VESDA by Xtralis
VESDA VFT-15 Product Guide
www.xtralis.com
5
2
Principle of Operation
Air samples are drawn from the protected area through a microbore pipe network towards the detector.
Microbore systems normally sample air from the end of the microbore tube via a microbore sampling point.
The pump draws air from the microbore tubes into the detector inlets where the samples are combined,
filtered, and directed to the laser detection chamber. VFT detectors have an Overall position which draws air
equally from all sectors.
The detection chamber consists of a laser beam directed across an optical chamber, through which the
subsample flows. A photodetector built into the optical chamber measures the amount of light scattering from
smoke particles in the air. A clean air sample will cause very little scattering but as the smoke density of the
sample increases, the amount of light directed onto the photodetector will also increase. The light signal is
processed such that it becomes a direct measurement of the amount of light scatter caused by smoke.
Information about laser chamber safety can be found in Appendix E.
If the detected smoke is higher than the preset alarm thresholds in the detector (Alert, Action, Fire 1 and Fire
2), an alarm will be reported. One or more Alarm relays, preset to activate at an alarm threshold will signal the
host panel after a preset time delay. Alarm states are also shown on the display panel, and an audible warning
is given. An optional alarm beacon can also be fitted.
VFT detectors have an additional preset trigger level, Trace, which must be set below the Alert level. When
Trace is activated, the rotary valve will sequentially scan the sectors, in order to determine the source of the
event. While in Trace mode, the default levels for Alert, Action, Fire 1 and Fire 2 are the same as used for
Overall monitoring. If required, different levels for Alert, Action, Fire 1 and Fire 2 may be set for each sector in
the Sector Alarms menu.
2.1
Flow Monitoring
VFT-15 detectors perform flow monitoring of individual sectors and of the combined airflow.
While scanning, the control system monitors for blockages or disconnections of the microbore tubes for each
sector by detecting when the air flow is above or below acceptable flow thresholds. Flow thresholds are
dependant on air flows measured during normalization.
The normalization process enables the detector to learn typical air flow characteristics of the system and sets
these expected flow readings to 100% for each sector. A normalization sequence must be performed at
installation. The detector has default high and low limits and associated delay times, which may be changed
in the Configure menu.
Detailed information on how to design and install an effective pipe network can be found in VESDA Pipe
Network Design Guide and VESDA Pipe Network Installation Guide.
More information on how to design and install an effective microbore network can be found in Appendix B.
