HVAC for on-air facilities

E! Entertainment’s E! News studio uses an overhead HVAC system with a tapered diffuser. Photos courtesy Syska Hennessey.

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 provide comfort and safety for a large group of people.

Live-audience television broadcasting studios typically range in size from 2500- to 5000sq ft, and include 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 can provide 24-hour cooling for technical equipment, a comfortable environment for an audience ranging from 50 to 200 people, and be able to accommodate a significant lighting load, all while meeting an NC rating of 25 or lower.

Minimizing noise and vibration

This combination of operational challenges requires an HVAC system to deliver low-velocity air to the studio and distribute it using convection and diffusion principles. And, the designers must locate the mechanical and electrical rooms as far as practical from sound-sensitive areas to avoid transferring noise and vibration.

Choosing the right HVAC system is usually the first crucial step. 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, this type of system must be placed as close to the space as possible, and is likely to transfer an unacceptable amount of noise and vibration. If an architect or HVAC designer recommends such a design, immediately begin asking tough questions.

Typically, packaged air-handling or air conditioning units are a good choice. These units should be self-contained and include fans, motors, chilled water coils or direct expansion (DX) coils with compressors and, in some cases, silencers. Such standard systems typically meet studio noise criteria. If not, they can be upgraded from a packaged unit to a custom or semi-custom manufactured unit.

In this typical on-air studio, ceiling elements such as the lighting grid, studio lighting, ductwork, plaque diffusers and acoustic installation are suspended above the stage and elevated live-audience seating.

Once a station has selected the design for the cooling unit, it's time to decide where to locate it. Usually, that requires a balance between having it close enough to the studio for effective design and having it far enough 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 100- to 300ft. To help minimize the transfer of sound and vibration, use a fan with a variable-frequency drive. This allows the fan speed, and therefore the air velocity, to be varied. 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 35- to 75W/sq ft, yet the mechanical system should be designed for an average operating condition of 25- to 50W/sq ft for practicality, diversity and economy. The size of an HVAC unit for a studio of approximately 4500sq ft and a live audience of about 125 people can range from 60 to 75 tons.

This occupancy level also increases the fresh-air requirement. Typically, such a room requires an air flow rate of 15- to 20cfm per person. Therefore, the size of the air-handling unit must increase proportionally. This is another good reason to locate the air handler on the rooftop, if possible. If a facility installs 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, new shafts are required to accommodate these requirements.

Air distribution vs. structural limitations

Air distribution presents structural challenges requiring 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 18- to 22ft to accommodate the lighting grid. The lighting grid needs to be kept on the same plane throughout to avoid shadows, and all utilities must be kept above this plane. This height requirement increases in proportion to the studio's area. As a result, 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 600ft/min) is required. This can be achieved with large ductwork. To achieve the acoustical requirement of NC 25, reduce the exit velocity at the diffuser of the air conditioning system even further, to 300ft/min or less. This increases the size of the ductwork significantly. Noise can also be reduced by using silencers and acoustical lining in the ductwork. Be sure to locate the silencers outside the studio in order to reduce noise.

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, engineers must further reduce air velocity at the diffuser, typically from 600ft/min to anywhere from 100- to 150ft/min. A custom-made plaque diffuser (see Figure 1) is the most practical solution for live-audience applications because it achieves thermal comfort and acoustical requirements. It is also cost-effective. Another custom-made diffuser, a tapered linear diffuser (see Figure 2) is heavier and costs more than a plaque diffuser. A third custom-made diffuser, a fully perforated duct/diffuser, is recommended only for studios with a ceiling height more than 24ft. Whenever possible, locate the diffusers so 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 system for 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. In both overhead and under-floor methods, return air ducts have to be sized to deliver the air at a velocity not exceeding 300ft/min. Also, the placement of return air ducting is crucial. Don'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 minute to minute 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 station staff adjust the air volume during a show to maintain adequate cooling (typically about 68°F or lower). The most cost-effective implementation would be a single, DDC-controlled air handler serving the studio.

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

Electrical systems in live-audience studios require special attention. The electrical engineer must design a power system that provides maximum flexibility for the user. Typically, a studio lighting designer or set designers define the lighting load. The electrical system must be able to handle loads as high as 35- to 75W/sq ft and the power system must be designed with enough flexibility to meet both permanent and temporary equipment loads.

Meeting variable loads

Life safety is a key issue in a live-audience studio. Emergency lighting can employ fluorescent house lights or battery packs.

Heightened electrical requirements

Finally, we get to the issue of providing clean technical and backup power. The dimmer room, control room, equipment room and certain AC outlets in the studios need clean power. 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's main service electrical ground.

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. A filter or continuous UPS system can be added to further reduce interference/harmonics in the technical equipment. Also, the technical power and AV cabling/signal requirements should be kept on separate distribution systems to prevent distortion. Because transformers, UPS and dimmers are noisy, it's a good idea to house this equipment in separate rooms located at least 100- to 150ft away from the studio. Or else, to reduce the length and associated cost of cable runs, the transformer and/or dimmer room can be lined with acoustical insulation and the transformer placed on vibration isolators.

Technical power is key

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 and facility designers alike. Nonetheless, carefully planned and skillfully designed mechanical and electrical systems will support successful live broadcasting today and in the future.

Plan for reliability

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