Management Issues
Equipment racks come in all shapes and sizes; this can make rack selection a rather difficult task. In this guide to equipment cabinets, we show you how to go about selecting the right rack for your gear. In the process, we present you with some useful tips on how best to mount equipment in rack enclosures.
We end this guide by discussing an issue that is often ignored completely especially by most home theater enthusiasts - that of thermal management in equipment cabinets. This is extremely important for the safety of your gear.
Salamander Synergy 329 A/V Cabinet
(available from amazon)
A versatile AV cabinet in a nice hardwood finish complemented by matching metal trims and optional aluminum castors or polished aluminum feet instead of the leveling feet included with the cabinet.
Selecting Your AV Equipment Rack
Selecting a suitable rack for your audio and video requirements can turn out to be a time-consuming and tedious process. However, this does not have to be so!
AV Equipment Rack
Future Proof your Installation
An equipment rack is like a projection screen - it is not something you have to replace every few years, as is the case with most AV electronics. If chosen properly, a component rack will last a lifetime.
Therefore, invest in your rack. Choose a rack solution that will take your present equipment while allowing for possible future expansion. Going for the wrong solution may prove expensive in the not-so distant future!
Look for an equipment rack that includes adequate cable management and enough space for a suitable AC distribution system with surge protection on the individual outlets.
And if you want to really invest in the protection of your expensive electronics, include enough space to house a power conditioner. To find out more on the subject of power protection in the home theater, refer to the series of guides appearing under the power projection section of the site.
Middle Atlantic ASR-48
Rotating Slide-out Component Shelving System
The ideal component rack should give you full access to both the front and rear panels, so all wiring and controls are easily accessible.
Some racks systems such as the Avrak (as reviewed on our site) or the Middle Atlantic ASR-48 (available from amazon) feature a pull-out rotating base to give you full access to the back of your gear from the front. Others like the Atlantic also include pull-out shelves that sit on slotted rails for ease of access to the back panel of individual components.
Another important issue is satisfactory ventilation. This is of prime importance especially with fully enclosed equipment racks. However, even with an open-frame equipment rack, having all your gear centrally in one location tends to result in a higher air temperature around your system components.
In particular, keep in mind that amplification gear can produce a lot of heat when in use - which if left to accumulate around your gear, will damage your system components. The use of suitable ventilation through correct thermal management - whether passive or active - is thus essential for the correct operation of your equipment. We cover the subject of thermal management in equipment racks further on in this article.
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The following equipment mounting tips should come in handy when mounting audio and video equipment in equipment rack enclosures; the aim here is to mount your gear in a safe, secure way:
1. Keep a low center of gravity; heavier components should be mounted on the lower shelves. This also means that when loading equipment, whenever possible, you should always begin at the bottom - moving upwards.
2. When working with EIA-standard 19-inch equipment racks, allow for 20 pounds per rack space (or RU).
3. When mounting system components in 19-inch racks, always support equipment from the bottom and fasten the two lower rack screws first. This will make it easier to install any remaining top screws.
4. Rear support is normally not needed unless the weight of the unit exceeds 10 pounds, in which case, the use of rear screws is recommended.
5. Rear support is usually required when either the center of gravity of the unit tends towards the rear, or when the unit dimensions are such that the use of front screws only would produce inadequate support.
6. When mounting system units in equipment racks, it is equally important to keep an eye on thermal management as further detailed below. This means you need to take some time to understand the different cooling processes used by the different system components you plan to install in an equipment rack.
For example, some amplifiers have built-in fans that draw air in through the front and vent to the rear, or to the sides, while others may work the other way round. Intermixing different units in this manner may easily create mixed convection currents within the rack; this would reduce rather than assist the overall cooling efficiency of the ventilation within the rack unit.
This article would not be complete without discussing the issue of thermal management in equipment racks. However, it is first important to understand the basics in thermal management. The basics of thermal management should be the following:
1. The primary objective is to remove the air heated by your system components rather than introduce cool air. The latter on its own - if not planed careful, may not be effective in dissipating the heat within the equipment rack.
2. When installing equipment, keep in mind that heat rises and move from hot areas to cooler areas; air flow design should enable the heated air to move freely from bottom to top. This is more effective in removing the heated air.
3. Forced airflow in a downward direction can create a mixed state of convection currents that may lead to hot spots within an enclosed rack.
So what can you do to keep your system cool?
You have two basic options, namely that of passive convection, and forced ventilation using suitable cooling fans; the latter can be further sub-divided in two different process - 'chimney' and 'localized' cooling.
Forced ventilation using the 'chimney' cooling effect makes use of a fully enclosed rack configuration with rear and side panels in place; intake cooling fans are placed at the lower part of the rack while exhaust fans are placed at the top rear part of rack enclosure. Any unused front slots and rear/side ventilation slots (other than those used by the fans) should be fully covered to enhance the airflow around the system components from the cooling fans at the lower part towards the upper exhaust fans at the top.
