Atriums: More Beauty Less Beast

The following is the first in a series of two articles focusing on HVAC and fire-protection strategies for large open-area spaces. This month's article deals with atriums. The second piece, to appear in the pages of CSE later this summer, will cover auditorium design. Consider a skylight-capped four-story atrium that also includes a large façade with a southern exposure.

By Byron Stigge, New York, Stuart Martin, MRICF, Manchester, U.K., Mark Owen, IEng ACIBSE, MASHRAE, Leeds, U.K. Buro Happold Consulting Engineers June 1, 2002

The following is the first in a series of two articles focusing on HVAC and fire-protection strategies for large open-area spaces. This month’s article deals with atriums. The second piece, to appear in the pages of CSE later this summer, will cover auditorium design.

Consider a skylight-capped four-story atrium that also includes a large fa%%CBOTTMDT%%ade with a southern exposure. Not only is the space devoid of shading, but a number of bridges and balconies overlook the space and the atrium’s large volume encourages a temperature gradient to develop and lodge in these areas. Also, due to cost issues, the skylights are not thermally broken, so condensation develops when warm, moist air meets the cold thermal bridge of a skylight mullion.

Such a scenario might cause many mechanical engineers to cringe, especially those striving for sustainability and indoor environmental quality. So what can be done to make the best out of a bad situation?

Managing the challenge

One of the ways engineers can deal with such nightmarish climate-control issues is by utilizing displacement ventilation . Literally operating on the principle that hot air rises, displacement ventilation schemes rely on natural thermal convection, where warm, stale air—with its impurities—ascends and is displaced by cool, fresh air. As people, lights and other heat sources warm this air, it then rises, to be exhausted near the ceiling. The result is that supply air constantly refreshes the space in a natural flow by displacing older air.

The main advantage of this strategy is that ducts, air handlers and chillers can all be sized smaller. Because displacement ventilation does not work on a mixing principle—where all air in a room is thoroughly mixed at the same temperature—lower supply air volumes can be provided directly to the occupied zone, allowing unoccupied areas to float outside the normal comfort range. Furthermore, displacement ventilation doesn’t necessarily have to take the form of underfloor systems. Rather, the ventilation system can utilize any low velocity supply sent directly to the occupied area.

In addition to being a functional solution, displacement ventilation is appealing to architects. From their perspective, one of the exciting features of the scheme is that supply air grilles can be discreetly integrated with the architectural design. Also, the outlets can be design features, and don’t need to be prepackaged diffusers. Consequently, any large surface with porosity or cracks that allow air to seep out at about 30 feet per minute (fpm) will suffice. The very nature of these systems mandates architects to become involved in designing the ventilation system, thereby integrating it with the rest of the design and elevating its priority, as well as making it easier to pass ducts, pipes, etc.

That being said, several criteria have to be carefully considered before specifying a displacement ventilation system. For example, in high occupancy spaces, lower volumes of supply air and higher supply temperatures may not sufficiently remove the latent load, causing humidity to build up over time. Also, diffusers are usually placed near or on the floor, so supply air velocities should be less than 30 fpm to avoid uncomfortable drafts around people’s feet and ankles. If a large porous plate is used as the diffuser, the porosity of plate for the displacement diffuser should be between 8% and 30%. Fortunately, diffuser manufacturers have designed and tested low-velocity diffusers, for which dimensions, flow-rates, throw-distances and velocities are all published. The properties of these manufactured diffusers can also be used to benchmark custom designed diffusers.

With regards to the movement of return air, it can travel through the atrium volume itself if:

  • Floor plates open up to the atrium.

  • The space is an open plan.

  • Return fans are located high in the atrium.

  • Floor plates are not more than 30 ft. deep.

This technique works particularly well for 100% fresh-air systems with fan coil units because small amounts of fresh air are supplied and all return air is exhausted.

Supplemental strategies

Additional atrium HVAC strategies include the following:

  • Radiant-floor heating. This is often an appropriate heating approach because loads in an atrium are relatively even, low and predictable from 8 a.m. to 7 p.m., radiant floor heating focuses on the ground level where people traverse, instead of upper areas that are typically unoccupied.

