Demand Performance

Editor's note: This is the second part of a two-part series on HVAC and fire-protection design for large open-air spaces, while last issue focused on atriums, this installment will examine criteria for auditorium and theater design. From the opening scene to the curtain call and beyond, it's important that theater and concert patrons not only be entertained, but are also afforded comfort and s...

07/01/2002


From the opening scene to the curtain call and beyond, it's important that theater and concert patrons not only be entertained, but are also afforded comfort and safety during performances. The "openness" of these facilities, however, makes the design of HVAC and fire-protection systems difficult to stage.

One must begin with the understanding that the criteria for auditorium design come in many forms, each depending on the type of facility, performance requirements and the space itself. For example, concert, opera, ballet and theater venues all have different environmental criteria. In fact, these requirements can vary from company to company.

In any case, a temperature of 68°F is frequently used as the design standard—73°F during actual performances to account for the heat rise associated with increased occupancy. Consequently, it is essential to build flexibility into the heating/cooling plant to vary the internal environmental temperature

Humidity control is not particularly a problem in general theaters—providing that humidity stays within a range of 40% to 60%. It is more critical in concert and operatic halls where the level of humidity can have an adverse effect on a vocal or instrumental performance. Humidification systems are often provided on stage.

Room noise criteria in auditoriums is also a subject of much debate and again will depend on the use of the space. It is common to seek the advice of an acoustic consultant. But the base preferred-noise criteria (PNC) for auditoriums used for concerts and operatic halls is around PNC15, and for general theaters, PNC20. On the other end of the spectrum, extremely quiet conditions can also result in acoustic discomfort.

The air up there

Traditionally, theater and concert hall auditoriums have ventilation schemes utilizing downflow mixed air. Generally, it's because ventilation is often part of a retrofit, and the only way to install the ductwork diffusers is via the roof void. However, new theater projects utilize more efficient methods, including displacement and natural ventilation.

A number of ventilation schemes are available, including traditional downflow air, displaced ventilation and natural ventilation. Following are the advantages and disadvantages of each:

Downflow mixed-air . These traditional systems use air volumes between 23 and 27 cfm per person, discharging at least 17 cfm of fresh air and exhausting air at a high level through high velocity diffusers.

Advantages:

  • Lower air volumes than a displacement system, if the supply air condition can be reduced.

  • Major duct installation is at a high level, negating the need for large risers to the auditorium floor level.

  • Varied seating and stage arrangements can be accommodated.

Disadvantages:

  • Uncomfortable down drafts can "dump" onto patrons during hot weather.

  • Careful grill/diffuser selection is required to provide correct throw without high noise levels.

  • Air distribution is somewhat hit-and-miss during colder weather, due to the natural buoyancy of warm air.

  • Poor comfort control, as a point of monitoring, is traditionally located in return-air ducting and is subject to gains not associated with the occupants.

  • Poorer air quality due to contaminants remaining at low level.

Displacement ventilation . This is a more suitable approach to ventilation in auditoriums, as the discharge air only treats the occupied zone with discharge velocities of around 318 cfm with air exhausted at high level. The air volumes can be as high as 25 cfm per person, but design volumes of approximately 17 cfm per person have produced suitable results, especially when utilizing exposed thermal mass.

Advantages:

  • Supply air directly treats the occupied zone.

  • Provides good comfort control.

  • Maintains low noise levels due to low air velocities.

  • Uses cooling capacity of external air for a greater percentage of the year.

  • Has lower operating costs.

  • Removes contaminants.

  • In small theaters of less than 200 seats, can be implemented without the cooling using thermal mass.

Disadvantages:

  • Riser space required to distribute air to low level.

  • Under-seat plenums required.

  • Less flexible for seating and staging options.

  • Large numbers of diffusers required to produce low velocity discharge.

  • Balcony fronts require upstands to avoid air "dumping" on the audience below.

For stage and seating flexibility, it may be necessary to provide a hybrid solution such as using the principles of displacement with the inclusion of sidewall discharge points in areas where seating is removable.

