Plasma TV & A/V Ventilation
by Frank Federman, President.: Active Thermal Management
Thermal problems and the custom audio-video designer/installer
There is a problem which has been facing the consumer and custom A/V Community for some time. Not a glamorous problem with a neat high-tech solution, but one that's real, getting harder to solve and even harder to ignore. The problem is HEAT. Consumers no longer necessarily want to show off their electronics on open freestanding shelves, nor do they always want to stare at a 60" black piece of glass when the systems are turned off. The trend today is to aesthetically conceal the components.
Many of the A/V Industry's problems have been addressed by the manufacturers; but little attention has been paid to the problem of the heat generated by the equipment they make and you install. As we add more equipment to installations; excess heat is a problem you must address.
Yesterday's "stereo" is today's "home theater", with 5 to 7 full-range channels, and very high powered amplifiers to handle low-frequency effects. While two 50 watt channels were considered adequate for music reproduction just a few years ago, six 100+ watt channels are the norm today, generating significant amounts of heat.. Add to this the equipment needed to complete the system; Plasma TVs, surround-sound decoders, tuners, CD changers, DVD players, VCRs, DBS receivers, and the list grows. If we're installing a multiroom system, there's frequently more than one of each source…. To paraphrase the auto industry, "It's not your father's stereo".
When the equipment is mounted in closed cabinets or racks, and the racks are tucked into recesses or closets, the heat generated by this equipment has nowhere to go, and temperatures can rise to levels that will shorten equipment life to a significant degree. All installers have come up against this problem in more than one installation, knowing that despite the severity of the problem, little meaningful help was available. In addition to knowing the theory and practice of audio and video, a basic knowledge of thermodynamics is becoming necessary, because the day when a muffin fan would solve the problem is long gone.
You know when you have a heat problem; You don't have to burn your hand to know that most of the power that goes into home entertainment products is dissipated as heat. But………….
· How do you get rid of the heat?
Every installation is different; there's no single approach that will work in every situation. And any successful approach has to be quiet enough to be a real solution; one that doesn't just turn a heat problem into a noise problem!
- How do you do it quietly?
Fundamental #1 : Electronic equipment "runs hot" because it's less than 100% efficient. If it were 100% efficient, all the power that went from the wall socket into an amplifier would come out as sound from the speakers and all the power drawn by a Plasma Display would reach the screen as light. All equipment would remain at room temperature.
In real life, typical power amps are about 40% efficient overall (depending on variables); that means about 60% of the watts used turn into waste heat. Practically all the power going into a plasma or liquid crystal display turns into heat, as it does with t signal processors and most signal sources. Power amps may dissipate the most heat, but the rest of a modern system frequently makes significant contributions to an overall heat load that could have the system operating in "sauna" mode, especially digital devices.
Fundamental #2: In an enclosure, the basic relationship between power input "and temperature rise for the system designer and installer is this: As heat is dissipated by the equipment in an enclosure, the temperature of the air (and the other equipment within that enclosure) will rise until the heat leaving the enclosure equals the heat entering it.
If the equipment is in a room, enclosure, wall space or closet with poor ventilation, it's only a matter of time before the air in the enclosure is about as hot as the equipment; putting fans within the enclosure will just stir the heated air; the heat must be removed from the enclosure
There are several ways heat can leave: Heat is transferred from a hot thing to a less-hot thing by one or more of three ways: conduction, convection, and radiation.
Conduction describes the situation in which one end of a piece of metal is heated and the other end gets hot a bit later because heat has traveled through the metal. Metals like copper and aluminum conduct heat very well; steel conducts it less well; air conducts heat poorly Little heat moves within or leaves an enclosure via conduction.
Radiation: Placement of equipment in an enclosure can result in significant heat being radiated from hotter to cooler pieces of equipment. Radiation of infrared energy is what makes your face feel hot when you drive an air conditioned car on a sunny day. The air in the car is cool, but radiant energy from the sun passing through the windows is felt as heat on exposed skin. Similarly, a DVD player near or below a power amplifier may start feeling a bit toasty after a while, unless a barrier interrupts the flow of radiation. Radiation will move heat around within an enclosure, but causes very little heat to leave; the problem of radiated heat in home entertainment systems has been largely ignored, while many a DVD player has slowly baked to death……
- Convection: Most heat problems in the typical home theater equipment rack or cabinet are caused by the third way heat can move: convection. Air that has been heated rises. The warmed air is lighter than the surrounding air, rises, and gives up some of its heat to anything cooler in its path. This mechanism can result in moving significant amounts of heat around and out of an enclosure, if the construction is sufficiently "open". The use of open shelving and cabinets without backs or doors can result in installations that are thermally cool, but may not be aesthetically acceptable to the client. Unfortunately, the openings typically provided for cooling in a cabinet or wall enclosure are far too small for efficient convection cooling.
Solutions: In most applications, mechanically removing the air which has been heated by use of fans has been the most effective way to reduce the severity of the heat problem. The system designer can use an air mover to exhaust hot air; the reduced pressure in the enclosure will pull cooler air in. Alternately, the fan can be turned around to push cooler air in, diluting the hot air and forcing it out of the enclosure. Both approaches are valid.
There's also the matter of noise -- fans make noise in several ways.
· The cheaper ones vibrate, and send the resulting "buzz" into whatever they're fastened to. If they're fastened to the thin back panel of a cabinet, that panel can act as a drumhead and amplify the noise.
If there are struts in front of or behind the fan blades (as with most of the "muffin" types), they produce a tone caused by the air column being "chopped" by the struts each time a blade passes.
Another source of noise is the "whoosh" of air moving. The faster the air moves, the louder the "whoosh". As you move the same amount of air more slowly, the noise of the air moving drops both rapidly and dramatically.
So you pay for your thrills; a fast-turning fan moving air quickly can produce a strong cooling effect, but with the penalty of fan noise to annoy the listener.
Serious air moving devices (quiet and way beyond muffin fans) suitable for attachment to racks, cabinets, and wall enclosures are available. Devices to quietly ventilate a small cabinet or a large closet are available, devices that don't just move the heated air around, but move it out of the enclosure, replacing it with cool room air.
Specific to the problem of cooling a plasma display that has been recessed into a wall, the Active Thermal Management Cool-vent, an attractive 3 ˝" by 15" grille, is fitted with 6 reversible (and very quiet) fans. Available in 25 species of wood, it can be finished to blend with the wall. Using one below the screen to bring room air in, and one above to exhaust heated air, they form a very quiet, very effective ventilation system. A thermally-activated switch controls the fans automatically. Depending on the individual installation, the ATM System 2 or System 3 modular enclosure ventilator kits may be more appropriate for cooling an in-wall video display.
While not all installations require forced ventilation, it is important to be aware of some of the solutions.