Air-duct smoke detectors are not the same as open-area room detectors. Area detectors’ main function is to sense smoke as a sign of fire. Duct detectors are designed to trigger a fire alarm and also to reduce the circulation of smoke through heating, ventilating and air conditioning ducts (HVAC). Why is this so important? Smoke and toxic fume inhalation is the leading cause of death and injury and can affect people much faster than the heat of the fire.
According to the National Fire Protection Association (NFPA), “Most fire deaths are not caused by burns, but by smoke inhalation. Often smoke incapacitates so quickly that people are overcome and can’t make it to an otherwise accessible exit. The synthetic materials commonplace in today’s homes produce especially dangerous substances. As a fire grows inside a building, it often will consume most of the available oxygen, slowing the burning process. This ‘incomplete combustion’ results in the release of toxic gases.”
Smoke’s components—particles, vapors, toxic gases—each can be lethal in their own way, according to the NFPA. The very systems that circulate the air will circulate poisonous substances in a fire, even a slowly smoldering one. Sensing that smoke is traveling through the HVAC ductwork and quickly reacting to it can save lives.
“It is imperative to control the spread of smoke in any structure,” said Steve Hein, general manager, Global Fire and Life Safety Systems, GE Security, Bradenton, Fla.
Controlling the spread of smoke
To control the spread of smoke, Hein said, a building needs a system engineered to integrate with its air circulation system. In response to smoke, the HVAC system must be shut down immediately, and appropriate dampers, pressurization and exhaust equipment must confine the smoke to the fire’s immediate area. If possible, the system should pump smoke to the outside.
“When there is a fire alarm, the fire alarm control unit takes over almost all of the building systems,” Hein said.
This procedure is advocated in NFPA Standard 90A, Installation of Air Conditioning and Ventilating Systems, and has been backed up by testing performed by the Fire Detection Institute in conjunction with the University of Maryland Department of Fire Protection Engineering and the National Research Council Canada, as detailed in the collective report, “Investigation of the Application of Duct Smoke Detectors in Heating, Ventilating and Air Conditioning Systems.”
A System Sensor, St. Charles, Ill., application note, “Duct Application Smoke Detectors,” makes a similar point: “National and local safety standards and codes recognize the ability of air duct systems to transfer smoke, toxic gases, and flame from area to area.”
It goes on to say that, in a typical single zone system, the dampers close and the fans shut down. But that is not the only approach; another possibility is to shut down just the supply fans, while keeping the fans that provide exhaust to the outside air running.
As Hein pointed out, the primary purpose of a smoke control system is to safely evacuate all of the building’s occupants. Smoke and toxic fumes often prevent people who are in no immediate danger of being burned from safely exiting. The Air Products and Controls, Pontiac, Mich., slide presentation, “Duct Smoke Detector Training, General Course: Codes and Standards,” by David L. Hall, points out, “You have less than 60 seconds to escape a smoke filled environment before inflicting potentially serious damage to your health.” It goes on to describe the hazards of a smoke-filled area: “Reduced egress speed due to sensory (eye, lung) irritation, heat or radiation injury (beyond that from the flames themselves); reduced motor capability, and visual obscuration; choice of a longer egress path due to decreased mental acuity and visual obscuration; and chronic health effects in firefighters and occupants.”
The best way to use smoke detection to save lives requires careful system design. For example, if smoke is detected on the fifth floor of a high-rise, the approach might be to seal off the air supply to the fourth and sixth floors, evacuate all three, send all elevators to the main floor for firefighter use, and selectively release electrically held door locks that could prevent access to stairwells to initially keep occupants of the other floors away from the stairs in order to minimize crowding and confusion.
To accomplish this “smoke control,” the fire system would automatically open exhaust fan dampers on the fifth floor and start the evacuation fans. On the floors above and below, the smoke floor (floors four and six), the fire system would start pressurization fans to “sandwich” the smoke and contain it to the fire floor. Additionally, the stairways (used to evacuate occupants) would start their exhaust fans to ensure the fire rated exits are free from smoke.
Duct smoke detectors are a vital part of a fire management system. The system must be designed to detect fire as soon as possible; to control the spread of flames, smoke and toxic gases; to minimize injury and loss of life; and to help occupants evacuate as quickly and safely as possible.
BROWN is an electrical engineer, technical writer and editor. He serves as managing editor for SECURITY + LIFE SAFETY SYSTEMS magazine. For many years, he designed high-power electronics systems for industry, research laboratories and government. Reach him at email@example.com.