HVAC for broadcast facilities

Designing mechanical and electrical systems for live-audience television studios presents unique challenges
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Keeping cool while remaining quiet in a control room or on-air studio requires sophisticated air handling systems. Photos courtesy Syska Hennessy Group.

On-air, live-audience television is becoming a benchmark of broadcasting and is attracting increasing numbers of viewers worldwide. Designing mechanical and electrical systems for live-audience television studios presents unique challenges. Such systems must meet stringent technical criteria for spatial function, noise level, cooling, lighting flexibility, power quality, emergency power, system redundancy and isolation grounding. Moreover, they must also be designed to provide comfort and life safety for a large group of people.

On-air live audience television broadcasting facilities typically consist of a studio ranging in size from 232m2 to 464m2, plus a control room, audio room, video room, green rooms, machine rooms and other support areas and amenities. The mechanical engineer must design a system that is capable of providing 24-hour cooling for technical equipment, accommodating a significant lighting load and providing a comfortable environment for an audience ranging from 50 to 200 persons, all while meeting a noise-criteria (NC) rating of 25 or lower.

Suspended above the stage and the elevated live-audience seating area in this typical on-air studio are ceiling elements such as the lighting grid, studio lighting, ductwork, plaque diffusers and acoustic insulation.

Minimizing noise and vibration

This combination of operational challenges requires an HVAC system that delivers air to the studio at a low velocity and distributes it using convection and diffusion principles. It also requires that the mechanical and electrical rooms are located as far as practical from the studio and other sound-sensitive areas to avoid transferring noise and vibration.

Defining and selecting the right type of HVAC system is usually the first crucial step toward meeting these requirements. A fan-coil system, which cools spaces by blowing air over coils that circulate chilled water, is not a good choice for an on-air live audience studio. Although some manufacturers claim otherwise, no one can guarantee that this type of system will meet stringent noise criteria or vibration-isolation requirements. This equipment must be placed as close to the space as possible and is likely to transfer an unacceptable amount of noise and vibration. If your architect or HVAC designer recommends such a design, you should immediately begin asking tough questions.

This control room uses low-level HVAC distribution as well as overhead distribution.

Typically, packaged air-handling units or air-conditioning units are a good choice. All of the components of air-handling or air-conditioning units should be self-contained to include components such as fans, motors, chilled water coils or direct expansion (DX) coils with compressors and, in some cases, sound silencers. This type of standard system typically meets studio noise criteria. If not, they can be upgraded from a packaged unit to a custom or semi-custom manufactured unit.

Once you've selected the cooling unit design, it's time to decide where to locate it. Usually, that involves tradeoffs between having it close to the studio for effective design or having it far away for maximum noise isolation. Typically, the roof is the only place available to hold it. Most facilities mount cooling units either on the rooftop or in a dedicated mechanical room indoors. For indoor installation, the HVAC units must be mounted on isolators in an acoustically lined mechanical room located as far away from the studio as practical — at least 30m to 90m away. To help minimize the transfer of sound and vibration, use a fan with a variable-frequency drive. This allows you to vary the fan speed and, therefore, the air velocity. It is also best to control the fan with direct digital control (DDC) instead of using mechanical or pneumatic sensors and controls. DDC systems use electronic sensors and microprocessors to provide precise, dynamic control of the variable-frequency drive.

When sizing the studio HVAC load, consider factors such as lighting load, number of occupants and, in some cases, envelope load, which consists of roof, walls and glazing. Lighting load can be as much as 377W/m2 to 807W/m2, yet the mechanical system should be designed for an average operating condition of 269W/m2 to 538W/m2 for practicality, diversity and economic reasons.

A live-audience studio with an occupancy of about 125 persons also increases the fresh-air requirement. Typically, such a room requires 7.08L/s to 9.44L/s per person, based on ASHRAE Standard 62-2001 (“Ventilation for Acceptable Indoor Air Quality”). Thus, you must increase the size of the air-handling unit. This is another good reason to locate the air handler on the rooftop. If you install the air handler within the building, shafts to the rooftop must be large enough to deliver the required volume of fresh air as well as relief/exhaust air. In an existing building, you'll have to provide new shafts to accommodate these requirements.

Air distribution vs. structural limitations

Air distribution presents structural challenges, and developing a solution requires ingenuity on the part of the mechanical engineer and close coordination with the architect, structural engineer and acoustical consultant. On-air, live-audience studios typically require a minimum floor-to-ceiling height of 5.5m to 7m to accommodate the lighting grid. The lighting grid needs to be kept on the same plain throughout to avoid shadows, and all utilities must be kept above this plain. This height requirement increases in proportion to the studio's area. As a result, typically there is often little space left above the lighting grid for ductwork because of the structural-support and seismic-restraint systems the lighting grid requires.

To meet noise criteria, slow-speed airflow (typically 3.048m/s) is required, which can be achieved by using large ductwork. Furthermore, to achieve the acoustical requirement for NC 25, reduce the exit velocity at the diffuser of the air-conditioning system even further to 1.524m/s or less. This increases the size of the duct-work significantly. You can also reduce noise by using silencers and acoustical lining in the ductwork. Be sure the architect locates the silencers outside the studio to effect the proper sound reduction.

Figure 1. Diffusers reduce air velocity and noise. Shown here are the details of a plaque diffuser. Click here to see an enlarged diagram.

