_______________________________________________________________
The
HISTORY of VESDA and MONITAIR ™
_______________________________________________________________
1.1
NEPHELOMETRIC SMOKE DETECTION
Ahlquist
and Charlson (1967) describe a large nephelometer and its use in air pollution
monitoring of urban environments, particularly for recording smog levels.
The device was improved and produced commercially in USA.
In
1970 the Commonwealth Scientific & Industrial Research Organisation (CSIRO)
used the nephelometer to conduct research into forest fires.
This was installed in a light aircraft and flown through smoke plumes,
principally during a bushfire outbreak in the Karri forests south of Perth
(Western Australia). During these
flights CSIRO collected data to calibrate the nephelometer using absolute
filters to weigh the mass of smoke monitored at each run.
They were able to correlate these readings with visual range and light
obscuration. The span of smoke
concentration was some 20 to 240 mg/m3
in a linear relationship. This
corresponded to a visual range span of 40 to 4 km respectively in a
hyperbolic relationship.
Subsequently
the Australian Post Office (APO) as it was then known, engaged CSIRO to help
investigate technologies that could prevent service interruption due to fire.
APO provided a test site so that every available type of fire detection
device could be installed, to discover the most appropriate technology for
telephone exchanges, computer rooms and cable tunnels.
The materials of risk were insulated cables in a wide range of sizes,
which were overheated with electric current or hot plates.
CSIRO
suggested that the nephelometer should be used as the benchmark for the APO fire
tests. This was installed to
monitor smoke levels within the return-air ducts of the mechanical ventilation
system, utilising a chart-recorder output display.
At
the conclusion of several weeks of testing, it was discovered that there was not
one commercially-available fire detection technology suitable for preventing
major damage to telephone equipment. The
currently-available detectors operated far too late, well into the fast flaming
stage, when the fire had become almost uncontrollable.
The
one technology that showed great promise was the nephelometer itself. Its available sensitivity was found equivalent to 0.1%/m
obscuration full-scale, with useful indications down to one-tenth this value.
Consequently its sensitivity could be hundreds of times greater than
available smoke detectors. With
this it could detect the early overheating stage of a potential fire, allowing
plenty if time for preventive action before the onset of flame.
As an installed, automatic
fire detector, the nephelometer was unsuitable because of its high cost,
long-term drift, lack of ruggedness and the lack of suitable alarm outputs
(chart recorder only). At the APO
workshops a prototype smoke detector was developed, which had adjustable
detection thresholds to activate an alarm system, and comprised a dual-chamber
design intended to compensate for drift in sensitivity as well as to compensate
for externally-introduced pollution. They then sought industry’s commercial development and
manufacture of this nephelometric smoke detector.
At
this time the broad concept for incipient fire detection was based upon the use
of an ultra-sensitive smoke detector monitoring air from a ventilation duct.
However, it remained to perfect a practical instrument of such
high sensitivity, having high reliability and low cost, and to develop
efficient, reliable and economic methods for gathering and conveying smoke to
the detector, such that the high sensitivity could be properly utilised in a
variety of environments.
1.2
INITIAL DEVELOPMENT
APO
approached Australia’s then-largest electronics manufacturer, AWA to
commercialise the detector. AWA
undertook to investigate this and approached Australia’s largest fire alarm
company, Wormalds for advice. However
Wormalds apparently feared a high false alarm rate from such sensitive equipment
and effectively discouraged AWA.
Some
three years passed and APO became frustrated by a lack of progress. In 1977 they opened a tender for development of the
technology, offering a $60,000 R&D grant, a blanket order for the first 60
products, and access to their patent rights relating to the dual-chamber
version.
Of
the five applicants, IEI Pty Ltd (then employing 50 people) was regarded as too
small. Fire Fighting Enterprises
had good fire industry knowledge and route to market, but lacked a credible
R&D capacity. British Aerospace
Australia had a good R&D capacity but lacked good fire industry knowledge.
APO therefore encouraged FFE and BAA to present a joint venture
submission, which was accepted.
However,
various individuals at APO and CSIRO were dissatisfied, fearing that the AWA
mistake would be repeated. They
preferred a small, flexible company to take-on the challenge. While giving IEI moral encouragement, they could offer no
financial support or blanket order, nor access to the dual-channel patent
rights.
