Research Proposal: Fire Technology

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¶ … Duct Smoke Detectors: The Impact of Various Factors on Their Effectiveness," Construction Technology Update No. 72, December 2008 by G.D. Lougheed

The use of smoke detectors in heating, ventilation and air conditioning (HVAC) ducts is a requirement of a majority of building codes in North America, including the National Building Code of Canada (NBC). As part of this requirement, the NBC stipulates that the HVAC system should be shut down during fire events in order to reduce the amount of smoke that is circulated in the affected premises by the HVAC's fans. Furthermore, the NBC requires the installation of the smoke detector must be in a location that is situated downstream of the fresh air intake vents, but in some regions there is also a requirement for the installation of a smoke detector in the return air duct. These requirements are in response to a 1939 recommendation from the National Board of Fire Underwriters in 1939 and installation requirements are set forth in various code standards, including CAN/ULC-S524. In this article Lougheed provides an overview of the requirement, a description of the study undertaken by researchers at the Fire Detection Institute and the University of Maryland, a review of the study's data concerning the comparative driving forces of the HVAC system vs. those created by a fire event, the impact of smoke dilution, smoke aging and air filter effects on the efficacy of the duct-mounted smoke detectors. A presentation of the study's approach to examining the impact of smoke stratification in lengthy straight runs of ductwork and the effectiveness of sampling tubes that are inserted into the air ducts to detect smoke levels was followed by a brief summary of the study's findings.

Statement of the Research Problem

Based on a paucity of timely research that supports the 1939 recommendation from the National Board of Fire Underwriters, the Fire Detection Institute and researchers from the University of Maryland (UMD) jointly conducted a study to develop relevant technical data for use by code developers, standards committees as well as system designers. The purpose of this study was to investigate the use of air duct smoke detectors in their primary capacity as a method of detecting fires and/or smoke that is being circulated in a building's HVAC system as well as the use of duct-mounted smoke detectors using sampling tubes inserted into air ducts as part of an overall smoke management system. Because the requirement for air duct detectors was based on a 70-year-old recommendation, the sponsors of this study wanted to determine whether there was sufficient data available to support the ongoing enforcement of this requirement for air duct smoke detectors in both the fresh air intake as well as the return air duct given the added costs involved in this dual arrangement as well as the additional possibility of false and nuisance alarms that have historically been the result of this requirement.

Description of Research Procedures

The researchers from the NRC Institute for Research in Construction (NRC-IRC) used the 10-story test building provided by the Fire Detection Institute for this study while the researchers at UMD were responsible for providing the results of modeling studies and small-scale experiments to examine the following study elements.

Comparative Driving Forces. Pursuant to the study's stated purpose, the researchers (presumably those from the NRC-IRC but this point is not stipulated by the author) investigated the need for the requirement to shut down a building's HVAC system during fire events by comparing smoke movement created by the HVAC fans vs. The smoke movement that is caused by the fire and related dynamic effects. According to Lougheed, "It was found that the HVAC-related pressure differences were generally larger than those stemming from other factors, including the fire itself. These greater pressure differences also led to higher flows and the distribution of smoke to floors where there was no fire" (p. 2). Based on these findings, it was the researchers' conclusion that absent an alternative active smoke management strategy, the requirement that the HVAC system should be turned off when fires are detected to prevent the spread of smoke and fire throughout the remainder of the building was justified, but cautioned that the benefits of this step relate to the characteristics of the specific building involved.

Dilution Effects. Based on a concern that smoke detectors are unable to detect the levels of smoke that they are designed to detect when they are mounted in air ducts based on the potentially lower concentrations that accumulate in ductwork, the procedures used by the NRC-IRC researchers to study the dilution effects of air-duct mounted smoke detectors employed four different types of air-duct smoke detectors that are commercially available (i.e., ionization, photoelectric, sampling and multi-sensor). To determine the viability of each of these types of duct-mounted smoke detectors, the researchers examined the analog signal that was provided by each detector type which is used to trigger the smoke alarm based on variable settings compared to the optical density of the smoke at an identical location within the HVAC system's return air duct.

Smoke Aging. To investigate the impact of smoke aging (the tendency of smoke to cool and alter its characteristics as it moves away from the original fire source as well as the condition of the air ducts that carry it) on the efficacy of duct-mounted smoke detectors, the researchers measured the number and size of smoke particles at the supply duct and compared it to a point 3 meters downstream in the air duct.

HVAC Filters. Because HVAC filters eliminate part of the smoke that is introduced into the HVAC system and the level of smoke that is eliminated differs depending on the type of air filter use, the researchers measured the amount of smoke that was eliminated in both the return air duct and in the supply air duct downstream of the filter at the NRC-IRC facility.

Stratified Flow. Smoke tends to differentiate itself when it travels along long uninterrupted straight flows of ductwork; therefore, relevant standards recommend that duct-mounted smoke detectors should be placed between 3 and 10 duct diameters from inlets, bends, and duct outlets to ensure that the smoke being detected by the duct-mounted smoke detectors has the opportunity to achieve a uniform mix. The only description of the research procedures used for this component of the study was Lougheed's observation that the researcher took "measurements in a duct system near the fire source" (p. 4).

Efficacy of Sampling Tubes. Duct-mounted smoke detectors are commercially available in two types: (a) duct-installed smoke detectors (these are identical in operation to ceiling-mounted detectors); and (b) smoke detectors that are mounted on the exterior of the duct with a sampling tube placed inside the duct. Based on concerns about the current standards for performance of the sampling tubes currently in use, the researchers surveyed 65 commercial buildings in the Baltimore/Washington area.

Flaws in the Procedural Design

By and large, the procedural design used by the researchers in this study was congruent with the study's stated purpose, but there were some flaws identified, though. For instance, as to the measurement of smoke dilution, Lougheed points out that, "The measurements related to dilution and aging effects were made in a return air duct. However, the results would be the same for detectors located in the supply duct" (2008, p. 2). This conclusion fails to take into account unexpected disruptions in the functioning of the HVAC system, including the return air dampers becoming inoperable. In addition, one of the stated purposes of the study was to provide a cost-benefit analysis concerning current requirements for duct-mounted smoke detectors which was provided in a superficial fashion only and which requires an extrapolation of the study's findings to determine. Finally, based on the results of this study, Lougheed provides a number of findings that he suggests are supported by the research by the UMD and NRC-IRC, but the author does not indicate whether these findings were provided by the principal researchers or were based on his own reading of the study and its results.

Analysis of the Data

Because each of the four different types of smoke detectors used in air ducts had a different range of outputs, the effectiveness of the duct-mounted smoke detectors in overcoming the effects of smoke dilution was accomplished by converting the respective outputs to a percentage scale using the maximum output for each detector as a basis. The results of this part of the study showed that the output of all four types of smoke detectors corresponded to changes in the optical density of the smoke levels being measured. The results of this portion of the study found that all four types of smoke detectors were capable of detecting smoke in the HVAC system when smoke concentration levels were reached that corresponded to those criteria used to initiate building evacuation.

The analysis of the smoke aging component determined that the amount of small particles at the supply duct was decreased by a factor of 10 compared… [END OF PREVIEW]

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