Building Design and Construction Handbook - Merritt - Ventech!

3.40 SECTION THREE
Photoelectric Detectors. These indicate a fire condition by detecting the smoke.
Sensitivity can be adjusted to operate when obscuration is as low as 0.4% per ft.
In these devices, a light source is directed so that it does not impinge on a photoelectric
cell. When sufficient smoke particles are concentrated in the chamber, their
reflected light reaches the cell, changing its resistance and initiating a signal.
These detectors are particularly useful when a potential fire is likely to generate
a substantial amount of smoke before appreciable heat and flame erupt. A fixedtemperature,
snap-action disk is usually included in the assembly.
Combustion-Products Detectors. Two physically different means, designated ionization
type and resistance-bridge type, are used to operate combustion-products
detectors.
The ionization type, most generally used, employs ionization of gases by alpha
particles emitted by a small quantity of radium or americum. The detector contains
two ionization chambers, one sealed and the other open to the atmosphere, in electrical
balance with a cold-cathode tube or transistorized amplifier. When sufficient
combustion products enter the open chamber, the electrical balance is upset, and
the resulting current operates a relay.
The resistance-bridge type of detector operates when combustion products
change the impedance of an electric bridge grid circuit deposited on a glass plate.
Combustion-products detectors are designed for extreme early warning, and are
most useful when it is desirable to have warning of impending combustion when
combustion products are still invisible. These devices are sensitive in some degree
to air currents, temperature, and humidity, and should not be used without consultation
with competent designers.
Flame Detectors. These discriminate between visible light and the light produced
by combustion reactions. Ultraviolet detectors are responsive to flame having wavelengths
up to 2850 A˚ . The effective distance between flame and detectors is about
10 ft for a 5-in-diam pan of gasoline, but a 12-in-square pan fire can be detected
at 30 ft.
Infrared detectors are also designed to detect flame. These are not designated
by range of wavelength because of the many similar sources at and above the
infrared range. To identify the radiation as a fire, infrared detectors usually employ
the characteristic flame flicker, and have a built-in time delay to eliminate accidental
similar phenomena.
(‘‘The SFPE Handbook of Fire Detection Engineering,’’ National Fire Protection
Association, Quincy, Mass.)
3.5.9 Smoke and Heat Venting
In extinguishment of any building fire, the heat-absorption capacity of water is the
principal medium of reducing the heat release from the fire. When, however, a fire
is well-developed, the smoke and heat must be released from confinement to make
the fire approachable for final manual action. If smoke and heat venting is not
provided in the building design, holes must be opened in the roof or building sides
by the fire department. In many cases, it has been impossible to do this, with total
property losses resulting.
Large-area, one-story buildings can be provided with venting by use of monitors,
or a distribution of smaller vents. Multistory buildings present many problems,
particularly since life safety is the principal consideration in these buildings.