Two
years later the FFE/BAA detector was launched at a price of $7000 each, and 60
units were delivered to APO. This
unit was of high quality (aviation standard), but the first IEI model had been
launched some six months earlier, at less than half the price.
Although
APO/Telecom National Headquarters felt contractually bound not to purchase IEI
units for five years, almost all of the original 60 FFE/BAA units were never
installed and no further deliveries were made.
It was primarily the support given to IEI by the State Electricity
Commission of Victoria, who had originally planned to manufacture their own
version of the detector, that gave IEI their initial market of any size.
Apart from a few sites within the Victorian Region, widespread adoption
of the system by Telecom came later. Ultimately
Telecom/Telstra became Australia’s largest single user.
To
provide the necessary reliability, features, miniaturisation and reduced cost
for export markets, during 1982 the detector was completely re-designed from
scratch at IEI, incorporating numerous patented inventions that were filed in
Australia and throughout the developed world (Cole, 1983 a to g).
The
world market for aspirated smoke detection was pioneered by IEI from 1983
onwards, to a very sceptical and conservative fire industry.
Marketing persistence and improving technical performance maintained the
company’s market leadership and gained more than 50,000 sites for the system
by mid 1998. From 1990 and again
from 1993 this success had stimulated direct competitors (being former IEI UK
distributors or employees), producing alternative brands of aspirated smoke
detector and capturing a fairly static 20-25% total market share between them.
1.3
ASPIRATED SMOKE DETECTION
The
new concept of aspirated smoke detection (as developed at IEI) involves the
forced induction and transport of a continuous sample of air to a single
detector. This air sample must
faithfully represent the air quality throughout a designated fire zone, which
may be as large as 2,000m2.
The
IEI detector was a form of nephelometer (air pollution monitor) retaining its
unusually high sensitivity, typically hundreds of times higher than conventional
smoke detectors. A true
nephelometer maintains this high sensitivity across the full spectrum of smoke
particle sizes or types that are produced at any stage of fire development,
possibly involving a wide range of combustible materials. Such high sensitivity is required for two purposes:
(a)
to detect the earliest traces of airborne particles or aerosols released due
to the overheating (decomposition or pyrolysis) of materials, and
(b)
to overcome the dilution of this “smoke” caused by the predominantly
“fresh” air within the zone.
A
modern aspirated smoke detection system includes a number of small-bore pipes
distributed across a ceiling (above or below).
Sampling holes (pipe-wall orifices) are drilled into each pipe at
suitable intervals. Air is
continuously drawn into the pipework via all of the holes, towards the centrally
located detector using an air suction pump (aspirator).
The
location of the pipework and the holes is generally governed by local fire codes
and standards such as Telecom Australia TPH1525 (1995) and Australian Standard
AS 1603.8 (1996). Typically, the
pipes and holes are laid out according to a square grid pattern that places each
hole where a conventional point detector would otherwise be located.
In the case of higher risk areas, the spacing of this grid pattern is
reduced for denser coverage. Such
a square grid is illustrated in Figure 1.1 (being a reflected ceiling plan).
A
particular advantage of aspirated smoke detection is the high prospect of
aggregation - that is, smoke from one source may enter several sampling holes at
once, thereby increasing the smoke concentration (reducing the dilution) and
causing an earlier warning. Aggregation
cannot occur with conventional point detectors.
Sampling
pipework is now being installed in a wide variety of configurations. If the pipework is at ceiling level, sometimes it is
surface-mounted in full view. In
areas where aesthetics dictate concealment of the system, the pipework is
mounted above the ceiling, with flexible “capillary” tubes each coupling the
pipe to a ceiling-mounted nozzle (which penetrates the ceiling). This
nozzle is much smaller than a conventional point detector, and is often made
invisible.
Figure
1.1 -
A modern, grided pipe layout for a typical fire zone (IEI, 1991)
1.4
THE NEED FOR ONGOING RESEARCH
Until
recent years, aspirated smoke detection system pipework was generally designed
in close cooperation with the manufacturer.
Nowadays, given the large annual volume of installations (with 50,000
systems installed by mid 1998), and a growing self-reliance within the fire
industry, there is increasing concern to ensure that the growing number of
system designers, installers and maintainers exercise sufficient competence to
ensure system reliability. This is
especially important because in most sites employing aspirated smoke detection,
it is the only form of active fire protection provided.
In
response to this concern, there has been increasing pressure for the provision
of approved system design tools (software).
