It’s a truism that almost nothing is 100% efficient; a measure of the inefficiency of most devices we deal with is how much heat they produce. Heat is energy that has been lost for one reason or another and is not available to do the job at hand, whether that job is moving our car along the road, moving a loudspeaker’s cone to produce sound, or moving large quantities of 1’s and 0’s around at very high speeds.
In many cases is not only energy that is lost but energy that must be paid for, and not just in dollars and cents. Heat not properly dealt with can cause problems ranging from making our car overheat to damaging the electronics in our A/V or IT system. Digital electronics—be they satellite receivers, DVD players, codecs, or computers—may “lock up” and become unresponsive when overheated; analog components appear to be more heat-tolerant, but in reality electrolytic capacitors are drying out and thinner-than-hair wires inside integrated circuits and transistors are being subjected to repeated thermal cycles of excessive expansion and contraction, leading to premature failure.
Racks on Racks on Racks
Modern A/V and IT systems consist of various electronic components frequently mounted in racks which may themselves be freestanding or in closets or other enclosures. The racks may be skeletal (i.e. simple open frames to which the components’ front panels are attached) or may be enclosed by sheet metal sides and front and back doors. Whatever the form, each electronic component in the system will generate some heat. The systems designer and ultimate user can ignore this at their peril.
The trivial case, in which a few devices mounted in a skeletal rack frame, in the open, in conditioned space, and consuming very low amounts of power, can safely be ignored. But such systems are few and far between today. More typical is the rack containing many power-hungry devices, all mounted in a rack either shrouded by side and back panels or located in a closet, millwork – or both. In these cases, ignorance of likely damage from heat will be far from blissful. Overheated components will express their displeasure in any number of ways, from sub-par performance to catastrophic failure.
There are several ways to reduce the temperature within a rack. One is through the use of passive thermal management; allowing natural convection currents to let heated air rise and exit at the top of the rack while cooler air enters through an opening at a low point. We will not discuss this technique at length.
Convection, while ‘free,’ is a very weak force. It is dependent on the small difference in density of hot and cold air, which is why a hot air balloon is huge, yet capable of lifting only light loads. Convection currents are easily blocked or disrupted should a vent be even partially obscured. Heat loads today, given the increasing use of digital devices and the tendency to install more equipment in smaller racks and enclosures, are too often beyond the ability of convection to even approach the necessary level of heat removal.
Cool Down Mode
Another way to cool a rack is through the use of air conditioning, or active refrigeration. Air conditioning systems, properly sized and installed, let us set rack temperatures as low as we want; The only caveats being that we don’t cool below the dew point and condense moisture on our equipment, or raise our energy bill to unacceptable levels. While expensive to buy, install, and operate, air conditioning systems that are dedicated to electronic systems may be the only practical solution when heat loads are large.
Be aware when the air conditioning system is shared with people, as when the supply and/or return ducts are an extension of an HVAC system that also serves the building and its occupants. The danger is that the thermostat may turn the system off when the occupants are comfortable or keep it from running at all in the cooler parts of the year, while the electronics are still generating the same amount of heat. There is also the extreme situation of HVAC systems installed in temperate areas. They can become the building’s heating system in cold weather. If these potential problems can be avoided, dedicated air conditioning is an effective cooling technique, and in some cases the only practical solution to avoid damage by overheating.
Guidelines are not complicated; cool air should be delivered via a supply point high and in front of the rack, while the return for heated exhaust air should be located high and behind the rack. Of the many types of analog and digital equipment being installed today, almost all fan-equipped components draw cooling air in front of their front panels and exhaust it to the rear. The arrangement described allows a ‘waterfall’ of cold air to fall in front of the rack where it can be pulled in, while a high-mounted exhaust fan in the top of the rack, or high on its rear panel, pulls heated air out into the return duct. We can accommodate those components without internal fans by placing passive vent panels below them. If the exhaust fan has been properly sized, it will pull conditioned air in. In some cases, it may be necessary to use one or more small fans inside the rack to prevent pockets of stagnant heated air from accumulating.