In some applications, you must further reduce air velocity at the diffuser, typically from 3.048m/s to anywhere from 0.5m/s to 0.762m/s. A custom-made plaque diffuser is the most practical solution for live audience applications because it achieves thermal comfort and acoustical requirements. It is also cost-effective. (See Figure 1) Another custom-made diffuser, a tapered linear diffuser, is heavier and costs more than a plaque diffuser. (See Figure 2) A third custom-made diffuser is a fully perforated duct/diffuser. This diffuser is recommended only for studios with a ceiling height more than 7.3m. Whenever possible, locate the diffusers so that they direct airflow toward the faces of audience members. This provides the most comfort because the audience will feel coolness but no breeze or drafts.

The overhead air distribution system described above is the most widely used distribution system in studio applications. Another distribution method is to deliver air through a raised floor plenum. With a low supply system, the return is located high for proper air circulation and effective cooling and comfort.

Figure 2. A tapered linear diffuser, shown here, is heavier and costs more than a plaque diffuser. Click here to see an enlarged diagram.

In both methods, overhead and under floor, return air has to be sized at low velocity not exceeding 1.524m/s. Also, the placement of return air ducting is crucial. You can't locate return and supply airflow at the same height or location. With overhead distribution, locate the return at a low level. With under-floor distribution, locate the return at a high level.

In a typical recording or broadcast studio, constant-air-volume control may be sufficient and effective. But, in an on-air, live-audience studio, the load generated by the audience can vary from day-to-day, show-to-show and even in a matter of minutes during the course of the broadcast. For this type of studio, the ventilation system that can provide the best combination of comfort and energy conservation is a dedicated air-handling unit with variable-air-volume (VAV) control. The VAV control lets you adjust the air volume during a show to maintain adequate cooling. The most cost-effective implementation would be a single, DDC-controlled air-handling unit serving the studio.

Meeting variable loads

Finally, proper air distribution is critical in technical-equipment rooms. Cold-air outlets should be located in the front of or above the racks and return ducts located at the back of the racks to carry the heat to the return, thoroughly maintaining the equipment at the proper temperature.

Some on-air, live-audience broadcasting facilities require additional support spaces, such as dressing rooms, green rooms and a living space for hosts and other talent. Some on-air facilities also include a radio broadcast facility that may be used simultaneously or during separate hours. Consider all these elements when designing the facility's heating and cooling system.

Live-audience studios also require more attention to their electrical system requirements. The electrical engineer must design a power system that provides maximum flexibility for the user. The lighting load is typically defined by a studio lighting designer and, in some cases, set designers. The electrical system must be able to handle loads of as much as 376W/m2 to 807W/m2. But be sure to design the power system with enough flexibility to meet both permanent and temporary equipment loads.

Heightened electrical requirements

Safety is a key issue in a live-audience studio. Emergency lighting can employ fluorescent house lights mounted above the pipe grid. The emergency fixtures serving the studio are usually put on a shunt-trip relay to allow for total blackout during shows if needed. But, in the event of a real power outage, the emergency lights will come on. It's best to power emergency lights with a generator, but battery packs will also serve well if the studio is not located within a high-rise building.

Finally, we get to the issue of providing clean technical and backup power. It is essential to provide clean power to the dimmer room, control room, equipment room and certain AC outlets in the studios. The typical solution involves installing isolation transformers with an isolated ground or uninterruptible power supply (UPS). The technical ground usually consists of a ground bus bar in each technical room and copper conductors all daisy- chained together and tied into the building main service electrical ground.

Technical power is key

If the total technical-load requirement is greater than 200A, it is often cost-effective to have a 480V, three-phase, four-wire service delivered to the facility, where it is stepped down to 277V or 120V. You can add a filter or continuous UPS system to further reduce the possibility of interference/harmonics in the technical equipment. In addition, you should keep the technical power and AV cabling/signal requirements on separate distribution systems to prevent distortion. Because the transformers, UPS and dimmers are noisy, it's a good idea to house this equipment in separate rooms located at least 30m to 45m away from the studio. Alternatively, to reduce the length and associated cost of cable runs, you can line the transformer and/or dimmer room with acoustical insulation and place the transformer on vibration isolators.

Also, design the electrical system to allow maximum flexibility in configuration of the various types of equipment in the control room, equipment room, video room, etc. A raised floor lets you run power to multiple connections — both outlets and hardwired connections — under the floor. Provide at least 215W/m2 to 322W/m2 — or more — to meet equipment loads. Additional outlets, say four to five per wall, will add flexibility. It's best to locate electrical panels within the space they serve, either in the control room, equipment room, machine room, video room, etc. This will also allow you to expand and redistribute the power supply.

Reliable backup power is crucially important for live-audience, on-air broadcasting facilities. A backup generator will maintain power to the lighting, cooling system, servers and routers. But generators need as much as eight seconds to start up. For some applications, that's acceptable. But, for broadcast, it's not. In a broadcast facility, a UPS is necessary for critical equipment — that is, servers, router hubs and a few light fixtures — to allow continuous broadcasting.

Reliable backup power

Broadcasting with a live audience is like performing a high-wire act without a net. The medium creates unique opportunities, But, with no room for error, it also presents challenges for television directors, producers, talent, and facility designers alike. Nonetheless, carefully planned and skillfully designed mechanical and electrical systems will support successful live broadcasting today and well into in the future.

Charbel Farah is associate partner and Hisham Barakat is senior vice president of Syska Hennessy Group, a consulting, engineering, technology and construction company.