AS 1603.8 - 1996 calls for an assessment of the design tool as an
essential part of new product approvals. Whereas
Australia has led the world in developing such Standards, both Factory Mutual
and Underwriters Laboratories in USA now have a similar requirement.
All
of this should be understood within the context of Performance Based Building
Codes, which are emerging worldwide, as an alternative to prescriptive codes.
This has the effect of placing increasing demands upon the competency
with which building systems are designed. The
end result will be to place greater emphasis on risk management techniques
applied specifically to each individual building, to obtain the most
cost-effective package of fire prevention and mitigation technologies.
1.5
THE DETECTOR
Cole
(1983 a to f, 1991 a, 1992 a & b, 1995 a & b) patented a series of 11
groups of inventions in Australia (extended worldwide).
These improved the nephelometer concept specifically for fire industry
applications, affecting its reliability, miniaturisation, fluid kinematic loss,
solid-state detection, signal processing, controls, displays, battery standby
operation, ruggedness, cost reduction, and volume manufacture.
This development resulted in the first practical, reliable and affordable
instrument for aspirated smoke detection, suitable for export.
Although
the nephelometer itself had originally been known as VESDA® (Very Early-warning Smoke Detection Apparatus), this term
subsequently became understood to embrace the complete aspirated smoke detection
system which includes the air-sampling pipework, pump, dust filter, associated
hardware, and controls (not including the fire alarm indicator panel or the
monitoring bureau).
Figure
1.2 -
Nephelometer sensitivity
1.6
ASPIRATION TECHNIQUES
To
describe the various air sampling arrangements then envisaged, Cole (1982)
commissioned a series of illustrations. Figure
1.6 shows a computer room with
suspended floor plenum, including a separate reference detector used to monitor
incoming “fresh” air quality (its reading being subtracted from that of the
other detector). Figure 1.7 shows
three detectors in a return-air ventilation duct application, Figure 1.8 shows
equipment racking (using holes in the pipe to draw-off samples of any rising
smoke) and Figure 1.9 shows a method using small flexible tubes extended into
hotel room air conditioners, used for sampling the air that recirculates
throughout the room.
Cole
(1983 g) published guidelines for system design, detailing the impact of pipe
length, pipe diameter, number of pipes, hole size and hole separation upon smoke
transport time for the VESDA Mk2 detector configuration, finding that multiple
pipes offer significantly faster response in a give zone size.
Figure
1.6 - Plenum smoke detection as proposed for “computer rooms, cleanrooms,
control rooms, operating theatres and radio/tv studios using under-floor
ventilation” (Cole, 1982)
Figure
1.7 - Ductwork smoke detection as proposed for “laboratories,
offices, supermarkets, hospitals, institutions, libraries, museums, art
galleries, studios and theatres” (Cole, 1982)
Figure
1.8 - Equipment bay smoke detection as proposed for “electronic racks,
telephone exchanges, cable tunnels, power generators, transformer halls, control
rooms, broadcast transmitters and switchboards” (Cole, 1982)
Figure
1.9 - Compartment or mechanical services smoke detection as proposed for
“hotels, apartments, hospitals, barracks, prisons, dormitories, schools,
trains and ships” (Cole, 1982)
Cole
established an optimum pipe diameter of 21mm and hole size of 2mm,
and a maximum practical pipe length of 100m.
He showed that two such pipes could be used either side of a
centrally-mounted detector, to cover a 200m section of a tunnel, while
four pipes up to 50m long could be employed to cover a large roof space
or floor void. Within the size of a
statutory fire zone, the aggregate length of pipe attached to one detector would
not exceed 200m.
Subsequently,
Cole (1991 a) characterised a range of pipe systems (such as Figure 1.1) in
greater detail. This was done for
the ultimate purpose of designing an efficient aspirator from scratch, optimised
for early smoke detection. He
developed a relationship between average velocity (providing the flow rate
required for aspirator design) and core velocity (affecting smoke transport
time) - a ratio of central importance to system simulation.
Cole’s
research resulted in the design and production of a new volute centrifugal air
pump type of aspirator (used in VESDA Mk3) which has an efficiency 10 times that
of the original muffin fan, exceeding expectations (based upon the literature)
for such a pump of low specific speed (8.4).
