Hot Stuff

Jan 1, 2004 12:00 PM, By Bob Schluter

You've put a tremendous amount of time and effort into designing the right installed A/V system for your client, integrating all of its many components and assuring elegance of operation and performance. But there is an old adage that accompanies every complex endeavor: any system is only as strong as its weakest link. Increasingly, how you manage the thermal aspects of all of those components can mean the difference between a successful project and one that's going to be a maintenance nightmare in the future.

Here's one key reason why thermal management is so important: heat problems will most likely manifest themselves as intermittent problems, as opposed to a complete system failure. That means that troubleshooting thermal-based problems is much more difficult. Addressing heat issues from a systems approach right from the beginning is the best way to solve them.

Anyone who has ever taken flying or skydiving lessons will remember instructors telling them, “The ground is your enemy; altitude is your friend.” A similarly simplistic but very real view of the world is useful when it comes to thermal management of audio equipment: heat is the enemy; heat management is your friend. Notice I didn't say, “Air conditioning is your friend.” There's a reason for that.

Designing thermal management into an audio installation requires the same sort of systems approach that the rest of a project needs. But you'll need to understand one critical concept first: managing heat is not just a matter of introducing cool air but rather removing — that is, managing — the heated air. It might seem counterintuitive, but it's basic physics. Heated air has behavioral characteristics in enclosed places, and simply injecting cooler air does not assure that they will magically merge and create an average temperature. Rather, pockets of hot and cold air can develop, isolating the cold air from where it's needed and concentrating the heated air into areas where it can do the most harm. For instance, putting an air conditioning vent into an equipment closet with the intent of cooling it will cause formation of thermal gradients. Putting in an air conditioning return — the suction grill — will pull heated air out of the closet space. That's managing a thermal environment, not fighting it. This is an approach that's been learned and absorbed by the computer industry, where heat management is critical to keeping huge computer installations running.

PASSIVE AND ACTIVE

There are two main philosophies to approaching thermal management inside the enclosure: passive and active. Passive solutions revolve around venting and understanding the physics of heat to maximize the efficiency of vents. Active solutions involve adding fans to move air in a predetermined path.

Two airflows are involved in a thermal system; one is how the heat travels through the rack, and the other is how the air moves throughout the room. The interaction between these two airflows is important and must be considered when taking a systems approach. All heat (expressed as BTUs per hour) generated by equipment must first be removed from the rack, and then the room itself must have the ability to remove the total heat from all racks. Many installations do not have the luxury of an air-conditioned environment, so consideration must be given to how the room itself will vent. You want to be sure that whatever heat is removed from the rack will not raise the room temperature significantly. Understanding how to quantify heat in this way will allow systems designers and installers to interact more efficiently with architects and builders.

For digital equipment, the room itself should be no hotter than 75 degrees Fahrenheit. This gives a 10 degrees Fahrenheit temperature difference between the room and the recommended 85 degrees Fahrenheit internal rack temperature for optimum equipment life. The cooler the room (above the dew point so condensation does not occur), the fewer vents or cubic feet of air per minute (CFM) the fan will need to push. The most accurate way to determine how much and what type of thermal management is necessary is to use a nomograph (See the sidebar “Taking the Heat Off.”).

Heat flows from hot to cold, and you cannot make the heat come out of a cabinet unless the outside air is cooler. Convection is the process of air passing over a hot object and carrying the heat away. As stated previously, it's always better to focus on removing heat from above rather than adding cold air.

Ambient temperature can build up in closets, and heat should be exhausted out if the ambient air inside the closet exceeds 75 degrees Fahrenheit; 85 degrees Fahrenheit is the maximum recommended constant operating temperature for most equipment. Most studies have shown that for every 10 degrees Fahrenheit rise above 85 degrees Fahrenheit, digital equipment life is reduced by about 40 percent. However, the Uptime Institute states that for every 18 degrees Fahrenheit increase above 70 degrees Fahrenheit, long-term reliability is reduced by 50 percent.

In the case of a single rack in a closet, it is important to use a fully louvered closet door and monitor the temperature when there is no air conditioning feeding the closet. For passive convection (that is, no fan) applications, wider racks are beneficial; a good chimney effect made possible by the spacious sides of mounted equipment draws heat upward effectively. Avoid locating the racks directly under supply ductwork. Cold air falls, and the flow of the hot air that rises from the top of the rack should have no impediments on its way back to the return air (intake) duct.