If the building’s HVAC system can accommodate the extra heat load, it may only be necessary to use the third rack cooling technique. This will provide active thermal management using only strategically-located fans, eliminating the cost and complexity of refrigeration.
Moving the necessary number of cubic feet or air through a rack every minute can be accomplished using ventilation systems available on the market. For freestanding racks, it is a matter of pulling heated air out from the top of the rack and replacing it with cool room air entering at the bottom (we have made the assumption that the rack is in a conditioned space, and that the building’s HVAC system can deal with the heat generated in the rack). Fan systems are available which can be mounted near or at the top of the rack. They draw heated air up from below and discharge it through their front panel into the room. Other systems discharge the heated air straight up through the top of the rack.
While effective, neither of these systems are effective when the rack itself is enclosed in a closet or millwork. In this case, we must first get the hot air out of the rack, and then get the hot air out of the closet. Systems are available that perform both functions; they pull air up from lower parts of a rack, then move it through flexible tubing to an area outside the closet. Better ventilation systems represent a trade-off between moving air and generating noise. When the system is in a remote equipment room, noise is not an issue; when it’s in the board room, noise from fan motors and air movement becomes bothersome. Consulting with cooling system makers’ technical personnel is a great help during the design process.
A Word About BTUs
A BTU, or British Thermal Unit, is the amount of heat required to raise or lower the temperature of a pound of water one degree Fahrenheit. We can relate the amount of air we have to move through our rack to allow no more than a safe temperature rise by using the formula: CFM (cubic feet of air per minute) = BTU’s (to be removed)/1.08 x the allowable temperature rise, or CFM = BTU/(1.08)(∆T).
This is a straight-forward math problem, but how do we know how many BTUs we have to remove? We can convert the electrical power (in watts) dissipated by the equipment in the rack to BTUs using the formula 1000 watts = 3400 BTUs/hr (rounded). To know the total number of watts being dissipated within the rack, excluding amplifiers, it is best to measure the actual power consumption of the equipment in operation.
Simply adding up ‘nameplate’ watts, or watts calculated from amperage listed on equipment, will almost always produce a high and inaccurate number. This is because the watts or amperes listed on equipment is always worst-case, absolute maximum, as required by Underwriters’ Laboratories, Canadian Standards Association, or other regulatory or safety agencies. It does not reflect power consumption or current draw under typical operating conditions. Therefore, it is best to measure the power being used by the rack’s components while in typical operating mode.
Measuring the power consumed by audio amplifiers is a bit more involved. While virtually all other electronic components convert essentially all the power they consume into heat, amplifiers deliver significant power to loudspeakers. Not all of the power going into them turns to heat. Complicating the matter is the fact that there are several classes of amplifiers in use today, with efficiencies ranging from very low to fairly high. These amplifier classes are an alphabet soup of A, AB, D, and on and on.
A further complication is what program material the amplifier is amplifying. Speech? Music? If music, is it classical or hard rock? These are not minor points; speech and classical music demand high volume levels (and therefore large amounts of power) only periodically, while hard rock plays at a constant level. Experience has taught us that for most systems it is reasonably safe to use 10% to 15% of an amplifier’s maximum rated output as its long-term contribution to heat within a rack. That is, a 2 channel, 100 watt per channel amplifier will likely contribute between 20 and 30 watts of heat, or 70 to 100 BTUs.
We add the non-amplifier watts to the (calculated) amplifier watts to get the total watts being consumed in our rack, then convert that to BTUs using the conversion factor discussed earlier; 1000 watts = 3400 BTUs. For “delta T”, the allowable temperature rise, experience has shown that in most installations within conditioned spaces a 10 to 15 degree rise is an acceptable trade-off between the ideal and the achievable.