Suffering from overheating equipment, dust ridden racks or poor airflow in your racks? Here’s some advice from Middle Atlantic Products on rack layout and positioning that will help you keep your cool.
Locked away in equipment cupboards and rack rooms, the equipment rack is the out-of-sight but indispensable heart of an AV System. Whether you have multiple racks full of DSP units and routers, or a single boardroom solution containing just a media controller, a couple of switchers and an amplifier, thermal design of racks and enclosures is essential to ensuring the functionality and reliability of the equipment and system.
There are two types of airflows to consider in all cases. The first is how heat travels through the rack, and the second is how air moves in the room around it. The interactions between these two airflows are important, and must be considered when taking a systems approach.
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In most integrated audio/video installations, the largest heat load will come from power amplifiers while they are driven. However, there are an increasing amount of devices that produce a considerable amount of heat. Microprocessors, which are often embedded to do signal processing, emit greater heat from equipment than ever before. Additionally, as the speed of these clocks increases, the heat continues to rise. Couple that with the continuing miniaturisation of electronics, and you can see that the trend is for more and more heat to be generated per rack space. Heat has been proven to reduce the life service of most equipment, making it vital to effect the removal of this heat.
All heat (BTU/Hr) generated by equipment must be removed from the rack, and then the room must itself have the ability to remove total heat from all the racks present. Middle Atlantic Products recommends that the room temperature is no hotter than 24ºC (75ºF). This is a 5º difference between the room and the recommended 29ºC (85ºF) internal rack temperature for optimum equipment life.
Before you even get to rack layout and cooling, consideration should be given to the placement of the rack. Customers often prefer to have equipment racks out of sight in cupboards, these are often poorly ventilated and doors should be fully louvered and the temperature monitored if air conditioning is not present.
Go with the flow
Whether you are opting for a passive or active cooling solution will decide the optimal layout of the rack. Proper planning of the cooling air path inside a rack ensures that no “hot-spots†occur, and the waste heat is effectively removed.
The most common airflow found in high current draw equipment (typically amplifiers) is that which pulls cooler air in the front and exhausts the heated air towards the rear or sides. Most other non-amplifier equipment that has internal fans, will draw in air through the rear or sides, and exhaust from 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.
Simulations and real-world testing have shown that moving air through a cabinet from bottom to top results in the lowest internal cabinet temperatures.
Passive cooling
At normal room temperature, a rack is able to dissipate 300 – 500 watts of heat through natural convection. This requires adequate vent openings at the top and bottom of the unit (none in the middle for effective “chimney†flow) and an unimpeded airflow inside the rack. One advantage of using passive convection is its intrinsic reliability.
Air movement in a properly configured cabinet is generated by thermal gradients. Proper configuration most importantly includes optimisation of component placement. Hotter equipment located lower in a rack will provide a greater natural airflow. Equipment that passively vents often has intake vents on the bottom, or vents on the top, so care must be taken not to block these with equipment stacked directly on top of each other. A common mistake amongst installers is to put vents between each piece of equipment without regard to the re-circulation of hot air. This can “short-circuit†the airflow because the vents are placed too close to fans or heat sources (DIAGRAM).
With any passive convection scheme, the general rule is the more venting up the top of the rack, the better.
Active cooling
In cases where there are too many BTU/Hr. for natural convection to properly perform this task, it is essential to force the heated air from the rack. Active thermal management involves the use of fans to effectively remove heat from an equipment rack.
In cases where fans are required, it is acceptable to put vents between equipment with front intakes. If a fan is necessary for the top of the rack, you must ensure that this fan’s CFM rating is at least the sum of the CFM ratings of all the equipment. Hot air will not “short-circuit†and re-circulate between equipment, as the fan will draw air from all openings. A solid rear door is recommended in this situation to control airflow from front to rear.
In active management solutions it is vital to give proper consideration to venting, to avoid starving the forced airflow. Sufficient venting, in appropriate positions, should be allowed to match the CFM of exhaust fans. Horizontal shelves in the rack, which would block internal airflow should also be vented.
Properly employed, fans will substantially reduce interior operating temperatures if intake vent placement, size and airflow are done correctly. However, fans will add little value over a good passive design if air short-circuiting occurs from having intake points close to the fan. (DIAGRAM). Care should also be taken using multiple fans mounted side by side. If one fan should fail it will provide a short-circuit path for the adjacent ones.
Proper attention to effective thermal management during system design results in systems are efficient and reliable, while minimising unnecessary service calls.