Audio Video Equipment Racks (2)
Rack Selection & Equipment Mounting Tips

In this second part of our guide to equipment racks, we cover the subject of rack selection and the process of mounting equipment in rack enclosures. We also discuss the important issue of thermal management in rack systems.

Selecting Audio/Video Equipment Racks

Selecting a suitable rack for your audio and video requirements can turn out to be a time-consuming and tedious process. Partly the culprit is the vast selection of rack solutions available on the market.

However, this does not have to be so!

Here are some tips that should prove useful when selecting an equipment rack.

Future Proofing your Installation: An equipment rack is like your projection screen - it is not something that you have to replace every few years, as is the case with AV electronics. If chosen properly, a component rack will last a lifetime. Therefore, invest in your rack. Choose a rack solution that will take all your present equipment while still allowing for possible future expansion. Remember - going for the wrong solution may prove expensive in the not-so long term.

Support Features: 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.

Equipment Access: 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 feature a pull-out rotating base to give you full access to the back of your gear from the front. Others feature pull-out shelves that sit on slotted rails for ease of access to the back panel of individual components.

Ventilation: 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 equipment. The use of suitable ventilation through correct thermal management - whether passive or active - is thus essential for the correct operation of your equipment. 

Equipment Rack-Mounting Tips

• 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.

• When working with EIA-standard 19-inch equipment racks, allow for 20 pounds per rack space (or RU).

• 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.

• 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.

• 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 mounting using the front screws only would produce inadequate support.

• Equally important when mounting system units in equipment racks, is to keep an eye on thermal management as further detailed below. This also implies that you need to take some time to understand the different cooling of your gear. 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.

Thermal Management - Keep it Cool!

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 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.

• Also, when installing equipment, keep in mind that heat rises and will move from hot areas to cooler areas. Therefore, air moving from bottom to top is more effective in removing the heated air.

• On the other hand, 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  ...and quiet?

You have two basic options - passive convection, and forced ventilation using suitable cooling fans. While the two may be used to complement each other, and therefore improve the overall cooling within the rack, yet care should be taken since what applies for passive convection does not always apply for forced cooling. The dos and don'ts listed below should help better clarify this issue.

Thermal Dos and Don'ts

Natural Convection

• Mount components that generate most heat near the top of the enclosure. This may appear in contradiction with the second principle of thermal management stated above. However, 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.

• If possible, use wider equipment racks as the extra space on either side of the rack will create a 'chimney effect' that will further help to draw the hot air upwards.

• DO install vented rack panels between equipment without fans

Forced Ventilation (with fans)

• Direct the airflow within the equipment rack by placing vented rack-panels only in the front near equipment. This in particular applies for equipment with built-in fans using front intake.

• 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 to assist the built-in cooling process of the individual units.

• Do not place any vented rack-panels in the top-most part of the equipment rack; instead cover completely to help improve the chimney effect.

• Similarly, do not use a ventilated rear-panel with a fan-cooled equipment rack. Doing so would simply reduce the effectiveness of the forced cooling within the overall volume of your rack system.

Keep it Quiet!

Fan-cooled systems, while highly efficient in removing the heated air from around system components, yet 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 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 keep the correct ambient temperature around your gear. Inadequate flow means that your cooling system would not be able to get rid of all that hot air fast enough to match up the heat being generated by your equipment.

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 replace the air in a 25 cubic feet empty equipment rack (a typical 42U high 19-inch rack), in just 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, 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 components and check that it is within the limits as specified in the accompanying literature that came with your home theater gear. If you do not have access to a suitable thermometer, the rule of thumb here is to aim at keeping the temperature of your respective system components comfortable to touch - if this is not the case, extra cooling is needed.

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