Evaluation on the performance of the probe type (duct type) smoke detectors in the mechanical ventilation systems

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Evaluation on the performance of the probe type (duct type) smoke detectors in the mechanical ventilation systems


Author: Ng, Ka-wah
Title: Evaluation on the performance of the probe type (duct type) smoke detectors in the mechanical ventilation systems
Year: 2001
Subject: Fire detectors
Hong Kong Polytechnic University -- Dissertations
Department: Multi-disciplinary Studies
Dept. of Building Services Engineering
Pages: xviii, 91, [164] leaves : ill. ; 30 cm
Language: English
InnoPac Record: http://library.polyu.edu.hk/record=b1597794
URI: http://theses.lib.polyu.edu.hk/handle/200/644
Abstract: Probe type (duct type) smoke detector in the mechanical ventilation system was used as an early warning device for fire protection. However, the performance of these detectors was not clearly understood. Experimental studies on the performance of the duct type smoke detectors were reported. Two types of duct smoke detectors, namely ionization and photoelectric type were investigated. A series of 81 number smoke tests with four numbers of smoke detectors were performed in the site and in the laboratory conditions. A rectangular ductwork of 600 (W) x 300 (H) mm was fabricated for the laboratory experimental works. The response time of the duct smoke detector was measured with various conditions of air duct velocity, temperature, relative humidity, detector's sensitivity level and air sampling tube mounting location. The measurements were also performed with different smoke density inside the ductwork. Smoke was generated by the smoke generator and by burning the joss sticks during the tests. Experimental results showed that smoke detection was only responded when the mounting configuration of the detector sampling tube was set facing the air flow direction. Dilution effect of air was found with increasing air velocities inside the duct. The smoke concentration decreased when the temperature inside the ductwork was increased from 26oC to 30oC during the experiment. Conversely, the smoke density increased when the relative humidity inside the ductwork was increased from 63 % R.H. to 74 % R.H. Various detector actuating times were measured in the site tests and laboratory tests with air velocities inside the ductwork regulated from 1.06 ms -1 to 6.44 ms -1 and from 1.2 ms -1 to 5.5 ms -1 respectively. It was found that the actuating time was dependant on the duct dimension, smoke type and the detector sensitivity level. The corresponding response time of detector was increased with the increase in air duct temperature. However, the actuating time was decreased with an increase in the relative humidity. Results showed that the response times of the photoelectric and ionization type smoke detectors were similar but the photoelectric one was not responded to the smoke generated from burning of joss sticks. Moreover, the sensitivity setting of the detector (Hi/Med/Lo) affected the response time significantly at low air duct velocities. Nevertheless, the readings were found close to each other at velocity over 2.9 ms -1 in the laboratory test. Based on the experimental results, the duct smoke detector was recommended to be installed in the exhaust/ recirculation ductwork as indicated in the Fire Services Department Codes of Practice for Minimum Fire Services Installation and Equipment and Inspection in Hong Kong, which can minimize the adverse effects caused by cooling, heating filtering and high velocity pressure after the supply fan in the downstream of the air handling/ ventilation equipment. Multiple regression studies on the empirical duct smoke detector response time formula showed that factors such as duct velocities, temperature, humidity and the mounting location of the probe detector should be closely monitored.

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