  • Natural ventilation . This scheme also works well in tall spaces, as the stack effect drives air pressures higher. In turn, ventilators, typically located up high, can move a good deal of air. Atriums usually have less stringent temperature and humidity requirements, especially when they are physically separated from adjoining floor plates. Outdoor air, however, has to be between 58°F and 68°F, which means that in more temperate climates, natural ventilation can replace mechanical systems for more than 10% of the operational hours.

If non-conventional strategies such as natural ventilation and displacement ventilation are decided upon for an atrium scenario, designers should familiarize themselves with computational fluid dynamic (CFD) modeling. Although a complicated technique, such modeling demonstrates airflows and temperatures in such spaces, ultimately verifying that the design, in fact, works.

Atrium fire design

Not only do atriums have unique HVAC issues, they pose fire-protection challenges as well—especially because fire marshals demand extra design and analysis for both evacuation and smoke exhaust. This mentality derives from the tendency of such officials to think of atriums in terms of being a large “hole” in a building, making it more dangerous than an equivalent non-atrium enhanced building due to the increased risk of fire and smoke spreading up and through the “hole.” To ascertain if there actually are additional risks, several factors need to be considered: Is the aim of the design to protect life during the escape period, or to increase the safety of firefighters? Or is it to protect the building itself, the property inside the building and the business activities within? Each level of safety requires a different strategy to deal with the effects of fire and smoke spread vs. containment. It is essential at the start of any design process to define these goals.

To do so, the type of building and profile of the occupier must be defined. The level of risk varies between those who are awake and familiar with the premises, those who are awake and unfamiliar with the premises, those who may be asleep and those receiving medical care, in the case of a hotel or hospital.

Clearly, the risk to people who are familiar with the premises and awake will be lower than those who may be asleep. The awake occupants’ response times to an alarm and the time taken to locate exits will be quicker. Also, if the proposed building’s fire compartment dimensions exceed those of the equivalent non-atrium building, there is a greater risk.

Finally, when the circulation zone passes along the perimeter overlooking the atrium, smoke concentrations can impede visibility and be dangerous to inhale. If no balconies are present, it is recommended by some codes that doors to escape stairs be sited so that they are at least 15 ft. away from any void edge.

Fire-protection systems associated with the various occupant profiles can vary greatly. The inclusion of a particular protective system will also be based on the ability of an uncontrolled fire to grow and spread through a space to the point where the conditions become untenable for escape.

The following fire- and smoke-prevention systems are recommended for use in atriums:

  • Automatic suppression , typically water based.

  • More extensive fire detection and alarm systems , which may incorporate voice annunciation.

  • Active fire dampers or doors that can close off the atrium space to reduce the flow paths of flame and hot gas.

  • Fire-rated glazing systems that incorporate radiant heat-reduction properties where architects desire open aspects of the design.

  • Smoke-control systems , either natural or mechanical in operation.

As with HVAC design, CFD can provide valuable data. CFD can model smoke generation, extraction and concentration levels in a three-dimensional space, with results used to determine appropriate mechanical extract capacities, locations and area sizes (See CSE Oct. 2001, “Model of Success,” pg. 54). Then, the results can be presented to fire officials for variances to codes or value engineering reductions.

Wrap up

In sum, fire engineering has a key role to play in atrium design, and ideally should be integrated into the design process, rather than treated in isolation from other engineering disciplines.

Together with HVAC design, these building systems may pose challenges, but make up a crucial part of these unique spaces. However, by addressing these systems with technical expertise, forethought, flexibility and innovation, occupants can continue enjoying the aesthetic beauty of atriums with comfort and security.

Skylight Side Effects

A common feature in atrium and auditoriums are skylights. While they supply wonderful daylight, they also bring in huge solar gains.

In order to block summer sun at high angles and accept some winter sun at lower angles, fixed louvers work effectively. Even though motorized shades seem to be an ideal solution, maintenance is difficult in deep atriums, especially those taller than 10 stories. A second, and somewhat diametric problem is the fact that sunlight will not penetrate to the floor level. As a result, a more sophisticated technique of using solar tracking mirrors may be necessary to bounce light evenly to the floor level on all sunny days throughout the year.

Also a concern, condensation appears on skylights when the inner surface of the glass is below the dew point temperature of the air next to it. This is always a concern for cold-weather climates, but particularly for atrium skylights as the air at the top tends to be more moist than room air because it collects moisture from more people, plants, water features, etc. High-level exhaust in sufficient quantity helps, but the use of thermally broken, double-glazed insulating glass can be a better tool.