The design of atrium smoke control systems provides a great opportunity for a Professional Engineer, typically a Fire Protection Engineer, to flex his/her muscles and demonstrate how decades of devotion to math, physics, and chemistry (and often lack of a social life) have paid off.
The goal of the atrium smoke control system is to prevent occupants from the harmful effects of smoke, when there is a fire in an atrium. The design criterion is straight forward. The goal is to keep the smoke more than 6 feet above the highest occupied floor inside the atrium. So if the atrium is 12 floors, our goal is to keep the smoke 6 feet above the 12th floor balcony.
As simple as that sounds, the design can become very complex due to the many different considerations (variables).
Design Fire
One of the first considerations is to evaluate the combustibles inside the atrium. Typically, the greatest fuel load is located at the bottom floor of the atrium. This is where people tend to gather for the ambiance of the atrium. As a result, you’ll often see tables, chairs, couches, and sometimes even vegetation at the base of the atrium for building occupants’ leisure. However, the volume of combustibles are only one of many factors that determines overall smoke production. Other factors contributing to smoke production include:
- Material being burned (wood, plastic, flammable liquids, combustible metals)
- Material density
- Number of air pockets in the material being burned
- Location of material with respect to walls
- Humidty
- Ambient temperature
- Configuration of materials being burned
All these fuel load possibilities can have a significant impact on the smoke production from a fire. For example, plastic typically burns much hotter and produces more smoke than wood. Additionally, combustible products located along walls tend to have greater difficulty burning than combustible products in the center of a room due to lack of oxygen flow into the fire. This can cause greater smoke production at early stages in the fire since there will likely be incomplete combustion (i.e. the fire is fuel rich and lacks oxygen for stoichiometric combustion).
Smoke Travel Distance
Another important factor to consider when designing an atrium smoke control system is how far the smoke has to travel before it reaches the atrium ceiling. The further smoke travels, both vertically and horizontally, the more air it sucks in, which simply creates more smoke. So the taller the atrium, the more smoke that will be produced.
Smoke Reservoir
It is also important to evaluate how much smoke can accumulate at the atrium ceiling before it comes down to a point where it will harm occupants. This acceptable amount of smoke inside the atrium is commonly referred to as the “smoke reservoir” or “safety smoke layer.”
Tenability Threshold
A vital step in designing an atrium smoke control system is determining how much smoke to which occupants can be exposed before they are overwhelmed. There are three major harmful elements of smoke to people:
- Temperature
- Carbon Monoxide
- Impaired Visibility
After being exposed to an elevated temperature for a prolonged duration, people begin to experience trouble breathing and lungs incur permanent damage. This temperature threshold is typically around 140 degrees Fahrenheit.
When occupants are exposed to prolonged levels of carbon monoxide, typically around 350 parts per million (ppm), they can become very sleepy and pass out, unable to make it out of the building on their own.
Studies have shown that when occupants visibility distance through smoke drops below 30 feet, they become confused, slow down, travel in wrong directions, and sometimes even stop moving, altogether.
Timed Egress Analysis
Another important factor to consider when designing a smoke control system is how long it takes occupants to leave the building after the fire alarm system activates. This time can differ tremendously, depending on the building use. For instance, it take much longer for occupants of an apartment building to evacuate than occupants of an office. It needs to be assumed that occupants of an apartment are asleep when the fire alarm system activates. Occupants of an office building are assumed to be awake and coherent. Considerations also need to be made for large numbers of people queuing at doors and stairs.
Fan Sizing
Once we determine the rate of smoke being produced (volume/time), the smoke reservoir we have to work with, how long it takes occupants to evacuate the building, and the amount of smoke to which occupants can be exposed, we need to determine how much smoke is required to be exhausted from the atrium ceiling before the smoke layer at the atrium ceiling drops down to evacuating occupants. The fans have to be sized just large enough to allow the last occupant time to make it out of the building. Fire department personnel are trained and equipped to deal with adverse smoke and fire conditions far beyond what civilians can handle.
Fire Alarm and Mechanical System Integration
After fan sizes for the removal of smoke in the atrium have been established, we then need to decide what will activate those fans to turn on. Will there be beams detectors (lasers that point across the atrium) that activate when they sense smoke passing by? Standard smoke detectors or the fire sprinkler system is another possible way to activate the smoke control system.
Other Factors
There are numerous other factors to consider when designing a smoke control system, such as the effects sprinklers have on the fire, make-up air into the atrium to prevent significant pressure gradients, stratification (buoyant smoke cooling down as it travels up the atrium), stack-effect (the effect that the HVAC system has on the building in relationship with outside conditions, wind pressure effects, elevator piston effects, and more. However, we’ll leave those topics for a more technical discussion on smoke control systems.
The bottom line is that some aspects of engineering can actually be quite interesting (or at least not terribly boring). Also, while engineers aren’t professionals who demand the spotlight, it’s worth noting the value they have on society for maintaining a safe public environment.
Next time you’re in an atrium building, look out for some of the features of the smoke control system noted in this article.