In most integrated A/V installations, the largest heat load will come from power amplifiers while they are driven. (The state of the amplifier load will have an effect on how its heat is measured.) However, add in signal-processing microprocessors, faster clock speeds, and the continuing miniaturization of electronics, and the amount of heat generated per rackspace of equipment, also known as increased heat density, is trending upward.

Proper planning of the cooling air path inside a rack ensures that no hot spots occur and that the waste heat is effectively removed. The most common airflow found in higher current draw equipment is that which pulls cooler air in from the front and exhausts the heated air toward the rear or sides (known as front-intake equipment). Simulations and real-world testing show that moving air through a cabinet from bottom to top results in the lowest internal cabinet temperatures. Some amplifier manufacturers still take the cabinet air through the rear and exhaust it out the front (known as rear-intake equipment). This presents some special thermal design challenges. Downward airflows are a bad idea, creating mixed convection (mixture of forced air and convection) during operation and in the event of fan failure (see Fig. 1). Most other nonamplifier equipment that has internal fans will draw air in through the rear or sides and exhaust out the sides or rear. This recirculates the cabinet air, and care should be taken as to its placement so the natural convective rise of heat is not disturbed. Hot air rises, and the hotter it gets, the more cubic feet of air per minute (CFM) flow occurs by natural convection. The friction of all vents gets in the way of the flow; more open area, in the form of slots or perforations, is always better. For multiple convection-cooled amplifiers (amplifiers with no fans), put vents in between, unless the amplifier manufacturer states otherwise.

PASSIVE CONVECTION

In an environment at normal room temperature, a rack is able to dissipate 300W to 500W of heat (not audio watts) through natural convection. That requires adequate vent openings at the bottom and top of the unit (none in the middle for effective chimney flow) and an unimpeded airflow inside the rack.

The main advantage of natural convection is its intrinsic reliability. Proper configuration most importantly includes optimization of component placement, and hotter equipment located lower in the rack will provide a greater natural airflow. That is especially true when you are using passive convection in high ambient temperatures.

Equipment that passively vents (without fans) sometimes 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. Otherwise, it is acceptable to stack equipment directly on top of each other (see Fig. 2).

YOUR BIGGEST FAN

In cases in which 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 forced-air applications, a narrower cabinet can be selected to save space. There are no exposed sides on racks in the middle of a multibay installation; therefore the only way to introduce air is through the face or rear. Additionally, the best way to exhaust the air is to incorporate a fan top.

In these cases, it is acceptable but not recommended to put vents between equipment with front intakes. Most front-intake equipment fans are between 25 and 50 CFM each. If a fan is required for the top of the rack, ensure that this fan's CFM rating exceeds the sum of the CFM ratings of all the equipment. Hot air will not short-circuit and recirculate between equipment, because the fan will draw air from all openings. A solid rear door is recommended in this situation to control airflow from front to rear (see Fig. 3).

Shelves are an important component of the internal airflow planning process. Shelf surfaces that overhang the internal natural rise of heat should be vented. Any obstruction to airflow will raise the temperature in the lower portion of the rack, possibly creating a stratification zone, which should be avoided if possible.

SIZE AND PLACEMENT OF FANS

Fans will substantially reduce interior operating temperatures if intake vent placement, size, and airflow are done correctly. However, fans add little value over good convection designs if air short-circuiting occurs from having intake points close to the fan. The solution? Careful placement of vent blockers, as part of your proper thermal management planning, will prevent the short-circuiting of airflow in rack enclosures. Blocking the upper vents will ensure that heated enclosure air will be forced out through topmounted exhaust fans (see Fig. 4). An important point to make here is to avoid placing side vents near fans. A topmounted fan will suck in air from the vents, not air from down inside the enclosure that needs to be vented. 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.

The ideal spot for fan placement is in the top, where the hotter air needs to be removed. This also aids the natural force of the hot air rising. Rackmounting (vertical placement) of fans is recommended where there is a likelihood of contaminants falling into the rack from above. Using multiple fans mounted next to each other requires that they be checked regularly for proper operation. Once one fan stops functioning, it provides a short-circuit path for the airflow. Don't be fooled by thinking two fans will help; when one fails, it acts as a vent near a fan and will not remove heat from the enclosure effectively. Additionally, fans help reduce condensation in colder environments.