In practical terms, the transport time for smoke in a 100m pipe
was reduced from five minutes down to one minute, while the electrical power
input was reduced from 5W down to 2W. This
was achieved by a purely theoretical design methodology without need for an
empirical prototyping phase.
After
IEI’s merger with Vision Systems Ltd in 1995, this new aspirator was
incorporated into the next model detector known as VESDA LaserPlus.
The full-spectrum Xenon lamp design of previous VESDA models was replaced
with a solid-state, infra-red laser light source, which offered improved
maintenance at the expense of reduced sensitivity to small particles.
Current
independent research by Cole has focused upon maintaining the high reliability
of using a solid-state light source, while providing some discrimination of
smoke particle sizes, leading to a better response to the prevailing fire
conditions. His new product is
known as “MONITAIR”.
1.7
MATHEMATICAL SYSTEM MODELLING
Taylor
(1984) is the first known researcher to mathematically model the air flows
through a single sampling pipe within an aspirated smoke detection system.
He wrote a computer program to determine the pressures and flow rates at
all points along the pipe, based upon the VESDA Mk2 pump characteristics and
established theoretical principles.
Taylor
produced the results of seven different pipe arrangements, but none of these was
tested experimentally. In modelling
pipes of 30mm internal diameter, he found that for a 100m pipe
with 11 sampling holes, the maximum transport time was 237sec. Although
such a lengthy delay would be unacceptable by today’s standards, it can be
shown that the more likely result for Taylor’s pipe arrangement would be an
acceptable transport time of 69sec with a not-so-acceptable hole flow (un)balance
of 50%.
Notarianni
(1988) used a different approach. As
distinct from minimising the transport time, a requirement for good balance was
Notarianni’s fundamental objective in system design.
She stated that to achieve balanced flows, the hole diameters must be
increased along the pipe in the direction away from the detector, in accordance
with the reduced local suction pressure.
Notarianni
wrote a computer program known as “SNIFF” to specify hole sizes and
positions. Unfortunately, her methodology assumes that each sampling hole flow
rate can be accurately controlled in a real installed environment.
In practice, the hole size is critical because the flow rate varies with
the fourth power of diameter, at a given pressure differential.
For example, a hole specified as 2mm could have an error in
diameter (due to a blunt drill) of 0.2mm or 10%, resulting in a +46%
error in flow. Despite such
criticality, Notarianni’s computer program specifies hole sizes in steps of
1/64” (which is 20% of 5/64”). This
could result in greater than -50% or +100%
error (binary orders of magnitude), defeating her objective to achieve perfectly
balanced flow and significantly undermining the efficacy of her model.
In
association with developing a new aspirator design, Cole (1991) modelled various
aspirated pipe system configurations. He
wrote a computer program known as “ASPIRE” based upon the known theory
governing flow in pipes, supplemented by experiments.
Cole defined the figures of merit for a system as
the
smoke transport time (which is the worst-case delay for smoke
to arrive at the detector from the least-favorable position),
the
balance (describing the variation in smoke-sensitivity
throughout the system due to the inequality of hole flow rates, and is again
the worst-case figure representing the least-favorable position), and
the
share (being the proportion of the total air flow in the pipe
contributed by all the sampling holes, excluding the end vent).
This last figure of merit is used to avoid excessive
smoke-sensitivity at the end vent compared with a typical hole, noting that
this vent also has a major effect on the smoke transport time.
In
various jurisdictions the time is required to be no more than 90sec
for general applications, although 60sec is often specified for high-risk
areas. In the interest of achieving
reasonably uniform smoke-sensitivity throughout the zone, the balance and
share are each required to be at least 70% (preferably 80%). It should be
noted that for simplicity, all sampling holes in a given pipe are usually
specified at the same size, but as an option, the balance can be set to
100% so that the required diameter of each hole is computed and displayed.
This often requires fine increments in drill sizes that are not generally
available. Therefore as a
compromise, it is also possible in software to set preferred steps in hole
diameter (according to available drill sizes) and to judge the suitability of
the balance obtained.
Subsequently, while retaining all the flexibility of this software, Cole (1999) wrote a more-accurate computer model of aspirated pipe systems known as “ASSIST”, based upon new theory he developed within a PhD research program. This produced a threefold improvement in accuracy over his previous method (1991), avoiding the need for embedded empirical corrections, and offering a much greater understanding of the complex fluid dynamics processes involved as well as higher reliability in the modelling results.