Natural ventilation . These types of schemes recently have been incorporated into new theater designs with success, but are generally limited to theaters with seating capacities of less than 300 seats.

Air is drawn in at a low level or below the seating and discharged at a high level through the auditorium roof, utilizing the natural stack effect generated by heat gains within the space. Note that such schemes are typically supplemented with mechanical ventilation to assist air movement when the natural effect is not sufficient.

Advantages:

  • Supply air directly treats the occupied zone.

  • Provides good comfort control.

  • Maintains low noise levels due to low air velocities.

  • Uses the cooling capacity of external air for a greater percentage of the year.

  • Has low operating costs.

Disadvantages:

  • Not suitable for large theaters.

  • Limited response to point loads.

  • Large areas of intake louvers must be located at a low-level.

  • May not be suitable for areas with high traffic or low-level pollutants.

  • Chimneys or stacks required to enhance stack effect.

  • Under seat plenums required.

  • Less flexible for seating and staging options.

  • Large numbers of diffusers required to produce low velocity discharge.

Again, the most effective method for conditioning an auditorium space is a displacement system—either naturally or mechanically driven—which is both quiet and treats the occupied space.

Whether to use a natural or mechanical system depends on theater size and load profile.

The drama of fire

Theatrical performances are staged in one of two configurations: either using an end stage or a thrust stage. With the end stage, the players and their props are viewed through a proscenium opening in the wall separating players from the audience. With a thrust stage, the audience becomes more intimate with the performers, but closer proximity to the stage increases the fire risk potential for the audience and more controls are demanded from the design. These two functional layouts have very different fire performance requirements based on the risks the audiences are exposed to from the props and scenery.

When designing for an end stage, the following guidelines should be followed:

  • The auditorium should be designed as one fire compartment.

  • Stage and side stage areas should be designed as a second fire compartment.

  • There should be no fire or smoke ventilation provision in the auditorium.

  • The fly tower should be provided with ventilation.

  • Fire control of materials on stage is considerably more flexible than for a thrust stage configuration; i.e., the levels of pre-treatment of the materials used is lower.

  • The fire-safety curtain is either a single fire-rated element, such as a fire damper, or is a water-drenched curtain. The water-drenching system is activated by either a mechanical link or manually by the stage manager.

  • Sprinklers are not provided on stage areas or within auditorium.

  • Suitable fire-detection and alarm system should be provided to both spaces.

When designing fire protection for a thrust stage:

  • Auditorium and stage areas are considered as one fire compartment.

  • Because there is no fly tower, no stage ventilation is required.

  • There should be more stringent control of the types of materials that can be used in the construction of scenery and sets. Also, pretreated materials should be used to reduce the rate at which a fire can develop. This extends to any side stage areas that may be present.

  • Uncontrolled materials on side stages must be fire-separated from the thrust stage/auditorium.

  • Because there is no safety curtain, no drencher system is required.

  • A suitable fire-detection and alarm system should be provided to the one "combined space."

In some design cases, when these requirements are tested against the form, functionality and flexibility requirements of an auditorium, conflicts in the performance requirements may be derived at the line of the proscenium wall and the side stages. This may be the case when the theater wishes to operate a combination of end and thrust stage performance types. Consequently, either the performances must adapt to one form of stage arrangement, or an alternative solution is to design both the fire safety curtain and the potential fire separation lines between the main stage and side stage areas. Such a solution must provide measures that compensate for the lack of fire separation between the areas.

By establishing the design's core performance requirements, the risk can be assessed and a set of goals defined for developing solutions. For example, provide an escape strategy based on the codes requirements or additional fire-control measures, such as sprinklers in fire risk areas. Another strategy is including a smoke- and heat-exhaust ventilation system designed to maintain safe conditions in the space.

This latter provision would also be beneficial to firefighters. If a clear layer smoke-control solution is adopted, they will be able to establish the seat of the fire more clearly than in a room or space without such ventilation systems.

Encore

Once key parameters and variables are identified within different types of auditorium facilities, engineers can then draw from a pool of strategies. And by knowing the pros and cons of the different mechanical approaches, engineers will be best equipped to specify accordingly.





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