All fans fail over time. Simply put, the faster a fan runs, the faster it wears out, and slower-running fans are quieter. Even if filters are not employed, the more unnecessary air that is forced through the rack will deposit dust inside the electronics, reducing its thermal transfer. Slowing the airflow down to the required amount will reduce the deposited dust.

There are other considerations when installing fans in enclosures. The installation of front doors in most cases has an effect on airflow. As a general rule, if a vented front door with less than 60 percent open area is chosen, fans are recommended. The exception to this rule is when the equipment has high static pressure front-intake fans built in, which is rare. In all other cases, the use of a fan in the top of the rack in series with the equipment's built-in fans 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.

Two terms are used to describe fan performance: airflow rate and static pressure. Airflow rate is the volume of air moved per unit of time, commonly expressed as CFM. Static pressure is the pressure or suction the fan is capable of developing. In a rack, it is the measurement of resistance to airflow. There is system impedance involved with forced-air cooling. As air travels through intake vents and filters, the air pressure drops. The system impedance is the sum of all pressure drops. The fan selected must be capable of operating at this static pressure, or the CFMs will drop. 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.

HYGROSCOPICS

There's dust, and then there's dust. Dry dust, the kind you get in a nonhumid environment (generally 65 percent relative humidity or less), will for the most part pass benignly through the venting system. In a humid environment, however, dust will absorb the moisture in the system and accumulate on circuit boards. Computers and other digital equipment using rapid clock rates will be most affected by this hygroscopic dust failure. Filtering helps prevent digital and other sensitive equipment from having hygroscopic dust failure, which occurs in humid environments. Inlet air filters are highly recommended to extend the service life of digital equipment, because most switchers, routers, hubs, and other processing equipment have their power supply fans in the rear, without any filtered front air intake. Filter loading and subsequent maintenance requirements can be greatly reduced with the use of a proportional speed thermostatic fan control circuit, which was explained previously, because the overall volume of air is lower when not required. Don't forget that, like on air conditioners, filters require maintenance or they will clog.

Thermal management is a complex issue. But there are standardized solutions, and understanding the basic physics involved will go a long way toward helping create a robust and problem-resistant rack installation.

Bob Schluter is president and chief engineer at Middle Atlantic Products. He is currently designing next generation enclosure systems and thermal management products.

Taking the Heat Off

Nomographs such as Fig. A can tell you at a glance the minimum ventilation (active or passive) required to provide an interior rack temperature of 85 degrees Fahrenheit. This nomograph takes into account important parameters such as heat transfer, distance between inlet and outlet openings, friction losses, and vent area.

Amplifiers vary greatly in waste heat output. This nomograph should be used only when waste heat data is available from the amplifier manufacturer.

To calculate total waste heat (column B):

Obtain total waste heat output by combining the published waste heat (BTU/per hour) of all amplifiers in the rack.
Add up total measured amperage draw from all other equipment and multiply by 400 (total amperage × 400 = total BTU per hour at 117V). Note that all amperage is figured at approximately 117V. It is best to use a current measuring device at the power strip inlet to measure BTU actual current draw, because nameplate ratings sometimes are fuse size and not what the equipment actually draws.
Combine BTU/Hr. totals from steps 1 and 2 to obtain total for all equipment. Mark total in column B.

To obtain minimum ventilation requirements:

Mark ambient room temperature in column C and connect points in B and C with a straight edge.
The minimum cooling required providing an interior rack temperature of 85 degrees Fahrenheit is shown on column A, where the straightedge intersects the minimum cooling requirements column.

System Requirements:

For passive and active ventilation, ensure adequate intake vents are installed.

Be certain no short-circuiting of air occurs.

This nomograph assumes full-range rock music is used as program material.

This nomograph assumes that amplifiers are driven to medium-light clipping levels.

BTU/Hr. Calculations

One hundred percent of the power consumed by communications equipment and computer products is converted to heat. Calculating BTU/Hr. output for equipment other than amplifiers is simple: the more current it draws, the more BTU/Hr. will be produced. At 117V, each ampere of current drawn produces exactly 400 BTU/Hr. of heat output.

Amplifiers are not as straightforward because of the different nature of circuit designs and other variables. The real-world BTU/per hour output can be estimated by taking into consideration which output design is found in the amplifier, the type of power supply, what type of program material is played, how many ohms the speaker load is, and at what level the amplifier is to be driven on average.