For effective cooling of the equipment within the AV rack, the chimney effect requires the presence of sufficient clear space between the equipment in the rack and the rear panel for the hot air from the lower shelves to flow unobstructed towards the exhaust fans. This means that equipment racks designed to use forced chimney cooling should be at least few inches deeper (or wider) than the equipment with the largest footprint installed in the rack.
Instead, 'localized' cooling, also referred to as spot cooling, makes use of cooling fans placed in close proximity to equipment that generates most heat—like power amplifiers and power conditioners—to help take away the heat from the most significant heat sources within the AV rack. This helps reduce the possibility of heat built-up and therefore hot spots, around other system components in the rack.
Generally this type of cooling relies on the use of a fully enclosed rack as in the forced chimney cooling method but with the rear panel removed. Forced spot cooling provides superior cooling effect to the forced chimney method as long as the equipment rack is placed some three to four inches away from the wall to enable the hot air to escape. Spot cooling represents a suitable alternative when the equipment rack footprint is such that it does not provide enough free airspace at the rear of the rack to allow for air from the lower parts of the equipment rack to flow freely towards the upper exhaust fans.
Note: While it is possible to house system components with integrated forced ventilation in a rack enclosure using passive cooling, yet care should be taken since what applies to passive convection does not always apply for forced cooling. The thermal do's and don'ts listed below should help better clarify this issue.
1. Mount components that generate most heat near the top of the enclosure. If you were to mount such components at the very bottom, the already heated air rising from the bottom will eventually overheat the rest of the equipment in the enclosure in a very short time.
2. If possible, use deeper/wider equipment racks as the extra space on either side of the rack and the rear will create a passive 'chimney effect' that will further help to draw the hot air upwards.
3. Install vented rack panels between equipment without fans. In the case of an open rack with passive cooling, care should be taken to ensure that heated air would not be drawn back into the rack enclosure as this would lead to a fast buildup of hot spots around system components.
Forced Ventilation (with fans)
1. Direct the airflow within the equipment rack by placing vented rack-panels only in the front near the equipment. This in particular applies for equipment with built-in fans using front intake.
2. Similar, study the cooling of the different components. This is necessary to ensure that you design the cooling within the rack in such a manner as to assist the built-in cooling process of the individual units.
3. Do not place vented rack-panels in the top-most part of the equipment rack; instead cover any vents on the upper part of a rack to help improve the chimney effect.
4. Similarly, do not use a ventilated rear-panel with fan-cooled equipment. Doing so would simply reduce the effectiveness of the forced cooling within the overall volume of your rack system.
But... Irrespective of any guidelines, it is important to realize that when it comes to thermal management in equipment racks, this has to be designed around your specific system components - by taking into account the heatflow around the equipment within the rack.
The following video clip (no soundtrack available) prepared by the Design Institute of the Faculty of Engineering and Computing at Coventry University on behalf of AVracks, presents an excellent graphics explanation of the heat buildup (as seen from the side) and the resultant heatflow in a typical AV rack installation with different cooling solutions. This video accurately identifies issues arising in the area of thermal management in equipment racks.
The conclusions arising out of this video were eventually used by avracks.co.uk in the design of their CoolRack range of cooling products. However, the video itself can also serve as a useful source of information to anyone aiming at designing an effective DIY AV rack cooling solution for home theater system components.
Video courtesy avracks.co.uk
Fan-cooled systems, while highly efficient in removing the heated air from around system components, have the disadvantage of being rather noisy. This may not be an issue if your rack is in a media closet outside your home theater room. However, if a fan-cooled equipment rack is within the same room as your big screen, then it is a different story. The noise produced by the fans may be too annoying during periods of silence when watching a movie.
We have found that the best solution to drastically reduce fan noise - while still maintaining adequate flow of air - is to increase the number of fans but run these at reduced speed. Ideal for the purpose would be 12-volt DC cooling fans operated at circa 50% of their rated supply voltage. It is correct to assume here that a DC fan operated at 50% the rated supply voltage would produce 50% of the fan rated airflow - hence the need to increase the number of fans.
Getting the correct airflow is important to maintain the ambient temperature around your gear at a safe level. Inadequate flow means that your cooling system would not be able to get rid of all that hot air fast enough to match up with the heat generated by your system components.
How much air displacement is needed to keep your equipment cool?
Fans are rated in 'cubic feet per minute' or CFM. A fan rated at 25CFM means it can circulate all air in a 25 cubic feet equipment rack (a typical 42U high 19-inch rack), in just under one minute. Airflow is cumulative meaning that to arrive at the total air displacement, you just add the CFM capacity of individual fans.
Under normal ambient temperature and working conditions, you need approximate 10 CFM every 100 watts of heat dissipated by your equipment. This however depends on the type and area of surface being cooled; in other words, it depends on how effective that flow of air is in getting rid of the heat.
Our best advice here is to monitor the temperature around the various system components; use an electronic thermometer with a suitable wired probe to measure the temperature while the system is in use. Check that the measured temperature remains within the limits specified by the respective equipment manufacturers in the accompanying literature that came with your system components. If you do not have access to a suitable electronic thermometer, the rule of thumb is to aim at keeping the temperature of your respective system components comfortable to touch; if this is not the case, extra cooling is required.
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